No abstract

A 90-wire, aluminum, z-pinch experiment was conducted on the Saturn accelerator at the Sandia National Laboratories that exhibited azimuthally symmetric implosions and two x-ray bursts, a main burst and a subsidiary one. These bursts correlated with two consecutive radial implosions and are consistent with predicted magnetohydrodynamics behavior. A variety of time-resolved, accurately timed, spectroscopic measurements were made in this experiment and are described in this paper. These measurements include ͑1͒ the pinch implosion time, ͑2͒ time-resolved pinhole pictures that give sizes of the K-shell emission region, ͑3͒ timeresolved K-series spectra that give the relative amounts of hydrogenlike to heliumlike to continuum emission, ͑4͒ the total and the K-shell x-ray power outputs, and ͑5͒ time-resolved photoconducting diode measurements from which continuum slopes and time-resolved electron temperatures can be inferred. Time-resolved Ly-␣ and Ly-␤ linewidths are obtained from the spectra and inferences about time-resolved ion temperatures are also made. All of these data correlate well with one another. A method is then presented of analyzing this data that relies on the complete set of time-resolved measurements. This analysis utilizes one-dimensional radiative magnetohydrodynamic simulations of the experiments, which drive z-pinch implosions using the measured Saturn circuit parameters. These simulations are used to calculate the same x-ray quantities as were measured. Then, comparisons of the measured and calculated data are shown to define a process by which different dynamical assumptions can be invoked or rejected in an attempt to reproduce the ensemble of data. This process depends on the full data set and provides insight into the structure of the radial temperature and density gradients of the on-axis pinch. It implies that the first implosion is composed of a hot plasma core, from which the kilovolt emissions emanate, surrounded by a cooler, denser shell, and it provides details about the structure of the temperature and density gradients between the core and shell regions. These results are found to be broadly consistent with an earlier, less detailed, data analysis in which plasma gradients are ignored. However, the ability to reproduce the full spectroscopic data in the present analysis is found to be sensitively dependent on the radial gradients that are calculated. ͓S1063-651X͑97͒06609-9͔

To examine prospects for gain in a Lyman-a recombination laser driven by a high-intensity, shortpulse laser, we calculate the residual energy in both hydrogen and helium during recombination after the ionizing pulse. The expected gain as a function of residual energy and density is then separately evaluated. The residual energy calculation includes above-threshold ionization (ATI) in the presence of a background plasma, as well as inverse-bremsstrahlung heating. At electron densities over 10" cm ' but below critical density, the plasma reduces the ATI energy by approximately a factor of 2, but without a previously reported dependence on the pulse width. Inverse-bremsstrahlung heating can be significant, but is not dominant for the parameters considered. Detailed recombination-laser gain calculations were performed for the Ly-a transitions of both H and He, using Stark profiles to represent the laser line cross section. To obtain gain of near 2 cm lasting at least a few ps, the H plasma temperature must be less than 3.5 eV and electron density between 4X 10" and 4X10" cm; for He, the temperature must be less than 15 eV and the electron density between 2X 10'8 and 2X10' cm '. Our calculations indicate that these conditions can be satisfied for H, if the driving laser intensity is above 4X 10' Wcm, and for He, if the laser intensity is above 1.7 X 10' W cm and the wavelength is below 0.6 pm.PACS number(s): 42.55. Vc, 32.80.Rm, 52.40.Nk, 52.50.Jm

Aluminum wire array, Z-pinch experiments have been performed on an 8 MA generator using arrays consisting of 24, 30, and 42 wires. These experiments were designed to scan through a region of ͑array mass, implosion velocity͒ space in which greater than 30% conversion of the implosion kinetic energy into K-shell x rays was predicted to occur ͓Thornhill et al., Phys. Plasmas 1, 321 ͑1994͔͒. Array masses between 120 and 2050 g/cm were used in these experiments. An analysis of the x-ray data taken using 24 wire arrays, shows a one-to-one correspondence between the observed kilo-electron-volt yields ͑5-64 kJ͒ and the fraction of initial array mass ͑0.3%-60%͒ that is radiating from the K shell. The 30 and 42 wire experiments demonstrated that tighter pinches with increased radiated powers were achieved with these larger wire number, improved symmetry arrays. In addition, increases in the implosion mass and array diameter in the 30 and 42 wire number cases resulted in increases in the radiated yield over the corresponding 24 wire shots, up to 88 kJ, which can be interpreted as due to improved coupling and thermalization of the kinetic energy. Moreover, spectroscopic analyses of the 30 and 42 wire experiments have shown that the increased wire numbers also resulted in K-shell radiating mass fractions of greater than 50%.

High-energy modes of oscillation in a zero-temperature relativistic electron gas in a strong background magnetic field are reported. The modes propagate parallel to the magnetic field and appear both in a longitudinal and in two transverse polarizations. The underlying mechanism is the binding between electrons near the Fermi surface and virtual positrons, which is enhanced by the presence of the filled Fermi distribution, in a Cooper-pair-like phenomenon. The energy of the mode is of the order of the pair energy (over 1.02 MeV), and the mode exists only for wave numbers k above a critical value, such that the mode group velocity exceeds the velocity of an electron on the Fermi surface. Damping of the mode is insignificant at the critical wave number and increases with k to a relatively small maximum value.PACS number(s): 52.25. Mq, 52.60.+h, 12.20.Ds, 97.60.Jd