 Physics experiments that we perform with a high-powered laser at the large plasma device in Los Angeles and The motivation for this work comes from from space physics. So we're trying to Simulate magnetized collision of shock waves. Those are shocks that mediated by magnetic reflection that you find in the heliosphere and Best example is a planetary bow shock such as the earth bow shock So this is a little schematic here if you look you have the solar wind coming in at high Supalphynic speed and when it meets the an obstacle here, which is the earth magnetosphere It's launching a shock wave and depending where you look at and what the angle is between the shock normal and the interplanetary magnetic field you can either have a Perpendicular shock, which is called the Magnetosonic shock wave where you have a well-defined shock ramp. That's relatively small Or if you look here whether the shock normal is more aligned with the magnetic field then you you create a so-called Parallel or alphanic shock wave, which is much more diffuse. There's a lot of turbulence and waves upstream and downstream and In the experiment we try to reproduce this situation by ablating a laser plasma So this is a picture. There's a target that is irradiated with a high intensity of laser and a Plasma of blades at supalphynic speed into a pre-formed Ambient plasma and so here the laser plasma acts as piston that is moving and the ambient plasma is stationary in Space the piston is magnetosphere and it's stationary and the the plasma is the solar wind which is moving but in the shock frame the situation is exactly the same and So my that the convenient thing about the experiment is we can change the orientation of the laser plasma Relative to the magnetic field so we can study both magnetosonic and alphanic shock waves And my talk is split in two parts first. I'll talk about the perpendicular case and then the parallel case the Main goal of the experiment is to investigate shock formation. This is something it can only be They can cannot be done easily in space since shocks in space are the steady state. They always exist Shock formation is relevant to shock reformation and also to the dissipation How does the shock decelerate the incoming plasma and then of course it's interesting for code validation? In particular, hybrid codes have been used in the past to study space shocks, but they are They're facing several challenges when they're applied to much smaller systems that are comparable to the to the relevant scaling Such as comets or man-man explosion and these laboratory experiments so The work is performed at the large plasma device LAPD in Los Angeles This is a user facility for basic plasma science and Walter Geklman is here. He's giving the next talk He will talk more about it I'm sure for the purpose of this talk. I just want to say it produces these large magnetized Plasmas, it's almost 20 meters long half a meter diameter The plasma density is is around 10 to the 13 per cubic centimeter at 10 electron volts and their magnetic field coils that produce an axial magnetic field of several hundred gauss along the machine it's pulsed on once a second by discharge But it is current free. It's essentially steady state so the lifetime is several milliseconds and it's highly reproducible and For these shock experiments. We use a high-power laser that is coupled to the LAPD the laser fires It's fired on a on a solid target inside the preform plasma and you're creating these exploding plasmas at superphonic speed The purpose of the laser plasma is to act as a magnetic piston to lunge shocks in the ambient plasma And because we have the spectrometry geometry, we can we can look at perpendicular oblique or parallel shocks Since the plasma is so large The shock can fully separate from the piston so eventually The shock is the piston is just energizing the background and the shock is propagating within the ambient plasma It's independent of the specifics of the driver and also since the plasma is large enough to support alpha in waves They can participate in the evolution of the shock, which is very important for the purpose for the parallel shock And then lastly everything is low pressure So we can stick in probes and do local measurements of plasma parameters So the first Part I want to talk about is the magnetosonic shock so perpendicular to the field and this is a Schematic of the experimental setup. This is One section of the LAPD so that the plasma is going this way There's a magnetic field is pointing along the plasma column and we have a solid target coming in from the top It's plastic so carbon and hydrogen and the laser beam is coming in from here at an angle It strikes the target the surface of the target is pointing to the right and the laser blades always perpendicular to the surface So exactly across the field, so we're only using about 50 centimeters of the plasma We don't do not take advantage of the full length of the ambient plasma in this case And we have probes such as magnetic flux probes or fibers coming in that can probe the magnetic field at different distances from the target And this is a picture of the laser plasma When it is very small just a couple of nanoseconds after the laser strikes You see it plates to the right at high speed At our laser intensity is around 10 to 13 bus per square centimeter the bulk debris is actually carbon plus 4 and And from now on I will call this the debris so the debris ions those are the laser plasma ions that explode into the Ambient so they move at 600 grams per second with the large velocity spread The coupling from the laser to the debris I can a energy is very efficient can get 50 percent into the debris ions All right, so before I show you some data was a few words about how these shocks work This is a schematic of a magnetosonic shock So here's the upstream region. This is the actual shock ramp And this is that the downstream the compressed region so plasmas coming in from the left that superfiniic speed It gets decelerated in the shock and then when it comes out it's the parthenic and This is that the jump across the shock so the density magnetic field temperature everything goes up Based on the idea that these shocks evolve from a magnetosonic Soliton that Steepens in time the thickness is relatively small It's on the order of the electron skin depth which is comparable to the electron gyro radius And it is much smaller than the iron gyro radius. So the incoming plasma leads to a charge separation the un magnetized Ion's keep going the electrons are retarded in this horizontal electric field leads to an E cross B drift of the magnetized electrons along the shock normal and it's exactly this relative drift between the electrons and the ions that provides the free energy for For instabilities to grow to provide that provide the dissipation in these shocks also the gradients Density magnetic field temperature can lead to additional cross field currents provide dissipation and then eventually When when the shocks get strong enough let's say Mach 2 feeling Mach 2 or higher Then this this horizontal field and the the potential gets so large that the irons some of the incoming irons get reflected And in the plasma rest frame They change momentum by a factor of 2 so they actually speed up They will return back to the upstream region But they gyrate once until they come back to the shock and now have enough energy to cross the shock so they can pass and only on the downstream side they're now gyrating which Effectively increases that the ion temperature and again it provides free energy to drive instabilities and turbulence to thermalize the plasma so in the experiment We have the additional complication. They have two different plasmas We have the laser plasma and we have the ambient plasma And what the purpose of the laser plasma is to act as a piston and shock the ambient And this is a picture of the laser plasma About a microsecond after laser fires. So this is 50 centimeters by 50 centimeters. This was filtered For carbon plus 4 so it only shows the debris irons and you can see that it has formed this drop shape bubble here The leading edge is is it's moving at about 600 kilometers per second, which is super alphanic The the black thing around is that's the ambient plasma. You can see it and if you look at the magnetic field shown here on the right the color is the magnetic field Then you can see that Inside the the plasma here inside the bubble the field has been expelled The same cross field currents that I was just pointing out in the shock They will actually create a magnetic field in the opposite direction that expels The field in the simple mhd picture you could say the highly conducting laser plasma is pushing the field out of the way and Compressing it ahead. So this is the diamagnetic bubble a field free region and the field gets compressed on the outside And this is also the magnetic pulse which eventually will will steepen into a shock if you if you let it grow long enough So it turns out for the purpose of coupling turbo lens isn't really that efficient yet effective So coupling between the laser plasma in the ambient here is due to large-scale laminar electric fields and if you look at the Electromomentum equation coupled with Amperes law, then you see that there's several Factors that can create an electric field for the coupling This is the pressure gradient which we typically ignore you can have Magnetic field gradients or curvature and most importantly in our case You can have this iron term So you have a term that depends on the iron current of the laser debris cross magnetic field This is called the Lamar term this dominates for supersonic explosions, and it will point downwards in this picture So the iron current J points to the right the magnetic field comes goes It to the parts that points down Okay, and if you look at the errors here the errors in this picture show the electric field you can see there's a strong radial field that he slows down the The debris that's actually from this term, but in the magnetic pulse everything kind of points downwards If you look very carefully, okay, so we can see this in the experiment and I just want to show you two sets of two movies here This again is a picture of the debris. This is filled up for carbon plus 4 50 by 50 centimeters the target is here and the plasma explodes to the right and there's a magnetic field going into the Board this picture is the same, but now it's filtered for the helium background and So we'll only shown to show the the ambient plasma and in fact for this experiment The ambient plasma was limited in size to this region here so there's neutral helium around it and Plasma sitting here and at this point you can't see it because it is it's too dim so now If I start this you can see the laser plasma Floating to the right And eventually it will it will propagate through the the ambient. There's some interesting flute modes that develop here Probably due to the low hybrid drift disability or some magnetic Rayleigh type in stability And what happens is when this debris cloud plows through the ambient it gets intensified By several orders of magnitude and so what happens is these flute modes They create fast electrons which intensify the self-emission okay, and If you look later That's the same picture for the ambient ions. He's running the same movie, but now it keeps running see after it has been intensified The ambient plasma gets pushed out and up and so this is quite Remarkable I'll say because if you look at the the mean free part of a debris ion relative to In the in the ambient plasma it is a hundred meters experiment is a fifty centimeters here So the debris ions you should just move through it but instead they do couple to the background they push it up and They couple through this Lama field, which is actually vertical so even though the electric field points down everything is moving to the right and You can see this here We did spectroscopy of the self-emission so we had a Viper looking up Integrating the self-emission along this line of sight and when you look near the target at early times Then you seeing this spectrum here shown in red. This is wavelength versus intensity. The blue curve is the helium background By itself, you just look at the LAPD plus mobile itself. It shows this helium plus line Okay, so with the debris cloud Coming by it is broadened and it is shifted to the blue But we're responding to about 200 kms per second speed So that means here at this location the ambient is actually moving downwards which is consistent with this Lama field and then The ambient I say they speed up They start generating in the background field and then when you look further away from the target They should be moving up and sure enough when you look further away at later times you seeing this spectrum here So now it is redshifted So the laser plasma bottom line the laser plasma is doing exactly what it's supposed to be doing It is coupling energy and momentum to the ambient without collisions and So now with this in mind now we can look at you look at the actual shock and this light is a little bit busy This summarizes what you have to do to launch a magnetosonic shock in the laboratory this is based on work by Bashur and more than 30 years ago and It it puts the Lama radius of the debris and the ambient in relation to these two scaling length This thing here. I am this is called the equal mass radius and it's essentially the size of the diamagnetic bubble okay, and What Bashur and found is that the debris ions must have a gyro radius that is not much Larger than the than this equal mass radius if they are larger. They will just leak out of the bubble and they want couple Okay, so if I in this plot here, I'm plotting the ratio of the The piston or the debris Lama radius over this equal mass radius In order to couple you have to be here kind of left of around one. You can't be at very high values okay the problem is now in in the laboratory we can use other gases in space everything is protons and to hydrogen but we can change the mass of the ambient plasma and For example, if you would go to to neon or argon then the the alphanic Mach number will go down and the alphane speed will go way up However, it would suggest you can drive shocks more easily. However, the ambient ions wouldn't be magnetized So that's where the second term comes in the so-called equal charge radius This is the distance over which the piston ions have overrun and an equal charge density And it turns out that this is essentially the length over which Lama coupling is effective If you had protons exploding into hydrogen these two scale length would be would be exactly the same But if you go to a heavier ambient masses, then this is going down And so the bottom line is that both the debris and the ambient must be magnetized So these two conditions have to be fulfilled and if I'm plotting everything in this one plot here It means we have to be in this lower left quadrant here Okay, also everything to the right of this line is super alphanic so everything here is not a shock anyway and These points here summarize all the the experiments that we performed over the years when we started out We could create super alphanic flows, but they wouldn't fulfill this these coupling criteria and there would be no shocks and Only once we had increased the laser energy and the laser density The ambient density where we able to go in in the right parameter regime here Okay, so the bottom line is from the experiment point of view You need laser energy to maximize the bubble size to maximize this quantity here But you also need ambient density so the alphanic speed goes down and you can slow down the ambient ions Otherwise, they are not magnetized Yeah, so I'm going to show you some data from this point here, which is a shock and This is these are magnetic profiles As a function of time measured with the B dot probe every line is one shot And the probe has been moved to different distances from the target here and what you can see is this is the magnetic magnetic sonic pulse and Then this is the piston the piston here the diamagnetic bubble From the time of flight you can calculate a speed of 600 kilometers per second Which is about Mach 2 and you can see that eventually this is stalls out and The piston has a maximum size here of maybe of a half a meter and most importantly this pulse At the edge of the bubble is separating eventually From the piston and it's deepens into a shock. So this would be the shock. This is the upstream region This is the downstream region and if you look at two of those curves Here where the shock has formed and you compare a case with the ambient plasma in black and a case without the ambient plasma in red You see there's a huge difference You will always get the diamagnetic bubble and a little bit of compression even in vacuum. However, only with the ambient plasma Will you get a compression that's consistent with the jump conditions? The leading edge is much faster So that shows has been has been energized the the piston is much slower And it's also smaller. So you can see how energy has been transferred to the ambient plasma And if you look at the profile at this time here, then you get this this is Space versus medical compression. So you see the unperturbed upstream region You see the actual shock ramp and then this is the shock downstream region which is several ten gyro edges and size and this would be the piston the cavity and We do to verify that this is indeed a shock we run hybrid simulation. So these are Pig irons and fluid electron simulations in two dimensions with three dimensions for the velocities This allows us to eliminate the shortest Time scale so we can run a problem of the size of the experiment on our small computer and What this shows here is again space about 70 by 70 centimeters the color is the magnetic field the target is here and The plasma uplates to the right and these little white dots are actually some of the debris ions and if I run this Then you see how the debris ions implode to the right They're forming this diamagnetic bubble shown in blue and at the edge of the bubble There's this magnetic pulse which eventually Steepens into a shock and propagates out to the right And you can see how the debris ions actually stop at the edge of the bubble eventually so this little strip shows the density the ambient density and You can see that this pulse is carried by the ambient plasma not by the debris ions See this these are the ambient ions So that's the shock and in fact if you look at this If you look at this in some more detail this plot here shows Again, this shows the magnetic field in color as function of time and space This is the bubble. This is the magnetic pulse at the edge which eventually steeped us into a shock The shock has formed when the speed changes here This is when the shock has formed and if you look at the line outs at this time shown up here You have the magnetic field in black so we could see is the bubble and then this is the shock and In blue you have the ambient ion density so you can see there They correspond to each other so that the shock is carried by the ambient ions The debris is shown in red the debris has already stopped and the shock and the aminites are keep going and This is the phase space Velocity as a function of distance from the target for the debris in red and for the ambient in blue And you can see how some of the debris has been stopped The ambient has been swept out from this region and has been sped up to Mach 2 and most importantly you can see this little Ring here. This is actually a signature of reflected ions which provide dissipation in this shock The hybrid code does not include The turbulence that that provides dissipation in the shock So the only only this patient that we have here is reflected ions Okay, so this is the perpendicular case. So now I'll talk about the parallel case. This is More interesting probably for this audience because the parallel shocks are formed by electromagnetic ion ion stability. So this depends much more on turbulence and It's essentially the same experiment we use the LAPD. We use the same laser, but in a different geometry So the target is now in the center of the machine and the surface is actually facing along the magnetic field and The beam is coming in here with the large mirror inside the vacuum vessel So that we can irradiate the surface the debris explodes exactly along the plasma column in the magnetic field and This gives us much more space. We now have say 15 meters of plasma to interact with In this case the formation depends on electromagnetic ion Instabilities, so there is for ions moving along the field. There is really no bubble There's no piston. There will be a little bubble because some ions are moving at angles but the Diomagnetic bubble is not really important here. What is important is that you have super affinity debris ions moving along the plasma and These shocks require more space and it requires about a hundred ion inertial length, but we also have a lot more space along this direction Okay, so There are two electromagnetic ion ion stability set up particularly important for alphanic shocks One is the so-called right hand instability or HI This is it's simply a gyro resonance between the debris ion and a magnetosonic wave so this is a right-hand right-handed instability and What it can do is it can pitch angles get the ambient ions which actually leads to a compression in the transverse direction So that's not really what we want. We what we would want is coupling to the background in the longitudinal direction However, this right-hand instability has a very has really no threshold for the onset of growth and so It's dominant when the debris ion density is less than the background density, which is which is the case in our experiment It still needs to be super alphanic and in fact this plot here shows the growth rate for this right-hand instability as a function of Mach number alphanic Mach number and ratio between beam density and ambient density and so The rather it is that the the first it will grow and so you can see these points indicate where the experiment is Depending on the laser energy. And so we should easily see it Even at less energy, we should see it More interesting later on will be this non-resonant instability or in our eye and this Requires more beam density It also needs to be super phoenix what this will do is it can Accelerate the background ions the ni is essentially a shear wave a shear alpha in wave that becomes non-linear While while this is a big data sonic wave Okay, so for now the goal of the experiment was to just create a field parallel debris cloud that super alphanic that travels through the LAPD over several meters and Is dense enough to excite the right-hand instability later on? We will try to incite the non-resonant stability and actually see a shock wave so Since this depends critically on the debris ions. I want to say a few words about the Debris I see an effect we can measure this with the lung may probe. So it's just a conducting tip several meters from the target that collects the iron current and so you can see the arrival of ions and From this we can just calculate the spectrum So this plot here shows the iron current as a function of speed and the black curve is this lung lung may probe data and you can see it peaks here At around 300 kilometers per second, which is Mach 2.5 or so But there will be fast lines all the way up to 500 or 600 grams per second there will also be many slower lines and In addition we we can look at several charge states with the with the spectrometer So for example, we can look at fluorescence from carbon plus 4 which is our bulk debris and Then we get this this distribution here. So what this shows you is Essentially that the bulk is indeed carbon 4 But there's faster stuff that has to be some other charge state. We can't see this at the moment It could be carbon plus 5 it could be carbon plus 6 it could be protons and When you look at carbon plus 2 it's it's here's the slower stuff. So apparently there's also some carbon plus 3 Okay, the reason why I'm telling you this is This velocity spread is even bigger because we have a spread of charge states and the velocity spread is is crucially in these experiment because it leads to a drop in density so if you look at at Different distance from the target if you're going to say to five meters seven meters the total iron current is significantly lower simply because of the longitudinal velocity spread and That means near the target The the conditions for the growth of the right and instability is fulfilled far away. It isn't and So now if I show you What happens to the magnetic field? This these are magnetic field traces a function of time for different distances from the target in meters Okay, and what you can see is at any position You're getting these high frequency oscillations at early times and there's actually a frequency Chirp starts at high frequency, and then it's low frequency and and then we see a shear wave later on and If you look at one of those traces do for it They're transform or a wavelength analysis you're getting frequency as a function of time Then you're getting this frequency chirp here the color the white the white color scale that that's actually the power and This color curve here This shows the dispersion relation for the right and instability and it agrees remarkably. Well, it even shows here in red That's the highest growth. It even shows Where would he expect the highest frequency? So at this point? We're very convinced that we already seeing the right and instability and I just want to show you one brief hybrid simulation of this case here. This is The direct direction along with the LAPD. This is transverse to it It's distorted. So this direction is much longer than this one in reality. Okay on the left. We have the debris density This is the magnetic field transverse to the external field and this is the ambient density and when i'm running this You can see The debris ions flying to the right. They gyrate around here most importantly we're seeing these right and polarized waves High frequency earlier on then lower frequency at later times And if you look at the ambient density, there's nothing happening at this point. So According to the simulation, we are launching waves, but Then not nearly large enough to actually accelerate the ambient plasma, which you need for shock to form but if you compare The magnetic probe data on the left This is magnetic field as a function of time at one given position far away from the target Right where you get the high frequency stuff and the lower frequency The simulation shows essentially the same It shows same dynamics. It shows the same amplitude. In fact, even several gauss here So bottom line is we are exciting the right instability, but it's at this point not enough to launch a shock and What we do want is The ambient to start moving. This is the hybrid simulation that we run out over longer times So again, this is the distance along the plasma column. This is the velocity of the ambient at different times This red curve here shows the current Length of the plasma in ion inertial length. So that's limited by the size of the lab So there's much happening But you wouldn't have to go further out, which means either we would need a longer plasma Or you need more density And then the ion inertial length goes down and then then we'll be able to see it in the LAPD But I want to point out that there's there's an upgrade going on for the LAPD that will do this Will boost the density And there's also another device At ucla that may become available at some point that that would have a much longer plasma So it seems feasible in the future to actually let the debris cloud propagate through the plasma long enough for for a shock to form And that's all I really have here. So bottom line So far we only studied the The formation the formation of magnetic sonic shocks we think is well understood Now what we haven't done at all yet is we actually look at the properties of these shocks in particular the turbulence But I hope I made the point that there is plenty of turbulence going on. That's worth looking at um in particular electromagnetic ion ion stabilities for alphanic shocks and We think in the near future we'll be able to to launch Also non-resonant instability and actually shocks and look at turbulence in more detail Thank you