A. Starobinets

Plasma dynamics in pulsed strong magnetic fields

Recent investigations of the interaction of fast-rising magnetic fields with multi-species plasmas at densities of 1013–1015 cm−3 are described. The configurations studied are planar or coaxial interelectrode gaps pre-filled with plasmas, known as plasma opening switches. The diagnostics are based on time-dependent, spatially resolved spectroscopic observations. Three-dimensional spatial resolution is obtained by plasma-doping techniques. The measurements include the propagating magnetic field structure, ion velocity distributions, electric field strengths, and non-Maxwellian electron energy distribution across the magnetic field front. It is found that the magnetic field propagation velocity is faster than expected from diffusion. The magnetic field evolution cannot be explained by the available theoretical treatments based on the Hall field (that could, in principle, explain the fast field propagation). Moreover, detailed observations reveal that magnetic field penetration and plasma reflection occur si...

A numerical test of activated rate theories for cusped and smooth potentials

A numerical study of the effect of dissipation on the radiationless transition rate in the adiabatic and solvent‐controlled limits is presented. For light particle reactions, the nonlinearity of the potential surface in the vicinity of the barrier top is important, and the potential may be approximated as a cusped double well potential, provided that the nonadiabatic coupling is small compared to the thermal energy. Three different theoretical approaches for calculation of the thermally activated rate are analyzed and compared with exact numerical results. We find that Variational Transition State Theory (VTST) with a planar dividing surface, as well as the approach of Calef and Wolynes (CW), provide a good description of the rate of symmetric reactions. A rate expression suggested by Dekker is found to be the least accurate. The CW approach is most accurate in the strong damping regime, while VTST is better in the weak damping regime. The accuracy of both methods improves as the potential is smoothed. VT...

Evolution of MHD Instabilities in Plasma Imploding Under Magnetic Field

Two-dimensional 3-ns-gated visible images, recorded at different times during the implosion of plasma under azimuthal magnetic field (Z-pinch), revealed ringlike instabilities followed by the development of axially and azimuthally nonuniform structures in the imploding plasma. Remarkably, the evolution in time of all structures was found to be highly repeatable in different shots, which should allow, through 3-D magnetohydrodynamics modeling, for systematically studying the development in time of these complex phenomena and correlating them with the initial plasma parameters. The data are also used to infer the time-dependent outer plasma radius and plasma radial velocities.

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...

Investigation of Ne IX and Ne X line emission from dense plasma using Ross-filter systems

We report on the application of well-balanced Ross-filter systems for the diagnostics of x-ray emission from a Z-pinch plasma. The composition and thicknesses of the filter layers were so selected to yield the intensities of neon H- and He-like emission lines separately with relatively good accuracy. The systems provide convenient absolute and time-dependent measurements of the emission intensities, and they are particularly useful for relatively low source-light intensities. The applicability of the systems is examined with the aid of time-dependent modeling of the stagnating plasma, using collisional-radiative and radiation transport calculations. The data and modeling are used to yield information on time evolution of the plasma density and temperature at stagnation.

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.

On the role of the plasma composition in the magnetic field evolution in plasma opening switches

High spatial- and temporal-resolution spectroscopic methods are employed to perform detailed studies of the interaction between the propagating magnetic field and a multi-ion-species plasma. The experiment is performed in a plasma-opening-switch configuration, in which a 150-kA current of /spl sim/400 ns duration is conducted through a plasma. Recent observations demonstrated a new phenomenon of simultaneous rapid magnetic field penetration into the heavy-ion plasma and specular reflection of the light-ion plasma, leading to ion-species separation. Additionally, noticeable inconsistencies between experimental results and theories were found. The current paper summarizes these recent results and discusses the aspect of the role of the plasma composition in the magnetic field evolution and ion dynamics. In order to systematically investigate the effect of the plasma composition, a method for producing plasmas with controllable compositions, based on spatial species separation and electrode heating, is presented. This method allows for achieving plasmas with varying proton-to-carbon ion ratios, however, at different electron densities. Measurements are described for studying the relation between the magnetic field propagation velocity and the plasma composition and density; however, this relation is not yet satisfactorily clear.

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.

Spectroscopic Investigation of the Particle Density and Motion in an Imploding z‐Pinch Plasma

Experimental investigations of the ion density and flow as a function of time and space in z‐pinch plasmas are of key importance for improving the understanding of z‐pinch dynamics. For such studies, measurements of emission‐line shapes can be highly useful.In the present experiment line emission of oxygen ions is used to investigate the ion density and motion in the imploding plasma in a 0.6‐μs, 220‐kA z‐pinch experiment. For the time period studied here (220 – 85 ns before the stagnation on‐axis), the plasma properties have been extensively characterized previously, employing various spectroscopic methods to determine the time‐dependent radial distributions of the ion velocities, the magnetic field, the charge‐state composition, the electron temperature, and the particle densities. In particular, the electron density was determined from the absolute intensities of spectral lines, from the ionization times in the plasma, and from momentum‐balance considerations, based on the previously measured time‐depe...

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.