Tania Shelkovenko

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

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.

A study of aluminum x-pinch assembly through x-ray abosrption spectroscopy

Summary form only given. Previous studies of plasma pinches, such as x-pinches or hybrid pinches, have thoroughly characterized the radiating hot spot formed at the center of the plasma in terms of size, temperature, and density. However, much of the plasma volume surrounding the hot spot has remained relatively unstudied. While point projection imaging and interferometry can be used to probe the density of this surrounding plasma, these diagnostics cannot provide measurements of plasma temperature. Emission spectroscopy can reveal temperatures, but the intensity of the hot spot outshines the surrounding plasma making spectral studies of nearby regions exceptionally challenging. We propose that a study of the surrounding plasma can be accomplished by means of absorption spectroscopy in spite of the high brightness of the x-pinch. Such an experiment has been designed for the XP pulser at Cornell University. The XP pulser, which is capable of delivering 500 kA of current in 100 ns, is used to drive an x-pinch as a source of continuum radiation. This radiation is dispersed by an astigmatic mica crystal before interacting with another x-pinch serving as the object plasma. The astigmatism of the crystal allows focusing to occur both at the sample location as well as at the detector for increased luminosity. To date, the experimental design for the study of plasma in an aluminum x-pinch has been completed. The object plasma under study will be Al 5056, an Al alloy containing 5% Mg. The H-like and He-like resonance and satellite lines in the spectrum from the Mg will be used as the basis for plasma diagnosis. The back lighting source will be tantalum due to its relatively flat continuum spectrum between the wavelengths of 8.2 and 9.5 angstroms. Preliminary results from this experiment will be presented.

Elliptical spectrometer for the study of x-pinch physics through absorption spectroscopy

Summary form only given. We discuss here the use of the x-pinch x-ray source together with an elliptical crystal spectrometer for determining plasma conditions in high energy density plasmas. The use of absorption spectroscopic techniques for the study of plasma conditions is often restricted to diagnosing non-radiating plasma samples. This is done to avoid radiation emitted by the samples being recorded along with the probing radiation. This can easily obscure or conceal the features of the absorption spectrum and introduce substantial error in inferred conditions.

Anode–Cathode Asymmetry in a Wire-Array

Presented is a laser-backlit image of a tungsten wire-array Z-pinch at 1 MA. This image shows highly resolved (to about 20-40 μm) anode-cathode-asymmetric wavelike structure on the outer edges of the ablating wires. The development of this structure implies that axial shear flow occurs in ablating wire-array Z-pinches.

Z

Summary form only given. Experimental results showing wire array z-pinch implosions on the 1-MA, 100-ns rise time COBRA pulsed power generator are presented. The principal diagnostic used for these studies was an optical streak camera system, while other supporting diagnostics include a time-gated framing camera, a laser backlighting system, time-integrated pinhole cameras with various filters, and silicon diodes and diamond photoconducting devices for monitoring X-ray production. The data produced by the entire suite of diagnostics is analyzed and presented to provide an overall picture of implosion dynamics and timing on COBRA. In particular, the implosion timing relative to the start of the current pulse on COBRA is compared to that which is predicted by the ablation/implosion model developed for wire array experiments on MAGPIE, a pulsed power generator that has a similar peak current to that of CORBRA, but with a longer, 240-ns rise time.