A. D. Cahill

Hybrid X-pinch with conical electrodes

A hybrid X-pinch configuration consisting of solid conical electrodes connected by a wire has been tested on a 45 ns risetime, 500 kA peak current pulsed power generator. Wires of different materials were loaded through holes in the cones; wire lengths were varied from 0.6 to 2 mm. Most of these hybrid X-pinches generated an intense single burst of soft x-rays and developed a single hot spot that was of micron-scale size. Hybrid X-pinches generate less hard x-ray intensity than standard X-pinches.

Hybrid X-pinches

Results from experimental studies of a hybrid X-pinch with an initial configuration in the form of a high-current diode with conical tungsten electrodes spaced by 1–2 mm and connected to one another with 20- to 100-μm-diameter wires are presented. The experiments were carried out at four facilities with a current amplitude from 200 to 1000 kA and front duration from 45 to 200 ns. It is shown that, in spite of their simpler configuration, hybrid X-pinches with a short rise time of the current pulse (50–100 ns) are highly competitive with standard X-pinches in the generated soft X-ray power and the formation of a single hot spot in them is much more stable, while hard X-ray emission is almost absent. The possibility of using hybrid X-pinches as soft X-ray sources for point projection X-ray imaging of plasma objects is considered.

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...

Helical plasma striations in liners in the presence of an external axial magnetic field

Awe et al. found on the 20 MA Z machine [Acta Phys. Pol. A 115, 956 (2009)] that applying an externally generated axial magnetic field to an imploding liner leads to a helical pattern in the liner when viewed with soft x-ray radiography ([Phys. Rev. Lett. 111, 235005 (2013)] and [Phys. Plasmas 21, 056303 (2014)]). Here, we show that this phenomenon is also observed in extreme ultraviolet self-emission images of 10 mm long cylindrical metal liners having varying diameters and varying wall thicknesses on a 1 MA, 100–200 ns pulsed power generator. The magnetic field in these experiments is created using either twisted return current wires positioned close to the liner, generating a time-varying Bz, or a Helmholtz coil, generating a steady-state Bz.

Dynamics of hybrid X-pinches

The dynamics of a new type of pinches—hybrid X-pinches (HXPs)—has been studied experimentally and numerically. The initial configuration of an HXP consists of a high-current diode with conical tungsten electrodes separated by a 1- to 3-mm-long gap and shunted with a 20- to 100-μm diameter wire. It was shown earlier that a hot spot (HS) with high plasma parameters also formed in the HXP, although its initial configuration is simpler than that of a standard X-pinch. Although details of the HXP dynamics still remain insufficiently studied, the main factors governing the HXP formation were investigated both experimentally and using magnetohydrodynamic simulations. The formation of a specific pressure profile in the electrode plasma after the wire explosion was investigated both experimentally and theoretically. It is shown that the effect of the pressure profile on the expanding wire plasma is similar for both standard X-pinches and HXPs, which allows one to assign them to the same class of loads of pulsed facilities. It is also established that the final stages of HS formation and the parameters of the HS plasma in standard X-pinches and HXPs are practically identical.

Study of New Configurations of Hybrid

A hybrid X -pinch (HXP) configuration consisting of solid conical electrodes connected by a wire as the load of a pulsed power generator has been tested on four different generators with currents varying from 200 kA to 1.2 MA and with rise times ranging from 50 to 170 ns. Wires of different materials, diameters ranging from 25 to 200 μm, and lengths from 0.5 to 3 mm were loaded through holes in the cone. It was possible to optimize the wire length and diameter for most materials tested so that the HXP would generate an intense single burst of soft X-rays with energy yield comparable with that obtained using standard X pinches. Furthermore, this soft X-ray source permits a greater variety of materials to be used, including combinations of different materials (e.g., parallel, twisted, or braided wires) or thin tubes with powders inside. Here, we report results obtained with one, two, or three parallel, twisted, or braided wires as well as with fine metal or plastic tubes with a wire inside or filled with melted metal with the goal of extending the spectral bands of HXP radiation and optimizing the energy yield in the soft X-ray burst.

X

X-ray absorption spectroscopy is a powerful tool for the diagnosis of plasmas over a wide range of both temperature and density. However, such a measurement is often limited to probing plasmas with temperatures well below that of the x-ray source in order to avoid object plasma emission lines from obscuring important features of the absorption spectrum. This has excluded many plasmas from being investigated by this technique. We have developed an x-ray spectrometer that provides the ability to record absorption spectra from higher temperature plasmas than the usual approach allows without the risk of data contamination by line radiation emitted by the plasma under study. This is accomplished using a doubly curved mica crystal which is bent both elliptically and cylindrically. We present here the foundational work in the design and development of this spectrometer along with initial results obtained with an aluminum x-pinch as the object plasma.

Pinches

Individual wires in a z-pinch were replaced with twisted pair “cables” of similar linear mass on the COBRA pulsed power generator, resulting in peak power and yield increases in radiation above 1 keV. A cable is defined here as two or more fine wires twisted together to form a continuous strand with a wavelength (λt) dependent on the twists per unit length. The magnitude of λt appears to play a strong role in these increases, with the largest gains found for a λt of ≈0.75 mm.

A doubly curved elliptical crystal spectrometer for the study of localized x-ray absorption in hot plasmas

X pinches are well known to produce very small, dense plasma pinches (“hot spots”) that emit sub-nanosecond bursts of 1–8 keV radiation. Hard X-ray radiation in the range from 8 to 300 keV or more is also emitted, and only a small portion of which is associated with the X-pinch hot spot. In hybrid X-pinches (HXP), the  10 ns hard X-ray pulse is terminated by fast closure of the gap between the two conical electrodes of the HXP by rapidly expanding electrode plasmas. The temporal, spectral, and spatial properties of this higher energy radiation have been studied. This radiation was used for point-projection imaging with magnification between 1.5 and 6, and spatial resolution of 20–100 μm was demonstrated.

Enhanced keV peak power and yield using twisted pair “cables” in a z-pinch

Summary form only given. We present a study of the formation of bow shocks in radiatively cooled plasma flows. This work uses the XP generator (260kA, 145ns) at Cornell University to drive an inverse wire array. This generates a quasi-uniform, large scale hydrodynamic flow accelerated by Lorentz forces to Ma > 1. This flow impacts a stationary object placed in its path, forming a well-defined Mach cone. Collinear interferogram and gated-self emission diagnostics demonstrate that the cone angle with distance from the wire decreases (increasing Mach number) and is indicative of a strongly cooling flow. Rapid density increase from the background flow into the object is indicative of a strong density jump at the shock. High resolution self-emission imaging shows the formation of a thin (<;60 μm) strongly emitting shock region where Te~50eV, indicating rapid cooling behind the shock. In addition, emission is observed upstream of the shock position which may be consistent with the formation of a radiative precursor. Data compare well to analytical calculations of the expected scale-lengths of both the precursor and cooling regions, and initial simulation work will also be presented.