Session YO6: Magnetized HEDP and HED Measurement/Diagnostic Techniques

Study of transport phenomena in laser-driven, non- equilibrium plasmas in the presence of external magnetic fields

We present experimental and simulation results from a study of thermal transport inhibition in laser-driven, mid-Z, non-equilibrium plasmas in the presence external magnetic fields. The experiments were performed at the Jupiter Laser Facility at LLNL, where x-ray spectroscopy, proton radiography, and Brillouin backscatter data were simultaneously acquired from sub-critical-density, Ti-doped silica aerogel foams driven by a $2\omega$ laser at $\sim5\times10^{14}\,W/cm^2$. External B-field strengths up to $\sim20\,T$ (aligned antiparallel to the laser propagation axis) were provided by a capacitor-bank-driven Helmholtz coil. Pre-shot simulations with \textsc{Hydra}, a radiation-magnetohydrodyanmics code, showed increasing electron plasma temperature with increasing B-field strength -- the result of thermal transport inhibition perpendicular to the B-field. The influence of this thermal transport inhibition on the experimental observables as a function of external field strength and target density will be shown and compared with simulations.   

Magnetic flux pile-up and ion heating in a current sheet formed by colliding magnetized plasma flows

We present data from experiments carried out at the Magpie pulsed power facility, which show the detailed structure of the interaction of counter-streaming magnetized plasma flows. In our quasi-2D setup[1,2], continuous supersonic flows are produced with strong embedded magnetic fields of opposing directions. Their interaction leads to the formation of a dense and long-lasting current sheet, where we observe the pile-up of the magnetic flux at the sheet boundary, as well as the annihilation of field inside, accompanied by an increase in plasma temperature. Spatially resolved measurements with Faraday rotation polarimetry, B-dot probes, XUV imaging, Thomson scattering and laser interferometry diagnostics show the detailed distribution of the magnetic field and other plasma parameters throughout the system. [1] Suttle et. al, PRL (2016), [2] Hare et. al, PRL (2017)   

Validation of non-local electron heat conduction model for radiation MHD simulation in magnetized laser plasma

In laser plasma physics, application of an external magnetic field is an attractive method for various research of high energy density physics including fast ignition. Meanwhile, in the high intense laser plasma the behavior of hot electron cannot be ignored. In the radiation hydrodynamic simulation, a classical electron conduction model, Spitzer-Harm model has been used in general. However the model has its limit, and modification of the model is necessary if it is used beyond the application limit. Modified SNB model [1], which considering the influence of magnetic field is applied to 2-D radiation magnetohydrodynamic code PINOCO. Some experiments related the non-local model are carried out at GXII, Osaka University. In this presentation, these experimental results are shown briefly. And comparison between simulation results considering the non-local electron heat conduction mode are discussed. [1] Ph. D. Nicolai et al, Phys. Plasmas 13, 032701 (2006)   

Laser-Plasma Modeling Using PERSEUS Extended-MHD Simulation Code for HED Plasmas

We discuss the use of the PERSEUS extended-MHD simulation code for high-energy-density (HED) plasmas in modeling the influence of Hall and electron inertial physics on laser-plasma interactions. By formulating the extended-MHD equations as a relaxation system in which the current is semi-implicitly time-advanced using the Generalized Ohm's Law, PERSEUS enables modeling of extended-MHD phenomena (Hall and electron inertial physics) without the need to resolve the smallest electron time scales, which would otherwise be computationally prohibitive in HED plasma simulations. We first consider a laser-produced plasma plume pinched by an applied magnetic field parallel to the laser axis in axisymmetric cylindrical geometry, forming a conical shock structure and a jet above the flow convergence. The Hall term produces low-density outer plasma, a helical field structure, flow rotation, and field-aligned current, rendering the shock structure dispersive. We then model a laser-foil interaction by explicitly driving the oscillating laser fields, and examine the essential physics governing the interaction.   

Development of experiments for high-intensity laser plasma interaction in a magnetic field of the pulsed power generator

Experiments were developed for investigation of the laser plasma interaction in the megagauss magnetic field of the 1MA Zebra pulsed power generator coupled with a 50TW laser. These experiments are relevant to astrophysical plasmas, particle and x-ray generation, and isochoric heating in a strong magnetic field. Magnetic fields in loads were measured with Faraday rotation in a glass sample placed near the load. 1-3MG longitudinal and transversal magnetic fields were measured in different loads. Impact of the fast rising magnetic field on metal laser targets was demonstrated. Focusing and targeting laser systems were integrated into the chamber of the Zebra generator. Shots at intensity of \textgreater 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}}$ demonstrated collimation of plasma and generation of jets on the front and rear sides of the foil target in the axial magnetic field.   

PDV based technique for probing AK gap plasma in Z Machine target loads

It is empirically known that the behavior of current loss in the convolute, feed, and load region on Sandia's Z Machine (Z) depends on the current pulse shape as a function of time during the experiment. Charged particles can be liberated from current carrying surfaces upstream of the load as a result of the time varying electric fields generated. Pulsed electric current from the four magnetically insulated transmission lines come together in the convolute to feed the target load. In this region, the magnetic insulation breaks down, which can lead to charged particle collisions and sputtering on conductive surfaces. The resulting low density plasma affects the load dynamics. Confirming the presence and behavior of low density anode-cathode (AK) gap plasma in the target load region of Z is thus a matter of great importance. In this work, we outline a new photon Doppler velocimetry (PDV) based technique to detect arrival time of upstream plasma in the AK gap of a cylindrically convergent target load. Analysis of the PDV deduced time varying refractive index of the plasma allows for an estimation of its electron number density.~   

X-ray Spectropolarimetry of Z-pinch Plasmas with a Single-Crystal Technique

When directed beams of energetic electrons exist in a plasma the resulting x-rays emitted by the plasma can be partially polarized. This makes plasma x-ray polarization spectroscopy, spectropolarimetry, useful for revealing information about the anisotropy of the electron velocity distribution. X-ray spectropolarimetry has indeed been used for this in both space and laboratory plasmas. X-ray polarization measurements are typically performed employing two crystals, both at a 45\textdegree Bragg angle. A single-crystal spectropolarimeter can replace two crystal schemes by utilizing two matching sets of internal planes for polarization-splitting. The polarization-splitting planes diffract the incident x-rays into two directions that are perpendicular to each other and the incident beam as well, so the two sets of diffracted x-rays are linearly polarized perpendicularly to each other. An X-cut quartz crystal with surface along the [11-20] planes and a paired set of [10-10] planes in polarization-splitting orientation is now being used on aluminum z-pinches at the University of Nevada, Reno. Past x-ray polarization measurements have been reserved for point-like sources. Recently a slotted collimating aperture has been used to maintain the required geometry for polarization-splitting enabling the spectropolarimetry of extended sources. The design of a single-crystal x-ray spectropolarimeter and experimental results will be presented.   

Picosecond Time-Resolved Temperature and Density Measurements with K-Shell Spectroscopy

The thermal x-ray emission from rapidly heated solid targets containing a buried-aluminum layer was measured to track the evolution of the bulk plasma conditions. The targets were driven by high-contrast 1$\omega $~laser pulses at focused intensities up to~1~$\times $~10$^{\mathrm{19}}$~W/cm$^{\mathrm{2}}$. A streaked x-ray spectrometer recorded the $\mbox{Al\thinspace He}_{\alpha } $ and lithium-like satellite lines with 2-ps temporal resolution and moderate resolving power ($E \mathord{\left/ {\vphantom {E {\Delta E\approx 1000}}} \right. \kern-\nulldelimiterspace} {\Delta E\approx 1000})$. Time-integrated measurements over the same spectral range were used to correct the streaked data for variations in photocathode sensitivity. Linewidths and intensity ratios from the streaked data were interpreted using a collisional radiative atomic kinetics model to provide the average plasma conditions in the buried layer as a function of time. Experimental uncertainties in the measured plasma conditions are quantified within a consistent model-dependent framework. The data demonstrate the production of a 330$\pm $56 eV, 0.9$\pm $0.3 g/cm$^{\mathrm{3}}$ plasma that evolves slowly during peak $\mbox{He}_{\alpha }_{\mathrm{\thinspace }}$emission. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

The Simultaneous Combination of Phase Contrast Imaging with In Situ X-ray diffraction from Shock Compressed Matter

Here, we present the simultaneous combination of phase contrast imaging (PCI) techniques with in situ X-ray diffraction to investigate multiple-wave features in laser-driven shock-compressed germanium. Experiments were conducted at the Matter at Extreme Conditions end station at the LCLS, and measurements were made perpendicular to the shock propagation direction. PCI allows one to take femtosecond snapshots of magnified real-space images of shock waves as they progress though matter. X-ray diffraction perpendicular to the shock propagation direction provides the opportunity to isolate and identify different waves and determine the crystal structure unambiguously. Here, we combine these two powerful techniques simultaneously, by using the same Be lens setup to focus the fundamental beam at 8.2 keV to a size of 1.5 mm on target for PCI and the 3rd harmonic at 24.6 keV to a spot size of 2 um on target for diffraction.   

Strategies for Time-resolved X-ray Diffraction of Phase Transitions with Laser Compression

As part of a program to document kinetics of phase transitions under laser-driven dynamic compression, we are designing a platform to make multiple x-ray diffraction measurements during a single laser experiment. Our plans include experimental development at Omega-EP and eventual implementation at NIF. We will present our strategy for designing a robust platform that can effectively document a wide variety of phase transformations by utilizing both streaked and multiple-frame imaging detectors.~~Preliminary designs utilize a novel CMOS detector designed by Sandia National Lab.~~Our initial~experiments include scoping studies that will focus on photometrics and shielding requirements in the high EMP environment close to the target.   

Studies of soft x-ray transmission through grid supported CH layers

Recent experiments have shown that it may be possible to use laser-heated high-Z foils to drive new radiation transport (RadTran) experiments in gas fill tubes. These tubes must be pressurized above 1atm and the x-ray source needs to be physically separated from the gas. To achieve this, a grid-supported CH seal is implemented. The grid reduces the total surface area of the gas-seal interaction region lowering the thickness requirements for the CH layer. However, as mesh spacing is reduced, hole closure from wire ablation may reduce the x-ray flux. To optimize the seal design, experiments were performed measuring x-ray transmission through CH layers supported by meshes composed of copper, gold, or stainless steel and using hexagonal or square mesh geometries. The x-ray source was formed by heating a 0.5 μm thick planar gold foil with a 4 ns laser pulse at an intensity of 2$\times$10$^{14}W/cm2$. Emission data was collected using an x-ray framing camera and a Dante photodiode array. Experiments show that the CH layers can reach effective temperatures of nearly 100 eV but mesh design significantly affects performance, with a nearly 20 eV difference between the best and worst performing seal targets. This talk will discuss our findings and their impact on future RadTran experiments.