Sonal Patel

Magneto-Rayleigh-Taylor experiments on a MegaAmpere linear transformer driver

Experiments have been performed on a nominal 100 ns rise time, MegaAmpere (MA)-class linear transformer driver to explore the magneto-Rayleigh-Taylor (MRT) instability in planar geometry. Plasma loads consisted of ablated 400 nm-thick, 1 cm-wide aluminum foils located between two parallel-plate return-current electrodes. Plasma acceleration was adjusted by offsetting the position of the foil (cathode) between the anode plates. Diagnostics included double-pulse, sub-ns laser shadowgraphy, and machine current B-dot loops. Experimental growth rates for MRT on both sides of the ablated aluminum plasma slab were comparable for centered-foils. The MRT growth rate was fastest (98 ns e-folding time) for the foil-offset case where there was a larger magnetic field to accelerate the plasma. Other cases showed slower growth rates with e-folding times of about ∼106 ns. An interpretation of the experimental data in terms of an analytic MRT model is attempted.

Technique for fabrication of ultrathin foils in cylindrical geometry for liner-plasma implosion experiments with sub-megaampere currents

In this work, we describe a technique for fabricating ultrathin foils in cylindrical geometry for liner-plasma implosion experiments using sub-MA currents. Liners are formed by wrapping a 400 nm, rectangular strip of aluminum foil around a dumbbell-shaped support structure with a non-conducting center rod, so that the liner dimensions are 1 cm in height, 6.55 mm in diameter, and 400 nm in thickness. The liner-plasmas are imploded by discharging ∼600 kA with ∼200 ns rise time using a 1 MA linear transformer driver, and the resulting implosions are imaged four times per shot using laser-shadowgraphy at 532 nm. This technique enables the study of plasma implosion physics, including the magneto Rayleigh-Taylor, sausage, and kink instabilities on initially solid, imploding metallic liners with university-scale pulsed power machines.

Determination of plasma pinch time and effective current radius of double planar wire array implosions from current measurements on a 1-MA linear transformer driver

Implosions of planar wire arrays were performed on the Michigan Accelerator for Inductive Z-pinch Experiments, a linear transformer driver (LTD) at the University of Michigan. These experiments were characterized by lower than expected peak currents and significantly longer risetimes compared to studies performed on higher impedance machines. A circuit analysis showed that the load inductance has a significant impact on the current output due to the comparatively low impedance of the driver; the long risetimes were also attributed to high variability in LTD switch closing times. A circuit model accounting for these effects was employed to measure changes in load inductance as a function of time to determine plasma pinch timing and calculate a minimum effective current-carrying radius. These calculations showed good agreement with available shadowgraphy and x-ray diode measurements.

Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator

Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigans low-impedance linear transformer driver (LTD)-driven generator (0.1 Ω, 0.5-1 MA, and 100-200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3-0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to 2 × 1020 cm-3. Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot.

Seeded Magneto-Rayleigh-Taylor instability experiments on A 1-MA LTD

Summary form only given. Recent research on the 1-MA Michigan Linear Transformer Driver, MAIZE, has focused on the Magneto Rayleigh-Taylor (MRT) instability and validation of analytic theory, developed at UM [1,2]. MRT is a concern to all forms of magnetically imploding experiments, most recently with the imploding liners anticipated in the MagLIF geometry.[3] Eliminating or mitigating MRT is crucial to success of these programs.

Experiments on electrothermal instability as a seed for Magneto-Rayleigh-Taylor instability on accelerating, ablating foils

Summary form only given. The electrothermal instability (ETI) arises whenever a current-carrying material has a resistivity that depends on temperature. When resistivity, η, increases with increasing temperature, ETI causes striations to form perpendicular to the direction of current. On pulsed-power-driven, ablating metallic loads, this process can cause sections of the target to ablate earlier than the bulk material, creating a macroscopic surface perturbation on the plasma-vacuum interface. Experiments are underway on the MAIZE 1-MA linear transformer driver at the University of Michigan to study surface perturbations produced by ETI as seeding for the Rayleigh-Taylor (MRT) instability on imploding liner [1] and accelerating foil plasmas [2]. Target foils are fabricated at the Lurie Nanofabrication Facility at UM by depositing ultrathin (200 to 500 nm) coatings of aluminum or titanium on 1.5 μm Chemplex Ultra-Polyester films. Foil thicknesses are chosen to maintain the same mass between shots, and the materials are chosen to provide substantially different values of dη/dt, which impacts the growth rate of the electrothermal instability. Targets are ablated and accelerated by driving a current of 500 to 600 kA on MAIZE, and the accelerated plasmas are imaged using a 12-frame laser imaging system. Images of these plasmas are compared to determine if initial plasma interface perturbations are measurably different on targets of different materials, with the same mass, but different ETI growth rates.

In-situ anode heating and plasma glow discharge cleaning and its effects on atomic constituents in the A-K gap in Self-Magnetic Pinch (SMP) experiments

Summary form only given. The RITS-6 inductive voltage adder (IVA) accelerator (3.5-8.5 MeV) at Sandia National Laboratories produces highpower (TW) focused electron beams (<; 3mm diameter) for flash x-ray radiography applications. The Self-Magnetic Pinch (SMP) diode utilizes a hollowed metal cathode to produce a pinched focus onto a high-Z metal anode converter. There is not a clear understanding as to the effects various contaminants such as: C, CO, H, H<;sub>2<;/sub>O, H<;sub>m<;/sub>C<;sub>n<;/sub>, O<;sub>2<;/sub>, and N<;sub>2<;/sub>, on the anode surface or in the bulk, may have on impedance dynamics, beam stability, beam spot size, and reproducibility.Various cleaning/outgassing methods have been explored such as: heating bulk Ta to temperatures of 1,000 °C for ~2000 s or more with and without thin films of pure Al or Au, pulse heating Ta foils (~100μm thick) to ~2,400 oC for ~1 s, and plasma glow discharge cleaning using an Argon-Oxygen 80/20 gas mixture for ~2000 s. The effects of in-situ cleaning were characterized via in-situ residual gas analysis, separate Temperature Programmed Desorption of witness samples, thermal modeling, and ultimately through radiographic and pulsed power performance of the diode. Initial experiments indicate a significant reduction in H and C as indicated by high-speed spectral analysis of plasmas at the converter and a reduction of back-streaming proton currents. Experiments are ongoing, and latest results will be reported.

Experimental investigation of the effects of an axial magnetic field on the magneto Rayleigh-Taylor, sausage and kink instabilities in imploding liner-plasmas

Summary form only given. Experiments are underway to study the stabilizing effects of an axial magnetic field on the magneto Rayleigh-Taylor (MRT), sausage, and kink instabilities in imploding liner-plasmas at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility at the University of Michigan (UM). The liners were fabricated from ultrathin aluminum foils and had thicknesses of 400 nm and 6.55 mm diameters, and were imploded by discharging a current of 600 kA using a 1-MA Linear Transformer Driver1. An independently triggered capacitor bank was used to drive a set of Helmholtz coils in order to generate axial magnetic fields of up to 5 T. The imploding plasma was imaged using a 12-frame laser imaging system, which captured both shadowgraphs and self-emission over a 120-180 ns period. Varying the external magnetic field enabled a controlled study of the stabilizing effects of axial magnetic fields on instability growth in imploding plasmas.

Experimental investigation of the effects of an axial magnetic field on the magneto Rayleigh-Taylor instability in ablating planar foil plasmas

Summary form only given. Experiments are underway to study the effects an axial magnetic field on the magneto Rayleigh-Taylor instability (MRT) in ablating planar foils on the 1-MA LTD at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility at the University of Michigan. In planar foil ablation experiments at UM, MRT is observed when the expanding plasma-vacuum interface decelerates as the magnetic pressure exceeds the plasma pressure during the drive current1. Theoretical investigation at UM has shown that an axial magnetic field along with magnetic shear may reduce the MRT growth rate in general2. To test this experimentally, axial magnetic fields are generated using helical return current posts. The axial field is proportional to the drive current and peaks at 13 T for 600 kA peak current. A 775 nm Ti:sapphire laser is used to shadowgraph the foil in order to study the MRT instability. Results indicate improved confinement in addition to significant anisotropy on the left and right sides of the foil when compared to experiments at UM using planar return current posts with no axial field. Recent work utilizes a new load configuration where return current plates run perpendicular to the foil current, producing an axial field that can be adjusted based on the proximity of the plates to the foil.

Z-Pinch plasma instability experiments on the UM linear transformer driver

Summary form only given. Experiments are underway on the 1-MA, 100-kV MAIZE linear transformer driver (LTD) z-pinch experiment to explore the characterization and stabilization of the magneto-Rayleigh-Taylor (MRT) and electrothermal instabilities (ETI. Instability experiments at UM utilize 400 nm thick, planar Al foil loads and Al-backed Mylar foils. Theory has shown that axial magnetic fields and magnetic shear can reduce the growth rate of MRT2. To test effects of axial magnetic fields, pseudo-helical return current electrodes are substituted in place of planar electrodes. Instability growth is measured by sub-ps laser shadowgraphy viewing across the foil plasma, perpendicular to the current. The electrothermal instability (ETI) is an important earlytime effect on foil ablation experiments due to its ability to seed the destructive magneto-RayleighTaylor (MRT) instability. In tandem with the axial B-field work, additional foil compositions are being studied to determine ETI growth rates for varied material properties. Similarly, the effect of surface roughness on the resulting ETI is being investigated. Spectroscopic diagnostics are also under development on the UM LTD, which use Zeeman splitting to measure local magnetic fields and current distributions in z-pinch plasmas. Initial experiments, involving Al foils seeded with sodium, have measured Zeeman splitting towards the end of the current pulse using Na D lines and an ICCD coupled to a 0.75 m optical spectrograph. Several ion lines are currently being studied, such as Al III and C IV, as potential emission line candidates to measure the magnetic field along the current rise when the temperature is higher.