E. Kroupp

Novel method for characterizing relativistic electron beams in a harsh laser-plasma environment.

Particle pulses generated by laser-plasma interaction are characterized by ultrashort duration, high particle density, and sometimes a very strong accompanying electromagnetic pulse (EMP). Therefore, beam diagnostics different from those known from classical particle accelerators such as synchrotrons or linacs are required. Easy to use single-shot techniques are favored, which must be insensitive towards the EMP and associated stray light of all frequencies, taking into account the comparably low repetition rates and which, at the same time, allow for usage in very space-limited environments. Various measurement techniques are discussed here, and a space-saving method to determine several important properties of laser-generated electron bunches simultaneously is presented. The method is based on experimental results of electron-sensitive imaging plate stacks and combines these with Monte Carlo-type ray-tracing calculations, yielding a comprehensive picture of the properties of particle beams. The total charge, the energy spectrum, and the divergence can be derived simultaneously for a single bunch.

Spectroscopic determination of the magnetic-field distribution in an imploding plasma

The time-dependent radial distribution of the magnetic field in a high density z-pinch plasma has been determined by observation of the contribution of the Zeeman effect to the spectral profiles of ionic emission lines. The dominance of the line profiles by the Stark broadening required high-accuracy profile measurements and the use of polarization spectroscopy. The plasma implodes in ≃600 ns, and the field distribution was measured up to 90 ns before stagnation on axis. During the implosion the plasma was found to conduct the entire circuit current. By comparing the data to the solution of the magnetic diffusion equation the electrical conductivity of the plasma was determined, found to be in agreement with the Spitzer value. These measurements, together with our previously measured ion velocity distributions, allowed for the determination of the time-dependent relative contributions of the magnetic and thermal pressure to the ion radial acceleration across the plasma shell.

Progress in line-shape modeling of K-shell transitions in warm dense titanium plasmas

Modeling of x-ray spectra emitted from a solid-density strongly coupled plasma formed in short-duration, high-power laser–matter interactions represents a highly challenging task due to extreme conditions found in these experiments. In this paper we present recent progress in the modeling and analysis of Kα emission from solid-density laser-produced titanium plasmas. The self-consistent modeling is based on collisional-radiative calculations that comprise many different processes and effects, such as satellite formation and blending, plasma polarization, Stark broadening, solid-density quantum effects and self-absorption. A rather strong dependence of the Kα shape on the bulk electron temperature is observed. Preliminary analysis of recently obtained experimental data shows a great utility of the calculations, allowing for inferring a temperature distribution of the bulk electrons from a single-shot measurement.

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) a...

Initial magnetic field compression studies using gas-puff Z-pinches and thin liners on COBRA

This magnetic compression of cylindrical liners filled with DT gas has promise as an efficient way to achieve fusion burn using pulsed-power machines. However, to avoid rapid cooling of the fuel by transfer of heat to the liner an axial magnetic field is required. This field has to be compressed during the implosion since the thermal insulation is more demanding as the compressed DT plasma becomes hotter and its volume smaller. This compression of the magnetic field is driven both by the imploding liner and plasma. To highlight how this magnetic field compression by the plasma and liner evolves we have separately studied Z-pinch implosions generated by gas puff and liner loads. The masses of the gas puff and liner loads were adjusted to match COBRAs current rise times. Our results have shown that Ne gas-puff implosions are well described by a snowplow model where electrical currents are predominately localized to the outer surface of the imploding plasma and the magnetic field is external to the imploding plasma. Liner implosions are dominated by the plasma ablation process on the inside surface of the liner and the electrical currents and magnetic fields are advected into the inner plasma volume; the sharp radial gradient associated with the snowplow process is not present.

Evolution of MHD Instabilities in Plasma Imploding Under Magnetic Field

Two-dimensional 3-ns-gated visible images, recorded at different times during the implosion of plasma under azimuthal magnetic field (Z-pinch), revealed ringlike instabilities followed by the development of axially and azimuthally nonuniform structures in the imploding plasma. Remarkably, the evolution in time of all structures was found to be highly repeatable in different shots, which should allow, through 3-D magnetohydrodynamics modeling, for systematically studying the development in time of these complex phenomena and correlating them with the initial plasma parameters. The data are also used to infer the time-dependent outer plasma radius and plasma radial velocities.

Effective versus ion thermal temperatures in the Weizmann Ne Z-pinch: Modeling and stagnation physics

The difference between the ion thermal and effective temperatures is investigated through simulations of the Ne gas puff z-pinch reported by Kroupp et al. [Phys. Rev. Lett. 107, 105001 (2011)]. Calculations are performed using a 2D, radiation-magnetohydrodynamic code with Tabular Collisional-Radiative Equilibrium, namely Mach2-TCRE [Thornhill et al., Phys. Plasmas 8, 3480 (2001)]. The extensive data set of imaging and K-shell spectroscopy from the experiments provides a challenging validation test for z-pinch simulations. Synthetic visible images of the implosion phase match the observed large scale structure if the breakdown occurs at the density corresponding to the Paschen minimum. At the beginning of stagnation (−4 ns), computed plasma conditions change rapidly showing a rising electron density and a peak in the ion thermal temperature of ∼1.8 keV. This is larger than the ion thermal temperature (<400 eV) inferred from the experiment. By the time of peak K-shell power (0 ns), the calculated electron d...

Mitigation of Instabilities in a Z-Pinch Plasma by a Preembedded Axial Magnetic Field

The effects of an axial magnetic field on the development of instabilities during a z-pinch implosion are studied using 2-D images and interferometry. The measurements clearly show mitigation of magneto Rayleigh-Taylor instabilities with increased magnitude of the preembedded axial magnetic field. Introducing the axial magnetic field also gives rise to new structures, indicating an interaction between the azimuthal and axial fields.

Beyond Zeeman spectroscopy: Magnetic-field diagnostics with Stark-dominated line shapes

A recently suggested spectroscopic approach for magnetic-field determination in plasma is employed to measure magnetic fields in an expanding laser-produced plasma plume in an externally applied magnetic field. The approach enables the field determination in a diagnostically difficult regime for which the Zeeman-split patterns are not resolvable, as is often encountered under the conditions characteristic of high-energy-density plasmas. Here, such conditions occur in the high-density plasma near the laser target, due to the dominance of Stark broadening. A pulsed-power system is used to generate magnetic fields with a peak magnitude of 25 T at the inner-electrode surface in a coaxial configuration. An aluminum target attached to the inner electrode surface is then irradiated by a laser beam to produce the expanding plasma that interacts with the applied azimuthal magnetic field. A line-shape analysis of the Al III 4s–4p doublet (5696 and 5722 A) enables the simultaneous determination of the magnetic field...

Characterization of the COBRA triple-nozzle gas-puff valve using planar laser induced fluorescence

We present neutral density measurements of argon (Ar) injected into the 70 mm outer diameter, 24 mm axial length, outflow region of the triple-nozzle gas-puff valve fielded for gas-puff z-pinch experiments on the (1 MA, 100–200 ns) COBRA generator at Cornell University. Measurements are obtained by planar laser induced fluorescence of (λ = 266 nm, E = 80 mJ, Δt = 3 ns) frequency-quadrupled Nd:YAG laser light, absorbed by acetone dopant introduced into the Ar at 7% by pressure. Results are acquired 500μs after valve opening, the time of current initiation during z-pinch experiments. Number density plots are obtained across the Outer (O), Inner (I) and Center (C) puffs, with nozzle backing pressures {O:I:C} = {1:3:8}psia and {4:0:10}psia, delivering ‘uniform’ and ‘hollow’ profiles respectively. The total mass per unit length in these puffs is 22±0.4 μgcm−1 and 47±1 μgcm−1. Density measurement precision is ±5×1015 cm−3.