WEBVTT 1 00:00:02.750 --> 00:00:32.109 Mark Kushner: Yeah, I hear you, seminar. It's my great pleasure to introduce today's speaker, the son of Michigan. 2 00:00:32.430 --> 00:00:41.929 Mark Kushner: Dr. Marcel Georgian, who's got his Phd. From the applied physics program, but it did his work in the laboratory for that. 3 00:00:42.260 --> 00:01:00.409 Mark Kushner: Since leaving Michigan he's worked at the Naval Research Laboratory versus a postdoc and then flipped that. And now, as a staff researcher, technically, I asked them to give me research scientists. And then they said, No, because I'm in an engineering division. 4 00:01:00.670 --> 00:01:14.450 Mark Kushner: Marcel has worked on a number of programs since leaving Michigan, and one area in particular, that we have quite a bit of topical interest right now is exploring cathodes and electron sources that are more compatible with 5 00:01:14.610 --> 00:01:22.010 Mark Kushner: propellant. But yeah, well, with every other competitors. 6 00:01:22.470 --> 00:01:32.179 Mark Kushner: With all that being said, it's a great pleasure to have you here, and thank you for joining us, and I will bestow upon you this ceremonial mipsy. Thank you. So, all right, I'll add to my collection. 7 00:01:33.820 --> 00:01:34.847 Mark Kushner: Thank you, everybody. 8 00:01:36.280 --> 00:01:39.140 Mark Kushner: Please take it away all right. Thank you. Ben. 9 00:01:39.711 --> 00:01:46.789 Mark Kushner: So the title of my talk here today is Rf plasma cathodes and other research activities in Plasma Propulsion group at Nrl. 10 00:01:47.270 --> 00:01:52.790 Mark Kushner: I'm hopeful that we'll get to the other research activities. But I did kind of focus on the rf, cap notes. 11 00:01:54.259 --> 00:01:55.389 Mark Kushner: So 12 00:01:56.300 --> 00:02:13.950 Mark Kushner: start off. We're going to talk a little bit about me. So, as Ben mentioned, I was here at the University of Michigan and did my applied physics. Phd. But 1st I was at Mcgill University to my bachelor's in Science in Physics, but I'm from Ohio, which I know in this crowd is maybe not the greatest thing. 13 00:02:14.050 --> 00:02:18.990 Mark Kushner: but on the top left there. I've kind of got the little stars of the different places that I've lived 14 00:02:19.555 --> 00:02:23.510 Mark Kushner: and then currently at the Naval Research Laboratory in DC 15 00:02:23.800 --> 00:02:31.780 Mark Kushner: on the top right, there are different examples. I can see that the it's not hmm 16 00:02:33.460 --> 00:02:41.410 Mark Kushner: anyways. I I can see that. The you can see a bunch of different examples, different things that I've worked on. Oh, yeah, perfect 17 00:02:41.995 --> 00:02:47.949 Mark Kushner: primarily in electric propulsion some examples of hull thrusters and thermionic cathode electron sources. 18 00:02:48.350 --> 00:03:06.040 Mark Kushner: And then the bottom right quadrant. There is to show that I'm also a human being that I do other things than just research. So I love to ski, although I haven't done it quite as much recently. Now that I have 2 kids. My one source of exercise is soccer on Wednesday nights, and then on Sunday mornings, I like to watch Formula One racing 19 00:03:06.300 --> 00:03:22.760 Mark Kushner: so a little bit about me. So now where do I do? Where do I spend 8 HA day or more of my time? So it's at the United States Naval Research laboratory. So it's about 15 to 20 min from downtown Washington, DC. A little dot there. 20 00:03:23.490 --> 00:03:46.830 Mark Kushner: And so the mission of the Naval Research Laboratory kind of is born out of this idea of Gosh, Thomas Edison, so that the Government should maintain a great research laboratory for military and naval progression to cut the costs. Basically. So we're supposed to be cheap. Only he has said that about the government 21 00:03:47.890 --> 00:03:48.709 Mark Kushner: but 22 00:03:48.920 --> 00:03:59.020 Mark Kushner: the Naval Research Laboratory was was founded in 1923 to basically out of that vision by the navy on the banks of the Potomac. So this is a picture of how we started. 23 00:03:59.380 --> 00:04:14.800 Mark Kushner: And then, since then, we've grown to about a billion dollar organization doing research with about 50% of our employees being Phd conducting basic and applied research. And then the mandate is really to look at everything from the depths of the ocean to the far reaches of space. So that's almost everything. 24 00:04:17.350 --> 00:04:26.279 Mark Kushner: so with that said within the Naval Research Laboratory. There are several divisions. I work at the Naval Center for space technology in the Spacecraft and Engineering Division. 25 00:04:27.000 --> 00:04:47.639 Mark Kushner: And so the Naval center for space research and projects is deep at the Naval Research Laboratory. So, starting in the fifties Project Vanguard, we were part of the team that put the 1st Us. Satellite on orbit, and they have the flight spares at the Naval research lab that you can literally go hold 26 00:04:47.970 --> 00:04:56.760 Mark Kushner: other 1st surveillance satellites in the sixties is something that they did in the seventies. They were naval research laboratories, heavily involved in the development of GPS. 27 00:04:57.547 --> 00:05:11.530 Mark Kushner: Then it's the tactical communication satellites. And then, most recently, in the 2,010 s. Working on robotic servicing of geosynchronous satellites. So this one is actually this guy here should be launching relatively soon. 28 00:05:13.033 --> 00:05:22.429 Mark Kushner: So we have a long history in space, technology and development. But I come from the propulsion group. So yet another level below. 29 00:05:22.750 --> 00:05:37.449 Mark Kushner: and our group kind of started building. You know, custom propulsion systems to meet a variety of different missions, but lately, probably in the last and probably the last 10 years have really focused on technology development 30 00:05:38.320 --> 00:05:45.009 Mark Kushner: and research. So really, these days, we focus on electropropulsion no longer really doing much chemical propulsion. 31 00:05:45.280 --> 00:06:04.590 Mark Kushner: And these here are kind of a list of areas that we have active research in. So electron sources for Hall and Ion thrusters which we'll talk a little bit about today, hall thruster testing or thruster testing all thrusters, for example, other advanced concepts, developing plasma diagnostics, not just for thrusters, but also for measuring patient space 32 00:06:04.850 --> 00:06:10.930 Mark Kushner: and as well as environmental simulation for materials testing in terms of space environment, hypersonics as well. 33 00:06:11.550 --> 00:06:14.239 Mark Kushner: We like to think ourselves of people that are 34 00:06:14.480 --> 00:06:23.300 Mark Kushner: capable of generating and diagnosing energetic flows more broadly. This kind of collection of works serves that need. 35 00:06:24.550 --> 00:06:43.649 Mark Kushner: Our group looks something like this. So you have myself there. I'm in the middle, and then you have Mike Mcdonald, Logan Williams, Jack Brookton, Nolano, who are the government staff scientists, and then we have a variety of test engineers and postdocs, Jeff Bynum, Logan, Fernandez Leanne Sue, who is also a Michigan grad, as as was Mike Mcdonald. 36 00:06:44.340 --> 00:06:46.339 Mark Kushner: And Anna Shepherd as well 37 00:06:46.690 --> 00:07:06.769 Mark Kushner: and on the right. There, as you can see, we host loads of interns from a variety of institutions. I only see a few names from the University of Michigan, and we do a good job overall in the State of Michigan. But I feel like Western Michigan is pulling ahead, so I feel like there should be a little more effort here from the great University of Michigan. 38 00:07:08.870 --> 00:07:18.860 Mark Kushner: so what I want to show here is kind of the centerpiece of our lab which just started up, or this was just delivered a few about a month ago. 39 00:07:19.160 --> 00:07:21.890 Mark Kushner: so we call it the large plasma test facility. 40 00:07:22.060 --> 00:07:25.959 Mark Kushner: just to not name, infringe the large vacuum test facility. 41 00:07:26.362 --> 00:07:34.229 Mark Kushner: You can see here getting delivered at the Naval Research Lab. So it's not as big as the one they have here, but kind of the 42 00:07:34.340 --> 00:07:37.499 Mark Kushner: the key value. And here you can see some of our group members 43 00:07:37.840 --> 00:07:48.120 Mark Kushner: is that it's got several antechambers that are connected to the main vacuum chamber that allow for parallel testing of a variety of things. And so the way we like to think of it as a good enough vacuum chamber. 44 00:07:48.380 --> 00:08:01.639 Mark Kushner: the good enough facility that'll allow us to get to the 80% solution pretty quickly, and having our smart people working in parallel and taking advantage of the large facility, but on the smaller side, on the smaller scale experiments. 45 00:08:01.860 --> 00:08:07.730 Mark Kushner: And then, when we need the really large vacuum facility, maybe we'll come here to Michigan and and run the test. 46 00:08:08.925 --> 00:08:09.850 Mark Kushner: So 47 00:08:10.320 --> 00:08:26.279 Mark Kushner: that kind of takes care of who I am, where I'm from, more or less what I do. But now we're going to talk a little bit more specifically about some of the research that I'm involved in. And so the topic for today is going to be Rf. Cathodes, for in situ resource utilization. And I'll cover a little bit about what that means. 48 00:08:27.040 --> 00:08:49.369 Mark Kushner: But first, st let's talk about what are electron sources for electropropulsion. So here I have a picture of a hull thruster. I'm not going to go into the details of how a hull thruster works. But for this situation, the critical thing to know is that a electro propulsion system tends to generate a beam of ions, and as a result, if you do not neutralize those ions your spacecraft will charge up by the release of positive charge. 49 00:08:49.670 --> 00:09:02.080 Mark Kushner: And so we need these electron sources to basically match the amount of current that we're generating in beam as well for generating the plasma and things like that. But by and large this is the purpose of electron sources for electric propulsion systems. 50 00:09:03.050 --> 00:09:11.999 Mark Kushner: And so a typical picture is that this thruster will get fed. Some amount of propellants, typically xenon for electric propulsion or some kind of noble gas. 51 00:09:12.140 --> 00:09:20.629 Mark Kushner: and we'll feed it both to the channel, which is the part that really produces the thrust, and we'll feed a little bit of it to the cathode to be able to generate the electrons 52 00:09:21.730 --> 00:09:28.289 Mark Kushner: within this cathode. The standard technology that we work with today is what we call a thermionic cathode. 53 00:09:28.550 --> 00:09:35.390 Mark Kushner: So this year I've stolen a picture out of the book by by Dan Gobel, Ira Katz. And now, Youngos Michelides. 54 00:09:35.800 --> 00:09:48.690 Mark Kushner: but the essential idea is that you're feeding a little bit of that propellant. You have this thermionic material, which is a special material that has what we call a low work function. Basically, it is going to take heat. 55 00:09:48.820 --> 00:09:55.779 Mark Kushner: to boil out the electrons into the gas phase, and those electrons will then interact. 56 00:09:56.100 --> 00:10:15.659 Mark Kushner: Imply heat. Electrons will interact with the gas to generate a plasma, and then we can extract those electrons overcoming space charge limits through as current electrical current. And this current is what ends up getting fed to the ion beam and to the rest of the thruster to make it all on. So it is a really essential part of the electropropulsion system. 57 00:10:16.367 --> 00:10:21.279 Mark Kushner: One thing to note here is that a typical work function is around 2 electron volts. 58 00:10:21.702 --> 00:10:30.540 Mark Kushner: and this is a kind of a key figure of merit for these systems. And kind of as a result, ends up kind of scaling when we go and compare with the Rf. Style. 59 00:10:32.060 --> 00:10:33.270 Mark Kushner: Now 60 00:10:33.920 --> 00:10:42.510 Mark Kushner: we're interested in in situ resource, utilization, and that kind of ties into this idea that Ben brought up this use of other kinds of propellants. 61 00:10:43.440 --> 00:11:03.200 Mark Kushner: And so what does that really mean? Well, say that we had some kind of base on the moon where we have people going around, and we need to be able to move things around in space around the moon, or we want to do Deep Space Mission to asteroids, and we'd be able to recover some propellants to come back, or some some sort of Conops like that 62 00:11:03.790 --> 00:11:16.149 Mark Kushner: turns out that there's a fair amount of water, for example, in in space in the solar system, and that is a potential useful propellant for doing lots or a potential useful material for doing lots of things. So we can. 63 00:11:16.400 --> 00:11:27.970 Mark Kushner: you know, make ourselves some water to drink. If we have people flying around we can use it for precision landing. We can use it, for, you know, electrical storage things like that. 64 00:11:28.140 --> 00:11:33.889 Mark Kushner: So one of the ideas is that we also want to try to use it for for propellant, for electric propulsion. 65 00:11:34.230 --> 00:11:41.600 Mark Kushner: And so the question that we were asking ourselves is, how can we make, you know, today's technologies, for example, hull thrusters that we like to use today. 66 00:11:41.930 --> 00:11:52.480 Mark Kushner: really compatible with this approach of in situ resource utilization, where we want to use, basically live off of the land and do everything that we can to not have to ship things from the earth in order to make our systems run. 67 00:11:54.214 --> 00:12:00.940 Mark Kushner: So a good example here is, instead of Xenon, we decided to run H. 2 0. Into the system. 68 00:12:01.310 --> 00:12:17.419 Mark Kushner: what would happen, and it turns out that you can more or less run H. 2 0. Into the thruster. And there aren't too many issues there per se. But if you were to explicitly try to run water into a thermionic cathode, the issue actually really ends up being the oxygen. 69 00:12:17.840 --> 00:12:40.300 Mark Kushner: So these thermionic emitters. They like to release electrons. Oxygen is a highly electronegative particle. So when I generate a plasma in here that H. 2 0. Is going to break down into hydrogens and oxygens, and that oxygen is going to react with the surface of my cathode. And it's going to basically destroy that work function, that special property that makes it great at boiling out electrons when you apply heat. 70 00:12:41.010 --> 00:12:51.539 Mark Kushner: And here's kind of the quintessential example is even at 20 parts per 1 million of oxygen fed into what would be a state of the art material lanthan hexaporide for thermionic emission. 71 00:12:51.720 --> 00:13:02.880 Mark Kushner: At 20 parts per 1 million you lose about 75% of the emission capability. So if you run the whole thing on water, for example, you're basically not going to be able to get anything out of it. 72 00:13:03.960 --> 00:13:09.829 Mark Kushner: So what can we do? Really, we need a a new technology to try to enable this kind of mission. 73 00:13:10.230 --> 00:13:13.710 Mark Kushner: And it kind of breaks down, particularly at this cathode problem. 74 00:13:16.185 --> 00:13:22.010 Mark Kushner: So our our goal here is to come up with basically a propellant agnostic electron source. 75 00:13:22.280 --> 00:13:29.450 Mark Kushner: And so when we kind of started this project we went and browsed the literature to try to find what are other people doing? 76 00:13:30.270 --> 00:13:40.789 Mark Kushner: And so the 1st one that came up was what they called an inductively coupled plasma cathode where they use Rf. Power to generate a plasma and try to extract some of the electrons. 77 00:13:41.570 --> 00:13:45.240 Mark Kushner: We found these microwave, or these Ecr cathodes, where 78 00:13:45.690 --> 00:13:53.270 Mark Kushner: they kind of use microwave power and magnetic fields to resonate with electrons, to generate a plasma and then extract the electrons. 79 00:13:54.330 --> 00:14:17.639 Mark Kushner: and then there are a couple of other variants that I've plotted here as a function of cathode current on the X-axis, and then the total input power that they required on the Y-axis, and you can see a couple of trend lines here where the Ecr type devices that people were creating ended up at about 180 watts per amp. So that's a useful figure of merit. That's the slope there. 80 00:14:19.480 --> 00:14:28.770 Mark Kushner: or 90 watts per amp for the Icps. And then the microwave Cathodes actually were the best where they cost the least amount of power per amp. 81 00:14:28.960 --> 00:14:31.850 Mark Kushner: 71 watts per amp. 82 00:14:32.640 --> 00:14:34.499 Mark Kushner: But it turns out that 83 00:14:34.790 --> 00:14:53.339 Mark Kushner: the large magnetic fields that you need to do for electron cyclotron resonance are a little bit challenging to generate in the 1st place, and then all thrusters have their own very specific magnetic field that they need to generate in order to be very efficient. And so these Icp Cathodes, you don't need a magnetic field inherently to make them run 84 00:14:54.565 --> 00:15:02.709 Mark Kushner: so they were kind of the the area or the technology of choice that we've focused on for the really for those integration reasons. 85 00:15:02.930 --> 00:15:16.520 Mark Kushner: The other thing I wanted to note in the plot here, though, is kind of that grayed out area, or sorry the this dot here, and this is where a typical thermionic cathode lives in terms of performance, and that's going to be about 20 watts per amp. 86 00:15:16.980 --> 00:15:21.830 Mark Kushner: So those are the figures of merit when comparing across technologies. 87 00:15:23.490 --> 00:15:31.020 Mark Kushner: So we've kind of focused on this technology and trying to understand it and trying to see how you could implement it more practically in the lab. 88 00:15:32.490 --> 00:15:41.480 Mark Kushner: So the Rf cathode fundamentally looks something like this, so it involves an inductive coil 89 00:15:41.910 --> 00:15:58.950 Mark Kushner: where you there, I guess there are a couple parts here. There's the inductive foil, there's the actual cathode body. This is the electrode that you will apply a bias to, and then there's an anode which is typically downstream of the device that represents the the thruster in in the test. 90 00:15:59.880 --> 00:16:04.560 Mark Kushner: But basically we'll have current flowing from the cathode to the anode in the system. 91 00:16:04.920 --> 00:16:18.199 Mark Kushner: This 1st step is to introduce a time varying current in the inductive coil. This generates a magnetic field that sloshes around this will whip the electrons around. I think it's Faraday's law. 92 00:16:18.973 --> 00:16:20.180 Mark Kushner: And then 93 00:16:20.930 --> 00:16:27.510 Mark Kushner: once the electrons get enough energy, they will start to ionize the gas and turn it into what we call an Rf. Plasma. 94 00:16:28.530 --> 00:16:44.170 Mark Kushner: Once we then apply electrical bias, you'll have ions, which are my big fat blue dots, and then the electrons are. The little yellow dots will start going to each electrode, the electrons going to the anode and the ions going to the cathode. 95 00:16:44.560 --> 00:16:53.740 Mark Kushner: So this is the basic picture that we have in our minds when thinking about these kinds of devices essentially, and one of the reasons that 96 00:16:54.520 --> 00:16:58.150 Mark Kushner: you'll see some of those numbers back here a few slides. 97 00:16:59.700 --> 00:17:09.359 Mark Kushner: One of the reasons why the Watts per ampere tends to be higher for all these microwave or Icp or ecr devices compared to the thermionic cathode 98 00:17:09.849 --> 00:17:20.639 Mark Kushner: comes from the the basic principle that we're ionizing things in the gas phase as opposed to releasing them from the material surface. So that work function was about a 2 volt. Cost 99 00:17:20.750 --> 00:17:22.579 Mark Kushner: to produce an electron. 100 00:17:22.890 --> 00:17:36.169 Mark Kushner: whereas to ionize any kind of propellant is going to be on the order of 10 to 20 ev of the ionization potential, and that that general scaling tends to hold and overall in the in the performance of these devices. 101 00:17:38.970 --> 00:17:46.789 Mark Kushner: So I guess. Kind of following up on on these the Rf cathodes. 102 00:17:47.680 --> 00:17:54.849 Mark Kushner: So they are essentially a a potential propellant agnostic device that you could basically drop in 103 00:17:55.010 --> 00:18:05.419 Mark Kushner: the replacement for the thermionic cathode. What we know from previous experiments is that they're relatively power hungry, largely because you have to pay that ionization potential per electron. 104 00:18:06.450 --> 00:18:19.040 Mark Kushner: Previous studies have largely focused on Argon and Xenon, which are noble gases. And a lot of the work that we did kind of started there as well. But we'd like to see how do they perform on something like water, for example. 105 00:18:19.370 --> 00:18:25.960 Mark Kushner: and there's a little bit of theory on how these things function, but not enough to really be able to design one from the ground up. 106 00:18:27.900 --> 00:18:36.750 Mark Kushner: So kind of the essential questions that we've been trying to answer is, how can we develop some of the theoretical tools to describe and and predict their overall? 107 00:18:39.344 --> 00:18:41.840 Mark Kushner: So where we started 108 00:18:42.010 --> 00:18:57.910 Mark Kushner: came from? Essentially noticing this similarity between what my Rf. Cathode looks like, which looks like this, a cathode that is largely, and encapsulating in our plasma, and an anode through an orifice and a asymmetric double probe. 109 00:18:58.520 --> 00:19:06.939 Mark Kushner: So, asymmetric double probe. The key difference is that the one leg of your probe is bigger than the other leg. 110 00:19:07.240 --> 00:19:11.020 Mark Kushner: and this asymmetry drives a slightly different behavior. 111 00:19:12.086 --> 00:19:17.089 Mark Kushner: But we we looked at this and said, well, our Rf. Cathode 112 00:19:17.200 --> 00:19:29.640 Mark Kushner: could maybe be treated like a asymmetric double probe in kind of an extreme case where the cathode is really really big, and the anode looks pretty small, probably because we're pulling directly through that orifice. 113 00:19:30.950 --> 00:19:42.739 Mark Kushner: And so what is convenient is that there's been a lot of work done on asymmetric double probes and trying to understand those Iv traces, particularly for trying to diagnose plasmas, extract information like density and temperature. 114 00:19:44.022 --> 00:19:57.770 Mark Kushner: And so there are some fundamental expressions that you can basically then plot the Iv curve. So the current that flows through the probe versus the bias potential that you put through the different legs. 115 00:19:58.080 --> 00:20:13.760 Mark Kushner: And you can see that as you change the size of the probe. So that's this parameter. A is that area ratio as you make it bigger and bigger and bigger on one side, basically, you have the same amount of current, no matter what, that you can collect the other side, you can start to collect more 116 00:20:15.623 --> 00:20:18.189 Mark Kushner: and physically, that's happening, because 117 00:20:19.730 --> 00:20:27.320 Mark Kushner: The the ends of these curves are always limited by the amount of ions that you can collect on the other side, because the electrons are incredibly mobile. 118 00:20:27.820 --> 00:20:39.389 Mark Kushner: And so, because and because of that, as you make the the cathode side bigger, you can start to pull more ions to them just by a sheer area, and that allows for more electrons to end up going to the anodes. 119 00:20:41.531 --> 00:20:44.299 Mark Kushner: So we basically took this theory. 120 00:20:44.400 --> 00:20:57.059 Mark Kushner: the probe theory and just tweaked it a little bit included a couple extra parameters where we could potentially allow for differences in the plasma density over the cathode surface versus the anode surface 121 00:20:58.460 --> 00:21:05.000 Mark Kushner: and then the the area ratio as well. That parameter a and then I cast it specifically in terms of 122 00:21:05.530 --> 00:21:14.479 Mark Kushner: discharge. Voltage equals. Some term is is in terms of an electron temperature. And then as a function of current. I 123 00:21:14.630 --> 00:21:22.369 Mark Kushner: because often in electro propulsion systems, the thruster demands a certain amount of current out of the cathode to neutralize the ion beams. 124 00:21:22.540 --> 00:21:26.010 Mark Kushner: And so when we run cathode tests, we're typically 125 00:21:26.160 --> 00:21:30.550 Mark Kushner: demanding a certain amount of current out of the cathode. And the voltage is kind of a result. 126 00:21:31.050 --> 00:21:32.510 Mark Kushner: Why, I cast it this way. 127 00:21:33.145 --> 00:21:42.830 Mark Kushner: But one of the key things to note here is that the discharge voltage is going to scale with the electron temperature as a as a critical parameter. 128 00:21:43.070 --> 00:21:45.989 Mark Kushner: And then what's a little bit encapsulated here is that this 129 00:21:46.210 --> 00:21:52.129 Mark Kushner: term? I is really the overall current running in the system normalized by the ion saturation current. 130 00:21:52.490 --> 00:22:03.109 Mark Kushner: And so when you have this term here on the bottom one minus, I, you have a kind of a limit where, as the discharge current approaches the ion saturation current, this whole thing tends to blow up. 131 00:22:04.020 --> 00:22:11.960 Mark Kushner: And fundamentally, what that's saying is that as I get to the Ion saturation current. There are no more ions for me to collect. I've sucked them all in, and so I cannot do more than that. 132 00:22:14.500 --> 00:22:25.379 Mark Kushner: Another key result here is that if you take this and basically normalize by the electron temperature, the the voltage, then you have a basically a completely normalized 133 00:22:25.530 --> 00:22:28.019 Mark Kushner: a normalized expression. 134 00:22:28.700 --> 00:22:35.539 Mark Kushner: So the way I read this is basically for a fixed geometry, I should expect that all of my 135 00:22:36.073 --> 00:22:41.030 Mark Kushner: Iv traces that I produce at different conditions, flow rates and powers 136 00:22:41.380 --> 00:22:49.020 Mark Kushner: should basically all collapse onto a single curve. As a result of this, if this is a good description for how the cathode functions. 137 00:22:51.356 --> 00:23:00.529 Mark Kushner: So for this to work, we're really, this is really a sheath theory like, how do the electrons and ions interact with the surfaces here. 138 00:23:00.690 --> 00:23:09.819 Mark Kushner: But these these expressions depend on both plasmas properties like electron temperature. And in here the plasma density. So how do we get those? 139 00:23:10.190 --> 00:23:13.230 Mark Kushner: And we started with a pretty simplified approach. 140 00:23:13.400 --> 00:23:15.819 Mark Kushner: where we basically did a global model. 141 00:23:16.570 --> 00:23:20.539 Mark Kushner: So we opened up our lieberman textbook and found the right one. 142 00:23:20.810 --> 00:23:25.820 Mark Kushner: And and so the one that they have there is the argon global model. And so we basically follow that 143 00:23:26.450 --> 00:23:34.709 Mark Kushner: it consists of basically a power balance and a particle balance where power goes into ionization and thermal energy for the electrons. 144 00:23:34.930 --> 00:23:40.520 Mark Kushner: and then the particles are also kind of balanced through ionization, and then the loss of particles to the walls. 145 00:23:41.682 --> 00:23:54.329 Mark Kushner: But if you kind of boil it down. The key takeaways from these expressions to know is that we expect that the density, and the therefore the Ion saturation current to largely scale linearly with the applied power. 146 00:23:56.050 --> 00:24:02.110 Mark Kushner: the applied power that you put into the system, and that for the for a lot of 147 00:24:02.260 --> 00:24:11.270 Mark Kushner: conditions. We expected the electron temperature to stay relatively constant. But it's not super sensitive. And the reason why we expect that is because 148 00:24:11.640 --> 00:24:15.660 Mark Kushner: the Ionization Cross section here is an exponential term. 149 00:24:15.830 --> 00:24:26.260 Mark Kushner: an electron temperature. And so any small changes over here, or even large changes over here can immediately be compensated by small changes in electron temperature. 150 00:24:29.850 --> 00:24:34.120 Mark Kushner: Alright. So first, st we started with a couple simulations. 151 00:24:34.600 --> 00:24:44.089 Mark Kushner: we took our global model and we calculate that power balance and particle conservation to solve for a plasma density and temperature. 152 00:24:44.340 --> 00:24:46.810 Mark Kushner: And then we also have to include 153 00:24:46.930 --> 00:24:54.990 Mark Kushner: some other factors like our coupling efficiency of Rf power. There's a scaling factor for effective temperature 154 00:24:55.280 --> 00:25:09.279 Mark Kushner: where we're expecting because of our geometry, and we have electrodes in close contact with both plasma as well as the inductive coil that you can get some effective enhancement of electron temperature at the sheath edge, due to stochastic heating of electrons. 155 00:25:10.544 --> 00:25:14.620 Mark Kushner: And so we kind of picked a number there that looked reasonable out of the literature. 156 00:25:15.674 --> 00:25:23.740 Mark Kushner: We have some experimental parameters like the area ratio A, which for our setup was about 7, 55. 157 00:25:24.500 --> 00:25:37.559 Mark Kushner: And then, because I've been thinking of this as the cathode kind of has a a clear surface that is in contact with the plasma. The anode is a little bit less clear. I've been basically treating the anode as 158 00:25:37.980 --> 00:25:39.170 Mark Kushner: the orifice. 159 00:25:39.290 --> 00:25:53.669 Mark Kushner: And so I've been using that area. And as a result. I've been thinking of the plasma density as being about the same. Because I'm thinking, if I if I treat this position as the anode, then the plasma that is, that it is in contact with is about the same 160 00:25:54.230 --> 00:25:56.270 Mark Kushner: as what is in contact with the Catholic 161 00:25:58.490 --> 00:26:03.439 Mark Kushner: So with those in mind, basically, we solve the the sheath properties. 162 00:26:03.580 --> 00:26:08.629 Mark Kushner: and then we get the discharge current, the discharge voltage as a function of discharge current. 163 00:26:09.060 --> 00:26:18.030 Mark Kushner: We'll see a lot of curves that look like this. And I'll explain the Red X's on the curve in a little bit. But the y-axis here is the voltage that needed to be applied to extract a certain amount of current 164 00:26:19.954 --> 00:26:30.009 Mark Kushner: here. I'm plotting that Watts per Amp, where I've taken the Rf. Power that we've put in plus the the DC power that gets put in divided by the amount of current that I can get out. 165 00:26:30.420 --> 00:26:35.659 Mark Kushner: And basically, you see, these curves start to fall down towards the minimum and then sharply rise. 166 00:26:35.970 --> 00:26:45.490 Mark Kushner: What's happening? And here this explains the Red X's where I've identified the minima, basically, the most efficient points of operation for these cathodes 167 00:26:46.390 --> 00:26:54.629 Mark Kushner: and those relate over here. So what's happening here is as we change the power going from left to right, 100 200 300 400 500 watts. 168 00:26:54.800 --> 00:26:59.380 Mark Kushner: You are able to extract more current by basically cranking up the plasma density. 169 00:27:00.260 --> 00:27:11.090 Mark Kushner: But you can only do it so much, because eventually you get stuck, you can only collect so many ions. And so you reach this asymptote here, this robot asymptote, where I can't possibly collect any more ions. 170 00:27:12.506 --> 00:27:21.390 Mark Kushner: The next step here kind of looking at what we might expect is that you know these points here, these X's are basically constant as a function of power 171 00:27:22.450 --> 00:27:31.619 Mark Kushner: is one of the is one of the results here, and then we expect that the the optimal current. So this value here on the x-axis. 172 00:27:31.940 --> 00:27:37.090 Mark Kushner: as a function of power goes up linearly and basically scales with the ion saturation current. 173 00:27:39.120 --> 00:27:55.910 Mark Kushner: we did the similar study where, instead of varying power. We varied flow rate. And I'll go through this quickly because it was wholly less interesting. Basically, everything stayed relatively flat. But one of the reasons being that I think, in our experiment, and the reason why I chose these parameters is to match an experiment that we did. 174 00:27:56.790 --> 00:28:08.909 Mark Kushner: but that if you were to run it, maybe at extremely low, slow rates, you might find some more interesting behavior, because in these cases, basically, there's always plenty of propellant to ionize. 175 00:28:09.020 --> 00:28:12.309 Mark Kushner: And it's as a result, it just doesn't make a whole lot of a difference. 176 00:28:14.500 --> 00:28:28.020 Mark Kushner: All right. So this is basically our our theoretical description of what we think is going on, and we have some expectations for what the Iv curve should look like, and how there should be some kind of minimum Watts per Amp that it's going to cost us to extract 177 00:28:28.330 --> 00:28:32.089 Mark Kushner: electrical current from the systems. We're going to try to put it to the test. 178 00:28:32.810 --> 00:28:43.990 Mark Kushner: And so here I have a beautifully grainy video or a picture of of our our test set up on our vacuum chamber here. So this is a small one that preceded the the large one that I showed later. Here earlier 179 00:28:44.890 --> 00:28:54.609 Mark Kushner: we have our Rf. Cathode sitting up here, where we have our inductive antenna kind of sitting here in the middle. We have Rf power systems sitting here on the right and the vector on the bottom. 180 00:28:54.950 --> 00:29:02.709 Mark Kushner: And basically, we're running Iv curves. We're biasing inside of here we have a little anode that is sitting downstream inside the vacuum chamber. 181 00:29:02.970 --> 00:29:11.480 Mark Kushner: And we're basically biasing the cathode that is sitting inside this quartz tube relative to the anode that's sitting downstream. 182 00:29:13.061 --> 00:29:26.779 Mark Kushner: But 1st what we did was treat the cathode as a probe like. If the theory is that the cathode should function, or that that the probe theory does describe the cathode, then maybe we can just use the cathode as its own probe. 183 00:29:27.690 --> 00:29:35.929 Mark Kushner: And so basically, what we did is we took these Iv traces and we curve fit them with the double probe theoretical description. 184 00:29:36.870 --> 00:29:51.179 Mark Kushner: And this allows us to basically back out density and temperature. And so when we look at those curve fit parameters, we extract out a, you know, electron density or electron temperature. And then I converted it into the density into an ion saturation current. 185 00:29:51.920 --> 00:30:06.149 Mark Kushner: And one of the key things that we we thought was going to happen in these systems is that the electron temperature should be about the same for the flow rates that we're interested in, at least, and that the Ion saturation current should increase linearly with Rf power, and it turns out that this is true. Born out in experiment here. 186 00:30:07.625 --> 00:30:12.650 Mark Kushner: The next piece is starting to look at that voltage cost. What is the Watts per Amp. 187 00:30:12.770 --> 00:30:19.859 Mark Kushner: What does that curve look like? And so here, these are actually the data points, not not the curve fits that I'm drawing here. 188 00:30:20.130 --> 00:30:20.990 Mark Kushner: But 189 00:30:21.250 --> 00:30:36.429 Mark Kushner: you can see a similar trend where we have. You know our best point is about 100 watts per amp, that the optimal voltage cost is that those X's are about at the same voltage as we expected from the theoretical description in the model. 190 00:30:37.100 --> 00:30:49.579 Mark Kushner: Something that is kind of interesting is that in ours. The shapes are very similar. But here the minima get broader and broader. That's not something that's really captured in the model. But it's interesting, because 191 00:30:49.860 --> 00:31:02.409 Mark Kushner: this means that the broader the minimum around the place that you want to be operating at means you have more flexibility in the amounts of or the levels of current that you can deliver at a optimal amount of voltage. 192 00:31:02.970 --> 00:31:05.300 Mark Kushner: so that seems to improve with power. 193 00:31:07.150 --> 00:31:31.639 Mark Kushner: Lastly, and the one that is my favorite is that you know, the theory predicts that if I normalize my curves by the discharge or sorry by the ion saturation current for the current and the voltage by the electron temperature that all these curves should collapse onto a single set of curves. And if you do that, it does turn out to be that case. So this is to me seems pretty convincing that our description is at least a reasonable description, for the Rf. Cathode 194 00:31:34.320 --> 00:31:39.880 Mark Kushner: So now here, kind of really doing. The the true comparison between experiment and theory 195 00:31:40.110 --> 00:31:50.099 Mark Kushner: is, I've I've taken essentially the actual experimental conditions and fed them to my my global model and my sheath model and it, and created myself some Iv curves. 196 00:31:50.430 --> 00:32:01.889 Mark Kushner: And so here you can see on the left the the comparison between the data and the experiment. You can see that there are some deviations right? So there's maybe some improvements to be made. 197 00:32:02.270 --> 00:32:20.009 Mark Kushner: But overall, actually, the minima that they predict look pretty good. And if you actually plot kind of these key parameters of interest the optimal voltage that you would run it at, and the current that you can extract. You can see they're actually pretty close and with a reasonable amount of agreement. 198 00:32:20.750 --> 00:32:30.700 Mark Kushner: So this was actually a pretty exciting finding that, you know, even if you have some deviations away that the tool, at least for understanding Rf. Cathodes, seems relatively useful. 199 00:32:32.032 --> 00:32:35.470 Mark Kushner: Now, I don't want to say that it is entirely predictive 200 00:32:36.226 --> 00:32:39.880 Mark Kushner: so my model, for example, does not 201 00:32:39.990 --> 00:32:48.820 Mark Kushner: like, does not determine a coupling efficiency on its own right. How much Rf power actually made it into the into the plasma. 202 00:32:48.960 --> 00:33:16.720 Mark Kushner: not something that I make an estimate of, and I think of myself as a decent plasma physicist. But I'm not Valerie Gadiak, so I don't make the world's greatest Rf devices. So I kind of put myself at about 50%. And we did some measurements to suggest that that was about the case, and that was into free space. And then, when we put the collector in its transparency was about 50%, which is where I came up to about a 2025% Rf power coupling. 203 00:33:17.060 --> 00:33:29.210 Mark Kushner: as my guess. And then there's a secondary effect on the electron temperature where I'm thinking that capacitive coupling to those electrodes isn't effectively enhancing that electron temperature by almost a factor of 2. 204 00:33:29.700 --> 00:33:33.740 Mark Kushner: But I think this is still an area that needs to be investigated and better understood. 205 00:33:36.000 --> 00:33:46.759 Mark Kushner: So all right. So we have a description that works pretty well. There may be a couple of factors that need to be better understood. But really we want to know, how does this thing run on water? Right? 206 00:33:47.210 --> 00:33:55.599 Mark Kushner: That was the whole point. So here I've got another one of these plots. We've got discharge voltage on the Y-axis discharge current on the X-axis. And here, in 207 00:33:56.120 --> 00:33:57.939 Mark Kushner: what do you call that red pink? 208 00:33:58.100 --> 00:34:04.580 Mark Kushner: We have where a typical range, for where a thermionic cathode should be able to deliver that amount of current. 209 00:34:04.730 --> 00:34:07.390 Mark Kushner: it's around 2025 30 volts. 210 00:34:08.252 --> 00:34:14.839 Mark Kushner: So when we run our Rf. Cathode on water. We see this. So this is not as good 211 00:34:14.960 --> 00:34:15.960 Mark Kushner: right? 212 00:34:17.190 --> 00:34:26.160 Mark Kushner: and these are different Rf powers that we put in, and we get these curves which share a lot of the same trends as what we saw with Argon. 213 00:34:26.330 --> 00:34:29.610 Mark Kushner: where kind of the curves sort of seem to line up with each other. 214 00:34:29.760 --> 00:34:34.139 Mark Kushner: inner shifting. Right? As we increase the power. This is basically what we're expecting. 215 00:34:35.555 --> 00:34:42.930 Mark Kushner: And really like. The nice thing is that, yes, it does work with water. That's nice. But it's not as efficient. 216 00:34:43.030 --> 00:34:50.540 Mark Kushner: But you know, maybe you've enabled a new type of mission that you couldn't do before with the thermionic Cathos. 217 00:34:50.750 --> 00:34:54.090 Mark Kushner: and so that has additional benefits outside of simply just performance. 218 00:34:54.439 --> 00:35:04.299 Mark Kushner: And so that becomes part of the trade right? Like, if I want to go. And you know deep space exploration and go to like 20 different bodies. And I'm gonna bring my propellant with me. And I'm going to harvest some stuff. 219 00:35:05.210 --> 00:35:07.840 Mark Kushner: Maybe this kind of technology makes sense 220 00:35:08.130 --> 00:35:13.119 Mark Kushner: as opposed to having to ship the Xenon around the solar system to make it work. 221 00:35:14.841 --> 00:35:25.990 Mark Kushner: So we took some of this data, and we tried to boil it down to try to understand how it's going to affect a hull thruster's performance. And so what can we expect from these results? Well. 222 00:35:26.360 --> 00:35:43.189 Mark Kushner: the overall, the overall higher voltages mean that we have a lower voltage utilization for the thruster essentially meaning that the voltage that I apply between the cathode and anode. Less of it is going to get used to accelerate ions. More of it is going to get used to extract electrons. So it is less thrust. 223 00:35:43.588 --> 00:35:48.490 Mark Kushner: So a typical health thruster running on Xenon has, like a 90 or more percent voltage efficiency. 224 00:35:49.190 --> 00:35:52.409 Mark Kushner: In our case it's going to be down to like 60. So this is gonna hurt us. 225 00:35:53.335 --> 00:36:00.790 Mark Kushner: There are additional frozen flow losses. Right? So water is going to have these rotational vibrational modes, association, and other things. 226 00:36:01.120 --> 00:36:05.689 Mark Kushner: and I pulled the value from the literature here that was looking at different kind of 227 00:36:05.920 --> 00:36:14.179 Mark Kushner: electro propulsion device on water, but had similar electron energies that I was expecting to what you might see in a hull thruster 228 00:36:15.721 --> 00:36:24.749 Mark Kushner: and then there are also other typical hull thruster losses that get you the typical hull thruster efficiency of somewhere around 50 to 70%. 229 00:36:25.610 --> 00:36:32.079 Mark Kushner: And so this, some of these points I basically took from from the literature here, or a compendium of performance data from all sources. 230 00:36:33.994 --> 00:36:45.909 Mark Kushner: So all right, I basically did an analysis of what does a 2 and a half kilowatt hull thruster look like, and a 5 kilowatt health ruster look like where 5 is basically state of the art of what you might find in space today. 231 00:36:46.280 --> 00:37:00.980 Mark Kushner: And so, assuming that those thrusters are running at 300 volts, which is a pretty typical hall effect, thruster voltage, we have a very consistent cathode coupling voltage right? So that optimal voltage for Rf. Cathode was consistently about 100 volts. 232 00:37:01.200 --> 00:37:11.840 Mark Kushner: and so I can use that as a constant which I have here in my table, and then that effective Watts per Amp changes only a little bit based on my expectations from the data. 233 00:37:13.545 --> 00:37:22.609 Mark Kushner: Those other frozen flow and other efficiencies for a hall thruster. And then next, we're going to look at you know. How much power did we need? How much less did we make ISP and so forth. 234 00:37:23.580 --> 00:37:36.410 Mark Kushner: So rf, power turns out that you need a lot of Rf power to make a considerable amount of current. So I don't remember what the number was. But we got. Yeah, we got about one and a half amps. Not quite. 235 00:37:37.030 --> 00:37:46.299 Mark Kushner: and that was with a few 100 watts. So now we need, like a kilowatts worth of Rf. Power to be able to generate 5 amps and almost 2 kilowatts to generate 10 236 00:37:47.712 --> 00:37:51.650 Mark Kushner: so the large, if power is going to is needed to run this 237 00:37:52.507 --> 00:38:01.729 Mark Kushner: which is getting close to 50% of the total power that you're putting into the system. So my, maybe a a point to make is that my 5 kilowatt. 238 00:38:02.160 --> 00:38:04.049 Mark Kushner: or my 2 and a half kilowatt. 239 00:38:04.470 --> 00:38:07.599 Mark Kushner: or sorry? The the total power that I'm putting in here 240 00:38:07.850 --> 00:38:10.530 Mark Kushner: is the sum of Rf. And DC. Powers 241 00:38:11.244 --> 00:38:16.799 Mark Kushner: to get to the 2 and a half and 5. I design the plot that way. 242 00:38:18.040 --> 00:38:25.109 Mark Kushner: So the thrust is relatively low. So, and this is largely because water has a low molecular weight. 243 00:38:26.276 --> 00:38:36.729 Mark Kushner: There's a xenon will typically have somewhere between 700 or 75 and 150 milliliters per kilowatt, whereas for water we expect it only be about 30 to 50. 244 00:38:37.440 --> 00:38:45.350 Mark Kushner: Our asp is relatively high, because the propellant is very light, so the voltage that it does see accelerates it a lot. Compared to Xenon. 245 00:38:45.990 --> 00:39:00.139 Mark Kushner: the thrust per kilowatt for the same reasons, is relatively low, both because we have more power needed to run the cathode we had to dump in Rf. Power, and it cost us more to get the electrons, but also the propellant is light. 246 00:39:00.810 --> 00:39:04.129 Mark Kushner: So the low thrust to power metric there. 247 00:39:04.320 --> 00:39:09.639 Mark Kushner: And this is basically telling me, like. You know how much oomph can I put in for a given size spacecraft? 248 00:39:10.800 --> 00:39:22.240 Mark Kushner: And then the last term here is the overall efficiency that you might want to expect. So the the efficiencies look rather low. You know, 18% is not great. A typical Xenon hull thruster is 50%. And above. 249 00:39:22.810 --> 00:39:27.959 Mark Kushner: But the nice thing is that you know, we didn't need to use Xenon, which is 250 00:39:28.350 --> 00:39:33.250 Mark Kushner: quite expensive these days, and not that easy to find elsewhere in the solar system. 251 00:39:34.260 --> 00:39:38.720 Mark Kushner: so that does enable certain things. 252 00:39:39.190 --> 00:39:50.120 Mark Kushner: So I would say that this number is not always lost. As a result of this, like, you could imagine a future where you just have lots and lots of power available to you, and being efficient with that power is maybe less important. 253 00:39:53.850 --> 00:40:02.870 Mark Kushner: So if we fast forward a few years, and this will be some abridged works of some work that Doctors Nolan, Uchizona, and Leah and Su did at the Naval research lab 254 00:40:03.390 --> 00:40:09.379 Mark Kushner: I contributed to. But so primarily here I've 255 00:40:09.570 --> 00:40:22.989 Mark Kushner: drawn this arrow of Rf engineering to go from my setup that I had over here to a cathode that runs inside of the vacuum chamber, and this arrow is doing a lot of heavy lifting here. And this was a lot of the work that Nolan had done. 256 00:40:24.010 --> 00:40:32.269 Mark Kushner: And then we've also started working on trying to improve that model looking at different propellants. So you know, we have a simple global model. 257 00:40:32.996 --> 00:40:38.550 Mark Kushner: What can we do? Going from Argon to Xenon to Krypton? Actually sorry this should be Krypton 258 00:40:39.614 --> 00:40:45.439 Mark Kushner: and then accounting for variations and plastic density that you might expect due to the 259 00:40:45.580 --> 00:40:50.159 Mark Kushner: basically, the coil being a finite length and the radius of the discharge 260 00:40:51.593 --> 00:41:01.809 Mark Kushner: so we're trying to make some improvements. And so we're trying to test those out both the hardware and then, as well as the the some of the theoretical descriptions, and see how well we do 261 00:41:03.301 --> 00:41:11.829 Mark Kushner: so specifically for noble gases. Leanne came up with these relatively simple scaling laws to try to get a quick test of what we think 262 00:41:11.940 --> 00:41:16.320 Mark Kushner: or how we think an experiment might go. So for propellants that have 263 00:41:16.750 --> 00:41:37.750 Mark Kushner: higher ionization potentials, we expected them to have higher electron temperatures basically to ionize the gas for the same amount of current you're going to try to generate, to ionize that gas. You need more. You need more energy in the electrons to overcome the ionization potential. So it makes sense that their temperature should be higher. 264 00:41:38.797 --> 00:41:48.350 Mark Kushner: In terms of the plasma density. There's a scaling with mass where the higher mass particles lead to higher densities. 265 00:41:48.610 --> 00:41:51.899 Mark Kushner: And they're the reason actually, is that 266 00:41:52.790 --> 00:41:57.060 Mark Kushner: that higher mass particles make it to the wall slower 267 00:41:57.330 --> 00:42:02.899 Mark Kushner: because they get they get there roughly, the ion sound speed which scales inversely with the mass. 268 00:42:04.353 --> 00:42:10.869 Mark Kushner: So the the heavier the particle, the more plasma you can make. Basically you've reduced your losses. 269 00:42:11.170 --> 00:42:20.220 Mark Kushner: And then the lower the ionization potential, I think, is pretty clear. You should probably get more more electrons out of it. If it's easier to ionize. 270 00:42:22.062 --> 00:42:24.149 Mark Kushner: There we go inwards. 271 00:42:25.210 --> 00:42:40.340 Mark Kushner: So our setup looks a little bit more like this now, where, instead of looking at it on the outside, we can see all the different components on the inside. We have our cathode with some Rf power probes and actual Rf power that's getting fed. We have our anode that's sitting in there, and then we have 272 00:42:40.620 --> 00:42:46.060 Mark Kushner: a collection of probes being able to diagnose the plasma at a couple different locations. 273 00:42:46.660 --> 00:42:55.229 Mark Kushner: and so we had a relatively simple setup here, just 1 1 power and flow rate condition, looking at 2 different propellants. 274 00:42:56.540 --> 00:43:03.550 Mark Kushner: And so we have a little, a few probes here. But really I'm going to focus on the linear probe diagnostic here. 275 00:43:06.300 --> 00:43:16.089 Mark Kushner: So here's the cathode in operation running on Argon and Krypton back here on the right. You can see there's a mirror to kind of see what's going on in the orifice region 276 00:43:16.970 --> 00:43:20.110 Mark Kushner: and operating at different discharge voltages. 277 00:43:22.384 --> 00:43:31.480 Mark Kushner: So just some for the models, the inputs and expectations. So if I'm doing Argon and and Krypton did I? Did. I say, Xenon, this this is Krypton 278 00:43:33.000 --> 00:43:48.149 Mark Kushner: Argon and Krypton. We have differences in Ion, mass. So we expect roughly a square root. 2 change in plasma density. As a result from from our, you know, our quick scaling laws. And then the ionization energy is off by less than 10% 279 00:43:48.685 --> 00:43:54.469 Mark Kushner: difference between the 2. So we expect the electron temperature to be pretty similar, but they're consistent between the 2. 280 00:43:55.590 --> 00:44:01.560 Mark Kushner: and so our our test properties and model parameters. So these are kind of coming from, you know. 281 00:44:01.750 --> 00:44:03.520 Mark Kushner: our actual experiment. 282 00:44:05.600 --> 00:44:20.610 Mark Kushner: So when we we feed it, the global model. Yeah, okay, this is, it makes sense that our global model spits out an answer that basically follows our scaling laws because our scaling laws came from our global law. So this basically shows that yes, we can do reasonable math 283 00:44:22.780 --> 00:44:26.209 Mark Kushner: But really, what we wish we could do is compare it to real data. So let's do that. 284 00:44:26.670 --> 00:44:30.160 Mark Kushner: So this is the Iv curve that we actually see an experiment. 285 00:44:30.370 --> 00:44:41.930 Mark Kushner: It looks actually quite different than what we saw in the past, which is quite interesting. So the blue dots are going to be. Those are argon as a function so voltage as a function of current. 286 00:44:42.070 --> 00:44:47.290 Mark Kushner: And then on the are those orange squares, orange? 287 00:44:47.520 --> 00:44:54.270 Mark Kushner: Yeah, we got you get you get this curve for for Krypton. 288 00:44:55.110 --> 00:45:03.060 Mark Kushner: And so what is what is nice is that? You can see that for Krypton you get more current than you would have thought for Argon. 289 00:45:03.550 --> 00:45:17.329 Mark Kushner: and that's kind of described at these high current. And you can see the the model when we put in our information. This is what the prediction is, and it looks pretty good, at least in this region. But then this whole region over here looks quite different. 290 00:45:17.610 --> 00:45:31.440 Mark Kushner: and that this is, I'm going to say, an active area of research and trying to understand why that is. But what we don't really know yet is, why is this Iv curve so different from before. And why is our model basically failing at these low currents now, when it didn't before? 291 00:45:32.951 --> 00:45:35.410 Mark Kushner: And so that is an 292 00:45:35.900 --> 00:45:41.610 Mark Kushner: an area of investigation. But we can do at least a little bit with our probes that we had as diagnostics. 293 00:45:42.430 --> 00:45:45.710 Mark Kushner: So our 1st probe was our eyeballs. 294 00:45:46.010 --> 00:45:50.060 Mark Kushner: And so we looked at it and saw that. Okay, when we're down on this side 295 00:45:50.740 --> 00:46:07.709 Mark Kushner: at low current, you don't see that much brightness in the Pluton. And then, when you look on this side all of a sudden, you see kind of, and it's not super noticeable, but you can kind of see some brightness kind of around the orifice there, and it was kind of distinct where you could notice it. If you're looking at it with your eyes. 296 00:46:09.209 --> 00:46:20.050 Mark Kushner: So there's something about, you know, as we increase as the voltage goes up, you're getting additional brightness in the plume region that is allowing you to somehow extract more pressure. 297 00:46:23.140 --> 00:46:27.819 Mark Kushner: Looking at the actual plume measurements of electron temperatures. 298 00:46:28.480 --> 00:46:31.870 Mark Kushner: So this is from the probe that is sitting closest to the cathode. 299 00:46:32.624 --> 00:46:38.819 Mark Kushner: So when you're at low voltage, you see relatively low electron temperatures values that you would probably expect. 300 00:46:39.850 --> 00:46:44.849 Mark Kushner: And then on the right hand side, once you get to higher voltages and you cross the mode. Hub. 301 00:46:45.903 --> 00:46:47.489 Mark Kushner: so yeah. 302 00:46:47.610 --> 00:46:59.460 Mark Kushner: you can see the higher electron temperatures start to show up, but that these data points are by and large kind of hanging together that the temperatures are roughly constant, even as we change voltage. Despite this mode hop. 303 00:47:00.530 --> 00:47:11.239 Mark Kushner: and then on the right hand side, you can see that pretty consistently. The density that we expect that we measure from krypton is higher than what we measured for argon, and with roughly the square root of 2 ratio 304 00:47:13.164 --> 00:47:23.595 Mark Kushner: so our our theory is actually looking pretty good. But it doesn't really describe this MoD hop behavior. It is individually describing the points 305 00:47:24.730 --> 00:47:27.879 Mark Kushner: the last part and something that our global model. 306 00:47:28.000 --> 00:47:32.160 Mark Kushner: by virtue of the fact that it is a global model doesn't really 307 00:47:32.340 --> 00:47:40.849 Mark Kushner: doesn't really count for changes in plasma potential. Everything is very flat in our model in terms of the plasma potential in bulk, plasma. 308 00:47:41.220 --> 00:47:42.969 Mark Kushner: Everything is happening at the sheets. 309 00:47:43.290 --> 00:47:56.709 Mark Kushner: And so here, what we see is that you know the plasma potential. Actually, they look very similar for each individual gas. But as you go across the mode, hop, you get a clear shift in the value of the discharge voltage. 310 00:47:57.020 --> 00:48:06.459 Mark Kushner: And so we need to be able to extend our description, to be able to capture this kind of behavior. And what is going on. And so this is the the active area of research that I'm really interested in learning more about. 311 00:48:07.770 --> 00:48:25.869 Mark Kushner: So we have a couple of different physical explanations of what we think we might be seeing. Maybe there's some kind of double layer formation happening in the orifice that's maybe contributing to more ionization and basically boosting the plasma density in the orifice region. And somehow this is helping us generate more current in the device. 312 00:48:26.664 --> 00:48:30.559 Mark Kushner: Or maybe there's there might be a couple other explanations as well. 313 00:48:31.195 --> 00:48:37.310 Mark Kushner: But suffice it to say that both from a theoretical and experimental standpoint, I think this is an area that we want to go into next. 314 00:48:38.799 --> 00:48:47.429 Mark Kushner: So this kind of concludes, the Rf. Cathode part where you know, we've been interested in this in situ resource utilization concept of operations. 315 00:48:48.260 --> 00:49:00.340 Mark Kushner: We've been investigating, particularly the propulsion aspects of it and trying to use existing electric propulsion systems. But by essentially hot swapping out the cathode technology to make them compatible with propellants like water. 316 00:49:01.170 --> 00:49:09.839 Mark Kushner: We've been studying the basic physics of how these things work. And then, you know, capturing experimental data and comparing it to that theory 317 00:49:10.680 --> 00:49:15.720 Mark Kushner: with the ultimate goal of being able to enable this kind of propulsive technology. 318 00:49:16.512 --> 00:49:26.210 Mark Kushner: So I wanted to do a time check, because I wasn't sure if I would have time to talk through some of these other things. How am I doing? What time am I supposed to be done? 319 00:49:28.540 --> 00:49:32.880 Mark Kushner: 4. It is 4. Yeah, I got 5 min. 320 00:49:33.100 --> 00:49:46.379 Mark Kushner: All right. I will blaze through these. So basically, 2 short topics here. We've also been interested in high speed plasma diagnostics as well as basically circuits, effects and emission in emissive sheets. 321 00:49:47.467 --> 00:49:58.160 Mark Kushner: So the probe that we've been interested in what we call the plasma impedance probe. So a typical linear probe, you basically put it into the plasma and you get a DC response in the current. 322 00:49:58.310 --> 00:50:13.209 Mark Kushner: And you measure that DC plasma response to extract density and temperature and potential information. But an impedance probe basically uses microwaves to target the electrons and then basically reflect those microwaves off them 323 00:50:13.510 --> 00:50:20.330 Mark Kushner: or absorb it, absorb or reflect and basically measure the spectral response of the plasma with an antenna. 324 00:50:21.420 --> 00:50:38.720 Mark Kushner: And so we've been developing this technique for a fair amount of time, and we figured out a way how to do it. Time resolved. And so here I'm showing a little bit of data that we collected where I'm comparing essentially one of these plasma impedance probes compared to a ion saturation current. Probe 325 00:50:39.261 --> 00:50:47.369 Mark Kushner: so the black is the plasma density that you extract from the impedance probe without going into the details of you know how the probe works. 326 00:50:48.144 --> 00:50:54.909 Mark Kushner: And the ion saturation current looks like this. And what you can see actually, is that they look qualitatively, at least quite different. 327 00:50:55.080 --> 00:51:13.490 Mark Kushner: And if you look at their spectra again, they actually look quite different, which I was honestly, quite surprised with, because in my experience I always kind of take the unsaturation current to be a good representative of what is going on with the plasma density. But in this case that didn't seem to be the that didn't seem to be true 328 00:51:13.760 --> 00:51:24.680 Mark Kushner: where these lower frequency oscillations, and even particularly these higher frequency ones, were not nearly as prevalent in the plasma density measured by the impedance probe 329 00:51:25.880 --> 00:51:37.889 Mark Kushner: compared to the Ion saturation. Probe. Now you got to ask yourself, right? As a scientist you got to question all the things is like, did I screw up with the impedance probe? Or is it actually telling me that this is really different? 330 00:51:38.080 --> 00:51:47.650 Mark Kushner: And so the way that I kind of went about trying to answer that question is to take the impedance probe, and I can use it also to calculate a temperature as well as the density. 331 00:51:47.910 --> 00:52:02.769 Mark Kushner: And with those 2 pieces of information I can recreate the Ion saturation current. So I have here now plotted the Ion saturation current calculated from the plasma impedance probe, and the current calculated from the Ion saturation probe, and you can see now, qualitatively look a lot better. 332 00:52:04.182 --> 00:52:09.987 Mark Kushner: And then, if you look at spectra, you can see that now these these features are well captured 333 00:52:10.620 --> 00:52:15.140 Mark Kushner: by by the impedance probe and the ion saturation current. 334 00:52:15.400 --> 00:52:36.210 Mark Kushner: And then there's a little bit of difference here at higher frequency, which again, could either be a result of errors in the plasma penis probe in our analysis or in the instrumentation, or is potentially due to something going on with the saturation probe. Now given that this is a relatively simple diagnostic. My guess is, it's something in here something related to bandwidth. 335 00:52:36.708 --> 00:52:47.209 Mark Kushner: But it is pretty clear that at these lower frequencies, you know, the the Ion saturation current was not necessarily a good representative of the density alone. 336 00:52:48.912 --> 00:53:02.049 Mark Kushner: So in this case the takeaways here are that the impedance probe and the saturation current agree up to 600 kilohertz, but that the electron temperature variations were critical to accounting for all those to get good agreement between the 2. 337 00:53:03.526 --> 00:53:07.889 Mark Kushner: And the last piece here was thinking about sheath models for 338 00:53:08.180 --> 00:53:13.220 Mark Kushner: hall thrusters running in different test facilities and how you electrically configure them. 339 00:53:13.580 --> 00:53:25.620 Mark Kushner: And basically, the the thought is, there's this unique test configuration that they call cathode tide, where they take the body of a thruster, and they electrically connect it to the actual cathode. 340 00:53:25.840 --> 00:53:33.679 Mark Kushner: And this allows for this current loop through the body and to the emitter in here, and I was trying to understand what happens when you do that. And 341 00:53:33.900 --> 00:53:44.760 Mark Kushner: so if you basically take and simplify this down, you have one wall, that's the cathode emitter, and the other one is the thruster body, and they're exposed to the plasma in between. It looks a little bit different. 342 00:53:44.980 --> 00:54:04.739 Mark Kushner: and here I can throw it all up all at once. But essentially it's right. The emitter is going to start emitting electrons, and this emission is going to affect because they are electrically shorted together, the thruster body and the cathode emitter. This is going to affect the potential that the thruster body sits at. 343 00:54:05.020 --> 00:54:20.870 Mark Kushner: but then it turns out that in certain circumstances, if your thruster body is large enough, or that it is in contact with enough plasma that the thruster body can then affect the sheath behavior that is going on at the emitter. But that's only in very extreme cases that you wouldn't actually expect to see an electric propulsion. 344 00:54:21.460 --> 00:54:23.590 Mark Kushner: What's kind of the conclusion of that study. 345 00:54:24.470 --> 00:54:33.800 Mark Kushner: Okay? So that that was a quick rundown of some, the other topics that we're looking at at the Naval Research lab, among others, that we didn't discuss here today. 346 00:54:34.470 --> 00:54:48.820 Mark Kushner: but that was kind of it for the science part, and the last part was going to be a little plug for all the different research opportunities that we have at the lab. So for undergraduates and graduates, you know, you've got the list of interests, but generally, we're interested in smart people. 347 00:54:49.110 --> 00:54:56.389 Mark Kushner: Really the answer, and there are different programs. Nrep is basically a 10 week summer internship 348 00:54:56.680 --> 00:55:12.769 Mark Kushner: that's paid. This Sep program is allows you to basically get, you are basically hired into the government. And you can come back essentially as many times as you want and take on and off breaks as you take classes, things like that. 349 00:55:13.750 --> 00:55:17.050 Mark Kushner: but allows you to come back without having to redo applications. 350 00:55:17.220 --> 00:55:26.270 Mark Kushner: We have postdoc opportunities as well. We're always seem to always be recruiting for postdocs. So if you have any interest in these kinds of topics, please talk to me. 351 00:55:26.530 --> 00:55:42.029 Mark Kushner: And lastly, for faculty, you know, Onr has these summer faculty programs that you know, provide some amount of funding for your summer salary, but you can also get access to the people of Nrl and all the different facilities that we have at our disposal. 352 00:55:43.210 --> 00:55:46.789 Mark Kushner: Alright. So with that, I will take any questions. 353 00:55:58.880 --> 00:56:09.060 Mark Kushner: Yeah. So you talked about kind of improvements, I guess, from the model. One of the significant ones was particularly in the earlier study, like the later studies captured 354 00:56:09.550 --> 00:56:38.459 Mark Kushner: saturation. But it seems like in the earlier work, that kind of offset and maybe kept going, even though I guess kind of the optimal power didn't change. Do you think that's maybe a viable avenue, or like what's going on there. Yeah. So my guess is what's happening. There is what's called sheath expansion. So the effective area of the cathode is probably going up. I I've fiddled with that a little bit, and 355 00:56:38.730 --> 00:56:39.810 Mark Kushner: I 356 00:56:40.750 --> 00:56:53.550 Mark Kushner: I don't know for sure yet, if that is the answer, but it might explain at least, that in part. So, depending on how you design your cathode collector, you can try to maybe take advantage of that sheep expansion. 357 00:56:57.720 --> 00:57:00.570 Mark Kushner: Yeah, thank you. 358 00:57:01.580 --> 00:57:02.669 Mark Kushner: People, don't you? 359 00:57:06.240 --> 00:57:18.559 Mark Kushner: Yeah, that's right, and that the actual measures. 360 00:57:20.790 --> 00:57:21.750 Mark Kushner: 5 of this. 361 00:57:21.900 --> 00:57:27.430 Mark Kushner: Yeah. So the way it works is basically it's resonating with the electron plasma frequency. 362 00:57:27.690 --> 00:57:40.650 Mark Kushner: And that gets you the density. And then, as part of the model, you actually have to try to model the sheath around the probe. And this can combine with the density information, get you an effective temperature. And so that's how extract a temperature. 363 00:57:40.800 --> 00:57:42.269 Mark Kushner: Since you can find. 364 00:57:45.070 --> 00:57:59.750 Mark Kushner: That's right, can you get like, do you have like a, you get like a time to solve temperature from this? Yeah. So I didn't include it here as one of the plots. But basically the transition from this, this plot here to this plot, here is a time resolved temperature. 365 00:58:00.220 --> 00:58:20.030 Mark Kushner: and it and that time resolved temperature actually is what really allows you to recreate the spectral response, which was honestly very surprising to me, because I thought it was mostly going to be in the density, because these kinds of oscillations that you see in cathodes, we always think are kind of ionization related, which, of course, is a process that's related to both density and temperature. 366 00:58:20.767 --> 00:58:25.540 Mark Kushner: But in this case it seems like the the temperature was critical. 367 00:58:27.450 --> 00:58:28.780 Mark Kushner: Here's the latest 368 00:58:32.190 --> 00:58:38.500 Mark Kushner: temperature steam just trying to as with this, which is good to see. 369 00:58:38.880 --> 00:58:51.982 Mark Kushner: Yeah. So like, when when we did those measurements back, when I was a student like, we ended up trying to do time resolve measurements of the temperature, and you could see it moving around for these kinds of oscillations. 370 00:58:52.630 --> 00:59:02.919 Mark Kushner: but this was kind of more, a little bit more direct from a single probe. Instead of needing to take a collection of probes, all trying to measures in a slightly different position and stuff like that. 371 00:59:03.650 --> 00:59:07.580 Mark Kushner: Yeah, yeah, we talked about this earlier. 372 00:59:07.840 --> 00:59:12.219 Mark Kushner: First, st I saw the data. So on the mode hopping stuff around, yeah, 51. 373 00:59:12.740 --> 00:59:28.499 Mark Kushner: So to me, that seems seems consistent with, like you're you're switching to a mode of non ambipolar current flow. Yeah, like that electron source non-emplar flow. So it seems like, once you get the bright spot 374 00:59:28.750 --> 00:59:36.440 Mark Kushner: you're you're forming some kind of add on spots at your orifice that it increases the effective electron collecting area. 375 00:59:36.570 --> 00:59:38.919 Mark Kushner: And then, like on the next slide you saw 376 00:59:39.880 --> 01:00:07.430 Mark Kushner: the plasma potential. Yeah, this is what I was thinking to the discharge voltage. So as you crank up the discharge voltage, you're just increasing the plasma potential. So that's like the characteristic of that. When I was seeing this plot again, while I was giving the talk, I was thinking back to the plots that you were showing me. And I was like, Oh, yeah, there we go. There's the plasma potential. You had a plot with the temperature jumping as well. Yeah, the temperature jumps, too. 377 01:00:07.640 --> 01:00:12.999 Mark Kushner: That's also a characteristic of that mode transition. Yeah, because when you're in the 378 01:00:13.698 --> 01:00:27.570 Mark Kushner: low discharge voltage mode, like the high energy electrons are escaping over the cathode sheath. But when you go to the Non Namipolar mode they're only leaving through the Anode, so you can find those high energy electrons, and that causes the temperature to jump. 379 01:00:27.950 --> 01:00:35.079 Mark Kushner: So all of this looks same as what we saw with just regular robes and cool. 380 01:00:35.230 --> 01:00:39.670 Mark Kushner: and then that not a polar electron source as well. Okay. 381 01:00:40.110 --> 01:00:42.099 Mark Kushner: right? We didn't break anything excellent. 382 01:00:44.550 --> 01:01:06.449 Mark Kushner: Sure. Just had a quick question about. I really like that. You can see very clearly the effect of the atomic mass between Argon and Krypton, and I think you mentioned that. And I was thinking about that in the context of water and the possible influence of chemical reactions, you know, breaking down water into maybe hydrogen. 383 01:01:06.450 --> 01:01:18.469 Mark Kushner: Have you thought about, you know, given the residence time, and you mentioned the mass flow rate doesn't have. At least you didn't look very closely at the mass flow rate effect. But have you thought about like residence time. And how can possible chemical reactions 384 01:01:18.700 --> 01:01:28.249 Mark Kushner: with water affect the operation of your arm? Yeah, we really have not had a lot of time to delve into trying to look at the chemistry itself. All that much. 385 01:01:30.250 --> 01:01:49.760 Mark Kushner: What I do know is that you seem to be able to run these kinds of devices at relatively low flow rates compared to regular thermionic cathodes. Basically, you can design the orifice to be quite small and get yourself enough pressure, and that will cause the neutrals and ions to basically recycle as the ions of the wall. They will come back as neutrals and get reionized. 386 01:01:49.880 --> 01:01:52.189 Mark Kushner: which is going to influence that chemistry. 387 01:01:52.370 --> 01:01:58.710 Mark Kushner: because, you know, whatever was ionized will then come back as neutral and get ionized again. And so 388 01:01:59.040 --> 01:02:04.829 Mark Kushner: my guess is, if you get a lot. If you design your cathode system, you get a lot of recycling. Then the 389 01:02:04.990 --> 01:02:13.550 Mark Kushner: the relative fraction of atomic, you know, neutral gas species is going to be higher than if you design your cathode to not be that way. 390 01:02:14.234 --> 01:02:16.940 Mark Kushner: But yeah, I you know 391 01:02:17.300 --> 01:02:20.890 Mark Kushner: there's a reason why we looked at Krypton and Argon is because it was easy 392 01:02:21.060 --> 01:02:29.209 Mark Kushner: or, relatively speaking, but yeah, we would love to to do a market, you know. Interesting model on on water. 393 01:02:31.670 --> 01:02:55.669 Mark Kushner: Have you ever tried going downward at the voltage and seeing if there's hysteresis, I have not tried to flip, flip the flip it negative. Yeah. So that it was like the other side of the Iv trace that you would get for a typical probe. I've not tried that. That is the thing that from data point to data point here, you're increasing discharge voltage. Right? That's right. Yeah. Oh, sorry. You mean going from high to low. No, yeah. We didn't get to that. 394 01:02:55.670 --> 01:03:01.930 Mark Kushner: That's right. Yeah. You were. You were seeing the Mr. Esus. That would be a good next thing to go check. Make sure that it's consistent. 395 01:03:04.120 --> 01:03:09.969 Mark Kushner: There are other options in the chat. Okay? Why don't we knock one of those out 396 01:03:13.210 --> 01:03:19.150 Mark Kushner: low voltage curves, maybe due to passion of voltage emissions. 397 01:03:20.000 --> 01:03:23.249 Mark Kushner: What was the material used for the Ion collector for water? 398 01:03:23.470 --> 01:03:31.709 Mark Kushner: Was there any oxidation observed? Yeah. So we use basically something very simple. We use 3 16 steel shim mostly. 399 01:03:32.209 --> 01:03:39.689 Mark Kushner: And yeah, you can. You can see that it does something to the surface. But it's not enough to make it stop working, I guess 400 01:03:42.270 --> 01:03:43.230 Mark Kushner: And 401 01:03:43.370 --> 01:03:56.230 Mark Kushner: did you check the energy distribution? Was it Maxwellian? So Icps usually don't make nice Maxwellians. So this is a a good good thing to notice that our our assumptions here that everything's kind of Maxwellian. 402 01:03:57.060 --> 01:04:02.849 Mark Kushner: So no, yeah, it is almost certainly not, Maximilian. Given that. It's an Icp discharge. 403 01:04:04.140 --> 01:04:07.569 Mark Kushner: So yeah, that is certainly a weakness in our in our model, for sure. 404 01:04:11.870 --> 01:04:13.819 Mark Kushner: Alright. Well, let's thank our speakers. 405 01:04:25.850 --> 01:04:26.720 Mark Kushner: So. 406 01:04:33.300 --> 01:04:34.120 Mark Kushner: yeah.