Session CP10: Poster Session II: Space and Astrophysical Plasmas. Magnetic Fusion: East, West, KStar & Other Tokamaks. High Energy Density Plasmas (2:00pm-5:00pm)

Application of the k-Filtering Technique to MMS Solar Wind Data

The k-filtering technique is applied to Magnetospheric Multiscale (MMS) satellite data of the solar wind. The MMS Mission consists of four satellites flying in tetrahedron formation to study Earth's magnetosphere; the spacecraft occasionally pass outside the magnetopause and into the solar wind for brief periods of time. The k-filtering technique allows for analysis of multiple linear waves modes present in the solar wind plasma. It has been applied to solar wind data from the Cluster II mission with interesting and varied results. In comparison to the Cluster mission, the data from MMS allow for significant improvement in resolution and increased frequency range in the analysis, as well as comparison with the previous Cluster findings.   

Particle-in-Cell Simulations of the Effects of the Electron Temperature Anisotropy on the Development of the Proton Firehose Instability in the Solar Wind

It is thought that kinetic instabilities in the solar wind act to limit the pressure anisotropies that would naturally develop if the plasma expanded adiabatically. First-principles simulations could provide a crucial insight on this open issue. However, these simulations are usually very challenging and out of the realm of traditional PIC codes, because of the multitude of spatial and temporal scales characterizing the processes of interest. Using our innovative semi-implicit PIC algorithm ECsim, we investigate the development and the non-linear evolution of the parallel firehose instability in the presence of anisotropic electron and ion distribution functions. The simulations are performed with a realistic electron-proton mass ratio and values of thermal velocities, magnetic fields and plasma densities characteristic of the solar wind around 0.5 AU. The effects of wave activities in scattering plasma protons and electrons and in reducing their anisotropies to marginally stable state is analyzed in detail. Our results show that the presence of an electron temperature anisotropy modifies the onset and the growth rate of the proton firehose instability, which develops earlier and grows faster, respect to the case where only ions are anisotropic.   

A Wave-Coherent Structure Duality in Plasma Turbulence: Are They Two Sides of the Same Coin?

The dissipation of turbulence in weakly collisional space plasmas remains controversial. Both fluid and kinetic turbulence simulations ubiquitously generate coherent structures---in the form of current sheets---at small scales, and the locations of these current sheets appear to be associated with enhanced rates of dissipation of the turbulent energy. The quest to understand the physical mechanisms by which the energy of turbulent fluctuations is converted to particle energy or plasma heat has driven vigorous debate about the relative roles of wave damping processes vs.~localized dissipation mechanisms associated with current sheets, such as magnetic reconnection. A major unanswered question is how these coherent structures arise in the first place. Recent analytical and numerical work has demonstrated that strongly nonlinear interactions among counterpropagating Alfven wavepackets---known as Alfven wave collisions---naturally generate current sheets self-consistently. Subsequent work has shown that the dissipation of the turbulent energy is localized near these current sheets but is clearly mediated through the process of collisionless Landau damping. Together, these results suggest that framing the debate as a choice between waves or coherent structures is a false dichotomy.   

Electric Fields of the Sun and Solar Wind

A simple model of solar electric fields explains the solar wind energetics and coronal "heating", invoking only gravito-electric and photo-electric forces. In the (collisional) solar interior, gravity \textit{necessarily} generates a radial electric field eE$=$(-1/2)m$_{\mathrm{p}}$g, so protons are 50{\%} levitated. At the surface, this gives eE(R$_{\mathrm{s}})=$1.4eV/Mm from displaced charge Q(R$_{\mathrm{s}})=$ -75.Coul. In the (weakly collisional) outer photosphere/corona, electron scattering of the photon energy flux G$_{\mathrm{E}}$ gives eE $=$ (G$_{\mathrm{E}}$/c) $\sigma _{\mathrm{\gamma e}}$. An estimated average photon-electron cross-section $\sigma_{\mathrm{\gamma e}}=$3x10$^{\mathrm{-24}}$m$^{\mathrm{2}}$ (typical of e-/p$+$ and e-/H correlations) gives eE$=$(4.eV/Mm) (r/R$_{\mathrm{s}})^{\mathrm{-2}}$, sufficient to generate the observed solar wind: protons are accelerated out of the 2.keV gravity well and up to 1.3keV kinetic energy within several R$_{\mathrm{s}}$, with total particle energy flux of G$_{\mathrm{E}}$/10$^{\mathrm{6}}$. This coherent proton/electron flow \textit{is} the K-Corona, obviating the T$=$100eV hydrostatic model (Van deHulst, 1950). Filamentation (1.Mm$^{\mathrm{2}})$ of the flow arises from the convection /recombination ("roiling") dynamics of surface granulations, with local electric fields generating strong currents and local magnetic fields. Statistical charge fluctuations, current filamentation, and neutral gas drag on the persistent proton/electron flow produces the pervasive \textit{fluctuating} magnetic fields observed by spacecraft.   

A Laboratory Model for the Parker Spiral and Magnetized Stellar Winds

A laboratory system that mimics the formation of magnetized stellar winds by driving Alfv\'enic plasma flows in a dipole magnetic field is presented. Plasma dynamics near the Alfv\'en surface are observed and involve magnetic reconnection and plasmoid ejection. These plasmoids are formed by ballooning perturbations that are driven by an accretion process. A radially inward Hall electric field is established by the magnetospheric ring current crossed with the poloidal magnetic field which causes the ions to accrete inwards until the pressure gradient becomes large enough to drive ballooning modes. These perturbations stretch the dipolar field lines into the current sheet until reconnection occurs, launching plasmoids into the stellar wind much like the quasi-periodic plasma blobs observed in the solar wind by LASCO. When the system is driven hard enough, the current sheet becomes thin and long, allowing for current driven instabilities to occur as well - forming much smaller plasma blobs at higher frequencies. Two-fluid simulations performed with the NIMROD code corroborate that in this helmet streamer-like magnetic geometry both pressure driven and current driven instabilities can be present and give rise to periodic plasma blob ejection amidst a bath of broadband fluctuations.   

Emission and Current Density Distribution in an Extended Magnetic Arcade

We report on observations and analysis of emission and current density distribution in the Wheaton Impulsive Reconnection Experiment (WIRX). The arcade-shaped plasma is formed between two parallel electrodes and is constrained by a magnetic coil surrounding the electrodes. This setup is geometrically similar to two-ribbon flares and magnetic arcades in the solar corona. ICCD images and a 1D, 20-channel custom photodiode camera are used to record emission from the plasma in space and time. A set of 76 probes samples the magnetic field throughout the plasma volume. Using the relative emission as a proxy for current density, we construct a model for the magnetic field and compare with the probe data. We find good agreement, indicating that the emission intensity is roughly proportional to current density in this gaseous arc discharge. The percentage of the electrode length filled by plasma current and the extent of the current-carrying region in the plasma is examined as a function of total plasma current and vacuum magnetic field strength. Higher plasma current and lower vacuum magnetic field leads to taller arcades and more diffuse current distributions, consistent with expectations from MHD force balance.   

Comparison of Electric Dipole and Magnetic Loop Antennas for Whistler Generation in the Inner Magnetosphere

There has been much interest over the years in the use of very low frequency (VLF) whistlers in the inner magnetosphere for remediation of trapped MeV electrons in the Van Allen belts. In preparation for future satellite-borne experiments, it is essential to differentiate the best antenna design to generate whistler modes in the magnetosphere. The two simplest and most studied antennas are an electric dipole and a magnetic loop antenna. It is well known that a loop antenna is much better at radiating whistlers than a dipole, but a dipole is easier to deploy in space especially when antenna dimensions have to be large for VLF wave generation. This study compares the performance between the two antennas which are modeled in a cold magnetoplasma. The radiation resistance, the efficiency of whistler generation, radiation patterns, and effective antenna dimensions are compared for any orientation of both antennas with respect to the magnetic field and over a range of altitudes.   

Landau Damping Signatures in Realistic and Down-Sampled Simulations of MMS Data: Characterizing the Use of Field-Particle Correlations

Field-Particle Correlations, which reveal velocity-space signatures of particle energization mechanisms in turbulent plasma, are applied to a dataset generated by the Astrophysical Gyrokinetics Code (AstroGK), and patterned after plasma conditions encountered by the Magnetospheric Multiscale (MMS) probes while collecting 70 seconds of data on October 16th, 2015. In early 2019, the Field-Particle Correlation method was used on this MMS data interval to measure secular energy transfer from the electric field, and revealed a signature congruent with electron Landau damping. Here, we corroborate this 2019 result by using Field-Particle Correlations to look for Landau damping in a dataset simulated to mimic the conditions of this MMS interval, and thus create a direct link between the method's use on both observational and simulated data. Following this corroboration, we analyze the degree to which the Field-Particle Correlation method tolerates deliberate down-sampling in time resolution of the simulated data, evaluated by persistent ability to identify the signature of Landau damping. Knowledge of this limit assists in characterizing the applicability of Field-Particle Correlations to future space-based missions, including Parker Solar Probe.   

Extensions of the magnetic pumping model for particle heating

One of the outstanding problems across a variety of astrophysical phenomena is the generation of electron and ion power-law distributions with superthermal tails. Most theories of particle energization rely on energy injection at a specific scale, such as the energy injection at the kinetic scale after passing through the turbulent cascade. We have shown that magnetic pumping, a model in which particles are heated by the largest scale magnetic fluctuations, is a complementary heating mechanism to the turbulent cascade, resulting in power-law distributions like those observed in the solar wind [1]. The ability of compressional Alfv\'enic turbulence to magnetically trap superthermal particles renders magnetic pumping an effective Fermi heating process for particles with $v\gg \omega/k$, and produces superthermal power-law distributions. Recent progress and extensions of this model will be presented, including the application of this model to differential ion heating near the solar corona. [1] E. Lichko, J. Egedal, W. Daughton, and J. Kasper. \textit{Astrophys. J. Lett.} \textbf{2}, 850 (2017)   

Determining the Velocity-Space Signatures of Magnetic Pumping Using the Field-Particle Correlation Technique

The mechanisms of particle energization are crucial to understanding space and astrophysical plasmas. However, because of the limited data available in in-situ measurements (often at only a single point in space), determining which process underlies the observed particle energization can difficult. Recent work has shown that correlations between the electric field and the plasma distribution function show distinct signatures for different mechanisms of particle energization. We extend this technique to consider magnetic field correlations, with the particular application of magnetic pumping models proposed to explain energization in the solar wind.   

Observational Analysis of Solar Coronal Plasma Clouds and Theory for Their Persistence

The inner solar corona, when observed in X-ray emission, consists of loops and unstructured clouds. The emission from the clouds corresponds to temperatures about 2 - 3 MK. These ``hot clouds'' are variable, but persist for many days. An observational analysis is presented involving magnetic field, density, and temperature measurements. Using these observations, a theoretical model is proposed where the macroscopic stability of the clouds is provided by the ambient magnetic field. In this model, at the more ``microscopic'' level, the plasmas associated with the clouds are considered as dominated by transport processes including particle inflow processes [1,2] that counteract relevant outward diffusion processes and can explain the lifetime of these clouds.\\ $[1]$ B. Coppi and C. Spight, Phys. Rev. Lett. 41, 551 (1978).\\ $[2]$ B. Coppi, MIT (LNS) Report HEP 13/07, Cambridge, MA, 2013.   

Modulation of magnetosonic waves by the background plasma density in a dipole magnetic field: 2D PIC simulation

Magnetosonic (MS) waves are important naturally occurring emissions in the Earth's magnetosphere. Previous theoretical calculations and PIC simulations have shown that the background plasma is a key factor to the excitation of MS waves. In this study, we investigate the MS waves modulated by background plasma density using a general curvilinear PIC simulation. The simulation model consists of three plasma components representing the background cool electrons and protons and tenuous ring distribution protons. It is found that MS waves can be locally generated by tenuous ring distribution protons in lower plasma density region. These waves are confined near the localized source region due to two mechanisms. Firstly, MS waves will be reflected because of the variation of background plasma density. Secondly, the waves leaving the source region are subject to damping from cool protons. The background plasma densities can modulate MS waves through controlling the wave growth rates in the source region. Our simulation results demonstrate that the background plasma density can modulate the MS waves and may play an important role in the spatial distribution of MS waves.   

Shear flow-interchange instability in nightside magnetotail causes auroral beads as a signature of substorm onset

A geometric wedge model of the near-earth nightside plasma sheet is used to derive a wave equation for low frequency shear flow-interchange waves which transmit $\vec{E} \times \vec{B}$ sheared zonal flows along magnetic flux tubes towards the ionosphere. Discrepancies with the wave equation result used in \citeA{KAL15} for shear flow-ballooning instability are discussed. The shear flow-interchange instability appears to be responsible for substorm onset. The wedge wave equation is used to compute rough expressions for dispersion relations and local growth rates in the midnight region of the nightside magnetotail where the instability develops, forming the auroral beads characteristic of geomagnetic substorm onset. Stability analysis for the shear flow-interchange modes demonstrates that nonlinear analysis is necessary for quantitatively accurate results and determines the spatial scale on which the instability varies.   

A multiscale semi-Lagrangian algorithm for fast-electron transport in the relativistic Vlasov-Fokker-Planck equation.

In tokamaks, relativistic runaway electrons traverse orbits at much faster time scales than collisional ones while dynamic scales of interest span over time scales much longer than those. Therefore, accurate and efficient modeling of orbit dynamics beyond collisional timescales is essential to model runaway dynamics in tokamaks. Common strategies to deal with this scale separation are based on bounce averaging, which is brittle and not generalizable to arbitrary 3D magnetic field configurations. In this study, we use a semi-Lagrangian scheme to bridge these scales. The approach reformulates the Vlasov equation as an integro-differential operator using Green’s functions, and then selectively approximates the integrals and uses operator splitting to make the method tractable (similarly to what was done in Ref. [1] for anisotropic diffusion). The proposed 1D-2V treatment is first-order accurate in time but promises to (i) preserve asymptotic properties associated with stiff Vlasov term, (ii) be uniformly accurate in $\epsilon$, where $\epsilon$ is the ratio of advection to collisional time scales, and also (iii) be optimal (i.e. scalable) with the total mesh points in the domain. We will demonstrate the algorithm with realistic applications of interest. [1] Chacon et al., JCP, 272 (2014)   

Slow Manifold Integrator for Electromagnetic PIC

Numerical Cherenkov radiation commonly plagues electromagnetic PIC simulations that attempt to use large implicit timesteps for jumping over light waves. We will present a new electromagnetic PIC algorithm designed to overcome this issue when the characteristic speed v for particles is much less than the speed of light c. The scheme is symplectic, implicit, and limits to a consistent scheme for electrostatic PIC at v/c tends to zero. Therefore the scheme is also asymptotics-preserving. We will show that the scheme possesses a discrete-time slow manifold. If a simulation is initialized near this slow manifold then Vlasov-Poisson, Vlasov-Darwin, and higher-order effects can be captured accurately without resolving the light waves. Moreover, knowledge of the discrete-time slow manifold can be exploited to eliminate the stiffness from the nonlinear solve defining a large timestep.   

Serendipity shape functions in NIMROD's delta-f PIC approach to energetic particle physics

Wave-particle resonances can have significant effects on plasma stability even with small, resonant sub-populations. For energetic particles (EPs) in tokamak plasmas, long-wavelength modes can interact with the second-adiabatic moment of EPs produced by neutral beams, external RF sources, or fusion-produced alphas leading to greater uncertainty in plasma stability boundaries. EP closures based on the PIC algorithm have long been used in extended MHD codes to capture this important physics. Extended MHD code NIMROD has both continuum and delta-f PIC [1] drift kinetic (DK) capability. Initially, bilinear shape functions were employed to incorporate the EP, anisotropic stress tensor into NIMROD's flow evolution equation. Serendipity shape functions, up to forth-order, have been implemented in delta-f PIC DK approach in an attempt to reduce the inherent noise in deposition of particle information onto NIMROD's higher-order finite element grid. Comparisons of growth rates and eigenfunctions for linear calculations using the old bilinear and new, higher-order delta-f PIC shape functions for giant sawtooth stability in DIII-D shot #96043 [2] are presented. [1] C.C. Kim, et.al., Comp. Phys. Comms. 164, 448(2004). [2] Choi, Turnbull, Chan, Chu, Lao, et.al., Phys. Plasmas 14, 112517(2007).   

A Topological Approach to Magnetic Nulls

The isotropes of a magnetic field, defined as the lines along which the direction of the magnetic field is constant, is introduced as a novel means to study the topology of magnetic fields. It is shown that the isotropes can be recovered as the stream lines of the isotropic field, which is defined via a geometric formula from the magnetic field. The behavior of the isotrope field in the vicinity of magnetic nulls resembles that of the electric field generated by point charges, and the index theorem for magnetic nulls can be reframed as a Gauss's Law on the isotrope field. \newline We demonstrate the isotrope field as a means of constraining the location of the nulls of a local magnetic field placed within a homogeneous guide field. Nulls will appear at the intersection of the surface where the local field's strength matches that of the guide field with the isotrope of the local field corresponding to a direction opposite the guide field. It is shown that, as the guide field is varied, nulls can form and annihilate in a fashion preserving topological index. The dipole field and Hopf field are used as example cases to demonstrate the behavior of the nulls formed when these fields are embedded in a static background field.   

Topanga: A Modern Code for E3 Simulations

We are developing the Topanga code for simulating the generation and propagation of the E3 electromagnetic pulse. The E3 component has a long pulse, lasting tens to hundreds of seconds. It is caused by a nuclear detonation's temporary distortion of the Earth's magnetic field. E3 EMP has similarities to a geomagnetic storm~caused by a solar flare and can produce geomagnetically induced currents in long electrical conductors, damaging components such as power line transformers. Our code's attributes include the following: spherical geometry for simplified boundary conditions and computational efficiency; couples a hybrid plasma model (fluid electrons and neutrals, particle ions, Ohm's law, and reduced Maxwell's equations) to a finite-difference time-domain electromagnetic solver (FDTD-EM); uses the IGRF magnetic field model, neutral atmosphere profiles from the US Standard Atmosphere or the NRL MSISE model, ionosphere profiles from the International Reference Ionosphere model; has ion-neutral, electron-ion, electron-neutral collisions; uses a fluid algorithm for motion of the neutral atmosphere; and has limited atmospheric chemistry. An overview of the code and simulation examples with some comparison to experimental data will be presented   

NRL SPADE plasma impedance probe measurements on the International Space Station

The Space PlasmA Diagnostic suitE (SPADE) instrument, developed by the U.S. Naval Research Laboratory (NRL), is a plasma impedance probe designed to monitor background space plasma conditions and provide early warning of the onset of hazardous levels of spacecraft charging, was delivered to the International Space Station (ISS) onboard the SpaceX CRS-17 launch on May 4, 2019 as part of the Department of Defense Space Test Program's STP-H6 mission. The SPADE experiment consists of two dipole antennas, one active antenna that is used to excite the local plasma and another passive dipole antenna that observes the excitation. The active probe is swept across a range of frequencies and DC voltage biases to determine the plasma impedance spectrum. The impedance measurements yield properties of the plasma, such as density, plasma potential, and electron temperature, while also providing data indicating the charging level of the ISS relative to the local plasma. SPADE responds to slight changes in the plasma sheath that forms around a charged object, providing a unique method for the early detection of charging. The year-long mission will test SPADE's ability to detect hazardous station charging events while providing long-term records of space weather conditions.   

CGA/NRL Impedance Probe as a ThinSat Spacecraft Payload.

Coast Guard Academy (CGA) space initiatives in partnership with the Naval Research Lab have required the development of a ThinSat impedance probe to make direct temperature and density measurements of space plasmas using a surface-mounted dipole antenna. The roughly 10 cm x 10cm x 2 cm payload platform is modified from the ThinSat platform provided by Virginia Commercial Space Flight Authority (Virginia Space) through a partnership with CGA. An onboard 75 MHz~ Direct Digital Synthesizer (DDS) is used to sweep the AC frequency applied to the dipole where time-dependent current and voltage measurements are recorded. Plasma temperature and density are subsequently determined from the resonant impedance frequencies. Measurements are then converted from digital to analog values and processed for transmission through the ThinSat bus. While the design calls for an ATMega328p, for ease of programming and implementation, an Arduino Mini Pro MicroController is used instead. Details of the unique ThinSat impedance probe circuitry architecture and lab preliminary findings will be reported.~~~   

Micro-Faraday Cups for Low Energy Ion Detection

We are developing an in-situ micro-plasma spectrometer for low energy ions in the plasma edge that requires modest resources and is easily replaced. Ions enter microscopic channels and are deflected based on their energy-per-charge ratio. Ten channels operate in parallel to increase the signal-to-noise ratio of the instrument. Because the ions of interest are typically low energy, less than 100 eV, conventional solid state detectors have too high an energy threshold and conventional microchannel plate detectors require too large a pre-acceleration potential. The key advance in the instrument described here is an integrated micro-Faraday cup structure designed to capture and record single ions. We will present initial measurements from the micro-spectrometer.   

Electromagnetic Emissions Induced by Small Meteoroids

Meteor fireballs produce strong electromagnetic (EM) pulses that result in audible sounds called electrophonics. These big fireballs, however, are rare phenomena, while much more frequent small, submilligram and submillimeter, meteoroids constantly bombard the Earth, depositing tons of extraterrestrial material in its atmosphere. Recent ground-based antenna observations during meteor storms have demonstrated that small meteoroids can also produce detectable electromagnetic pulses. Naturally produced EM emission could serve as a complementary diagnostics of meteor-related phenomena. These observations, however, are not free of controversy, so that it is important to understand whether detectable EM emission caused by small meteoroids may really follow from a first-principle theory of meteor plasma. To this end we need to understand and quantify the physical nature and dispersive properties of the EM emissions. Based on our recent analytic theory and simulations of the meteor plasma, we have analyzed possible kinds of EM emissions that may originate from meteor plasma. We will present the outcome of this analysis and its implications for radar observations and meteor diagnostics.   

Radial Magnetic Field Measurements in the Princeton Magneorotational Instability Experiment to Detect the MRI

Simultaneous, time-resolved, radial magnetic field measurements at various locations along the inner cylinder of the liquid-gallinstan-filled Princeton MRI apparatus have been carried out in order to detect and characterize the MRI instability in a magnetized shear flow that is otherwise hydrodynamically stable. The MRI-induced $B_r$ of the fastest-growing-mode is expected to be low $k_z$ and $m=0$ in contrast to the $B_r$ induced by Rayleigh centrifugal modes that can occur when the local shear parameter $q=-\partial \ln r/\partial r$ exceeds 2 due to shear induced by line-tying of $B_z$ to the conducting axial boundaries. Experimental results show the changing character of $B_r$ as the rotation of the system, $\Omega$, and $B_z$ are varied. The measurements are in only rough agreement with the expected MRI stability boundary from the linear WKB analysis, and so the results are compared with results from the SFEMaNS code which includes realistic material properties and boundary conditions. Over a broad range of parameters, both MRI and Rayleigh likely contribute to $B_r$ (as well as possibly magnetized Ekman flow) and so additional analyses using a global eigenmode code were carried out to artificially vary $B_z$ in order to disentangle the various contributions to $B_r$.   

Isolating Magnetorotational Instabillity (MRI) Using Eigenmode Analysis in the Numerical Simulation of Princeton MRI Experiment

The behavior of Magnetorotational Instability (MRI) in the Princeton MRI experiment can be further isolated from background effects through global eigenmodes analysis. The analysis is done by artificially changing of the vertical magnetic field $B_z$ from the nonlinear MHD simulated flow profiles. Along the low $B_z$ boundary of the MRI unstable region of $(\Omega_1, B_z)$ parameter space (where $\Omega_1$ is the rotational speed of the inner cylinder), the calculated growth rate will exhibit double peaks which correspond to two competing effects: Rayleigh instability and MRI, which are comparable in strength. This eigenmode analysis will enable us to sensitively map the boundary of the MRI unstable regime. This new method can be used to optimize other experimental parameters, such as end caps inner ring speed $(\Omega_3)$, to further isolate the effect of MRI in the system.   

Towards a fully kinetic dynamo simulation

Dynamo amplification of seed magnetic fields is believed to have produced the currently observed magnetization of galaxy clusters. Due to its inherently three dimensional nature, dynamos have almost exclusively been studied within the framework of magnetohydrodynamics. In order to produce the first fully kinetic dynamo simulation, we have considered a forced Roberts flow in a physical mass-ratio electron-proton plasma with inter and intra-species Lenard-Bernstein collisions. We target a low fluid Reynolds number and high magnetic Reynolds number regime in simulations with the continuum kinetic-Maxwell solver, Gkeyll [J. Juno et al 2018 J. Comp. Phys. 353, 110]. We observe an anomalously high resistivity compared to the Spitzer value, that hinders reaching sufficiently low magnetic diffusivity necessary to produce a growing magnetic field.   

The Magnetic Shear Current Effect in the Solar Dynamo

Recently, the magnetic shear current (MSC) effect has been identified as a possible dynamo mechanism in accretion disks driven by the magnetorotational instability[1]. We present a study incorporating the MSC effect into the dynamics of a model solar tachocline. Using the Dedalus framework[2], we solve the anelastic MHD equations in a box containing a convective layer on top of a stratified radiative layer in which a vertical shear flow is present. We argue that the MSC operates in the upper shear region driven by convective overshoot turbulence and present the effects of the generated large scale magnetic fields in our model. Discussion of the presence of the MSC effect will also be presented. Studying the efficiency of magnetic field generation and/or storage in the tachocline is crucial for understanding the its role in the global solar dynamo cycle. [1]Squire, J., & Bhattacharjee, A. (2015). Generation of large-scale magnetic fields by small-scale dynamo in shear flows. PRL, 115(17), 175003. [2]Burns, K., et. al. (2019). Dedalus: A Flexible Framework for Numerical Simulations with Spectral Methods. arXiv preprint arXiv:1905.10388.   

Electron Densities Inferred Using Optical Images of Standing Wave Patterns Observed in Plasmas Created with Focused, High-Power Microwave Beams.

Plasmas are initiated and sustained in the focal region of a high-power microwave beam operating with up to 9.5 kW CW beam power at 4.7 GHz. Under certain conditions the plasmas formed are steady-state and may be maintained for several hours. Direct measurements of the electron density using electrostatic probes have proven problematic because of the disturbance to the beam caused by the probe. An appealing alternative has been found using optical imaging. The stable plasmas exhibit regular axial variations in temperature and density believed to result from standing-waves within the plasma. Because the wavelength of the standing-waves is determined by the refractive index of the plasma careful imaging of the waves may be used to calculate the density.   

Temperature Anisotropy of an Expanding Helicon Plasma within a Diverging Magnetic Fields Region

Temperature anisotropy in an expanding magnetized plasma was investigated using laser induced fluorescence (LIF). Ion velocity distribution functions (IVDF) were measured downstream from the source region in an area of diverging magnetic field. An approximately 5 cm\texttimes 5 cm square area was interrogated at a radial (R) resolution of 2 mm and an axial (Z) resolution of 4 mm. IVDFs were measured parallel and perpendicular to the machine axis simultaneously using a mechanical scanning probe. The second moment of both the parallel and perpendicular IVDFs were compared to reveal a thermal anisotropy of nearly 10. A simple Boris stepper code was used to simulate ion motion from the plasma source through the expansion region. The code uses the measured electric field structure. The model reproduces the essential features of the measured IVDFs.   

Radiation from Weibel turbulence with PIC simulations

The filamentation (e.g., Weibel) instability is typical of high-energy-density environments like laser-produced and astrophysical plasmas, such as in collisionless shocks of gamma-ray bursts and supernovae, accretion shocks in galaxy clusters and others. It is generated in unmagnetized (or weakly-magnetized) plasmas with an anisotropic temperature, or, generally, with anisotropic particle distribution function. Radiation from the Weibel-generated, sub-Larmor-scale magnetic fields, known as the jitter radiation, differs significantly from the cyclotron or synchrotron radiation. In particular, its spectrum carries wealth of information about the magnetic field properties as is shown in both theoretical and numerical studies. Our goal here is to study such radiation from the first principles using the state-of-the-art PIC simulations. Here we discuss the techniques and some tentative results of the project.   

A machine-learning checkpoint/restart algorithm for particle-in-cell simulations.

With ever-increasing computing power and memory capacity, particle check-pointing for fault recovery of particle-in-cell simulations is stressing I/O subsystems, and becoming prohibitive. Given that future exascale computers are expected to be significantly more vulnerable to hard faults than current HPC systems, the availability of a fast and accurate recovery strategy is absolutely essential. In this study, we consider compression of the particle distribution function (PDF) by unsupervised machine-learning techniques.\footnote{G. Chen and L. Chac\'on, ``A machine-learning checkpoint/restart algorithm for particle-in-cell simulations'', in preparation} Specifically, we approximate the PDF with a Gaussian mixture.\footnote{Geoffrey McLachlan and David Peel. Finite Mixture Models. John Wiley \& Sons, 2004.} The Gaussian mixture is found by employing maximum likelihood principle with an information criterion, the minimum-message-length principle, for determining an optimal density estimation of the PDF.$^2$ Restart is conducted by moment-matching sampling of the Gaussian mixture, which strictly conserves charge/mass, momentum, and energy. We demonstrate the effectiveness of the method with various electrostatic and electromagnetic particle-in-cell simulations in 1D and 2D.   

On the origin of FRBs

Fast Radio Bursts (FRBs) is still an enigmatic phenomenon, even after more than a decade from their discovery. These are short radio pulses of tens of milliseconds duration (intrinsic) in the frequency range around a gigaherz and exhibiting very large dispersion measure, which is indicative of their extragalactic origin and, hence, of their exceptional brightness. Conventionally, FRBs are attributed to the cyclotron/synchrotron maser instability exciting an X-mode in a strong shock environment. We will discuss viability of this mechanism. We will also discuss whether FRBs can originate deep inside a magnetosphere of a magnetar and the role of QED and plasma effects.   

Mechanical Faraday effect to inform on pulsar rotation direction

Pulsar polarimetry is widely used to infer interstellar magnetic fields. These magnetic field estimates typically rely on the assumption that the observed polarisation rotation stems entirely from Faraday rotation in the magneto-optic plasma found between the linearly polarized point source pulsar and our planet. Yet, the magnetosphere that surrounds pulsar can in principle also affect polarization. While the classical Faraday rotation in the pair plasma magnetosphere itself may be negligible, we show that a mechanical polarization rotation (or mechanical Faraday) effect should arise from the pulsar magnetosphere rapid rotation [1]. Importantly, while this mechanical effect can be mistaken for Faraday rotation in the interstellar medium (ISM) due to its identical wavelength-square signature at typical GHz observation frequency, it can in principle be disambiguated in the sub-GHz thanks to a different wavelength scaling near a cut-off frequency. Separating these two contributions through low-frequency pulsar polarisation observations could thus permit correcting for possible systematic errors in ISM magnetic field estimates. It also offers a conceptual means to determine the rotation direction of pulsars, which remains otherwise inaccessible. [1] Gueroult et al, arXiv 1903.01193   

Radiation in equilibrium with plasma and plasma effects on cosmic microwave background

The spectrum of the radiation of a body in equilibrium is given by Planck’s law. In plasma, however, waves below the plasma frequency cannot propagate, consequently, the equilibrium radiation inside plasma is necessarily different from the Planck spectrum. We derive, using three different approaches, the spectrum for the equilibrium radiation inside plasma. We show that, while plasma effects cannot be realistically detected with technology available in the near future, there are a number of quantifiable ways in which plasma affects cosmic microwave background (CMB) radiation.   

Nonlinear Simulations of the Parker Instability with Cosmic Ray Streaming

The Parker Instability is a Rayleigh-Taylor like mode where the thermal gas is partially supported by magnetic and cosmic ray pressure. This instability occurs in the gas of galactic disks and possibly contributes to the dynamo effect in galaxies. We have performed a linear stability analysis on the Parker Instability where the stability of the system depends on how the cosmic rays couple to the thermal gas through microscopic instabilities. Since adding the additional effects of radiative cooling and a realistic gravitational potential, we have also been running numerical simulations of the Parker Instability in order to observe how the system evolves nonlinearly. In addition, we are investigating if a galaxy can produce enough cosmic rays to drive a galactic wind and blow itself apart. In order to study this, we evolve a system of magnetic flux tubes in a vertical gravitational potential into which we inject cosmic rays and observe how the flux tube evolves. This work aims to determine if the gas, magnetic fields, and cosmic rays are able to reach a stable equilibrium or if a threshold of cosmic ray injection can be reached where the flux tube continues to rise against gravity due to cosmic ray and magnetic buoyancy. We have and continue to use theory and simulations.   

Stochastic acceleration by the ion-cyclotron instability in a growing magnetic field

Using 1D and 2D particle-in-cell (PIC) simulations of a plasma with a growing magnetic field $\vec{B}$, we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As $\vec{B}$ grows, an ion pressure anisotropy $p_{\perp,i} > p_{||,i}$ arises, due to the adiabatic invariance of the ion magnetic moment ($p_{||,i}$ and $p_{\perp,i}$ are the ion pressures parallel and perpendicular to $\vec{B}$). When initially $\beta_i = 0.5$ ($\beta_i \equiv 8\pi p_i/|\vec{B}|^2$, where $p_i$ is the ion isotropic pressure), the pressure anisotropy is limited mainly by inelastic pitch-angle scattering provided by the IC instability, which in turn produces a non-thermal tail in the ion energy spectrum. After $\vec{B}$ is amplified by a factor $\sim 2.7$, this tail can be approximated as a power-law of index $\sim 3.4$ plus two non-thermal bumps, and accounts for $2-3\%$ of the ions and $\sim 18\%$ of their kinetic energy.Although we focus on cases where $\vec{B}$ is amplified by plasma shear, we check that the acceleration occurs similarly if $\vec{B}$ grows due to plasma compression. Our results are valid in sub-relativistic regimes where the ion thermal energy is $\sim 10\%$ of the ion rest mass energy. This mechanism can be relevant in low-luminosity accretion disks   

A variational approach to study plasma coupling with gravitational waves

Existing theories of gravitational-wave (GW)-plasma coupling are not directly applicable to the GWs of interest that are inhomogeneous in space and have more general polarization than in vacuum. We propose an alternative, variational formulation of this problem, which also leads to the prediction of the nonlinear ponderomotive force that a GW pulse exerts on massive particles. This force is calculated explicitly for the first time. Developing on our variational method, we also propose a geometrical-optics theory for collective matter oscillations in self-consistent metric with general polarization. Electromagnetic interactions can be added similarly, which, in the future, will lead to a generalized theory of plasma waves in astrophysical context.   

Towards an hybrid representation in the KSDFT simulations at high temperature

Orbital-based Kohn-Sham resolution of density functional theory is now a very popular method to describe condensed matter systems. Low temperature disordered systems can be considered through the finite temperature extension of the method and combined with molecular dynamics (KSMD). At high temperature, however, this approach becomes prohibitively expensive due to the huge number of quasi-empty orbitals to consider. KSMD simulations are just unattainable beyond a few tens of eVs, and must be replaced by their semi-classical counterpart, namely the orbital-free molecular dynamics method (OFMD). Even if the transition from KSMD to OFMD is well documented, a continuous transition inside a unique frame would be desirable. Inspired by a recent work1 we have implemented in the Abinit code a scheme that substitutes high energy orbitals by plane waves allowing for a much smaller orbitals basis sets and thus much faster calculations at high temperatures while keeping the orbital based precision at low temperature. A rewriting of the Kubo-Greenwood conductivity should allow for a global formulation valid in all regimes of temperatures. We intend to apply this methods to the calculation of Jupiter's isentrope keeping the same level of precision and statistics between computations at the surface of Jupiter, mainly made of molecular hydrogen, and at the core of the planet with metallic hydrogen at few tens of Mbar. [1] Shen Zhang, Hongwei Wang, Wei Kang, Ping Zhang, and X. T. He. Physics of Plasmas, 23(4): 042707, April 2016.   

Current-filamentation and Plasma Heating during Eruption of a Laboratory Arched Magnetized Plasma

Arched magnetized plasmas ubiquitously exist in the solar atmosphere and laboratory. We study dynamics and energetics of a laboratory arched magnetized plasma to gain a better understanding of processes governing their eruption. The arched plasma is produced using a hot-cathode lanthanum hexaboride (LaB$_6$) source and it evolves in an ambient magnetized plasma produced by another LaB$_6$ source [1, 2]. Typical plasma parameters are: $\beta \approx 10^{-3}$, Lundquist number $\approx$ $10^2$ - $10^5$, B $\approx$ 1000 Gauss at footpoints, plasma radius/ion gyroradius $\approx$ 20, B $\approx$ 0-50 G in the ambient plasma, and 0.5 Hz repetition rate. We present recent results on measurements of plasma density, electron temperature, and three-dimensional magnetic-field. These results demonstrate formation of multiple current channels associated with magnetic reconnection, excitation of fast waves, and plasma heating. Role of ambient magnetic field in generating the three-dimensional structure of current-channels and energy release from the arched magnetized plasma will be presented. \\ References: \\ (1) Tripathi and Gekelman, Phys. Rev. Lett. 105, 075005 (2010) \\ (2) Tripathi and Gekelman, Solar Phys. 286, 479 (2013)   

Instabilities of a relativistic electron beam in the Earth's magnetosphere

A relativistic electron beam emitted from an orbiting satellite along the Earth\textquoteright s magnetic field lines may be used for mapping of magnetic field lines [1,2]. Previous work has shown that the impact location of such a beam will shift hundreds of kilometers during different phases of a magnetospheric storm, and this impact will be detectable by ground stations provided that instabilities do not cause the beam to spread [2]. In this work we investigate the interaction of a relativistic electron beam pulse with the magnetospheric and ionospheric background plasma through which it propagates using particle-in-cell simulations. We analyze the influence of a variety of instabilities on beam scattering in the background plasma, examining whether or not the instabilities will have deleterious effects on the proposed scheme of magnetic field mapping.\\ \\1.Ennio R. Sanchez \textit{et al.} 2019, Particle beams as a resource to solve outstanding problems in space physics, submitted for publication in \textit{Frontiers in Astronomy and Space Sciences}.\newline 2.Andrew T. Powis \textit{et al.} 2019, Evolution of a relativistic electron beam for tracing magnetospheric field lines, submitted for publication in \textit{Frontiers in Astronomy and Space Sciences}.   

Magnetohydrodynamic simulation comparisons of cylindrical implosion experiments for Rayleigh Taylor instability

The Rayleigh-Taylor (RT) hydrodynamic instability degrade the performance of inertial confinement fusion (ICF) experiments. During both the acceleration and deceleration phases of the implosion, the RT instability grows and mixes materials at the interfaces. Mitigating the RT instability enables ICF implosions to achieve hot spot pressures necessary to reach ignition conditions with respect to the Lawson criterion. The current approach is to reduce seeds and match Atwood numbers to reduce instability growth. An alternate approach would be the application of magnetic fields. Here we present computational design efforts using radiation-hydrodynamic codes with magnetohydrodynamic capabilities (FLASH) to study RT inhibition under the influence of magnetic fields on a cylindrical imploding target. Since ablative stabilization has the strongest impact on the inhibition of RT, the key design challenge for cylindrical experiments is achieving relevant temperatures in the fill material. We explore ideas for validation experiments.   

Observations and Modeling of Magnetized Plasma Jets and Bubbles Launched into a Magnetized Background Plasma

The interaction of magnetized flowing plasmas with background plasma is a topic of wide interest in astrophysics as well as high temperature laboratory plasmas. In this work, hot, dense, plasma structures launched from a coaxial plasma gun into a background low density magnetized lab plasma. The dense gun plasma drags frozen-in magnetic flux into the chambe'rs background magnetized plasma, providing a rich set of dynamics to study magnetic turbulence, force-free magnetic spheromaks, shocks, as well as CME-like dynamics possibly relevant to the solar corona. Vector magnetic field data from an eleven-tipped B-dot rake probe and images from an ultra-fast camera will be presented in comparison with ongoing MHD modeling. Recent experiments show a possible magnetic Rayleigh-Taylor (MRT) instability that appears asymmetrically at the interface between launched spheromaks (bubbles) and their entraining background magnetic field and plasma. Efforts to understand this instability using in situ measurements, new chamber boundary conditions, and ultra-fast camera data will be presented.   

Iron Opacity at 156 eV as Measured on the National Ignition Facility (NIF)

The x-ray opacity of materials is important for regulating the flow of energy in the high energy density regime, but very few experimental measurements exist. To obtain opacity data, an experimental platform has been developed on the NIF. This platform consists of a hohlraum to heat the opacity sample, a source of x-rays to backlight the sample, and a spectrometer to measure the spectrally resolved transmission of the sample. The density and temperature of the sample are also measured. The first measurements on iron at a temperature of $\sim$156 eV and electron density of $\sim$8.4x10$^2$$^1$ per cc have been obtained. These measurements will be compared to theoretical calculations and future improvements to the platform will be discussed. This work was performed under the U.S. Department of Energy LANL contract 89233218CNA000001, LLNL Contract DE-AC52-07NA27344, Sandia Contract DE-AC04-94AL85000, and NNSS Contract DE-AC52-06NA25946.   

Impactful Times: Memories of 60 Years of Shock Wave Research at Sandia National Laboratories

Sandia National Laboratories' origin~began during World War II. In July 1945 our forerunner, Sandia Base,~was established to develop, test, and assemble non-nuclear parts of weapons. Shock~wave research became essential in the 1950s with the advent of supersonic and exoatmospheric missiles. A major concern was effects of radiation-produced shocks on materials. As a result, we developed a wide range of experimental, diagnostic, modeling, and computational capabilities. These have addressed complex issues related to both weapons and basic science. Notable applications have included analysis of the cause of the turret explosion aboard the USS Iowa, predicting the response to the Shoemaker-Levy comet impact on Jupiter, and evidence for an abrupt transition of dense liquid hydrogen from an insulator to a metal~at high pressures.~Six decades later, our research encompasses all aspects of material science from~high energy density physics to low density plasma surface interactions.   

Kinetic Effects and Neutron Generation in Converging Fully-Ionized Plasma Jets

In this work, the use of laser-driven convergent plasma fusion targets is investigated for the study of counter-streaming and converging rarefied plasma flows. The scheme consists of a fuel layer lined along the interior surface of a hohlraum that is laser-ablated and expands inward towards the hohlraum center. Previous experiments have demonstrated the potential of such targets as neutron sources. The plasma streams generated in these targets are initially nearly collisionless as they converge, leading to wide interaction length scales and long interaction time scales as the jets interpenetrate. Such interactions are difficult to accurately model using standard magnetohydrodynamic (MHD) simulations, which assume high collisionality. Instead we model the system kinetically using the hybrid particle-in-cell (PIC) code Chicago to explore the importance of kinetic ion effects during stagnation. Predicted neutron yields and stagnation properties (density and temperature) are compared to the results from experiments conducted at the OMEGA Laser facility for both vacuum and gas-filled targets.   

Comparing Single-Channel and Multi-Channel Ball Switches for HADES

In pulsed power technology, high pressure spark gap switches are vital to creating repeatable current wave forms on the timescale of hundreds of nanoseconds. Typical spark gap switches, formed by a single current channel, have several drawbacks, such as high inductance and excessive electrode etching. Unlike single-channel spark gap switches, multi-channel ball switches allow multiple current channels to form---this reduces the inductance of the switch while increasing its lifetime. To reduce the overall inductance in and minimize maintenance on HADES; the High Amperage Driver for Extreme States, built at the University of Rochester; we designed and tested two ball switch designs: a single-channel one-ball design and a multi-channel six-ball design. Both designs use pressurized, synthetic air as its insulating medium and are tested at $+$/- 100kV using a single brick set-up. The switches will be able to sustain 30 kA for hundreds of nanoseconds and can be triggered using high voltage-high current systems---like HADES. Our overall goal was to minimize cost, machining time, and inductance. Both designs were compared, and the more suitable design will be considered for general LTD brick integration.   

Studies of the effects of radiation trapping and radial gradients in short-pulse laser heated targets

Short-pulse, laser heated buried layers have demonstrated a promising capability in determining opacities of high temperature, high density plasma. A critical aspect of utilizing these data is inferring the plasma conditions using a K-shell tracer. Collisional-radiative models are often employed to determine which plasma conditions can generate a spectrum that will match observations. These models invoke approximations to increase computational tractability, such as escape factors, and require assumptions about the uniformity of the emitting plasma. We test these by comparing calculations of optical trapping using escape factors and full radiation transport against angularly and temporally resolved K-shell data from buried silicon dots at 2 g/cc and 600 eV. Additionally, we explore the impact of radial gradients on the inferred temperature evolution using 2D simulations with various radial temperature gradients.   

\textbf{An Investigation of Photoinjector-Generated Electron Beams for High-Energy-Density and Inertial Confinement Fusion Diagnostics}

Modern photoinjector electron sources can now regularly generate high-luminosity, low\textunderscore bandwidth relativistic electron beams with tens of femtosecond pulse lengths. Electron beams provide a unique diagnostic source because of their high elastic scattering cross section and ease of detection. Relativistic electron beams can also provide extra diagnostic capability in the form of electron radiography and bright broadband or monoenergetic x-ray generation via bremsstrahlung or Compton processes. Pairing an electron beam with the OMEGA EP laser would allow for a tunable source of nearly monoenergetic x rays in the 4- to 60-keV range using Compton scattering. This talk will detail an investigation into the utility of modern relativistic electron-beam sources for diagnosis of laser-driven high-energy-density (HED) and inertial confinement fusion experiments with subpicosecond time resolution. A focus is given towards relativistic electron diffraction diagnosis of HED physics targets and inverse Compton scattering x\textunderscore ray sources.   

\textbf{Spectroscopic Diagnostics Using Line-Radiation in Laser Driven } \textbf{Non-equilibrium Plasmas}

We investigate to diagnose plasma conditions of experiments performed at the Jupiter Laser Facility at the Lawrence Livermore National Laboratory, where X-ray spectroscopic measurements were acquired from sub-critical-density, Ti-doped silica aerogel foams driven by a 2$\omega $ laser at \textasciitilde 5x10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}}$. The ultimate objective is to study the effect of an external B-field in thermally insulating the hot plasma and investigating line-radiation in multi-keV, non-equilibrium plasmas. However, the near-term goal is to infer a time-integrated temperature at several positions along the laser propagation axis for several B-field cases and observe any sensitivity to density with 4.5{\%} of Ti by atomic fraction in SiO$_{\mathrm{2}}$ foam target. We use our non-LTE atomic model with a detailed fine-structure level atomic structure and collisional-radiative rates to investigate the Ti spectra at the estimated plasma conditions of density and temperature conditions. Synthetic spectra are generated with a detailed multi-zone, 1D multi-frequency radiation transport scheme from the emission regions of interest to analyze the experimental data and compare and contrast with the existing simulations at LLNL.   

Diagnosing Electrode Surfaces on the Z-Machine Using Optical Spectroscopy

Currently, optical spectroscopy is used on the Z-machine to characterize electrode surface conditions and plasma formation during the Z power pulse. Such measurements are needed to inform theory and simulation efforts to design next-generation pulsed power machines. Several diagnostic techniques and resulting measurements will be discussed, including surface electron densities using Stark broadened line emission from passive dopants, radiance estimates from absolutely calibrated streak spectra, and low temperature (under 5000 K) measurements of cathode surfaces using high gain calibrated avalanche photodiodes. Additional capabilities using laser activated dopants that are presently being developed to probe regions with lower electron densities (less than 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}})$ will also be described. * Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.   

Development of a novel dual view, four frame imaging system and other diagnostics to study electrothermal instabilities on Mykonos*

The electrothermal instability (ETI) is a Joule heating-driven instability that can initiate in solid liner-driven fusion targets, generating azimuthally correlated (striated) temperature and density perturbations. These perturbations may seed the magneto Rayleigh-Taylor (MRT) instability and can limit stagnation pressure and implosion uniformity. These experiments will observe ETI growth from diamond-turned, 99.999{\%} pure aluminum rods in a z-pinch configuration by monitoring characterized ``engineered'' defects machined into the rod surface. Experiments will be conducted on the \textasciitilde 1 MA Mykonos driver at Sandia National Laboratories. A novel multi-camera splitter system will be used to simultaneously image these scaled defect patterns on opposing sides of the target, in order to examine visible-light emission from the surface. Laser shadowgraphy and interferometry diagnostics are also being developed and will be compared to 3D-MHD simulations.   

VISRAD, 3-D Target Design and Radiation Simulation Code.

The 3-D view factor code VISRAD is widely used in designing HEDP experiments at major laser and pulsed-power facilities, including NIF, OMEGA, OMEGA-EP, SLAC, ORION, LMJ, Z, and PLX. It simulates target designs by generating a 3-D grid of surface elements, utilizing a variety of 3-D primitives and surface removal algorithms, and can be used to compute the radiation flux throughout the surface element grid by computing element-to-element view factors and solving power balance equations. Target set-up and beam pointing are facilitated by allowing users to specify positions and angular orientations using a variety of coordinates systems (e.g., that of any laser beam, target component, or diagnostic port). Analytic modeling for laser beam spatial profiles for OMEGA DPPs and NIF CPPs is used to compute laser intensity profiles throughout the grid of surface elements. We will discuss recent improvements to the software package and plans for future developments.   

Machine Learning for Classifying Hydrodynamic Breakdown in HED Plasmas

Correct prediction of shock width is important for evaluating atomic mixing in high energy density plasma experiments, e.g. shell-fuel mixing in inertial confinement fusion capsules. In this study we use machine learning to attempt to identify when 1d fluid descriptions of a multi species plasma reminiscent of a capsule interface fail to adequately describe a standing shock, and determine when a kinetic description is appropriate. We compare the effectiveness of various combinations of several different metrics for non-equilibrium behavior, such as shock width, species mixing, temperature anisotropy, Knudsen numbers, and deviations from Maxwellian distributions between a fluid and kinetic model to efficiently train a perceptron identify the breakdown regime.   

Computational investigations of x-ray illuminated foam spheres of different opacities

The radiation hydrodynamics of x-ray illuminated foam spheres of different densities were investigated experimentally on the OMEGA-60 laser system to study radiatively-imploded molecular gas clouds relevant to star formation. In the optically thick case, prediction of the shock location and strength is important for experimental design. We investigate a number of experimentally-relevant conditions for different densities (and opacities) by comparing semi-analytical self-similar solutions for equilibrium radiation diffusion to three-temperature simulations using the flux-limited diffusion radiation hydrodynamics code CRASH. Although the theory captures some of the salient features of the problem, we find that the three-temperature simulations are necessary for accurate heat-front and shock location predictions in the majority of cases considered.   

Numerical Improvements and Verification Problems for Hall MHD in the Ares Multiphysics Code

Hall MHD captures important effects occuring in weakly collisional astrophysical and laboratory plasmas. In the laboratory setting, Hall MHD has attracted recent activity for modeling power flow and magnetically driven targets, such as MagLIF liners. In this work, we compare several numerical schemes for Hall MHD in 2D configurations with out-of-plane magnetic fields. We show that numerical instabilities arising in second-order centered differencing schemes can be eliminated using donor cell advection. The methods are tested using a zero-velocity Hall drift wave, a Hall magneto Rayleigh-Taylor instability, and a new verification problem involving the self-consistent evolution of the magnetic field and fluid velocity. The newer verification problem derives the linear response of a perturbed Hall MHD equilibrium; eigenfunctions are numerically determined from a resulting ODE in Cartesian or cylindrical geometry. We show numerical generated by Ares converge to the analytic solution in the linear regime.   

The Effect of Anomalous Resistivity on Electrothermal Instability Growth

The current driven ETI (Electrothermal instability) forms when the material resistivity is temperature dependent, occurring in nearly all Z-pinch-like high energy density platforms. Simulations of ETI typically use a collisional based form of the resistivity as provided, e.g., in a Lee-More Desjarlais conductivity table, but in regions of low density collisionless transport needs to be incorporated to properly simulate the ETI growth. Anomalous resistivity is an avenue by which collisionless micro turbulent effects can be incorporated into a collisional resistivity, allowing for a better representation of these low density regions during ETI. Such low density plasmas may be present due to ablated liner material, plasmas injected by the powerflow, or in gas puff targets. Because the ETI growth rate depends on the derivative of resistivity with respect to temperature, anomalous resistivity may have a significant impact on the evolution of the ETI through its own temperature dependence. Comparisons are shown using the Ares multiphysics code with and without the incorporation of various anomalous resistivity models during ETI growth.   

Nonlinear Instabilities due to Drifting Species in High Energy Density Plasmas

Relative drifts between particles species are fundamental driving forces behind many plasma instabilities. For example, the Buneman instability arises due to an election-ions drift. We study the nonlinear evolution of these processes in the presence of externally imposed transverse magnetic fields. Our results are primarily drawn from simulations using both Vlasov--Maxwell and macro-particle methods. We compare electrostatically driven modes to full electromagnetic treatments. Ion to electron mass ratios of 1, 10 and 100 will be included.   

Hydrodynamic Design Simulations of XPIV-Compatible Targets using FLASH

We are developing an X-ray Particle Image Velocimetry (XPIV) technique at the University of Rochester to study the dynamics of high energy density (HED) materials by combing dynamic compression drivers with coherent light sources. Our recent experiments executed on OMEGA-EP tested titanium-seeded epoxy targets to develop a platform for tracking tracer particles in HED laser-driven flows. The experiments were designed using the one-dimensional (1D) radiation-hydrodynamics code LILAC. Here we present two-dimensional (2D) axially symmetric FLASH simulations of the laser-driven shock compression. The simulations investigate the shock wave's speed as it traverses the material along with the downstream density and pressure. We compare our 2D results with 1D simulations using FLASH and prior LILAC results. These are used to guide improvements in future experimental designs.   

Small-amplitude theory of the perturbation evolution caused by isolated surface defects in laser-driven targets.

Rayleigh-Taylor perturbation growth in laser fusion capsules can be triggered by isolated target non-uniformities, such as fill tubes, mounting stalks, tents, etc. Recently, the first experimental study of the perturbation evolution caused by isolated non-uniformities, straight grooves machined on planar plastic targets, has been carried out on the Nike KrF laser at NRL. Detailed analysis and numerical simulation of these experiments are in progress. We present a linear theory of the perturbation development caused by isolated small-amplitude indentations on planar laser targets. Advancing the earlier theory, we demonstrate how the shock-front, ablation-front, rear-target-surface, and areal-mass/opacity perturbations develop in time when the source of the initial perturbation is localized in space. If the initial shock-front perturbation is an azimuthally-symmetric Gaussian indentation, later, as the perturbed shock front flattens, its central part lags, and the shock front perturbation acquires a rotationally-symmetric ring shape. For an initial straight Gaussian groove, the shock front perturbation shape evolves into a shelf with a flat bottom and peaked edges. When a locally perturbed shock front breaks out at the flat rear surface of the target, a collimated jet is produced.   

Turbulent Transport in Magnetized HED Plasmas

Anomalously fast diffusion of plasma across magnetic fields has long been recognized in magnetic fusion devices and laser plasmas. Micro-instabilities driven by gradients in plasma parameters give rise to convective flow patterns on meso- to global scales, which leads to correspondingly enhanced diffusion coefficients. While some experiments have demonstrated aspects of anomalous transport in HED plasmas, many aspects remain unknown, and this physics is typically not included in MHD design codes for ICF. We present results from recent experiments on turbulent transport in magnetized HED plasmas on the OMEGA laser facility. A plasma was ablated from a plastic target into a pre-existing magnetic field powered by MIFEDS. The evolution of the global topology of the magnetic fields was imaged with proton radiography, and local plasma parameters including electron temperature and density were measured with 2$\omega$ Thomson scattering. The interaction of the laser plasma with the background field was measured for different laser energies, and for different target orientations relative to the field.   

Interaction of relativistic magnetized electrons with obstacles

Using a laser pulse from the OMEGA EP laser system focused to an intensity of $\sim10^{19}$Wcm$^{-2}$ we generate hot electron plumes on the surface of 25$\mu$m thick Al targets with high magnetization due to self-generated fields, given by $\sigma_{cold} = B^2/\mu_0n_em_ec^2 \approx 1$. These plumes expand at $\sim$c and interact with obstacles in the form of holes, or ``blobs'' of glue on the target. This interaction is probed using time-resolved proton radiography which allows for the measurement of fields in the plane of the target. The proton radiographs are analyzed using standard radiograph inversion codes and are compared to 2D and 3D particle-in-cell simulations.   

Simulations of deceleration phase Rayleigh-Taylor instabilities using the FLASH code

Experimental results and simulations of imploding fusion concepts have identified the MRT (magneto-Rayleigh-Taylor) instability as the largest inhibitor to achieving fusion. Understanding the origin and development of the MRT instability will allow for measures to be taken in order to dampen the instability growth, thus improving the chance that fusion concepts such as MagLIF (Magnetized Liner Inertial Fusion) are successful. A study of imploding geometry in 1D and 2D is presented using FLASH, an adaptive mesh refinement code. Two cases of implosion geometry, laser driven and magnetic flux driven, are used to study late stage deceleration-phase MRT development on the interior of the imploding liner. FLASH's MHD modeling capabilities are used and updated to employ SESAME tabulated resistivity for the liner material.   

Particle transport in laser-produced HED plasmas in the presence of a megagauss magnetic field

We present a planned experimental effort to study two basic features in previously unexplored regimes in a laser-generated HED plasma embedded in a strong magnetic field: 1) radial confinement of the bulk hot ion population, and 2) cross field transport of an energetic electron minority. We will generate a cylindrical plasma with variable electron and ion energies using a laser-irradiated clustering gas jet where a 50 TW, 130 fs laser pulse can be efficiently absorbed to generate keV temperatures. We will use a portable current source developed at Sandia National Laboratories to generate the megagauss-scale magnetic field required to produce a $\beta _{\mathrm{mag}}\sim $1 condition where electrons will be magnetized and ions will not. The subsequent plasma dynamics will be examined using time-resolved optical interferometry and time-integrated diagnostics for energy spectra. We have already made initial Rayleigh scattering measurements using a ns-pulsed laser that confirmed that the clusters from our pulsed gas jet source survive the shock which might be induced by the magnetic coil. Sandia National Laboratories is refurbishing and improving the pulsed power source, and we will also show progress on that effort.   

Expansion of a relativistic ionization wave launched by a sheath field in a strong magnetic field

MG-scale magnetic fields are now available through pulsed power technology, which opens the possibility of radially confining laser-generated plasma filaments in clustering deuterium jets. However, a long-lived relativistic ionization wave driven by a population of hot electrons created by an intense laser pulse, can be launched into the surrounding gas by the sheath field of the filament [PRL 112, 045002 (2014)], potentially affecting the transport of the entire plasma and negating the advantage of the confinement. In this work, we use 2D kinetic simulations to examine the impact of the applied magnetic field on the propagation of the ionization wave and to determine the criterion that must be satisfied to prevent the wave from being launched.   

Multiple Scattering Effects inWarm and Hot Dense Matter

Warm and Hot dense matter occurs in the interiors of giant planets and stars, as well as in inertial fusion experiments. It is matter that has been heated to tens of thousands or millions of Kelvin, and with a density similar to solid density. Due to a multitude of complicated and completing strong physics effects, like partial ionization, strong ion-ion coupling, and quantum mechanical electrons, warm and hot dense matter is very challenging to model. Here we investigate the role of multiple scattering, where electronic states are strongly influenced by the neighbouring atoms (i.e. electrons scatter multiple times and don’t become asymptotically free). To do this we have developed the multiple scattering Green’s function method for high temperatures. This method has favorable scaling properties at high temperatures and includes all the physics mentioned above. This presentation will demonstrate these scaling properties and give initial, promising results.   

A direct temperature measurement of resistively heated diamond using inelastic X-ray scattering.

Warm dense matter is a system in which the electron kinetic energy is comparable to the potential energy of interaction between the electrons and the nucleus. This property makes this system hard to analyze theoretically as it is too dense to be considered as a plasma and the electron temperature is too high to be analyzed using condensed matter theory. In such systems it is then of primary interest to measure its thermodynamics properties (i.e. temperature, density) to guide the development of models. Here, we present the development of a platform using inelastic X-ray scattering in a Johann geometry to measure temperature by the use of the principle of detailed balance. The experiment was conducted at the HED beamline at the European XFEL on resistively heated single crystal Diamond at 500 K. Such a platform may be readily combined with high energy lasers to generate and probe warm dense matter.   

New Prism Opacity Tables with Hot Electron Effects

We present new features of the PROPACEOS code, which generates equation-of-state (EOS) and opacity tables for radiation-hydrodynamics and spectroscopic simulations. In addition to existing capabilities to produce tables for LTE and optically thin NLTE plasmas, these new features allow PROPACEOS to perform calculations that include other effects of NLTE atomic kinetics. The primary purpose of this development is to facilitate efficient spectroscopic simulations for short-pulse laser experiments. The simulations are based on post-processing of PIC calculations and focus on the analysis of K-alpha/K-beta emission signatures. PROPACEOS can now produce emissivity and opacity databases on a grid with up to six independent parameters, e.g. plasma temperature, plasma density, and analytic function parameters for hot electrons. These new tables can be used to post-process PIC simulation hydro data containing binned hot electron distributions, resulting in greatly improved speeds for SPECT3D spectroscopic simulations. We will present simulation results relevant to ongoing experiments at the Omega EP laser facility. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Fusion Energy Sciences (FES) under Award Number DE-SC0018105   

Electrical properties of free electron laser generated warm dense water through reflection and transmission measurements

Driven by the breakdown of Spitzer theory in WDM regime, electrical conductivity is of great importance as an essential transport coefficient. Besides, broadband AC conductivity is also widely used to examine the relevant theoretical models. Nowadays, powerful X-, XUV-sources such as FEL enable improved accuracy in WDM diagnosis. We present in this study a pump-probe experiment ran at Desy, in Germany. \textasciitilde 100nm thick water sheets were isochorically heated by \textasciitilde 100fs FEL, and probed by an optical laser with \textasciitilde 500-850nm tunable wavelengths. Water in its liquid phase is transformed by increasing the temperature from a molecular to a dissociated/ionic regime. Reflection and transmission measurements were carried out in order to infer AC conductivity of the warm dense water to test the DFT-MD model. More precisely, we focus here on the temporal evolution of the interferometry measurement in transmissive mode: derived phase shift values were used to obtain the complex refractive index and then the AC conductivity at a given optical frequency.   

Nonlinear Polarization Mixing of Laser Beams Interacting with a Plasma

Understanding the interaction of overlapping laser beams in a plasma is important for precise control of laser energy flow at the National Ignition Facility (NIF) and for creating plasma-based high-power optical components. These overlapping beams modify each other’s energy and polarization through a ponderomotive interaction with the plasma [1]. The interaction of the beams is fundamentally a two-dimensional problem [2-4], which has important consequences for both NIF and plasma photonics (due to the effects on polarization and energy exchange). We present theoretical and numerical results detailing the effects of the interaction geometry on the beams. [1] Michel, P., et al. Physical review letters 113.20 (2014): 205001. [2] McKinstrie, C. J., et al. Physics of Plasmas 3.7 (1996): 2686-2692. [3] Lazarow, M., et. al. 49th Annual Anomalous Absorption Conference. Peaks Hotel, Telluride, Colorado. 10 June 2019. Poster Presentation. [4] Kur, E., et. al. 49th Annual Anomalous Absorption Conference. Peaks Hotel, Telluride, Colorado. 10 June 2019. Poster Presentation.   

\textbf{Ion Wave Plasma Optic Concepts for NIF and Other Lasers }

Recent demonstrations at NIF of plasma optics produced with ion waves driven by Cross Beam Energy Transfer (CBET) [1,2,3] have motivated work to develop concepts for similar optics to enhance laser performance at NIF and other laser facilities. The success of CBET models based on the linear response of ion waves in plasmas with minimal inverse Bremsstrahlung absorption [3,4], now motivates their use to design new plasma optics to produce beams with high performance in other respects, including: a beam combiner transferring energy to a beam with reduced focal spot size, a short pulse amplifier that transfers the power from many 1 ns beams to a single beam with \textless 0.1 ns duration, and a pulse compressor that uses a plasma combined pump with \textgreater 40 kJ in 1ns compressed to a duration of 10 to 100 ps in a second stage of interaction in a \textasciitilde 15 cm plasma. The challenges associated with designing and fielding such optics at NIF and elsewhere, as well as the requirements plasma optics place on new or upgraded facilities will be discussed to identify the most promising concepts. [1] P. Poole in preparation [2] R. K. Kirkwood et al Nat. Phys. 14 , 80 (2018). [3] R. K. Kirkwood et al Phys. of Plas. 25 056701 (2018). [4] A Colaitis et al Physics of Plasmas 25, 033114 (2018)   

Benchmarking non-LTE physics with experiments on gold plasmas at the OMEGA laser

Non-Local Thermodynamic (NLTE) physics of high-Z plasmas plays a crucial role in the coupling of laser energy to hohlraum x-radiation drive. Recent experiments to benchmark NLTE models of gold at electron temperatures of 1.5 to 2 keV and electron densities of few \( 10^{20} \) to few \( 10^{21} \) \( cm^{-3} \) have shown that the gold is more ionized than predicted by the best models. These experiments are designed to produce a uniform plasma by using a buried layer platform at the OMEGA laser. The temperature is measured with a K shell tracer and the density by imaging the sample in two perpendicular directions. Detailed spectra of gold 4, 5, 6 \(\rightarrow\) 3 transitions are used to measure the gold ionization states. Current results will be shown including work on resolving the discrepancy. The extension of this platform to higher temperatures and to radiation field and to direct measurements of NIF hohlraums will be discussed.   

Investigating flow strength in Tin through plasma-driven Rayleigh-Taylor instabilities

The Rayleigh-Taylor instability occurs when a lower density fluid pushes against a higher density one, leading to the growth of any surface perturbations. In the case of solids, this growth is mitigated by material strength (1). Consequently, it has been used as a way to determine the strength of various materials such as Cu, Fe, Ta, and Pb under ultra-high pressure conditions, by comparing against a strength-free system (2-4). Here, we use three lasers at the Omega EP laser facility, University of Rochester, to generate a Be plasma that drives a ramped compression to 1.5 Mbar into a rippled Sn target. The growth of the Sn ripples against the less dense CH is measured using face-on radiography. We then compare these results to hydrodynamic simulations using a Steinberg-Guinan strength model, allowing us to determine the flow stress of Sn at these pressure conditions. 1.Park, H.-S. et al. Phys. Rev. Lett. 114, (2015). 2.Huntington, C. M. et al. Bulletin of the American Physical Society (American Physical Society, 2017) 3.McNaney, J. M. et al. Bulletin of the American Physical Society (American Physical Society, 2019). 4. Krygier, A. G. et al. Phys. Rev. Lett., submitted (2019).   

Ion acoustic turbulence and anomalous laser absorption due to temperature gradient in ICF plasmas

Hot plasmas with strong temperature gradients, as the one obtained in inertial confinement fusion (ICF) experiments, may present electron heat flux able to generate ion acoustic instabilities. Return current instability (RCI) due to electron heat flux could be the source of stationary ion acoustic turbulence (IAT). Therefore, anomalous laser light absorption due to enhanced anomalous collisionality could occur in plasma where Landau damping of ion is negligible. Such effects are expected to occur inside hohlraum in gold plasma where ZTe/Ti>>1. Anomalous absorption and electron heat flux limitation due to RCI has been included as a reduced model in hydro code. Analysis of RCI in gold hohlraums experiments is presented. A specific experiment has been done on the Omega laser facility to measure and identify this instability. For this experiment, a gold plate heated by many beams generates expanding plasma, on which a probe beam is send to measure its absorption. At the same time, Thomson scattering is done along the path of the probe beam to measure plasma conditions and to measure ion acoustic wave (IAW) of the ion acoustic turbulence. Implications of all these processes for laser plasma interaction experiments are described and discussed.   

Design of a pulsed-power driven platform to study photoionization fronts in the laboratory

Photoionization fronts play an important role in many astrophysical environments, including the development of galactic structure at the end of the Cosmic Dark Ages [1] and the formation of stellar-wind bubbles around O-type stars [2]. Despite their importance, predictions of photoionization front behaviour have yet to be tested in laboratory experiments [3]. We present designs for a new experimental platform for studying photoionization fronts using the MAGPIE pulsed-power facility (1 MA, 500 ns). A wire array Z-pinch will be used to produce an intense burst of X-Rays (10$^{\mathrm{4}}$ J in 20ns) which will drive a photoionization front through a target. The target, a vaporised Al wire expanded to 10$^{\mathrm{18}}$ -- 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$, will be driven by a separate generator (10kA, 20ns). Front properties will be diagnosed using 2-color laser interferometry, Thomson scattering and X-Ray absorption spectroscopy. [1] B. E. Robertson, Nature, 2010 [2] J. Mackey, Astronomy {\&} Astrophysics, 2016 [3] R. P. Drake, The Astrophysical Journal, 2016   

Transport of Charged Particles in Laser-Driven Magnetized Turbulence

The interaction of charged particles and turbulent magnetic fields is key to understanding how cosmic rays traverse space. In this poster we report on numerical simulations and laser-driven experiments at the Omega Laser Facility at the Laboratory for Laser Energetics that measure the propagation of energetic particles through random magnetic fields in a turbulent plasma. We characterize their angular diffusion and recover their mean free path and associated diffusion coefficient. These experiments constitute the first laboratory probe of particle diffusion through magnetized turbulence in the absence of mean background fields and validate theoretical tools that are widely used in propagation studies of high-energy cosmic rays through the intergalactic medium.   

Radiative shock experiments on the SG-II laser

We present first results on the formation of radiative shocks on the SG-II laser at SIOM in China. The experiments build upon previous studies of piston-driven radiative shocks in Xenon [1] and Neon [2] using large-aspect ratio gas-cells, allowing the shocks to propagate unimpeded. The SG-II experiments looked at the dynamics of single and counterpropagating shocks in Argon at a pressure of \textasciitilde 1 bar with time-resolved, point-projection X-ray backlighting using a Scandium backlighter (\textasciitilde 4.3 keV probing energy). A new target design was used to study the late-time evolution of these shocks at times \textasciitilde 100 ns, allowing the development of spatial features at the head of the shocks to be investigated. [1] F. Suzuki-Vidal et al., ``Counterpropagating Radiative Shock Experiments on the Orion Laser'', Physical Review Letters 119, 055001 (2017), [2] T. Clayson et al., ``Counter-propagating radiative shock experiments on the Orion laser and the formation of radiative precursors'', High Energy Density Physics 23, 60-72 (2017).   

The effect of laser-generated fields on the focusing of high density relativistic positrons

High intensity lasers interacting with high Z targets facilitate a relatively high yield of MeV positrons. However, higher positron densities in the laboratory experiments is needed to achieve astrophysics relevant conditions and pair plasmas than the current experiments provide. Here, we report on novel target designs to increase the escaping positron yield and positron focusing effects for the same existing laser energy. The target utilizes strong fields in structured voids within a solid target, where laser-accelerated electrons are focused by self-generated magnetic field and positron are pushed toward target rear by electric fields. Numerical modeling predicts \textasciitilde 10x increase in escaping positrons depending on the number of voids. It is also seen that escaped positrons can be focused by a curved target surface resulting in increased local positron density. Detailed simulation results and experimental measurement will be presented.   

\textbf{Supra-thermal electron acceleration in collisionless shocks on OMEGA}

Acceleration of high energy particles in collisionless shocks have studied through theory and simulation for years but experimental validation has been lacking. This shock acceleration in astrophysical environments has been a long-standing candidate for the origin of cosmic rays. Recent OMEGA experiments using supersonic plasma flows have created astrophysically relevant collisionless shocks and shown evidence of electron acceleration. A laser-driven supersonic plasma flow collides with a gas bag of H2 to create a magnetized collisionless shock. Proton radiography measurements show Weibel filamentation, which leads to the generation of upstream magnetic turbulence. Electron spectra show a power law high energy tail indicative of acceleration. Here, we explore a number of electron acceleration mechanisms present in collisionless shocks and their connection to the measured electron energy spectra. This work was supported in party by the U.S. DOE, NLUF, and LLE.   

Improved accuracy in the COAX diagnostic for measuring supersonic radiation wavefront profiles

LANL's COAX (``co-ax'') is a diagnostic on Omega to infer the spatial profile of a supersonic radiation wave. It has a laser-driven halfraum to drive a Marshak wave down a cylinder of Ti-laden aerogel and uses a side-view Kr backlighter and Ti absorption spectroscopy to infer a radiation temperature profile. Recently, the process for analyzing the experimental data has been improved and automated using a generalized Boltzmann plotting technique, an inverse method, to determine the temperature and to resolve discrepancies between spectroscopy and atomic physics models. The technique, along with drive perturbation analysis and more stringent experimental tests of the COAX diagnostic, have improved the accuracy of the diagnostic.   

The Generation of Magnetized Jets Using 3D Printed Loads on a Pulsed-Power Driver

Astrophysical jets are diverse yet ubiquitous structures, typically associated with a gravitational engine that generates the axial flows from an accretion disk. However, the processes that maintain jet collimation and stability remain poorly understood. To explore the mechanisms at play, we propose to conduct a stability study of magnetized jets generated by pulsed-power drivers. Making an argument of magnetohydrodynamic stability, we may justify scaling our laboratory system by matching dimensionless parameters of the plasma jet: $Re>10^3$, $R_M\sim10^3$, $M>1$, and $\beta\gg1$. Our experiment will use a quasi-axisymmetric load, driven by 1MA pulsed power driver and capable of producing converging flows that transition into a strongly collimated, magnetized plasma jet. Using 3D extended MHD simulations from PERSEUS, we explore how the jet properties can be controlled by changing the load dimensions, the flow angular momentum and the overall magnetization of the system.   

Recent Experiments on the MAGPIE Pulsed Power Generator.

We present an overview of recent work on the MAGPIE pulsed power generator at Imperial College London. We use a suite of spatially and temporally resolved laser probing diagnostics, including interferometry, Thomson scattering and Faraday rotation imaging to diagnose high energy density, supersonic plasma flows which are generated by the ablation of plasma from wire arrays and radial foils or are driven using the X-Ray flux from stagnated wire array Z-Pinches. We study a variety of astrophysically relevant phenomena such as hypersonic jets; rotating plasmas; magnetic reconnection; the formation of bow shocks around magnetised and unmagnetised obstacles; and the dynamics of photoionization fronts.   

Radiation temperature measurement of a long duration x-ray source for steady-state laboratory photoionized plasma experiments

Long duration, i.e. tens of ns, broadband x-ray sources are important for steady-state laboratory photoionized plasma experiments relevant to astrophysics. In a series of experiments performed at the OMEGA EP laser facility, we have used the Gatling-Gun source to produce an x-ray drive that lasts for 30ns with a radiation temperature T$_{r}$ =90eV. The Gatling-Gun source is comprised of three Cu hohlraums; each is filled with TPX foam and driven by a separate 10ns square pulsed laser beams with 4.4kJ of UV laser energy\footnote{D. Martinez, 2017 Annual OLUG Workshop.}. The total source duration of 30ns is achieved by driving the three hohlraums sequentially in time. The radiation temperature was monitored with a VISAR diagnostic over a series of shots to check performance and consistency, and compare with previous miniDMX data\footnote{miniDMX reference}. We present measurements of the radiation temperature obtained from VISAR data from two series of experiments performed at OMEGA EP, in conjunction with radiation-hydrodynamics simulations of the VISAR diagnostic package.   

Exploring the Parameter Space of a Laboratory Astrophysical Experiment Modeling a Molecular Cloud Experiment using CRASH

Recent laboratory experiments explored radiation hydrodynamics relevant to irradiated molecular clouds, by using X-rays from a laser-driven gold foil to irradiate a foam sphere. The primary goal of this study is to identify conditions under which the foam sphere target will implode, explode, or be blown away. We used CRASH, an Eulerian code developed at the U. of Michigan, which includes block adaptive mesh refinement, multigroup diffusive radiation transport, and electron heat conduction. We explored different regimes of molecular-cloud photoevaporation proposed by Bertoldi 1989. The dynamics of the experiment depend on the shock velocity produced, the speed of sound in the target, the opacity of the target, the size of the target, and ionizing flux incident on the target. We present a parameter study varying the foam target and X-ray flux showing the conditions under which an experiment could access the different regimes.   

High-energy-density Targets Fabricated by The University of Michigan

The Center for Laboratory Astrophysics at the University of Michigan is unique among universities in that we have been fabricating targets for high-energy-density physics experiments for well over the past decade. We utilize the process of machined acrylic bodies and tightly toleranced mating components that serve as constraints, enabling our group to build repeatable targets. We favor traditional machining, utilizing 3D printing when it suits, taking advantage of the very best part of both of these methods of creating precision parts for our targets. Here we present several campaigns shot at the OMEGA, Titan and Trident facilities and methods used to fabricate those targets.   

Investigation of Lower Hybrid Wave Interaction with the Edge Plasma in WEST through Electric Field Vector Measurements

The WEST tokamak’s main mission requires it to establish and sustain long pulse H-mode operation in an all-W wall environment using only RF waves for heating and current drive. To drive plasma current, two lower hybrid (LH) launchers are installed and operate at 3.7 GHz with a combined power of up to 7 MW for 1000 s. A new experimental arrangement on WEST aims to measure the spatially resolved LH wave electric field vector, $\mathbf{E}_{LH}$, in front of one of these launchers. The main motivation is to improve understanding of LH wave interaction with the scrape off layer (SOL) plasma. The experimental measurement of $\mathbf{E}_{LH}$ is obtained by fitting the Schr\"{o}dinger equation to the $\sigma$-polarized ($\perp$ to $\mathbf{B}$) $D_{\beta}$ spectral line profile. The $D_{\beta}$ spectrum are acquired passively from optical emission observed near the lateral protection limiters. The measured $\mathbf{E}_{LH}$ is then systematically compared to simulations using a 3D full-wave COMSOL model. Details of the experimental arrangement will be described and initial $\mathbf{E}_{LH}$ results will be presented. Future work focused on studying $\mathbf{E}_{LH}$ as a function of LH power, confinement mode, Greenwald fraction, and magnetic geometry will be discussed.   

Model-based Linear Quadratic Integral Control Design for q-profile Shaping in EAST

In order to achieve advanced modes of operation, characterized by confinement improvement and possible steady-state operation, control capabilities for shaping the spatial profile of the toroidal current density, or equivalently the safety factor $q$ or the gradient of the poloidal magnetic flux, are essential. A linear quadratic integral (LQI) control-design approach has been followed in this work to further develop such control capabilities in EAST. The controllers, which have been designed based on a first-principles-driven control-oriented model of the poloidal magnetic flux profile evolution, have the capability of regulating several points of the q profile and its integral properties such as the internal inductance $l_{i}$. Moreover, by controlling the plasma current \textit{Ip} and the powers of both the low frequency (2.45 GHz) and the high-frequency (4.60 GHz) lower hybrid wave sources, the controllers can also regulate $\beta _{N}$. Nonlinear simulations show that the controllers can effectively regulate a combination of $q$(0.1), $q$(0.5), $q$(0.9), $l_{i}$ and $\beta_{N}$. The proposed control laws have been implemented in the recently developed Profile Control category in the EAST Plasma Control System (PCS) with the ultimate goal of testing them experimentally.   

Negative-Triangularity Configuration on EAST: Analysis of engineering limitations on superconducting, D-shaped, target-diverted plasmas

In recent years, tokamak research has repeatedly shown that the edge magneto-hydrodynamic stability is critical for handling the power to the walls and the divertor plates which is now and will mostly likely continue to be a limiting factor in the International Thermonuclear Experimental Reactor (ITER) and the DEMOnstration Power Station (DEMO). Recent experiments at Tokamak \`{a} Configuration Variable (TCV) [Medvedev et al.] and DIII-D [Austin et al.] have shown that a Negative-Triangularity Configuration (NTC) has a larger power handling area on the Low-Field-Side (LFS) divertor target plate [Medvedev et al.] and improved edge stability. However, there have been relatively few NTC experiments performed so far and none of them have been performed on a superconducting tokamak with shaping capabilities similar to ITER. To expand upon the previous experiments on TCV and DIII-D this paper addresses an initial test of the NTC capability of the Experimental Advanced Superconducting Tokamak (EAST) which has achieved a 7 s ohmic discharge Upper Singular Null (USN) target-diverted plasma with a lower traingularity of $\delta_L \le -0.09$.   

Study on optimized structure of the divertor suitable for KSTAR tokamak

An innovative divertor concept is required to construct a future fusion reactor of high duty cycle. In the environment of the fusion reactor with long drive time (high duty cycle), the power load on the target surface of the divertor should be limited when the steady state is assumed. In addition, erosion must also be suppressed to near zero. Constructing a dissipative divertor that satisfies these constraints, while at the same time not corrupting core quality, is a challenging task. To accomplish this goal, various attempts have been made in many tokamak devices. We proceeded to study the optimization of divertor structure suitable for KSTAR by applying the high dissipative SAS (Small Angle Slot) structure used in DIII-D. Simulation modeling was performed through the SOLPS-ITER package.   

Simultaneous injection of lower hybrid power at two frequencies on EAST

EAST operates a number of electron heating sources, including two lower hybrid (LH) systems at 2.45 GHz and 4.6 GHz, and a 140 GHz electron cyclotron (EC) source. This offers a flexible platform to study the interactions between EC and the two different frequencies of LH, and the possible synergy where the absorption of one radiofrequency (rf) wave increases the fast electron population needed for higher efficiency current drive of another rf wave. Because the DC electric field affects the fast electron distribution function, it is important that the magnetic equilibrium and the LH current drive are evolved self-consistently. For this reason, time-dependent modeling is a necessary tool to guide experiments on EAST dedicated to access and sustainment of steady-state. The various possible combinations of the three waves have been tested in experiment, in which a slight decrease in measured loop voltage suggests stronger non-inductive current drive, and is an indicator of possible synergy between the rf waves. Simulation is in progress to reproduce the experiment and better study the synergy between of the waves.   

Current Density Profile Evolution with Lower Hybrid Current Drive on EAST

Weak or reversed magnetic shear plasma scenarios with internal transport barriers (ITB) are considered to be prime candidates for steady-state (or long pulse discharge) high-confinement plasma operation. This can be achieved using an optimized q profile by controlling the heating and current drive systems in tokamaks. The eleven chord POlarimeter-INTeferometer (POINT) system on EAST can provide internal magnetic field measurements with fast time response (up to 1 MHz) thereby allowing realtime current and q profile monitoring using fast equilibrium reconstruction. High beta$_{\mathrm{N}}$ (1.8-2), H$_{\mathrm{98}}=$1.1 plasmas with good confinement are achieved with Neutral Beam Injection (NBI) and Lower Hybrid Wave (LHW). The central q profile is shown to be flat by POINT measurements. A new error correction method to decrease the stray light error for Faraday effect measurement is applied and stray light contamination is reduced to 0.5-1 degs. from 2-5 degs. Further efforts to reduce stray light are underway and essential for realtime q profile measurement required to control and extend high-performance scenarios developed on EAST.   

Experimental and modeling study of divertor particle flux width on EAST and DIII-D

The study of divertor particle flux width is carried out for different types of H-mode plasmas with neutral beam (NB) and low hybrid wave (LHW) heating schemes on EAST. For the experimental part, the particle flux width decays with the increase of plasma current. The amount of the heating power seems to have no effect on the particle flux width in pure NB or LHW heated plasmas. However, the heating scheme is found to have enormous influence on the particle flux width, the width tends to be larger in plasmas with higher ratio of LHW heating power. Comparisons among the particle flux width in type-I ELMy plasmas, inter type-I ELMy plasmas and grassy ELM-averaged plasmas show that the width for type-I ELMy plasmas is much larger than those of the other two types of plasmas while the width for the grassy ELM-averaged plasmas is a little larger than those of the inter type-I ELMy plasmas, which is probably due to the different intensities of background turbulence in these three types of plasmas. Simulation work with BOUT$++$ code is on going to figure out the potential causes of the differences of particle flux width among these three types of plasmas on EAST. For the DIII-D part, a L-mode discharge is employed for the simulation with BOUT$++$ code. The detailed analysis of the experimental and modeling results for both DIII-D and EAST discharges will be presented.   

Measurements of Low Intensity W I Light from WEST Tokamak using Spectrally Separate Narrow Band Pass Filters

Within the core of a magnetically confined fusion plasma, impurities contribute to energy confinement losses via radiative cooling. With enough radiative cooling, the plasma will disrupt and extinguish. Due to these losses, tracking impurities is critical to furthering fusion energy science. Eroded atoms from the wall that emit photons when they become excited/ionized after interacting with the plasma are simple to measure impurities. Using spectrometers, photons are collected over a wavelength range, which limit acquisition rates. With a filterscope, the emission line is isolated from the spectra and only light from a narrow band (1-2 nm) is collected thus increasing the rate of acquisition, leading to a current max rate of 100 kHz. With a full tungsten (W) wall on WEST, the need to track W impurities is paramount. Measurements with two filters were used to confidently measure the W-I emission from 400.9 nm on WEST. One filter, centered at 400.55 nm, collects the W-I light and the other at 403.55 nm collects background continuum radiation. These signals are subtracted and calibrated to produce a photon flux. We will present the results of this staggered filter method.