Laser Shearing Interferometer Research Articles

Study of gas-puff Z-pinches on COBRA

Gas-puff Z-pinch experiments were conducted on the 1 MA, 200 ns pulse duration Cornell Beam Research Accelerator (COBRA) pulsed power generator in order to achieve an understanding of the dynamics and instability development in the imploding and stagnating plasma. The triple-nozzle gas-puff valve, pre-ionizer, and load hardware are described. Specific diagnostics for the gas-puff experiments, including a Planar Laser Induced Fluorescence system for measuring the radial neutral density profiles along with a Laser Shearing Interferometer and Laser Wavefront Analyzer for electron density measurements, are also described. The results of a series of experiments using two annular argon (Ar) and/or neon (Ne) gas shells (puff-on-puff) with or without an on- (or near-) axis wire are presented. For all of these experiments, plenum pressures were adjusted to hold the radial mass density profile as similar as possible. Initial implosion stability studies were performed using various combinations of the heavier (Ar) and lighter (Ne) gasses. Implosions with Ne in the outer shell and Ar in the inner were more stable than the opposite arrangement. Current waveforms can be adjusted on COBRA and it was found that the particular shape of the 200 ns current pulse affected on the duration and diameter of the stagnated pinched column and the x-ray yield.

Magnetic Field Diffusion and Enhanced Resistivity in 12-cm-Diameter 200-ns 3.5-MA <formula formulatype="inline"> <tex Notation="TeX">$Z$</tex></formula>-Pinch Implosions

Investigations of magnetic field diffusion and plasma resistivity in 12-cm-diameter triple-gas-puff Ar <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> -pinch implosions were carried out by using planar laser-induced fluorescence (PLIF), a laser shearing interferometer (LSI), and a laser wavefront analyzer (LWA) on a 3.5-MA 200-ns generator. The PLIF measurements gave the initial Ar gas distributions. The implosion velocity and electron density profiles were measured from LWA and/or LSI. From these, the implosion plasma sheath thickness, ion density, mean ion charge states, temperatures, and implosion velocity are obtained, which allows us to calculate the classical plasma resistivity. A 1-D analytic magnetic field diffusion model is constructed and used to predict the imploding plasma sheath thickness and its resistivity. Based on comparisons of the experimental measurements and the diffusion model prediction, we found out that plasma resistivity is enhanced by the cross-field diffusion above the classical value, as high as 60 times the Spitzer's value. Details are given in this paper.

Magnetic Rayleigh–Taylor instability mitigation in large-diameter gas puff Z-pinch implosions

Recently, a new approach for efficiently generating K-shell x-rays in large-diameter, long-implosion time, structured argon gas Z-pinches has been demonstrated based on a “pusher-stabilizer-radiator” model. In this paper, direct observations of the Rayleigh–Taylor instability mitigation of a 12-cm diameter, 200-ns implosion time argon Z-pinch using a laser shearing interferometer (LSI) and a laser wavefront analyzer (LWA) are presented. Using a zero-dimensional snowplow model, the imploding plasma trajectories are calculated with the driver current waveforms and the initial mass distributions measured using the planar laser induced fluorescence method. From the LSI and LWA images, the plasma density and trajectory during the implosion are measured. The measured trajectory agrees with the snowplow calculations. The suppression of hydromagnetic instabilities in the “pusher-stabilizer-radiator” structured loads, leading to a high-compression ratio, high-yield Z-pinch, is discussed. For comparison, the LSI and LWA images of an alternative load (without stabilizer) show the evolution of a highly unstable Z-pinch.

Two-dimensional MHD simulations of a neon Z pinch on Hawk

Two-dimensional magnetohydrodynamic (MHD) simulations using MACH2 are benchmarked against laser shearing interferometer (LSI) images of the evolving electron-density sheath during 250-ns neon gas-puff Z pinch implosions on the Naval Research Laboratory Hawk generator. The initial density distribution for the MHD simulations is calculated using a ballistic-flow-model fit to the measured initial gas-density distributions. The implosion is modeled using an applied current profile, single-temperature energy equations, and black-body-limited optically thin radiation. For consistency with the radiation model, neon ionization and equation-of-state models have been added to MACH2. Computed MHD ion-density distributions compare well with LSI images as the snowplowed plasma channel evolves during implosion. The MHD results also show features that may be helpful for understanding early and weak K-shell radiation observed near the nozzle in Double-Eagle gas-puff experiments. Current-channel evolution derived from one-dimensional snowplow calculations compare well to the MHD results for the same current history and initial density distribution, indicating that axial mass flow does not strongly impact the implosion dynamics.

Z pinch imploding plasma density profile measurements using a two-frame laser shearing interferometer

A laser shearing interferometer (LSI) was used to make spatially and temporally resolved measurements of the electron density profile in an imploding z pinch. Experiments were conducted on the 0.7-MA/250-ns Hawk machine, the 2.5-MA/100-ns ACE-4 machine, and the 3.8-MA/190-ns Double Eagle machine. Time and space resolved measurements of the current and plasma density are needed for better understanding of the implosion dynamics and stagnation physics of z pinches. The electron density profile can be obtained using an LSI. The LSI passes a short pulse, collimated laser beam across the imploding z pinch, which distorts the laser wavefront. the maximum wavefront distortion occurs where the density gradient is highest, such as across the current sheath. After passing through the pinch, the distorted wavefronts are split into two beams that are laterally displaced relative to one other. This shearing causes interference between these two wavefronts and produces an interferogram, from which the plasma density profiles are derived. In the experiments, a 150-ps laser pulse was split into two pulses with an interpulse delay of several tens. of nanoseconds. This pulse pair gave two snap shots of the electron density profiles during the 100-300-ns implosions. From these interferograms, electron densities and implosion velocities of the imploding plasmas were derived, the current sheath was observed, and the plasma ionization states, growth rates, and wavelengths of instabilities were estimated. The results motivate construction of an upgraded instrument with four or more frames and with an added laser polarimetry measurement (Faraday rotation) capability to obtain both electron and current profiles.