J. C. Valenzuela

Study of instability formation and EUV emission in thin liners driven with a compact 250 kA, 150 ns linear transformer driver

A compact linear transformer driver, capable of producing 250 kA in 150 ns, was used to study instability formation on the surface of thin liners. In the experiments, two different materials, Cu and Ni, were used to study the effect of the liners resistivity on formation and evolution of the instabilities. The dimensions of the liners used were 7 mm height, 1 mm radius, and 3 μm thickness. Laser probing and time resolved extreme ultraviolet (EUV) imaging were implemented to diagnose instability formation and growth. Time-integrated EUV spectroscopy was used to study plasma temperature and density. A constant expansion rate for the liners was observed, with similar values for both materials. Noticeable differences were found between the Cu and Ni instability growth rates. The most significant perturbation in Cu rapidly grows and saturates reaching a limiting wavelength of the order of the liner radius, while the most significant wavelength in Ni increases slowly before saturating, also at a wavelength clo...

Measurement of pulsed-power-driven magnetic fields via proton deflectometry

Measuring magnetic field and current distribution in Z-pinch plasma systems is crucial to the validation of Z-pinch theory. In this letter, the demonstration of proton deflectometry to pulsed-power-driven loads at the mega-amp scale is presented, which is capable of making more detailed field maps in high-density regions of plasmas. In this method, a laser-driven, broad-spectrum, MeV-energy proton beam is directed through a pulsed-power-driven plasma system, and the resulting deflections are measured to examine configuration of magnetic fields and to infer the currents that support them. The technique was first demonstrated on simple short-circuit loads, and the results are in excellent agreement with numerical simulations providing reliable estimates of the field and current configurations. It was then applied to a more complex—radial foil—plasma load. The measurements show unexpected proton deflections that exhibit the complexity of the plasma load and that with further analysis will reveal details about the current and magnetic field topology in this complex configuration.

Shock formation in Ne, Ar, Kr, and Xe on deuterium gas puff implosions

1- and 2-D simulations of 1-cm radius, gas-puff liners of Ne, Ar, Kr, and Xe imploding onto a deuterium target are conducted using the discharge parameters for the Zebra (1 MA, 130 ns) driver using the resistive MHD code MACH2. This is an implementation of the Staged Z-pinch concept, in which the target is driven to high-energy-density first by shock compression launched by a diffused azimuthal magnetic field ( J×B force), and then by the adiabatic compression as the liner converges on axis. During the run-in phase, the initial shock heating preheats the deuterium plasma, with a subsequent stable, adiabatic compression heating the target to high energy density. Shock compression of the target coincides with the development of a J×B force at the target/liner interface. Stronger B-field transport and earlier shock compression increases with higher-Z liners, which results in an earlier shock arrival on axis. Delayed shock formation in lower-Z liners yields a relative increase in shock heating, however, the 2...

Characterization of laser-cut copper foil X-pinches

Quantitative data analyses of laser-cut Cu foil X-pinch experiments on the 150 ns quarter-period, ∼250 kA GenASIS driver are presented. Three different foil designs are tested to determine the effects of initial structure on pinch outcome. Foil X-pinch data are also presented alongside the results from wire X-pinches with comparable mass. The X-ray flux and temporal profile of the emission from foil X-pinches differed significantly from that of wire X-pinches, with all emission from the foil X-pinches confined to a ∼3 ns period as opposed to the delayed, long-lasting electron beam emission common in wire X-pinches. Spectroscopic data show K-shell as well as significant L-shell emission from both foil and wire X-pinches. Fits to synthetic spectra using the SCRAM code suggest that pinching foil Xs produced a ∼1 keV, ne ≥ 1023 cm−3 plasma. The spectral data combined with the improved reliability of the source timing, flux, and location indicate that foil X-pinches generate a reproducible, K-shell point-proj...

Measurement of temperature and density using non-collective X-ray Thomson scattering in pulsed power produced warm dense plasmas

We present the first experimental measurement of temperature and density of a warm dense plasma produced by a pulsed power driver at the Nevada Terawatt Facility (NTF). In the early phases of discharge, most of the mass remains in the core, and it has been challenging to diagnose with traditional methods, e.g. optical probing, because of the high density and low temperature. Accurate knowledge of the transport coefficients as well as the thermodynamic state of the plasma is important to precisely test or develop theoretical models. Here, we have used spectrally resolved non-collective X-ray Thomson scattering to characterize the dense core region. We used a graphite load driven by the Zebra current generator (0.6 MA in 200 ns rise time) and the Ti He-α line produced by irradiating a Ti target with the Leopard laser (30 J, 0.8 ns) as an X-ray probing source. Using this configuration, we obtained a signal-to-noise ratio ~2.5 for the scattered signal. By fitting the experimental data with predicted spectra, we measured T = 2±1.9 eV, ρ = 0.6±0.5 gr/cc, 70 ns into the current pulse. The complexity of the dense core is revealed by the electrons in the dense core that are found to be degenerate and weakly coupled, while the ions remain highly coupled.

Injector design for liner-on-target gas-puff experiments

We present the design of a gas-puff injector for liner-on-target experiments. The injector is composed of an annular high atomic number (e.g., Ar and Kr) gas and an on-axis plasma gun that delivers an ionized deuterium target. The annular supersonic nozzle injector has been studied using Computational Fluid Dynamics (CFD) simulations to produce a highly collimated (M > 5), ∼1 cm radius gas profile that satisfies the theoretical requirement for best performance on ∼1-MA current generators. The CFD simulations allowed us to study output density profiles as a function of the nozzle shape, gas pressure, and gas composition. We have performed line-integrated density measurements using a continuous wave (CW) He-Ne laser to characterize the liner gas density. The measurements agree well with the CFD values. We have used a simple snowplow model to study the plasma sheath acceleration in a coaxial plasma gun to help us properly design the target injector.

Characterization of a compact gas-puff nozzle and plasma gun assembly for staged Z-pinch experiments

Summary form only given. We discuss the design and characterization of a compact gas-puff nozzle and plasma gun assembly that will be used in Staged Z-pinch experiments1, where gas liners are imploded onto an on-axis target-plasma column. Previous work on the Staged Z-pinch demonstrated that gas liners can efficiently couple the energy and produce a uniform implosion on a target-plasma.The valve assembly produces an annular, high-atomic number, neutral gas-puff, co-axially with a target-plasma. The neutral gas-puff outlet consists of an annular solenoid valve and a converging-diverging nozzle, designed to achieve a Mach number around M=5 in the annular gas jet. The on-axis, cylindrical plasma gun is powered by a high-voltage capacitor. Breakdown occurs spontaneously when high-pressure gas is puffed between the electrodes, i.e., the deflagration mode3. The discharge current generates an annular magnetic field, producing the axial J×B force, which accelerates the plasma out of the gun. Measurements of the gas-puff and plasma-jet density distributions and ejection velocities by open-shutter camera, Mach-Zehnder interferometry, and Faraday cup diagnostics are made and compared with CFD simulations.

Design and optimization of a liner-on-target injector for staged Z-pinch experiments using computational fluid dynamics and MHD simulations

Summary form only given. Previous Staged Z-pinch experiments have demonstrated that gas liners (or puffs) can efficiently couple energy to a target plasma and implode uniformly, producing plasmas in High Energy Density (HED) regimes. In these experiments, a 50 kJ, 1.5 MA, 1 μs current driver was used to implode a magnetized, Kr liner onto a D+ target, producing 1010 neutrons per shot. Time-of-flight data suggested that primary and secondary neutrons were produced. MHD simulations show that, using optimized liner and plasma target conditions, neutron yield could be further increased in Staged Z-pinch implosions using the Zebra machine, a 1.5 MA and 100 ns rise time current driver. In this work we present the design and optimization of an injector for these experiments. The injector is composed of an annular high atomic number (e.g. Ar, Kr) gas-puff and an on-axis plasma gun that delivers the ionized deuterium target. The gas-puff nozzle optimization was performed using the computational fluid dynamics (CFD) code Fluent and the MHD code MACH2. The CFD simulations produce density profiles as a function of the nozzle shape and gas. These profiles are initialized in MACH2 to find the optimal liner density profile for a stable, uniform implosion that produces high neutron yield.

Investigation of Current Transport in

Here, we present an experimental and computational study using planar wire arrays to examine the transport of magnetic fields in wire array plasmas. Two groups of wires are placed adjacent to each other at a fixed distance, while the local spacing between the wires is varied. Experimental results employing the electrical B-dot probes are compared with the simulations performed with the numerical results from the resistive MHD code, Gorgon, and show that the magnetic field generated by each pair of wires determines the amount of current advected with the plasma flow to the central axis.

2\times 2

We report on counter-propagating plasma flows produced by two vertically opposing conical wire arrays using a compact, low inductance Linear Transformer Driver (LTD) “GenASIS” capable of producing 250 kA in about 150 ns. Laser interferometry and laser schlieren were performed with the use of a Nd:YAG 532 nm laser, with a pulse width of 5 ns. A shock wave formed by jet interaction was clearly observed and remained stationary for at least 50 ns. Interferometry data showed that the ion density of the jets prior to collision was of the order of 2×1017cm-3 and a jump in density of ~ 5 was observed at the shock wave region. A lower limit of ~ 100 km/s has been measured for the jet velocity. The ion mean free path has been estimated to be ~ 12 mm, which is larger than the shock wave scale ~ 5 mm, and hence the shock wave approaches the collisionless regime. Magnetic field advection, which can drastically modify the conditions for shock wave formation, will be discussed. Kinetic particle-in-cell modeling using LSP code has also been implemented and benchmarked against the experimental results in order to study the underlying physics of formation and evolution of the shock.