WEBVTT 1 00:00:00.000 --> 00:00:14.109 Mark Kushner: To introduce Professor White from the University of Nevada, Reno. Professor White obtained his Phd. From the University of Oxford in 2013, did a postdoc there as well. 2 00:00:14.690 --> 00:00:18.570 Mark Kushner: Yeah, I'll stop you if you say something horrendously wrong. 3 00:00:18.860 --> 00:00:21.650 Mark Kushner: A few small lies that's okay. 4 00:00:21.930 --> 00:00:37.954 Mark Kushner: Recipient of many distinguished honors and awards, including the column thesis Prize Nsf career award, and then is currently he is the chair of the Hydro Density Science Association and Jupiter Laser Executive committee. 5 00:00:38.470 --> 00:00:56.450 Mark Kushner: so we are honored to have you here to talk about temperature and heat, transport and warm, dense matter. But before you do so we would like to honor you with the distinguished Nipsey mug. 6 00:00:57.880 --> 00:01:08.170 Mark Kushner: Okay, okay, it's a 1 small eye. I'm the vice chair of this group, not the chair. But I'll let it slide, because one day it will have a power grab. I will be the chair. 7 00:01:08.630 --> 00:01:17.870 Mark Kushner: Okay. Today, I'm gonna talk a little bit about a few areas of my research. I want to give a sort of overview of what I do 8 00:01:19.130 --> 00:01:41.159 Mark Kushner: for those who don't know. I work in this area called warm, dense matter, and it sort of sits at the intersection between plasma physics and solid state physics or condensed matter. What that means is that to a plasma physicist. I'm a solid state physicist, and to a solid state physicist I'm a plasma physicist, and neither community will really accept me for who I am. 9 00:01:41.160 --> 00:01:57.910 Mark Kushner: It's an interesting area of the phase diagram to study, because all of the theories that work in one area don't work in this area so perturbative techniques that you might use when the potential dominates or where the kinetic energy dominate, they don't work anymore. 10 00:01:58.090 --> 00:02:24.429 Mark Kushner: The electrons in this regime are strongly quantum mechanical. Right? So to a plasma physicist, an electron is a point like particle that's flying around to us. It's a quantum mechanical wave packet that we've got to treat properly, and for that reason not only are the theories difficult to do, but all of the computation as well takes a long time and are quite difficult. So this is why it's a very interesting area to study. 11 00:02:24.640 --> 00:02:49.640 Mark Kushner: It also appears in lots of really interesting places. So there are lots of things you can do with lasers now. But the kind of things that I'm going to talk about today really are sort of like in the planetary interior regime, which is where you find these states of matter inside planetary interiors, and also in things like reactors and other places where you might get these extreme states. 12 00:02:50.425 --> 00:03:09.880 Mark Kushner: I would say, I don't like the name of the area that I work in warm, dense matter. This is not, you know, inspire anyone to think. Wow! What an amazing thing to study and warm means different things to different people. To us it's like 10,000, Kelvin. So that's what we mean by by warm. 13 00:03:12.170 --> 00:03:33.429 Mark Kushner: Okay? So today, I'm going to talk mostly about 2 different experiments. And I'm going to give you an overview of them. The 1st is talking about how heat flows through materials at these extreme states. And I'm going to talk about experiments that we conduct at the Omega laser facility. So I know a lot of people here use the Omega laser facility so hopefully, you will sort of 14 00:03:33.430 --> 00:03:47.689 Mark Kushner: be able to follow along that. And that's really I'm going to take the credit from my 2 students, Sarah and Cameron. So maybe some people in here know these people or have worked with them before Cameron sadly. It's now left and joined all 15 00:03:47.690 --> 00:03:51.059 Mark Kushner: which is devastating for me, but really good for him. 16 00:03:51.630 --> 00:04:04.720 Mark Kushner: And then I'm going to talk about some experiments that we've been performing at free electron lasers. So sort of switch it up a little bit, and if there's time we'll get to these extra ones. But probably we will not. So I just talk about 2 experiments today. 17 00:04:07.620 --> 00:04:09.489 Mark Kushner: Okay, so the 1st one 18 00:04:10.210 --> 00:04:21.530 Mark Kushner: I'm going to talk about transport. And I'm going to talk about heat transport. And why is this interesting? If we want to answer questions like, how long does it take a planet to cool down? 19 00:04:21.529 --> 00:04:43.579 Mark Kushner: Right then we want to know how fast heat will flow through that material, and it forms what's called one of the transport properties. So things like viscosity, which is the transport of momentum, electrical conduction, stopping power, all of these kind of complicated things to try to study in these extreme states of matter. 20 00:04:44.250 --> 00:05:09.970 Mark Kushner: And there are these transport properties, workshops every now and again. So here's a picture from one of the workshops. Some of you are in this picture. So you were there, and at these workshops all of the theorists come together, and they all run their codes, and they try to predict the same thing. And then there's a wild spread of results, and that's great for an experimentalist, because you can come in, and whatever you measure, you'll make one theorist happy. Right? So that's the goal. 21 00:05:11.080 --> 00:05:31.890 Mark Kushner: Here, for example, is thermal conductivity calculated with all of the codes at the workshop, and these bars represent the spread that the different codes calculated, and you can see that the in the regime I'm interested in, which is sort of 10,000 Kelvin-esque situations. 22 00:05:31.910 --> 00:05:52.620 Mark Kushner: We get 2 orders of magnitude spread in the results from the computational codes with no real agreement on what the right answer is. So these transport properties really make a great area to try and study and try and like hone in on what the actual thermal conductivity is in these extreme states. 23 00:05:52.930 --> 00:06:01.810 Mark Kushner: And so that's what I did. I thought, I'm going to go measure thermal conductivity. That's how I'm going to make my name. And so I'm going to tell you a little bit about the process we went through in developing this platform. 24 00:06:01.930 --> 00:06:05.860 Mark Kushner: and I'm going to start by saying we didn't do anything very clever. 25 00:06:05.900 --> 00:06:15.280 Mark Kushner: I was like, how do I measure heat flow? I know I'm going to get something hot, and I'm going to get something cold, and I'm going to put them together. 26 00:06:15.280 --> 00:06:36.039 Mark Kushner: and I'm just going to watch the heat flow from the hot thing to the cold thing, and then I'm going to measure how fast that was. And that's how I'll PIN this down, which sounds like really dumb idea, like you might, a kindergarten kid might come up with that idea. And it turns out it was a stupid idea. Okay, so that's the punchline here. But we actually found some amazing new physics on the way. 27 00:06:36.300 --> 00:06:39.760 Mark Kushner: But we just said, Let's let's do this really simple experiment. 28 00:06:40.200 --> 00:06:53.639 Mark Kushner: and we'll just watch how the heat flows from one to another, and it's not trivial to do at these extreme states. And if you do a back of the envelope calculation for these warm, dense matter materials. 29 00:06:54.256 --> 00:07:05.369 Mark Kushner: You find out that you know the warm, dense matter materials that we create only last, for like a nanosecond because they're at Megabar pressures, there's nothing around them. So they very quickly explode. 30 00:07:05.780 --> 00:07:33.030 Mark Kushner: So they last for a nanosecond, and then, if you do a quick calculation, you find out that the conduction from one hot material into a cold material is on the micron scale in a nanosecond. So you get like one micron or 2 micron of heat flow from the hot thing into the cold thing, and that's hot, that is, that is small. A hair is about 100 microns. So this is sort of a 100th of a hair time like scale, that the heat is going to flow. 31 00:07:33.718 --> 00:07:46.869 Mark Kushner: But we thought, Okay, we will take this challenge. Let's take some hot, warm, dense matter, and put it next to some cold, warm, dense matter. Whatever that means. Watch the heat flow, measure it on the one micron scale. 32 00:07:47.150 --> 00:07:52.670 Mark Kushner: And so we developed a platform for the Omega laser to do just that. 33 00:07:52.860 --> 00:07:56.749 Mark Kushner: And the idea is kind of simple. 34 00:07:57.020 --> 00:08:10.130 Mark Kushner: You take a tiny metal wire, and you surround it by a thick plastic cladding, and so in. This is on the order of a few microns, and this plastic is on the order of 100 microns. 35 00:08:10.850 --> 00:08:18.200 Mark Kushner: and you heat it all up with a load of X-rays, and the thing in the middle will be hot, and it'll be surrounded by something cold. 36 00:08:18.330 --> 00:08:23.400 Mark Kushner: and the hot thing, because it is extremely high, pressure, will explode 37 00:08:23.640 --> 00:08:36.669 Mark Kushner: and push into the plastic, and at some point the pressure of the middle thing will decrease as it expands, and it will equilibrate with the material around it, and it will just create a nice interface between the 2, 38 00:08:37.110 --> 00:08:39.950 Mark Kushner: and it will stop moving hydrodynamically. 39 00:08:40.400 --> 00:08:54.659 Mark Kushner: And then the way in which it evolves after that time is predominantly through thermal conduction. So heat flowing from this thing into this thing over the nanoseconds. After that initial hydrodynamic motion. 40 00:08:56.600 --> 00:09:00.150 Mark Kushner: The problem is, we can't measure temperature. 41 00:09:00.300 --> 00:09:18.529 Mark Kushner: and that is a statement that is almost entirely true. For this warm, dense matter samples. There is no temperature measurement in this regime until the second experiment I'm going to talk about today. But there was no temperature measurement. Why? Because these samples are opaque to visible light. 42 00:09:18.620 --> 00:09:32.990 Mark Kushner: So they're sort of partially ionized, opaque, divisible light. That means, if you want to know what the temperature of your sample is, or measure anything about your sample. Really, you're going to need to use X-rays and X-rays don't really measure temperature. Very well. 43 00:09:33.170 --> 00:09:37.040 Mark Kushner: you will find a handful of experiments that have measured structure 44 00:09:37.160 --> 00:09:41.210 Mark Kushner: and use that as a proxy for temperature. But there's no real temperature. 45 00:09:41.620 --> 00:09:45.910 Mark Kushner: So the question is as the heat flows from one material to the next. 46 00:09:46.270 --> 00:09:50.319 Mark Kushner: How does that change the density profile across the interface? 47 00:09:51.660 --> 00:09:52.690 Mark Kushner: And 48 00:09:53.630 --> 00:10:01.020 Mark Kushner: it does something really weird, and you would not believe me if I told you so. I'm going to show you a video, and then you will believe me. 49 00:10:01.450 --> 00:10:07.079 Mark Kushner: So here I have 2 materials next to each other in like a standard hydro stimulation. 50 00:10:07.653 --> 00:10:11.970 Mark Kushner: The left clot is the pressure of the 2 materials. You see, they're at the same pressure. 51 00:10:12.390 --> 00:10:14.689 Mark Kushner: This is temperature. 52 00:10:14.950 --> 00:10:18.760 Mark Kushner: So here's a hot material next to a cold material. And this is density. 53 00:10:19.000 --> 00:10:25.919 Mark Kushner: It doesn't really matter which 2 materials they are. We just kind of want to look at the general behavior. What's going to happen 54 00:10:26.790 --> 00:10:38.259 Mark Kushner: is this temperature profile is going to evolve according to the diffusion equation. The temperature is going to go from this one, or heat's flowing from here to here. It's going to smooth out, and you'll see this one smooth out. 55 00:10:38.720 --> 00:10:40.960 Mark Kushner: Nothing really will happen in this one. 56 00:10:41.140 --> 00:10:48.569 Mark Kushner: But watch what happens to the density profile. As I let this system evolve. And I'm going to pray that the video works. 57 00:10:51.570 --> 00:10:55.130 Mark Kushner: Okay, we have to click. 58 00:10:57.600 --> 00:10:58.810 Mark Kushner: Okay, good. 59 00:10:59.150 --> 00:11:02.039 Mark Kushner: Okay. They did not all play at once. But never mind. 60 00:11:02.470 --> 00:11:07.289 Mark Kushner: We'll play these 2. Okay, so this is what happens when you let this system evolve. 61 00:11:07.590 --> 00:11:11.389 Mark Kushner: You see the heat flow from the left material into the right material. 62 00:11:11.660 --> 00:11:21.830 Mark Kushner: You see different scale lengths on the left and the right. That's because they have different values of thermal conductivity, so the heat will flow more easily on the left, on the right. But look at this density profile. 63 00:11:22.410 --> 00:11:25.120 Mark Kushner: This is what happens. What is going on here? 64 00:11:25.330 --> 00:11:30.530 Mark Kushner: Well, what is happening is that the material on the left, as it cools down. 65 00:11:30.850 --> 00:11:49.319 Mark Kushner: is moving to ensure that the pressure is equilibrated everywhere, so as soon as the pressure starts to unequilibrate or change, the material will flow in response to that pressure gradient and the material will be back to being equilibrated. And so, as the material cools on the left, its density increases. 66 00:11:49.480 --> 00:12:07.340 Mark Kushner: and on the right, as the material heats up, its density decreases. So once you're on these long enough timescales that the material can flow in response to the thermally induced gradients, you actually see this sort of crazy, funky density profile appear at the interface between the 2 materials. 67 00:12:08.070 --> 00:12:21.029 Mark Kushner: And so our goal is to see this density profile right here in the experiment, and you can't quite see on this X-axis. But, like I said earlier, the length scales here are of order, 1, 2, or 2 microns. 68 00:12:21.330 --> 00:12:24.669 Mark Kushner: And, as I also said, these materials are opaque to visible light. 69 00:12:24.800 --> 00:12:32.480 Mark Kushner: that we need to take an X-ray of this interface with a 1 micron resolution in order to see this. 70 00:12:36.540 --> 00:12:59.819 Mark Kushner: and we have to do that at a big laser facility, where I can also generate these crazy states of matter. Right? So at the Omega laser facility. And so we've developed this X-ray radiography platform for the Omega laser facility. And it's pretty simple. You have some lasers here to generate an X-ray backlider. So we fire some of the Omega lasers and a vanadium foil. 71 00:13:00.380 --> 00:13:06.480 Mark Kushner: and this generates vanadium helium alpha at 5.2 Kv. Predominantly at least. 72 00:13:06.650 --> 00:13:19.470 Mark Kushner: But of course, the spot size here is 600 microns. This is too big a source to see our one micron physics that we're trying to get at. So we pass it through a slit to create a very tiny source. And then we image 73 00:13:19.840 --> 00:13:21.640 Mark Kushner: cool thing that we're trying to image. 74 00:13:21.920 --> 00:13:31.559 Mark Kushner: And one of our great developments is the creation of these slits which we cut at the University of Nevada, using a focused ion beam. 75 00:13:31.720 --> 00:13:34.369 Mark Kushner: and these slits are one micron wide 76 00:13:34.510 --> 00:13:42.969 Mark Kushner: by 50 microns long, and they are able to generate a small source, so we can look at tiny, tiny features in our X-ray radiography. 77 00:13:43.310 --> 00:14:12.659 Mark Kushner: I used to compare things to the size of a hair. My students have now found a red blood cell is too big to fit through these slits. So that gives you an idea of how small these slits are. And you know what else doesn't really go through this slit. Very well, many photons. So if you're trying to make an image of something right, and we put this in the way, there aren't many photons that get through this slit. So we're always trying to play a game of narrowing the slit, to have a small source 78 00:14:12.820 --> 00:14:18.709 Mark Kushner: and trying to get enough photons to create a nice image. But it does work. 79 00:14:18.940 --> 00:14:21.219 Mark Kushner: and I'll show you some pretty pictures in a minute. 80 00:14:21.460 --> 00:14:29.809 Mark Kushner: The other problem is what's called essentially interference effect. As you make your source smaller and smaller and smaller. 81 00:14:29.940 --> 00:14:33.759 Mark Kushner: you increase the spatial coherence of your being. 82 00:14:34.000 --> 00:14:39.170 Mark Kushner: and that means you start seeing things like diffraction and refraction 83 00:14:39.330 --> 00:15:05.099 Mark Kushner: from your object, and that can be good, or that can be bad, right? So I don't know if you've heard of like phase contrast imaging, then you would explicitly utilize that effect to see interfaces that you might not otherwise see. It can be bad, too, because it can hide the physics that you're trying to get at, because your image now is not a direct translation to the linear density, but now it's something much more complicated. 84 00:15:06.850 --> 00:15:27.910 Mark Kushner: and I can sort of just demonstrate that here. What I have. Here is an X-ray radiograph prediction of our target. On the top you see what we would see in an Omega experiment on the bottom. I have the same X-ray radiograph. But if you were at a free electron laser, so if you had an extremely coherent source. 85 00:15:27.920 --> 00:15:42.459 Mark Kushner: and I just put that to here to say, this is the image you get if you put your camera right up against the target, so you don't have any distance to allow propagation and interference effects. And this is, if you move your camera away to different distances. 86 00:15:43.020 --> 00:15:59.469 Mark Kushner: if you look at the bottom one. This is, if you went to a free electron laser. So you had very coherent source. Look, that doesn't look anything like what you have anymore. Right? This is diffraction effects from the X-rays passing past the target, giving you all of these crazy fringes 87 00:16:00.250 --> 00:16:09.500 Mark Kushner: on the top. But, Omega, it's not quite a spatially coherent, but you still get all of these diffraction fringes and things like that appearing around your physics that you're interested in. 88 00:16:09.660 --> 00:16:13.290 Mark Kushner: So that's the downside is, it's hard to work out. What's going on 89 00:16:13.520 --> 00:16:29.220 Mark Kushner: the upside is that this diffraction pattern is extremely sensitive to small changes in density gradients in your sample. So you can actually see 100 nanometer or 50 nanometer changes in the target 90 00:16:29.550 --> 00:16:53.650 Mark Kushner: will make quite big changes in this diffraction pattern. So if you kind of know what your sample will look like, you can use this to your advantage to get like sub micron resolution. If you don't know what's happened in your sample, you're like, Oh, my God! I've hit it with this laser, and now I've done something, but I don't know what. It's very hard to work out what has happened from the diffraction profile. So there's pros and cons to this. 91 00:16:54.970 --> 00:17:13.000 Mark Kushner: But we've been working on this platform now for a couple of years. There's a couple of sort of instrumentation papers on the platform. If you're interested. If you want to do high resolution, X-ray radiography, we can share our information on these slits and how to set it up and how to align everything. 92 00:17:14.220 --> 00:17:41.769 Mark Kushner: Oh, I always showed this picture, too. I don't know. I did this in school. I don't know if you guys did this in school, but I measured the width of a hair in school by putting my hair into like the laser like this right? And looking at the diffraction pattern. It's the same experiment, except our target is 100 times smaller than a hair, and instead of optical light, we're using X-rays. So the wavelength is 100 times smaller. So it's basically 100 times smaller version of this sort of 93 00:17:41.850 --> 00:17:47.680 Mark Kushner: classic. Oh, so 1,000 times smaller version of this classics like school experiment. Okay? 94 00:17:51.980 --> 00:17:56.300 Mark Kushner: So what we do is we take our imaging platform, which you see vertically here. 95 00:17:56.400 --> 00:18:24.520 Mark Kushner: and then we drive our system into some extreme state by using more laser beams from the Omega laser to create copper helium alpha. So this time at 8.3 Kv. That drives this pretty isochorically, so uniformly into this extreme state. And here's some simulations. You can see the wire in the middle in this case was tungsten with plastic. It explodes for about 2 nanoseconds. After 2 nanoseconds it sits there 96 00:18:25.170 --> 00:18:36.580 Mark Kushner: doing not very much, not very much, not very much. And then at 6 nanoseconds. This rare faction wave from the outside catches up with it, and it explodes so essentially, the whole physics package in the middle 97 00:18:36.590 --> 00:18:55.599 Mark Kushner: doesn't know that it's exploding until the rare faction wave reaches it right? So it still sits there doing nothing for a long time. And we're really interested in this time between 2 and 6 nanoseconds, where the thermal transport from the hot region to the cold region drives that sort of funky density profile that I showed you earlier. 98 00:18:57.880 --> 00:19:13.449 Mark Kushner: Here's some cool pictures of our target. This is the cold target with the wire in the middle and the plastic around the outside. Here's an exploded version. You can see a lot of striations here which I'll talk about in a minute. When I do a line out. 99 00:19:13.650 --> 00:19:37.059 Mark Kushner: You can see we're having terrible issues. This is back in 2022 with alignment. Look at the top. This is in the middle of the detector, but this one's like on the right. Someone's a bit on the left. This is the one where it's far too far on the left, right way off the edge of the detector. And that's because we're trying to line up these few micron, thick targets, meters apart in the Omega target chamber. And this was challenging. 100 00:19:37.658 --> 00:19:59.609 Mark Kushner: My student Sarah has fixed this now, so this has completely been solved with these cool 3D printed target frames, with all these like alignment features sticking out of it that holds everything in place. This alignment feature here sort of sticks out the bottom. This is 200 microns wide. So this is extremely small. 101 00:20:00.150 --> 00:20:13.759 Mark Kushner: 3D printing is just completely revolutionized. What we've been able to do in these experiments. Now. So these 3D printed frames are just are completely incredible. And now every single shot is in the middle of the detector. So it's problem solved. 102 00:20:15.460 --> 00:20:22.570 Mark Kushner: Okay? And so yeah, so here's a cold. Here's a hot shot. Here's the raw data 103 00:20:22.890 --> 00:20:44.389 Mark Kushner: you can see here the outside expanding, which is predicted by the hydro simulations. This is the shock wave moving out, due to the rapid expansion of the material in the middle, and then this is the interface between the 2 materials that's evolving according to that weird density profile. So we see all the features that we would expect to see. 104 00:20:44.700 --> 00:20:53.589 Mark Kushner: We also flip this data left right, and overlay it with itself, just to show how uniform the heating is on both sides of the target. 105 00:20:58.810 --> 00:21:05.900 Mark Kushner: As I said, the one of the issues with diffraction, I'll skip the next one is that it? 106 00:21:06.827 --> 00:21:08.740 Mark Kushner: Obscures the physics. 107 00:21:08.960 --> 00:21:20.230 Mark Kushner: and that's annoying right? And so we can only use forward models right now. So if I have a density profile, I can calculate what the diffraction pattern would look like. I can calculate the radiograph. 108 00:21:20.460 --> 00:21:36.530 Mark Kushner: and then I can check if it matches what I see in the experiment, and that's pretty much the only way you can do this right. This is the information problem in phase. Contrast. Imaging, for example, is the same idea. And so we create a test density profile. 109 00:21:36.670 --> 00:21:40.609 Mark Kushner: And then we play with all the parameters until it matches. 110 00:21:41.690 --> 00:21:48.140 Mark Kushner: And so here's the data. And you can see, here's raw data. Sorry 0 nanoseconds. 111 00:21:48.490 --> 00:22:08.260 Mark Kushner: 2 nanoseconds and 4 nanoseconds. We see the shock going out, we see the interface between the material evolving, and then on the right, you see the density profiles that create these diffraction patterns here. So we fitted these to these. And so this is how the material evolves. So even though here the shock is kind of hard to make out. Here you see quite clearly 112 00:22:08.440 --> 00:22:17.889 Mark Kushner: the shock moving out in the sample and the interface. If you look right here, this is this funky density profile that comes from that thermal diffusion. 113 00:22:18.680 --> 00:22:29.739 Mark Kushner: And then we've done a load of boring stuff with Bayesian analysis and error analysis and things like that. So I'm just gonna skip over that boring stuff, but we have done it. And 114 00:22:29.850 --> 00:22:42.220 Mark Kushner: the the next step is to calculate from the density profile a temperature. Okay, and so here are the density profiles at 2 times that we measured in the experiment. 115 00:22:42.710 --> 00:22:43.859 Mark Kushner: And then. 116 00:22:43.980 --> 00:22:52.800 Mark Kushner: of course, how do you get temperature from density? We have to assume an equation of state, and we have to calculate a pressure in the sample. So this 117 00:22:52.910 --> 00:23:09.569 Mark Kushner: is where, you know, we become a bit more model dependent on the experiment in the experiment, right? So we have to use an equation of state to calculate temperature. And if we do that, you get this thing here, so we can see the heat flowing out of one material into the other material. 118 00:23:10.000 --> 00:23:13.769 Mark Kushner: and I was completely stuck at this point. If you look on the right. 119 00:23:13.980 --> 00:23:32.590 Mark Kushner: you can see the prediction that I showed earlier. So this is the hydro simulation. And do you notice anything different between the experimental data? And then the prediction from the hydro. And I mean, I already put the answer. Here there is a jump or a discontinuity in the temperature at the interface. 120 00:23:32.710 --> 00:23:39.849 Mark Kushner: and this should be impossible. Right? Because if you just solve the diffusion equation, the 1st thing it does is smooth everything out. 121 00:23:39.960 --> 00:23:54.549 Mark Kushner: So I spent so long trying to get rid of this jump. I was like, this cannot be physical. This cannot be wrong. Can I do something to the equation of state model to make this work. And I did completely unnatural things to that equation of state model. And I could not get it to work. Okay. 122 00:23:55.780 --> 00:24:05.599 Mark Kushner: And then I was thinking I was thinking about it. Why isn't it working? And I was talking to a friend of mine? I don't know if you know Siegfried Glenzer. 123 00:24:05.840 --> 00:24:24.090 Mark Kushner: he's kind of a famous physicist in my field, very outspoken, and he was like, everything you've done is bullshit. And that's what he said to me. I was like, wow! And he said, It's because you forgot something called interfacial thermal resistance. I was like, What is that? And he says, Oh, he gets stuck at the interface, and I was like, Oh, does it 124 00:24:24.310 --> 00:24:25.789 Mark Kushner: so? I googled it. 125 00:24:26.430 --> 00:24:31.039 Mark Kushner: And I saw this picture right here on Wikipedia. 126 00:24:31.310 --> 00:24:32.810 Mark Kushner: I was like, Oh. 127 00:24:32.970 --> 00:24:51.444 Mark Kushner: this is a well known effect, right? That he doesn't like to go through interfaces. And this is actually talking about from more of an engineering point of view like there might be a gap or something if 2 things aren't touching. But there is actually something else. And you can write that 128 00:24:52.220 --> 00:25:15.470 Mark Kushner: even for atomically perfect interfaces. Heat doesn't like to go through the interface. Why is that? That's because the heat carriers scatter off the interface. Whether that be phonons scattering because of different vibronic properties on either side, or whether that is electrons. Because of the different electronic properties in our case, high ionization means it's probably electron driven conduction. So 129 00:25:15.730 --> 00:25:18.319 Mark Kushner: it will be different electron properties. 130 00:25:19.020 --> 00:25:38.869 Mark Kushner: And then, even worse than finding something on Wikipedia, I would say, is finding that it was actually predicted in 1822 by Fourier. So this is not a new idea. But I'm going to claim that it is a new idea to my field, at least because I don't know anyone else who really has thought about this in the depth that we have now thought about it. Okay. 131 00:25:39.210 --> 00:25:40.260 Mark Kushner: so 132 00:25:40.530 --> 00:26:00.189 Mark Kushner: he doesn't like to go through interfaces. That is a known fact. It's well known, for example, people who make computer chips. They spend a long time worrying about this because they want to get heat out of them, the chip right the CPU. And so if the heat doesn't want to go from the copper into the silicon, or whatever, then this is an issue. 133 00:26:01.530 --> 00:26:03.370 Mark Kushner: And so 134 00:26:04.770 --> 00:26:17.250 Mark Kushner: we're actually able to measure the thermal resistance of our interface. In this case, I just say this again, this is between tungsten at 20 ev and plastic at like one ev. 135 00:26:17.430 --> 00:26:23.269 Mark Kushner: and the heat does not want to go through this, so that the interface between these 2 metals, even at these extreme conditions. 136 00:26:23.450 --> 00:26:32.800 Mark Kushner: So you might think that if I have loads of electrons flying around like an abundance of free electrons, that you would not get this effect. But that is not true. You do. And 137 00:26:33.360 --> 00:26:58.900 Mark Kushner: if you compare what we get to sort of known values, you actually find it quite similar to metal metal interfaces, which is not a surprise, right? Because these samples are sort of with all their free electrons, might be thought of as sort of metallic. And so this is quite an important effect. I think right, because anytime you have an interface in one of these warm, dense matter experiments. Heat is not going to be where you think it will be 138 00:26:58.940 --> 00:27:10.280 Mark Kushner: if you think about like Ife, like the capsules that they're imploding in these fusion, experiments are layers and layers and layers of interfaces right? And I don't think the heat is going through the interfaces like they think. 139 00:27:10.420 --> 00:27:25.920 Mark Kushner: and that can have knock on effects for like shock impedance. If you change the thermodynamic properties either side of the interface, the shock's not going to pass through that interface. Like you think so? I think this is a very important phenomena, and it is not included, as far as I know, in any hydro 140 00:27:25.980 --> 00:27:46.709 Mark Kushner: code. So if you're running a hydro code. I don't think it has this sort of this Itr, this interfacial thermal resistance in a hydro code. So I'm very much looking forward to writing a hydro code to put this in and then running everything anyone's ever done and being like, Oh, you were wrong. So that's going to be great great fun for the next, like 5 years. 141 00:27:46.860 --> 00:27:56.879 Mark Kushner: there is an equation to calculate it, at least for metals. It's this equation here, some integrals over some density of states. It is beyond my ability 142 00:27:57.010 --> 00:28:09.429 Mark Kushner: to solve this equation. So I always throw this up. If there's a theorist in the audience who wants to help me calculate this. I'd love to calculate this for 2 warm, dense matter samples and see where we end up. 143 00:28:10.870 --> 00:28:25.690 Mark Kushner: Okay, so yeah, that's the end of this 1st experiment. Essentially, I like to say that we've solved it, you know, 200 years ago Fourier predicted that this would happen, and it has now been seen in solid, solid, solid, liquid, solid gas. And now 144 00:28:26.000 --> 00:28:37.820 Mark Kushner: plasma, plasma, warm, dense matter, warm, dense matter, interfaces, too, and I think we should all be thinking about that. If we're looking at sort of layered targets, buried layers, any kind of experiment like that. 145 00:28:39.500 --> 00:28:40.570 Mark Kushner: Okay. 146 00:28:40.840 --> 00:28:54.170 Mark Kushner: Next experiment I want to talk about sort of follows on very nicely. And this is direct temperature measurements. So in that one right, I will admit we inferred temperature from the equation of state and density measurements. Right? 147 00:28:54.370 --> 00:29:11.309 Mark Kushner: The question is, can we directly measure temperature in one of these systems? And the answer is, yes, we can. And I'm going to tell you about that experiment now, and I'm going to talk about a different physics problem here, which is bond strength 148 00:29:12.070 --> 00:29:14.509 Mark Kushner: in non-equilibrium materials. 149 00:29:15.660 --> 00:29:25.349 Mark Kushner: So if you've never heard of that. That's fine. It's a very simple idea. If I take a short pulse, laser 40 femtoseconds, fire it at a metal 150 00:29:25.520 --> 00:29:29.269 Mark Kushner: that laser will dump all of its energy into the electrons. 151 00:29:29.490 --> 00:29:44.889 Mark Kushner: and it will create these hot electrons, and the ions will remain cold. That happens sort of on a sub picosecond time scale. Then, on the next 10 picoseconds, those electrons will heat up the ions, and they will equilibrate and come together right? 152 00:29:46.460 --> 00:29:52.240 Mark Kushner: When you have that initial non-equilibrium system with the hot electrons and the cold ions. 153 00:29:52.410 --> 00:30:02.550 Mark Kushner: the interatomic potential between the ions is altered by the hot electrons. So if you heat all the electrons up, the bond strength will change. 154 00:30:04.040 --> 00:30:26.019 Mark Kushner: And there's this paper, by Bonina recalls in 2,006. This is sort of quite an important paper in our field. And Bonina said, Yeah, if you heat the electrons up to ev 4 ev. 6 ev the debye temperature which we use as a proxy for bond strength. So if the debye temperature gets higher, the bonds get stronger. 155 00:30:26.430 --> 00:30:38.780 Mark Kushner: she said, yeah, they get stronger. So if you heat it up to 80 v. You sort of double the divide temperature. So you can sort of think about that. You make the bonds twice as strong. So this sort of received a lot of attention at the time. 156 00:30:38.890 --> 00:30:47.489 Mark Kushner: This idea that you fire a laser at something, and it gets stronger sort of as a cool idea. So people sort of gravitated towards that. 157 00:30:49.770 --> 00:30:56.409 Mark Kushner: The problem is, in the resulting years. There were a load of experiments trying to 158 00:30:56.610 --> 00:31:25.809 Mark Kushner: measured this bond, hardening. 2 of them, I think, were published in science. This this guy got a Prb. I don't know what he was doing, really, you know, should have tried a bit harder. But what's great about this set of experiments, at least from where I'm standing. Is this one found bond hardening, this one found bond softening. This one found neither. So we were calling this like the Goldilocks problem for a long time. Right? So which one is the correct answer published over decades. 159 00:31:26.180 --> 00:31:28.699 Mark Kushner: and then another one that came out last year. 160 00:31:28.840 --> 00:31:42.309 Mark Kushner: Oh, no, 2 years ago. Now that's sad, isn't it? 2 years ago, and said, it's bond hiding again. So there's this sort of Goldilocks problem. If I fire a short pulse laser at piece of metal, does it get stronger, or does it get weaker? 161 00:31:42.670 --> 00:31:44.299 Mark Kushner: That's what we're trying to solve? 162 00:31:46.360 --> 00:31:49.430 Mark Kushner: Why does everyone have different answers? 163 00:31:49.660 --> 00:31:59.529 Mark Kushner: And the answer to that is really easy. Everybody uses this equation, and you don't have to look at any parts of the equation. Apart from the 2 circled things, nothing else matters. 164 00:32:01.100 --> 00:32:10.590 Mark Kushner: In an X-ray diffraction experiments you send some X-rays in, and you scatter the X-rays or diffract the X-rays off the sample. The intensity of the Bragg peaks 165 00:32:11.140 --> 00:32:16.620 Mark Kushner: is related to the temperature of the ions over the debye temperature. 166 00:32:16.730 --> 00:32:37.490 Mark Kushner: That's because if the atoms are vibrating around more. The Bragg peaks decrease in intensity. If the atoms are frozen in place in a strong lattice, then the Bragg peaks are the maximum intensity. And so if you measure the Bragg Peak decay, you can say something about how much the atoms are vibrating around. That's sort of well known. 167 00:32:38.337 --> 00:32:42.750 Mark Kushner: But how much the atoms are vibrating around depends on 2 things. 168 00:32:43.330 --> 00:32:58.349 Mark Kushner: their temperature right hotter things will vibrate more in the crystal, but also the strength of the potential. Which is this to buy temperature. So if I make the potential stronger, the atoms will vibrate around much smaller amount, and if I make the potential weaker, they can vibrate around more. 169 00:32:59.330 --> 00:33:08.259 Mark Kushner: Why does everyone have different answers for this experiment? It's because, while they measure this thing very well, this is very easy to get the Bragg Peak intensity in experiment. 170 00:33:08.960 --> 00:33:17.649 Mark Kushner: They guess the debye temperature. I'm sorry. No, they guess the ion temperature. They don't really know what their iron temperature is because, like I said, there's no temperature measurement. 171 00:33:18.130 --> 00:33:30.249 Mark Kushner: And so they just guess this, how do they guess it? Well, they assume how much energy gets dumped in they don't really know. Then they assume how fast the ions heat up, which we call the electron Ion equilibration rate. 172 00:33:30.460 --> 00:33:43.010 Mark Kushner: They don't know. There are predictions that span an order of magnitude here, so they sort of make a guess there and then they pull out an answer, and everyone made different assumptions about all those things. And so they all came to differences. 173 00:33:43.880 --> 00:33:51.749 Mark Kushner: So we can solve the entire problem because we can directly measure the ion temperature in our experiment. 174 00:33:51.970 --> 00:34:02.280 Mark Kushner: and then we'll be able to solve this equation. We measure this thing and this thing, and then we get out the bond strength. Then there'll be no questions, and we can finally put this sort of decades old controversy to rest. 175 00:34:04.100 --> 00:34:10.049 Mark Kushner: So here I plot just the solution to this equation. So this is the Rag Peak intensity. 176 00:34:10.350 --> 00:34:32.040 Mark Kushner: And this is the ion temperature for a range of divide temperatures for a range of different potentials. So I'm saying, if you measure this and this, and put a point on here, then you will know what the divide temperature is. So what we need to do is measure the Bragg Peak intensity and the ion temperature. And then we will solve this Goldilocks problem that has existed for, like all these 2 decades. Now. 177 00:34:33.460 --> 00:34:53.640 Mark Kushner: how do we measure ion temperature. So this is sort of the main thing pretty easily in terms of the idea, pretty difficult in terms of the actual experiment. So we essentially use doppler coordinate. So we have 2 samples here, a cold sample and a hot sample. 178 00:34:53.780 --> 00:34:57.829 Mark Kushner: and we send some X-rays in, and we scatter the X-rays off the sample. 179 00:34:58.220 --> 00:35:18.070 Mark Kushner: These are monochromatic X-rays. I'll talk about how we make them in a minute, but they're extremely monochromatic. They have a very very narrow bandwidth, and we scatter them off the target, and they essentially get doppler broadened by the velocity of the atom. So if the atom's moving towards the you will get an upshift away, you will get a downshift. 180 00:35:18.320 --> 00:35:47.969 Mark Kushner: Now our experiments are on gold. Gold ions are very heavy. They don't move very fast. That means the shift you expect in the Doppler shift is really, really tiny, like 50 milli electron bolts. And so we have to work really hard to get a narrow pulse where we can measure these tiny, tiny broadenings due to the velocity distribution. You can ignore this orange box unless you're a connoisseur of X-ray scattering, in which case you can look at this orange box. 181 00:35:48.270 --> 00:36:07.799 Mark Kushner: But mainly you can see here, here's a cold sample. We get this nice sort of tight Gaussian. It's actually a void, but not very much broadening. And then here, after some time, you see, it's broadened and we get a much broader spectrum, and that's just due to the velocity of the gold ions. 182 00:36:11.030 --> 00:36:16.779 Mark Kushner: and we do all of our experiments at Lcls. So we need a narrow 183 00:36:17.310 --> 00:36:28.609 Mark Kushner: pulse of X-rays, and we also needed to be pretty short temporally, because our sample does not last very long, it lasts of order 50 picoseconds in this case. 184 00:36:28.770 --> 00:36:43.249 Mark Kushner: so we go to the brightest X-ray source on the planet, which is a free electron laser. We use Lcls in slack in California, but we also go to the European expel in Hamburg, in Germany, and 185 00:36:43.430 --> 00:36:53.070 Mark Kushner: if you've never gone, they are the coolest facility on the planet, and I will stand by that. It's a billion dollar X-ray laser. It's 3 kilometers long. 186 00:36:53.070 --> 00:37:13.719 Mark Kushner: and it works by firing electrons, accelerating electrons, and then passing those electrons through what's called an undulator, which is an alternating array of magnets, and as the electrons move or wiggle through those magnets releases Bremstrah radiation, and you can get these very cool pulses of X-rays. 187 00:37:14.070 --> 00:37:38.100 Mark Kushner: and like I said, it's the brightest source on the planet. You get 10 to the 12 X-ray photons in 40 femtoseconds and, unlike optical lasers, X-ray lasers work all of the time with no issue. There's just a hole in the wall, and the X-rays come through it, and it works constantly 24 HA day. It never seems to break. It's really a marvel of engineering. 188 00:37:39.900 --> 00:37:54.380 Mark Kushner: the issue is that the X-rays, actually, they have a bandwidth which is still too wide for what we need. So we have to pass our X-rays through a monochromator to get an even narrower bandwidth. And then we can get that down to 50 million electron volts enough to see the 189 00:37:54.800 --> 00:37:56.810 Mark Kushner: to see the photons 190 00:37:57.340 --> 00:38:06.909 Mark Kushner: shift due to the gold iron motion. We scatter those X-rays off some sample. We have a detector to measure the broadening. 191 00:38:08.790 --> 00:38:10.619 Mark Kushner: Oh, there's this cool sort of 192 00:38:11.220 --> 00:38:17.089 Mark Kushner: picture here. Yeah. So the X-rays come in. They go through this monochremator. They scatter off the gold. 193 00:38:17.500 --> 00:38:35.990 Mark Kushner: These silicon dice crystal analyzers up here. I was just saying earlier, these are extremely cool, these crystals. They're $30,000 each. And normally, when you make an X-ray spectrometer. You take a crystal and you bend it into the shape that you want, but in this case the bending of the crystal will introduce strain. 194 00:38:35.990 --> 00:38:53.570 Mark Kushner: and you will ruin your resolution. So this is actually not one crystal, but it's thousands of tiny, tiny crystals, each of which is a planar crystal. So there's no strain introduced in bending, and they're all placed perfectly in the right place, and we'll spend a lot of money on these 195 00:38:53.920 --> 00:38:57.233 Mark Kushner: and that scatters onto a target here, and we can measure the 196 00:38:57.930 --> 00:39:25.670 Mark Kushner: measure the broadening of the spectrum. In that way we look at 170 degrees. So the X-rays come in, and they go back almost exactly the same way they came out. So this is backscattering geometry. So in and out almost directly. Here's the monochromatas that we pass the X-rays through in order to get on our bandwidth, and we also have a big area detector measuring the Bragg peaks which we'll use as well. 197 00:39:25.790 --> 00:39:34.549 Mark Kushner: Here's a picture of our gold. We're actually getting better and better. It says one Hertz. Here we can now go at 10 Hertz. Why is that important? 198 00:39:34.810 --> 00:39:44.429 Mark Kushner: Well, even with 10 to the 12 photons in each X-ray pulse we measure on our detector each time we shoot this laser. One photon. 199 00:39:44.780 --> 00:39:45.910 Mark Kushner: just one. 200 00:39:46.380 --> 00:40:04.950 Mark Kushner: So you can imagine this is the experiment. You have your gold sample. You throw 10 to the 12 photons at it. One comes back, and you make a note of how fast the ion must have been going that it scattered off, and then you do that again. You do that again, and you do that again, and you build up a distribution of how fast those ions must have been moving, and we can go at 10 hertz 201 00:40:05.260 --> 00:40:18.830 Mark Kushner: 10 Hertz is pretty fast. You can get a lot of photon statistics at 10 Hertz, even with one photon per shot. Our limitation now is that these targets cost about $7,000 each, which is about a dollar per shot. 202 00:40:18.970 --> 00:40:40.160 Mark Kushner: and I just can't afford to go any faster. I'm already spending $7,000, or even, you know, quite often we'll spend $30,000 on targets, and they'll all get blown up at 10 Hertz, and I just see the dollars melting away. So if someone asked me how to improve the experiment, I just need cheaper targets, basically at this point. 203 00:40:41.180 --> 00:41:10.050 Mark Kushner: But yeah, we are able to look at the ions, increasing temperature over time, the different laser fluences. Oh, I should say that we pump this with a frequency double thai sapphire laser which creates the extreme states of matter that we're interested in. I know everyone in this room loves like Petawatt lasers, and they're good, too. But I realized if you make your sample just 50 nanometers wide. 204 00:41:10.630 --> 00:41:17.740 Mark Kushner: one millijoule of a laser. Energy is plenty to bring you up to ev temperatures and extreme states. 205 00:41:19.340 --> 00:41:25.809 Mark Kushner: so we are able to see the temperature increase versus time here for different laser fluences. So 206 00:41:25.980 --> 00:41:40.340 Mark Kushner: see, in each case the temperature goes up, and this is the ions equilibrating with the electrons. Right? So we dump all the energy in the electrons. And then over these picoseconds the ions are warming up due to electron ion collisions. 207 00:41:42.130 --> 00:41:49.130 Mark Kushner: This line in each of them represents when the sample melts, and you'll notice there's a kink in the data when it melts. 208 00:41:49.560 --> 00:41:59.169 Mark Kushner: I don't know what that is. Don't ask me about it. I'm going to talk about the left of that line right now. That's not strictly true. I have some ideas, but 209 00:41:59.620 --> 00:42:06.359 Mark Kushner: it's pretty tricky what this kink is gonna be. But you see the ions heating up in each case. 210 00:42:10.170 --> 00:42:29.129 Mark Kushner: we're unable to overlay our data onto this theory plot. So the different optical laser intensities. And in each case, you see, this is this is the data here, and this is the temperature of the ions got from the broadening of my spectrum, and just the ratio of the Bragg peaks from the diffraction. 211 00:42:29.510 --> 00:42:46.559 Mark Kushner: And then we take these points, plug that into that horrendous equation. I showed you at the start, and overlaid it with the theory that my friend Venina published in 2,004. And this is what we got. So you can see, this is the theory prediction for what should happen. 212 00:42:46.790 --> 00:43:05.759 Mark Kushner: And then, you see the points which is our experimental measurement, and, unlike any other experiment that has been done to date, we directly measure everything in this. There's no free parameters. There's no calibration to think of. There's nothing like that. This is just this is the answer, and it's not up for debate. Right? 213 00:43:06.388 --> 00:43:08.141 Mark Kushner: So we're sort of 214 00:43:09.050 --> 00:43:18.450 Mark Kushner: Puzzled at this, I presented this once in front of Venina, who published this line, and I apologize to her for what I'm about to do. 215 00:43:20.050 --> 00:43:21.909 Mark Kushner: which has just shifted down 216 00:43:22.230 --> 00:43:30.170 Mark Kushner: like this right. Theorists love it when you do that and take their carefully position line, just shift it down right? So we shifted it down. 217 00:43:30.370 --> 00:43:38.860 Mark Kushner: and we see that it matches pretty well, and the reason we think that is is because the simulations were done assuming bulk 218 00:43:39.300 --> 00:44:02.740 Mark Kushner: gold. So an infinite amount of gold, right periodic boundary conditions in the simulation. But we have these very narrow 50 nanometer samples with very small grains in them, and what we think is happening is those small grains are weakening the bonds in the gold, so it starts weaker than if you had bulk gold, anyway. 219 00:44:02.740 --> 00:44:17.070 Mark Kushner: But still, once you fire the optical laser at it. You see a dramatic increase in the strength of the gold which increases with electron temperature. So it's an interesting effect that the stronger you are more intense. Your laser 220 00:44:18.150 --> 00:44:29.789 Mark Kushner: the higher the electron temperature will get in the initial picoseconds, and then the stronger the ion bonds will become, and the material will sort of strengthen in the 1st few picoseconds. 221 00:44:29.950 --> 00:44:39.629 Mark Kushner: and that gets stronger, the more powerful your laser. So something gets it doesn't blow apart. It actually gets stronger for a couple of picoseconds before it eventually blows up. 222 00:44:41.500 --> 00:44:49.739 Mark Kushner: I also think this weird graph could be one of the reasons for sort of the Goldilocks problem. Because if you were to come and just measure. 223 00:44:49.970 --> 00:44:57.900 Mark Kushner: even if you did it correctly, like the temperature, the bond strength at 2 Ev. And you just had this 1 point. 224 00:44:58.350 --> 00:45:18.830 Mark Kushner: what would you conclude? You might conclude that you've weakened the bonds because it's much weaker than standard bulk gold. But you don't get to see this trend right? So you would just assume that this is lower than the bulk gold. And so it must have got weaker. But actually, I think it was already weaker because you chose a very thin sample with very tiny grains. 225 00:45:21.290 --> 00:45:50.910 Mark Kushner: okay, so that's the sort of 2 big experiments I wanted to talk about one right, which is that this interfacial thermal resistance, that this heat gets trapped at the interfaces. So we've just had this accepted into nature communications so hopefully, we'll be out. You can read that paper soon, and then this bond strength. One is en route. We are writing it up right now, but we're pretty much done on that one. I just want to take 5 min to share a crazy 226 00:45:50.950 --> 00:46:07.370 Mark Kushner: idea with you, which is something we've just come across. And we're we're trying to get published right now, but is perhaps one of the craziest ideas I've ever had in my career, and we're trying to get it published in nature. So that gives you an idea of the crazy level of this. Right. 227 00:46:07.520 --> 00:46:08.410 Mark Kushner: Tom. 228 00:46:08.870 --> 00:46:11.630 Mark Kushner: I don't know if you know anything about superheating. 229 00:46:12.000 --> 00:46:37.370 Mark Kushner: I didn't 6 months ago. This is a video I stole from Youtube. So superheating is when you heat something up past the phase transition. But it stays in the original phase. Right? And why would that be? Well, oftentimes we talk about heating like melting, or something like that, needing to nucleate at some point in the system, and then you have a phase wave going through it. 230 00:46:37.840 --> 00:46:40.990 Mark Kushner: And the I don't know the famous superheating. 231 00:46:42.480 --> 00:46:47.070 Mark Kushner: the famous superheating example that you might know is this, don't put. 232 00:46:47.570 --> 00:47:00.840 Mark Kushner: Don't put water in the microwave in a glass, because you can overheat it beyond the boiling point, because the glass is so smooth, and then when you take it out, if you do anything to disturb it. 233 00:47:02.420 --> 00:47:14.560 Mark Kushner: it's going to explode because it will violently boil. And so this is a video where they do that, and then they drop like a penny or cent in, and then it suddenly causes it to explode. Right 234 00:47:17.740 --> 00:47:22.619 Mark Kushner: pretty cool. Right? So this is why you mustn't boil things in the microwave. But 235 00:47:22.990 --> 00:47:28.689 Mark Kushner: this is this idea of superheating. You can drive something past the point where it should mount. Okay. 236 00:47:33.350 --> 00:47:49.130 Mark Kushner: Short history of superheating. In 1948, this guy Kausman, wrote this very influential paper, and he actually said he looked at super cooling, he said. If you take the entropy of a liquid and a solid, and you just. 237 00:47:49.620 --> 00:47:52.940 Mark Kushner: you know, extrapolate wildly downwards. 238 00:47:53.080 --> 00:48:12.360 Mark Kushner: there's a point at which they cross, and that point at which they cross the entropy of the solid becomes more than the entropy of the liquid, which is thermodynamically impossible. You cannot have like a more disordered solid than a liquid. And he said, at that you can never super cool something beyond that point. Thermodynamics does not allow that. 239 00:48:13.070 --> 00:48:14.949 Mark Kushner: and for the next 240 00:48:15.050 --> 00:48:35.520 Mark Kushner: this, 1948, 70 years this paper has been cited. 4,000 times. People have been looking at this transition. No one has ever surpassed it. A pretty solid theoretical grounding. You just cannot get cooler than this limit. In 1988, these 2 other guys wrote this paper in nature, and they just extrapolated upwards. 241 00:48:35.970 --> 00:48:51.179 Mark Kushner: And they said that you can never heat something beyond 3 times the melt temperature because of the same thermodynamic instability, the entropy of the solid and the liquid cross, and it just cannot ever exist. Right? So 3 times the melt temperature is what they predicted in 1988, 242 00:48:51.630 --> 00:48:53.730 Mark Kushner: and, unfortunately for them. 243 00:48:54.530 --> 00:48:59.129 Mark Kushner: a load of other people came along in the resulting year. Then they lowered this limit 244 00:48:59.240 --> 00:49:17.609 Mark Kushner: so that you will never, ever get to this 3 times the mountains be totally impossible. One of the reasons this is the cool area is because they use the word catastrophe a lot in these papers, which is nice. There's this hierarchy of catastrophes, they call it like different ways in which things are going to mount. 245 00:49:18.080 --> 00:49:28.359 Mark Kushner: and I don't know if anyone noticed when I showed the data earlier. But if you noticed what temperatures our samples got to when we fired that laser at them 246 00:49:30.960 --> 00:49:34.309 Mark Kushner: significantly higher than 3 times the melt temperature. 247 00:49:34.450 --> 00:49:53.889 Mark Kushner: So here I just plot our time versus temperature for our samples. And if this is the plot here, and so if you just look at this point, for example, which we measured at 3 picoseconds, we get to 20 kilocalvin, so 20,000 kelvin for gold, and this is 14 times the amount temperature. 248 00:49:53.950 --> 00:50:18.800 Mark Kushner: So my argument is, is this the hottest crystal that anyone has ever measured? Have we made something which is hotter than anyone has ever measured before? And have we blown through this entropy catastrophe that was predicted in 1988 as the ultimate limit of superheating. You know nothing can ever go beyond. 3 times the melt temperature? Have we blown through that limit and created something which is actually 14 249 00:50:19.000 --> 00:50:20.790 Mark Kushner: times the melt temperature? 250 00:50:21.370 --> 00:50:45.210 Mark Kushner: And I think the answer is yes. Do I think I've broken thermodynamics? No, that would be crazy of me to suggest right, and I would probably be laughed out of any room with a load of scientists in it. What do I think is happening? Well, I agree with this 1988 paper that these 2 lines cross at 3 times the melt temperature these entropy lines if 251 00:50:45.680 --> 00:50:56.809 Mark Kushner: you let everything expand like things do when they're hot. So I think when things are hot, they expand, and that expansion contributes to the entropy of the system. If you turn this off 252 00:50:56.920 --> 00:51:07.389 Mark Kushner: and you don't let things expand. Actually, the solid entropy line falls from this little light blue one to this dark blue one, and the lines don't cross anymore. 253 00:51:07.590 --> 00:51:19.439 Mark Kushner: So I think there's a very simple answer for why we're allowed to heat something up to be on 3 times the melt temperature. I don't think I've broken thermodynamics. I think everything sort of is fine, but it does raise the question now 254 00:51:19.830 --> 00:51:37.629 Mark Kushner: of what is the ultimate limit of superheating like, how hot can you make something before it will spontaneously melt? Is there a limit of superheating. And I think that's now might be a more of an open question. Again, thanks to the results of this experiment. 255 00:51:37.740 --> 00:51:43.210 Mark Kushner: Okay, that's just one crazy idea. I wanted to share with everyone at the end. Kind of fun. 256 00:51:43.710 --> 00:51:50.369 Mark Kushner: Last thing, I'm just gonna do a plug. I hope that's okay. I am running a conference 257 00:51:50.410 --> 00:52:16.299 Mark Kushner: next this summer this summer in July, strongly coupled Coulomb systems. So if you love strongly coupled plasmas, I know some people in this room love strongly coupled plasmas, or any of these other topics that we're going to be talking about. Then you should try to come to my conference should be good. It's in South Lake Tahoe, in Nevada. So it's in a beautiful location. 258 00:52:16.300 --> 00:52:44.330 Mark Kushner: I also have $45,000 from the Nnsa to sponsor students to attend the conference. It's only for what do they call it? American people, us people, us persons which means a citizen or a green card, basically. But yeah, there is money available to help students attend the conference. So I'm currently working on how to make that appear on this tab. But it will appear in the next week. I think some kind of like 259 00:52:44.330 --> 00:52:54.540 Mark Kushner: system to sign up for that, but otherwise please attend should be fun in a very beautiful location, and you can stay an extra week and have like a vacation or something. 260 00:52:54.970 --> 00:53:11.389 Mark Kushner: It's also very easy to get from Reno to here, as I found out yesterday, very, very easy, just like 3 h to Salt Lake City, and then 1 h to Reno. Not too bad. Right? So yeah. Think about coming? Thank you very much. 261 00:53:17.020 --> 00:53:19.536 Mark Kushner: Thank you. Are there questions? 262 00:53:21.310 --> 00:53:23.641 Mark Kushner: Yeah, thanks for the next thought. 263 00:53:24.140 --> 00:53:37.210 Mark Kushner: my question was on the thermal conductivity measurement. So you kind of described how the interfacial. But now that you know about it, can you incorporate that into your 264 00:53:37.450 --> 00:53:47.259 Mark Kushner: model? Yeah, that's a great idea. Yeah, we actually did get the thermal conductivity of each material out as well. 265 00:53:47.758 --> 00:53:58.369 Mark Kushner: But it's not like as exciting as this new discovery. Right? So I think the we got the thermal conductivity right here for tungsten on this graph. 266 00:53:58.510 --> 00:54:01.759 Mark Kushner: where I compare against some models that I can find 267 00:54:02.680 --> 00:54:11.210 Mark Kushner: the issue or the utility of that information is low, because the 2 materials we use were tungsten, which 268 00:54:11.670 --> 00:54:23.010 Mark Kushner: no one cares about, I think, or is, you know, not not as interesting. Certainly, in the regime where we think we are right, which is way up here in temperature? Certainly not where these theory plots were. 269 00:54:23.960 --> 00:54:27.829 Mark Kushner: The other thing is, the other material is c. 8 h. 4 f. 4, 270 00:54:28.370 --> 00:54:43.999 Mark Kushner: which we use for experimental reasons, and I've never regretted having fluorine in anything. It's just so disastrous when it comes to trying to understand what's going on. So we've now repeated these experiments with Ch. We've also 271 00:54:46.100 --> 00:54:54.760 Mark Kushner: I didn't have time to show it, but we have these cool, cool, new targets. 272 00:54:54.780 --> 00:55:16.940 Mark Kushner: thanks to my student Sarah, which is iron nickel alloys, which is very similar to what you find inside planets. And then this is a glass surrounding it. So we've got these new targets with much more astrophysically relevant materials that I think people will be interested in. We also have these cool, amazing, 2 PP. Print, 3D. Printed targets. 273 00:55:16.940 --> 00:55:32.370 Mark Kushner: So this is going to be deuterium in the middle, liquid cryogenic deuterium surrounded with plastic. So we can look at deuterium, which is a very interesting material. I just put this in. This is this 2 PP printing technique. They printed this cylinder for us. 274 00:55:32.711 --> 00:55:49.089 Mark Kushner: This is as wide as like a couple of human hairs, and they 3D. Printed it like astounding what they can do now. But on the website they have this small picture here of these champagne glasses, and it says underneath, filled with one nanolitre of champagne. 275 00:55:49.290 --> 00:55:57.769 Mark Kushner: I don't know if they really did that. But that's what they claim. Anyway. My point is that I did get thermal conductivity of tungsten and CAH. 4 F. 4. 276 00:55:57.960 --> 00:56:00.450 Mark Kushner: If you're interested, I can give that to you. 277 00:56:01.014 --> 00:56:05.259 Mark Kushner: But hopefully, we will do more interesting materials imminently. Yeah. 278 00:56:09.090 --> 00:56:11.300 Mark Kushner: So I think I understand. Oh, sorry 279 00:56:11.980 --> 00:56:32.090 Mark Kushner: when you measure that temperature, jump in your 1st experiment. You said you used an equation of state to get that. And you just like ran a code to see how it expanded on one side and how it was expanded. How did you actually get the jump? I missed it? No, right? So we have density as a function of radial distance. 280 00:56:32.310 --> 00:56:47.579 Mark Kushner: We calculated the pressure of the system, which we did by looking at the late time behavior where the pressure equilibrated, and we just sort of calculated what pressure that must be at. So we had the pressure, and then we fed the pressure and the density into an equation of state to get temperature. 281 00:56:49.840 --> 00:57:05.879 Mark Kushner: It's different on the 2 different sides. I guess it was just it just didn't match up. Yeah, exactly. And we, I mean, we? Really, I spent a long time trying to make it match up by trying different equation of state models and things like that. And I just could not get it to work. So I think the 282 00:57:06.130 --> 00:57:24.439 Mark Kushner: the fact that there is a temperature jump, I'm 100% confident that there must be a temperature jump if it's exactly that. And you change different equation state models. Maybe I'm not so sure on that. We had to mess with the equation of state models by multiplying like some factor of 3. And it's just not possible that it could be off. 283 00:57:25.215 --> 00:57:26.970 Mark Kushner: By that much. 284 00:57:27.500 --> 00:57:31.514 Mark Kushner: The other thing about that, I will say is, I think. 285 00:57:33.910 --> 00:57:41.870 Mark Kushner: like the idea that this interfacial thermal resistance exists. I think I could have had the idea without doing the experiment. To me. It's obvious 286 00:57:41.990 --> 00:58:08.169 Mark Kushner: that it should be there. If it's there between 2 metals, why would it not be there between 2 plasmas? Right? And so that's sort of why I really want to calculate the value that I get from 2 sort of warm, dense systems. So I just need to find some electron density of States for these 2 systems and plug them into this equation and get a number. And if it's sort of ballpark what you get in metals. 287 00:58:08.190 --> 00:58:18.690 Mark Kushner: then I think I mean, I would have loved to have written that paper in advance, so I could predict this and then find it and look even cleverer right? But I haven't done it that way around. So 288 00:58:18.920 --> 00:58:24.569 Mark Kushner: yeah, I think it should be the experiment or not. I think it's a it's obviously the. 289 00:58:25.630 --> 00:58:27.370 Mark Kushner: So in your second experiment. 290 00:58:29.090 --> 00:58:33.550 Mark Kushner: It's it is well known that you know with laser you cannot hit uniformly. 291 00:58:33.730 --> 00:58:39.370 Mark Kushner: you know. Solids. Yep, and you know, even your you know film was relatively thin. 292 00:58:40.050 --> 00:58:47.649 Mark Kushner: I mean, maybe you need to have it even thinner, or you use. Why didn't you use X-ray Lcls X-ray to hit your center. 293 00:58:47.850 --> 00:58:55.350 Mark Kushner: Well, the problem with using the X-rays at Lcls is, it's right now, it's just one pulse. 294 00:58:55.960 --> 00:59:10.579 Mark Kushner: So we want to delay like 2 picoseconds or 3 picoseconds. So you would need 2 pulses of X-rays with few picosecond delay. And they're starting to be able to do this. This is sort of new, but yeah, they can split it. And things like that. 295 00:59:10.700 --> 00:59:26.560 Mark Kushner: So in our case, we have to. We use the optical laser, and we rely on 2 effects to get the uniform heating one is that the optical laser really excites these ballistic electrons that fly through, and the length of the 296 00:59:26.680 --> 00:59:36.690 Mark Kushner: so the mean, free path of these ballistic electrons has been measured to be about 150 nanometers, and so we chose our sample to be 50 nanometers so much smaller than that. 297 00:59:36.750 --> 01:00:02.739 Mark Kushner: The other thing is that even after these electrons have heated it up, you get this hot electron, population, maybe like 7 or 8 Ev electrons and cold ions, and the thermal conductivity of those hot electrons is very, very high. So even if there is any initial temperature gradient within, like a sub picosecond time scale. We expect the electrons to thermalize between the front and back of the film. 298 01:00:02.920 --> 01:00:16.859 Mark Kushner: But you're right. X-ray heating is much more optimal. That would allow us to go to thicker samples, too. That would increase our X-ray scattering volume, and we would get more than one photon per shot. 299 01:00:17.320 --> 01:00:30.409 Mark Kushner: And we're actually going to do the same experiment this summer, but with thick samples and long pulse laser drivers to shock it. And I'm sure that experiment is going to have way more difficulties with uniformity than this one right here. 300 01:00:33.980 --> 01:00:39.010 Mark Kushner: So in your sort of buried wire experiments, I know being a a 301 01:00:39.660 --> 01:00:49.769 Mark Kushner: because have you thought about instabilities forming along the length of the wire striations. And you showed one d simulations. But I've got a lot 302 01:00:49.770 --> 01:01:11.799 Mark Kushner: about instabilities forming mess with your diffraction. Yeah, yeah. So there are a few things that I've worried about in addition to thermal connection, the one which I also get asked quite a lot. But it's not your question is the particle diffusion? Did the particles just diffuse into each other? And my answer? There is no, because the tungsten ions are very heavy. 303 01:01:12.620 --> 01:01:13.550 Mark Kushner: Then. 304 01:01:13.820 --> 01:01:37.389 Mark Kushner: in terms of instabilities, there are 2 instabilities that people like to bring up Rayleigh Taylor, Rigmashkov right? So on the Rigmay Meshkov. We're safe because the shocks don't pass through the interface. The interface pushes and sends a shock off, and as long as that doesn't come back you're pretty much okay on the Rayleigh Taylor. We also think we're okay because we have a denser material pushing on a light material. 305 01:01:37.470 --> 01:01:46.339 Mark Kushner: And then, even in the deceleration phase that switches and we have the dense material slowing down the line of material. So we don't think that we're going to get any 306 01:01:46.660 --> 01:02:05.239 Mark Kushner: any instabilities in the system. We have done some 2D Rad hydro simulations in polar coordinates to see if we can see it like around and along, and we don't see it. But these are hard simulations to do, and it's hard to prove to anyone that you don't have instabilities. 307 01:02:06.440 --> 01:02:10.890 Mark Kushner: Thanks any other questions. 308 01:02:12.250 --> 01:02:15.630 Mark Kushner: Thank you very much. Thank you.