V. Fisher

A scaling of multiple ionization cross sections

On the basis of average experimental data we demonstrate scaling laws of electron-impact multiple ionization cross sections and propose expressions for the cross sections for arbitrary atoms and ions.

Electron-impact excitation cross sections for allowed transitions in atoms

We present semiempirical Gaunt factors for the widely used Van Regemorter formula [Astrophys. J. {bold 136}, 906 (1962)] for allowed transitions from {lt}{ital ital}{gt}{ital l}=0 or {lt}{ital ital}{gt}{ital l}=1 levels in atoms with the {ital LS} coupling. Cross sections calculated using these Gaunt factors agree with measured cross sections to within experimental error. {copyright} {ital 1996 The American Physical Society.}

Visible-light spectroscopy of pulsed-power plasmas (invited)

We describe the investigations of the plasma behavior in three pulsed‐power systems: a magnetically insulated ion diode, and plasma opening switch, and a gas‐puffed Z pinch. Recently developed spectroscopic diagnostic techniques allow for measurements with relatively high spectral, temporal, and spatial resolutions. The particle velocity and density distributions within a few tens of microns from the dielectric‐anode surface are observed using laser spectroscopy. Fluctuating electric fields in the plasma are inferred from anisotropic Stark broadening. For the plasma opening switch experiment, a novel gaseous plasma source was developed which is mounted inside the high‐voltage inner conductor. The properties of this source, together with spectroscopic observations of the electron density and particle velocities of the injected plasma, are described. Emission line intensities during the switch operation are discussed. In the Z‐pinch experiment, spectral emission‐line profiles of various charge‐state ions ar...

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...

Converging shock wave focusing and interaction with a target

Converging shock waves in liquids can be used efficiently in the research of the extreme state of matter and in various applications. In this paper, the recent results related to the interaction of a shock wave with plasma preliminarily formed in the vicinity of the shock wave convergence are presented. The shock wave is produced by the underwater electrical explosion of a spherical wire array. The plasma is generated prior to the shock waves arrival by a low-pressure gas discharge inside a quartz capillary placed at the equatorial plane of the array. Analysis of the Stark broadening of Hα and Hβ spectral lines and line-to-continuum ratio, combined with the ratio of the relative intensities of carbon C III/C II and silicon Si III/Si II lines, were used to determine the plasma density and temperature evolution. It was found that during the first ∼200 ns with respect to the beginning of the plasma compression by the shock wave and when the spectral lines are resolved, the plasma density increases from 2 × ...

Electron density and ionization dynamics in an imploding z-pinch plasma

The time-dependent radial distributions of the electron and ion densities during the implosion phase of a gas-puff z-pinch plasma are determined from measurements of continuum radiation, as well as time-dependent collisional-radiative analysis of the observed particle ionization history in the plasma. It is shown that during the 140‐ns-long time interval close to the end of the ∼620‐ns-long implosion phase, the total imploding-plasma mass increases by ∼65%, found to be consistent with the continuous ionization of the gas ahead of the plasma shell. Furthermore, the densities obtained, together with the previously determined radial distributions of the electron temperature, magnetic field, and particle radial velocities, are used to analyze the energy terms that support the radial propagation of the ionization wave seen in the plasma, thereby explaining the time-dependent radial distribution of the ion charge states in the plasma.

High-resolution spectroscopic X-ray diagnostics for studying the ion kinetic energy and plasma properties in a Z-pinch at stagnation

Doubly-curved-crystal spectroscopic systems are used to obtain time-resolved measurements of Ne K emission from the stagnating plasma in a Ne-puff Z-pinch experiment. These systems, with a spectral resolving power of /spl cong/ 6700 (only limited by the crystal rocking curve) and simultaneous z-imaging with a resolution /spl cong/ 0.1 mm, are used to obtain the time history of the ion kinetic energy at stagnation from the line profiles of Ly/spl alpha/ satellites, which were verified to be optically thin. The measurements allowed for tracking the ion energy throughout the entire K-emission period. It was found that the ions lose most of their kinetic energy during the K-emission period, i.e. before the electrons cool down enough to terminate the K-emission, and before the ions recombine to Li-like charge state. Also in this study, the profile of the optically thin intercombination line was used to investigate the velocities of the He-like ions. Together with the determination of the electron density from satellite ratios, absolute line and continuum intensities, time resolved observation of the plasma size, and collisional-radiative and radiation-transport calculations, these data are used to study the various contributions to the energy deposition and energy losses of the plasma at stagnation.

High-resolution spectroscopic X-ray diagnostics for studying the ion-kinetic energies at the stagnation of a Z-pinch plasma

Summary form only given. Spectroscopic systems with toroidally bent crystals, giving resolving power of 7000 and time-gated (1 ns) spatially-imaged spectra with 0.1 mm resolution, are used to obtain the kinetic energies of [H] ions as a function of time throughout the stagnation phase in a neon-puff, 350 kA Z-pinch experiment. The spectrographs were so designed to allow for Rocking-Curve-limited spectral resolutions, verified by double-grating measurements. Optically thin lines (mainly satellites) are utilized in order to obtain the Doppler contribution to the line profiles. The data show that the mean ion kinetic energies drop drown to the electron thermal energy at the end of the radiation phase of the plasma. The data also yield the electron density from the satellite-intensity ratios, and the total ion densities from line-shape measurements and analysis of the opacity effects. This allows for the investigation of the ion-energy conversion at stagnation and comparisons to the radiated energy, using collisional-radiative and radiation-transport calculations. The variations observed in the plasma structure and in the ion velocities along the pinch column allows for studying the relation between the ion-kinetic-energy history and the ionization processes in the plasma. In addition, the shapes of the resonance and intercombination lines of He-like ions are used to obtain the properties of the colder parts of the plasma. The energy balance in the plasma and inferences of temperature and density gradients will be discussed.

Interaction of a converging shock wave with a plasma

Summary form only given. Experimental studies of a spherical converging shock wave (SW) interaction with preliminary formed plasma are presented. The SW was generated by an underwater electrical explosion of a spherical Cu wire array (radius of 15 mm, number of wires of 40, and wire diameter of 0.1 mm) supplied by 240-kA amplitude current pulse with rise time of ~0.8 μs. The plasma was formed by a 40 Pa propane discharge between the cathode and anode electrodes, placed at the distance of 1.5 mm apart, inside of 2 mm inner diameter quartz capillary located at the equatorial plane of the array. The light emission from the plasma was transferred to the inputs of two spectrometers by optical fiber installed from the one side of the capillary and beam splitter with lenses. The spectral lines intensities at the output of spectrometers were recorded prior and during the SW interaction with plasma by intensified framing 4QuikE cameras and by a photomultiplier tube when one of the spectrometers was used as a monochromator. Hα and Hβ spectral lines and line-to-continuum ratios, combined with the ratio of relative intensities of C III/C II and Si III/Si II lines, were used to determine the plasma density and temperature evolution. It was found that during the first -200 ns with respect to the beginning of the plasma compression by the SW, the plasma density increases from 2×1017 cm-3 to 5×1018 cm-3 while the plasma temperature remained almost unchanged, 3-4 eV. Further, following a model of an adiabatically imploding capillary, the plasma density increases >1019 cm-3, resulting in continuum spectrum obtained in the experiment with temperature exceeding 50 eV at radii of compression ≤50 μm.

Analysis of implosion and stagnation of stainless steel wire array z-pinches at 18MA on the Z generator

We present detailed analysis of the implosion dynamics and stagnation of a Stainless Steel wire array z-pinch on the Z generator. Z2011 was a 0.68mg/cm, 70mm on 35mm nested wire array. In this presentation we use the complete set of data available for this shot to discuss in detail the implosion dynamics and conditions in the final pinch.