Session GP17: Poster Session: HED: Hydrodynamics and Magnetized Plasma (9:30am - 12:30pm)

The Role of Radiation in Shock-Driven Shear Instabilities

Although the hydrodynamics of interfacial instabilities has been the object of numerous studies in high-energy-density physics, the role of radiation on perturbation growth is poorly understood. We present a computational study of the role of radiation on shock-driven shear instabilities. Using CRASH, a block-adaptive Eulerian radiation-hydrodynamics code with flux-limited multigroup diffusion, we conduct two-dimensional simulations based on the shock-shear experiments of [1]. We consider two different laser drive profiles to isolate the effect of radiation on the growth of the mixing layer. The low-drive case gives rise to primarily hydrodynamic growth. In the high-drive case, the radiative precursor preheats the upstream material before the arrival of the shock, thus slowing the mixing layer growth. [1] K. A. Flippo, et al., Phys. Plasmas 25, 056315 (2018).   

Continuum Kinetic Studies of the Rayleigh-Taylor instability and nonlocal electron heat conduction.

Continuum kinetic simulations offer a method of capturing non-Maxwellian behavior without requiring the tracking of individual particles. In this work, the continuum kinetic code \texttt{Gkeyll} is used to study nonlocal plasma transport and its effect on the Rayleigh-Taylor (RT) instability. To study nonlocal electron transport, the Vlasov-Maxwell system is solved to examine changes in heat flow and conductivity related to electron collisionality. When the electron mean-free-path becomes large relative to the temperature gradient scale length, electrons can escape local temperature gradients and deposit their energy elsewhere in the plasma, leading to non-Maxwellian distributions and reduced heat flux around the temperature gradient. Following a 1x3v (1 spatial dimension, 3 velocity space dimensions) study of nonlocal electron transport, plasma transport in the RT instability will be studied kinetically in 2x2v. RT simulations are traditionally performed using fluid models, but kinetic effects on RT growth may be relevant to astrophysical and laboratory high energy-density regimes. Results from this study are compared to other kinetic models.   

Studies of Rayleigh-Taylor and magneto-Rayleigh-Taylor instabilities in planar and cylindrical geometries

Inertial confinement fusion experiments have identified the RT (Rayleigh-Taylor) instability as one of the largest inhibitors to achieving fusion. Consequently, understanding the impacts of externally applied magnetic fields on the growth of RT during implosion deceleration may allow for methods to mitigate the instability growth. A study in Cartesian and cylindrical geometry presents significant RT growth during the deceleration phase of imploding liners. Additionally, the impact of an externally applied magnetic field on MRT (magneto-Rayleigh-Taylor) growth and magnetic field strength is explored in an effort to mitigate the MRT growth. Cylindrical parameters are derived from experimental designs for Omega and NIF shots. FLASH's MHD and resistive-MHD capabilities are used to model the imploding cylinders. This work was supported by a subcontract from the Los Alamos National Laboratory and US DOE grant SC0020055.   

Two Experiments for Studying Feedthrough of Instabilities and Mix

Understanding the impact of hydrodynamic perturbations transmitted through thin, dense layers is important for inertial confinement fusion ignition schemes, particularly double- or multi-shell systems. Experiments to validate our understanding are challenging as they necessarily involve multiple interfaces, materials, transmitted and reflected shocks, etc. Two recent and ongoing experiments, one laser-driven design fielded at the National Ignition Facility, and one pulsed-power-driven design for the Sandia Z Machine, test and validate aspects of transmitted instability theory and feedthrough, including the qualitative difference in behavior between long and short wavelength modes: the buckling of the layer by long modes into imprinted shapes, and the cumulative impact of short modes leading to mix. Recent improvements in analysis have demonstrated how, beyond integral measures such as dominant wavelength and mix width, higher-order metrics such as material variances may also be extracted from the diagnostics and compared with theory and simulation.   

The Effect of Anomalous Resistivity on Electrothermal Instability Growth

The current driven ETI (Electrothermal instability) forms when the mate- rial resistivity is temperature dependent, occurring in nearly all Z-pinch-like high energy density platforms. High mass density ETI growth is predominantly striation form, magnetically perpendicular modes, because the resistivity tends to increase with temperature in this regime. In contrast, low mass density ETI growth is mainly filamentation form, magnetically aligned modes, because the resistivity tends to decrease with temperature. Simulations of ETI typically use a collisional form of the resistivity as provided, e.g., in a Lee-More Desjarlais conductivity table. However, in regions of low density, collisionless transport needs to be incorporated to properly simulate the filamentation form of ETI growth. Anomalous resistivity is an avenue by which these collisionless micro-turbulent effects can be incorporated into a collisional resistivity. For this work, 3D simulations of filamentation ETI explore the effect of anomalous resistivity. Additionally, newly derived theoretical forms of the filamentation ETI growth rate including anomalous resistivity are reproduced through simulation.   

Simulation of Electrothermal Instability Growth During Pulsed Power Driven Implosions of Metallic Liners Coated with Dielectric Coatings

Pulsed power experiments drive megaampere (MA) currents through metallic rods in nanosecond timescales, resistively heating the outer layer into plasma. Preliminary studies indicate that microscale material inhomogeneities can cause uneven heating, which leads to current filamentation or striation if the rods' electrical resistivity is temperature dependent. This creates a feedback loop that results in electrothermal (ET) instabilities that seed magneto-Rayleigh-Taylor (MRT) instabilities. The Plasma Dynamics Computational Laboratory at Virginia Tech has run simulations using the Ares code developed by Lawrence Livermore National Laboratory to model 0.8 MA currents pulsed through 0.8mm diameter metallic rods, in support of an effort by University of Nevada Reno (UNR) collaborators on the MYKONOS-V driver at Sandia National Laboratories (SNL). The magnetohydrodynamics code uses the Los Alamos National Laboratory SESAME equation of state tables and the SNL Lee-More-Desjarlais conductivity tables to model material effects. To better understand ET mitigation techniques, the computational study also investigates rods coated with dielectric material of varying thicknesses and compares the surface expansion velocities to photonic doppler velocimetry (PDV) of the UNR-SNL experiment.   

Fundamental studies on the electrothermal instability on the 1 MA Mykonos driver*

The electrothermal instability (ETI) is a Joule heating-driven instability that can initiate in the solid state in magnetically driven fusion targets. The ETI generates azimuthally correlated (striated) temperature and density perturbations. These striations may seed the magneto Rayleigh-Taylor instability, which can limit stagnation pressure and implosion uniformity. To better understand how local perturbations might seed ETI, experiments to observe ETI growth from diamond-turned, 99.999{\%} pure Al rods in a z-pinch configuration have been executed by monitoring 12-48um diameter ``engineered'' defects machined into the rod surface with ICCD images and a photodiode. Experiments are conducted on the \textasciitilde 1 MA Mykonos driver at Sandia National Laboratories. Shadowgraphy and PMT diagnostics are being developed and will be presented with ongoing experimental results.   

Photonic Doppler Velocimetry on Thick-Wire Surfaces Driven by Intense Current.

Photonic Doppler Velocimetry (PDV) was used to measure the surface motion of thick-metal (6061 and 5N Al, 5N Cu, 4N Ni, and Ti) wires driven to 0.8 MA by the Sandia Mykonos generator. Magnetic compression of the solid wire at the start of the current pulse was observed for the first time.~ Untamped pure (5N) Al loads compressed 40 nm radially before expanding.~ In addition, the surface magnetic field at the start of expansion was found, and the distribution of surface velocities was measured for the first time. Surface velocity distributions were observed to broaden later in time, before plasma forms. Accurate magnetohydrodynamic (MHD) modeling of electrical explosions is currently challenging due to uncertainties in the equation of state (EOS) and electrical conductivity, especially during the metal-insulator transition. The experimental measurements are being used to benchmark MHD modeling, and thereby inform the choice of EOS and conductivity tables for modeling.   

Spectroscopic and MHD modeling of magnetized cylindrical implosions using a laser-produced seed B-field

We present a comprehensive simulation study of magnetized cylindrical implosions at OMEGA using laser-driven capacitor-coil targets to produce a seed, pre-compressed B field. Ar-doped, D2-filled cylindrical targets will be symmetrically imploded using a 15 kJ, 1.5 ns laser drive. The plasma dynamics are numerically investigated in 2-D with the MHD code GORGON, which predicts a B-field exceeding 10 kT over the entire compressed core and significantly higher temperatures compared to the unmagnetized case. Synthetic X-ray emission spectra computed with the NLTE atomic kinetics code ABAKO and detailed Stark-Zeeman broadening codes (MERL, PPP-B, DinMol) show distinctive spectral features for a magnetized implosion and the case without seed B-field. These results suggest the use of Argon K-shell spectroscopy to extract plasma conditions throughout the implosion and bring information about changes in the hydrodynamic behaviour due to the impact of the B-field.   

Not Participating

Pulsed power facilities enable a variety of high energy density physics experiments. Modeling the interactions between the low density “vacuum” plasma that forms in the powerflow region and the high density target is challenging, especially in a single-fluid resistive MHD framework. We present a collection of improvements made to a resistive MHD code to better capture the vacuum contaminant plasma dynamics and its interaction with pulsed power targets. The macroscopic impact of kinetic microturbulence can be approximated through reduced models such as anomalous resistivity; care should be taken to propagate modified collision frequencies across collisional transport processes in the strongly magnetized plasma. Additionally, we emphasize the importance of energy balance in the vacuum region to prevent unphysical runaway heating; for instance, density floors should be enforced in an energy-conserving manner and discretizations of the diffusive heat flux from the vacuum to the target must behave well in the presence of material interfaces and the high temperature, low density, strongly magnetized vacuum contaminant plasma.   

Plasma Expansion from a Pulsed Power Electrode Surface

The status of measurements of plasma expansion from a current-carrying electrode surface in a pulsed power feed will be discussed. Experiments on COBRA at Cornell show slow expansion while current is rising, and rapid expansion coincident with driving voltage reversal and the beginning of current reduction. Interferometry of the expanding plasma density, visible light spectroscopy to give plasma temperature, and magnetic field measurements by Bdot probes and polarization Zeeman technique will be shown. The possible importance of inverse skin effects in driving the rapid expansion withvoltage reversal will be discussed.   

High-Voltage Solid-State Trigger for HEDP Applications Phase I Results

Eagle Harbor Technologies (EHT), Inc. is developing a solid-state thyratron replacement that can be used to trigger higher voltage spark-gap switches at Sandia and other laboratories. The current trigger generators used at Sandia are marginally reliable and have a long manufacturing and delivery time, and there is concern about the long-term availability of these thyratrons. When measured over short timescales, thyratrons typically have a jitter of a few nanoseconds; but over longer timescales, they can have a much larger drift. Additionally, thyratrons need stable, high-current, low-voltage power sources, have long warm-up times, and require conditioning shots to achieve a stable operating point. EHT recently completed a Phase I program to develop a first-generation prototype solid-state thyratron replacement. This unit produced 20 kV into 50 $\Omega $ with a sub-10 ns rise time and 100 ns e-folding fall time. EHT will present the design tradeoff study, selected topology, and key waveforms results.   

PUFFIN: a new microsecond, mega-ampere pulser for magnetised HED plasma physics

Many interesting plasma physics phenomena in the universe develop over long time scales. One example is magnetohydrodynamic turbulence, which must be driven over many hydrodynamic time scales to reach the statistical steady state typical of astrophysical plasmas. Existing pulsed-power generators are usually optimised for very fast rise times (\textasciitilde 100 ns), which can drive the rapid implosions which generate bright x-rays sources. However, for basic plasma physics or laboratory astrophysics studies it is desirable to drive the plasma over longer timescales. In this poster we discuss the design of PUFFIN, a medium sized pulsed power facility with a 1-2 MA peak current and a 2 us rise time, based on the LTD-5 modules developed at CEA Gramat. PUFFIN will be constructed at the Plasma Science and Fusion Center at MIT starting in January 2021. It will be a versatile driver of magnetised HED plasmas and will provide a testbed for diagnostic development. Particularly topics of interest include magnetised turblence, magnetic reconnection, and transport and instabilities in magnetised plasmas.   

Testing of the First BLUE Linear Transformer Driver (LTD) Cavity at the University of Michigan

BLUE is a 4-cavity linear transformer driver (LTD) system currently being constructed in the University of Michigan's Plasma, Pulsed Power, and Microwave Lab. The four 10-brick cavities were previously part of the Ursa Minor experiment at Sandia National Laboratories. When fully assembled, the BLUE system should be capable of delivering 8 kJ to a proper load in an 800-kV (open circuit), 100-ns pulse. Dual 100-kV, 12-kW Spellman power supplies allow a theoretical rep-rate of \textgreater 1 Hz for high-power microwave experiments using a GW-class magnetically insulated line oscillator (MILO), also under development at UM. The first prototype cavity has been assembled and single-cavity testing has begun. A polycarbonate lid allows operation of the first BLUE cavity as an impedance-matched Marx generator (IMG), though a pre-magnetization pulse generator has also been developed for operation as a traditional LTD. The construction status of the BLUE system will be presented in addition to experimental results with the prototype cavity.   

Generation of magnetic fields at the 1MA pulsed power machine

Generation of the magnetic field by different loads at the Zebra pulsed power machine was studied. The longitudinal B-field of 1.7 MG in the half-turn coils and transverse field of 4 MG on the rod loads were generated using a load current multiplier with current of 1.3 MA. The magnetic fields were measured with the back-reflected Faraday rotation diagnostics. A cutoff of the Faraday signal in spiral loads was studied. Inter-coil discharge in the spiral loads produced x-ray/UV burst and induced opacity in glass samples located 3-12 mm from the load. Opacity blocked the Faraday laser signal and magnetic fields. Half-turn Cu and Ta coils and rod loads did not impact closely located targets. Using half-turn coils, penetration of the 1 MG magnetic field in the stainless steel tubes is demonstrated. Experiments with plasma in the axial and transverse magnetic fields are presented.