K. M. Chandler

Point-projection x-ray radiography using an X pinch as the radiation source

Using an X pinch as a source of radiation for point-projection radiography, it is possible to project a high-resolution (1–10 μm) shadow image of dense plasma or test objects onto x-ray-sensitive film. The emission characteristics of X pinches composed of a wide variety of materials have been studied using several diagnostics. The pulse duration and shape of the x-ray bursts were measured in the 1.5–6 keV band using fast diamond PCDs and an x-ray streak camera with sweep speeds as fast as 10 ns for the full sweep (3.5 cm). To investigate the line and continuum radiation emitted by the X pinches, a convex spectrograph using a mica or KAP crystal, and a spectrograph based on a spherically bent mica crystal were used. Summarizing the data, including radiography results, wires known to have slower expansion rates and high boiling temperatures (NiCr, Ti, Nb, Mo, Pd, Ta, W, and Pt) appeared to yield the smallest x-ray source sizes, i.e., gave the best spatial resolution in radiographs and provided subnanosecond...

Time-resolved spectroscopic measurements of ∼1 keV, dense, subnanosecond X-pinch plasma bright spots

Bright, ∼1 μm x-ray sources (micropinches) produced within exploding wire X pinches are found to be near solid density and ∼1 keV electron temperature. For example, with a Ti X pinch, a 90 ps lifetime, 1.5–1.8 keV electron temperature, ∼1023/cm3 electron density plasma was observed. These plasma characteristics were determined using time-resolved x-ray spectra produced by 2- and 4-wire X pinches and collected by an x-ray streak camera with <10 ps time resolution. Together with a spherically bent mica crystal spectrograph, the streak camera recorded the 1–10 keV radiation emitted from X pinches made from different wire materials. Some spectra were dominated by continuum and others by line radiation. Spectral features varied on time scales ranging from 10 to 300 ps, depending on the wire material. Results are presented that demonstrate the necessity of time-resolved data for determining plasma conditions from micropinch x-ray spectra.

The relationship between exploding wire expansion rates and wire material properties near the boiling temperature

The energy deposited by a submicrosecond, /spl sim/1-kA current pulse in a 25-/spl mu/m diameter metal wire prior to its explosion, correlates directly with the expansion rate of the wire after the explosion. Energy deposition by resistive heating is terminated by the formation of plasma around the wire and a collapse of the voltage along the wire, and is evidently facilitated by the desorption of gases from the wire and/or the evaporation of the wire material (or impurities within it) as it heats up. Data presented here implies that the relationship between materials with the lowest resistivities and high exploding wire expansion rates found in earlier work (D. B. Sinars et al. 2000) is a result of the reduced voltage delaying the gas breakdown along such wires. This, in turn, increases the energy deposited resistively in the wire before the current shifts to the surrounding plasma. If gas breakdown does not occur until close to the full vaporization energy is deposited in the wire, the expansion rate will be more rapid than if a small fraction of the vaporization energy is deposited before voltage collapse.

Electron-beam-generated x rays from X pinches

X pinches are well known to produce very small, dense plasma pinches (“micropinches”) that emit short bursts of 1.5–8keV radiation [Shelkovenko et al., Phys. Plasmas 9, 2165 (2002)]. X-ray radiation in the 8–100keV range is also emitted, only a small portion of which is associated with the micropinches. Beginning immediately after the soft x-ray burst, higher energy x-ray emission is observed that is attributed to energetic electrons accelerated in the gaps that appear in the X-pinch plasma structure. The temporal, spectral, and spatial properties of this higher energy radiation (8–100keV) have been studied using two ∼0.1μs pulsed power generators, one operating at up to 450kA peak current and the other up to 270kA. This radiation was also used for imaging in a low magnification configuration, and spatial resolution of a few tens of micrometers was demonstrated.

Diagnostics on the COBRA pulsed power generator

The COBRA pulsed power generator has a variable current pulse wave form and amplitude (95–180ns rise time, up to 1MA peak current). It was designed to study wire array Z pinches and X pinches, including plasma formation, pinch implosion dynamics, and pinch plasma parameters as a function of current rise time. These loads have been studied using an extensive set of diagnostics with spatial and/or temporal resolution. The set of electrical diagnostics on the COBRA generator includes Rogowski coils to monitor the total load current and the current through individual return current posts, and there is also an inductive voltage monitor. A set of extreme ultraviolet and x-ray detectors is used to study the load radiation. Wire array and X pinch plasma formation and dynamics are studied using two-frame, point projection X-pinch x-ray imaging as well as with multiframe laser probing. Flat potassium acid phtalate crystal (KAP), convex, extreme luminosity imaging conical spectrograph, and focusing spectrograph with...

Time-resolved spectroscopy of Al, Ti, and Mo X pinch radiation using an X-ray streak camera

Abstract Time-resolved spectroscopic measurements of the radiation emitted from Al, Ti, and Mo X pinches have been made with ps time resolution. The radiation is emitted from micropinch plasmas with sizes of order 1 μm in times in the 10– 100 ps range. Spectra implied that dense, ∼1 keV plasmas were produced, such as a 90 ps lifetime, 1.5– 1.8 keV electron temperature and near solid-density Ti plasma. The experimental systems and analysis methods are described in detail, including line ratio calculations for μm-scale Ti and Al plasmas with ion densities of 1019–1024 cm−3 and ∼1 keV electron temperatures.

Temporal parameters of the X-pinch x-ray source

The X pinch has proved to be an excellent source of 2.5–10 keV radiation for point-projection radiography with spatial resolution of 2 μm or even better. The pulse duration of the x-ray bursts has been investigated for a wide variety of wire materials in the 1.5–10 keV energy range using a set of fast diamond photoconducting detectors with different filters, and using an x-ray streak camera to observe the source through four different filters on each pulse. All wires tested have intense continuum up to at least 6 keV, and the duration of the pulse is shorter for the harder radiation component for all materials. However, there are substantial differences between materials. For example, the pulse duration for Al with filtering for energy ⩾1.5 and ⩾5 keV are about 1 and 0.5 ns, respectively. By contrast, for Mo with filtering for energy ⩾2.5 and ⩾5 keV, the pulse durations are about 200 ps and ⩽80 ps, respectively.

Accelerated electrons and hard X-ray emission from X-pinches

The generation of accelerated electrons in the X-pinch minidiode is studied experimentally. It is well known that the explosion of an X-pinch consisting of two or more wires is accompanied by the formation of a minidiode, in which electrons are accelerated. The subsequent slowing down of electrons in the products of wire explosion causes the generation of hard X-ray (HXR) emission with photon energies higher than 10 keV. In this work, the spatial and temporal characteristics of X-pinch HXR emission are studied, the specific features of HXR generation are discussed, and the capability of applying this radiation to point-projection X-ray imaging of various plasma and biological objects is considered. The parameters of the electron beam produced in the X-pinch are measured using a Faraday cup and X-ray diagnostics. The experiments were performed with the XP generator (550 kA, 100 ns) at Cornell University (United States) and the BIN generator (270 kA, 150 ns) at the Lebedev Physical Institute (Russia).

Spectroscopy and implosion dynamics of low wire number nested arrays on the 1 MA COBRA generator

Low wire number nested array Z-pinch experiments have been carried out with wires made of aluminum, stainless steel (uniform), and combinations of these two materials (mixed) on the 1MA COBRA generator at Cornell University [J. D. Douglass, J. B. Greenly, D. A. Hammer et al., in Proceedings of the 15th IEEE International Pulsed Power Conference (IEEE, Piscataway, NJ, 2005)]. The outer array consisted of eight wires, whereas the inner array had four or eight wires. The 10μm Al wires were alloy 5056 and the 6.25μm stainless steel wires were alloy SS304. The diagnostic suite included fast-x-ray and extreme ultraviolet (EUV) detectors, a time-gated x-ray pinhole camera, x-ray spectrometers, and laser shadow imaging. The main focus was made on the spectroscopic study of plasma evolution after the main x-ray burst though the data from photoconducting detector (PCD) and EUV signals over the whole period of current, and in addition laser shadowgraphy images before the main x-ray burst were analyzed. Modeling of t...

Phase-contrast x-ray radiography using the X pinch radiation

The application of the X pinch x-ray source for phase-contrast x-ray radiography of low absorption materials is demonstrated. The X pinch is a source of radiation in the 1-10 keV x-ray band with extremely small size and short pulse duration. The small source size provides high spatial coherence of the imaging x-ray beam, enabling it to be used to image low absorption, low contrast objects with excellent spatial resolution. Images with spatial resolution better than 3 micrometers of exploded, insulated 25 micrometers W wire and biological objects are presented. The advantages of the X-pinch over other x-ray sources are discussed.