Sergei Pikuz

Cavitation and formation of foam-like structures inside exploding wires

Large-scale molecular dynamics (MD) simulations are used to study explosions of aluminum wires heated by electric current pulses. It is shown that the observed nonuniform radial expansion of the heated wire is associated with a liquid-vapor phase transition, which is caused by convergence of a radial tensile wave towards the center of the wire. Tension within the wave leads to cavitation in stretched melt that subsequently forms into a low-density foam-like material surrounded by a dense liquid shell. The foam decays into liquid droplets before the outer shell breaks apart. Simulated density profiles demonstrate good qualitative agreement with experimental high-resolution X-ray images showing the complex hollow structures within the long-living dense core.

X-pinch source size measurements

The X pinch plasma emits subnanosecond bursts of x-rays in the 3 - 10 keV energy range from a very small source. As such, it has been used for high-resolution point-projection imaging of small, dense, rapidly changing plasmas, as well as submillimeter thick biological samples. The very small x-ray source size of the X pinch provides high spatial coherence of the x-rays, enabling the X pinch to be used for imaging low absorption, low contrast objects with excellent spatial resolution by incorporating wave-optics effects. The reverse procedure has been used to determine the X pinch x-ray source size: well-defined micro-fabricated slits were imaged by point-projection radiography, and the detailed patterns were compared with wave-optics calculations of the expected image patterns on film as a function of x-ray source size and energy band. In addition, an x-ray streak camera was used to study the X pinch source size as a function of time. Dynamic shadow images of a boron fiber with a tungsten core and glass fiber sheathed in plastic were compared with a time-integrated radiographic image. Source sizes as small as 1.2 μm (full width at half maximum, assuming a Gaussian spatial intensity profile for the source) have been inferred.

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

Experiments on the Low Current Pulser LCP3 at Cornell University are exploring the properties of high energy density plasmas generated by current-driven explosions of single fine metal wires. These experiments are employing non-perturbing emission spectroscopy at visible wavelengths to obtain plasma conditions, including temperatures, electron density, ionization state, and magnetic field. A new diagnostic technique is being developed to determine the magnetic field which makes use of Zeeman-effect-produced differences in the line shapes of two fine structure components of a multiplet that are equally broadened by Stark and Doppler effects. This technique has been demonstrated in experiments performed at the Weizmann Institute of Science in plasmas with lower energy densities [1]. We are studying the time integrated and time resolved visible spectra to determine appropriate spectral lines for measuring magnetic field strength, including the Al III 5696Å and the Al III 5722Å transitions used in previous work[l]. Preliminary results will be discussed.

-Pinch: Highly Resolved Axial-Shear-Flow Structure Observed on the Outer Edges of Ablating Wires

This final report for Project 117863 summarizes progress made toward understanding how X-pinch load designs scale to high currents. The X-pinch load geometry was conceived in 1982 as a method to study the formation and properties of bright x-ray spots in z-pinch plasmas. X-pinch plasmas driven by 0.2 MA currents were found to have source sizes of 1 micron, temperatures >1 keV, lifetimes of 10-100 ps, and densities >0.1 times solid density. These conditions are believed to result from the direct magnetic compression of matter. Physical models that capture the behavior of 0.2 MA X pinches predict more extreme parameters at currents >1 MA. This project developed load designs for up to 6 MA on the SATURN facility and attempted to measure the resulting plasma parameters. Source sizes of 5-8 microns were observed in some cases along with evidence for high temperatures (several keV) and short time durations (<500 ps).

Low current single wire optical spectroscopy experiments

Streaked visible-light spectroscopy measurements are presented for aluminum (Al) wire-array z-pinch experiments on the 1-MA, 100-ns rise-time COBRA pulsed-power generator. For these measurements, a half-meter Czerny- Turner spectrometer was used in conjunction with the COBRA visible-light streak camera system. This allowed us to record visible-light spectra emitted from Al coronal plasma as a continuous function of time throughout the initiation, ablation, and implosion phases of the given wire-array z- pinch experiment. When using thick wires (~100-mum in diameter), the visible-band spectra observed consisted solely of continuum emission, which began at the moment of resistive voltage collapse. (Resistive voltage collapse occurs when coronal plasma first forms around the wire cores, and marks the transition from the initiation/resistive-heating phase to the ablation phase.) The continuum data collected are now being used to determine electron density. To determine electron density from this continuum data, an absolute calibration of the detection system was required. The details of these experiments and the absolute calibration technique are presented.

Scaling of X pinches from 1 MA to 6 MA.

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.