K-shell radiation physics in the ultrahigh optical depth pinches of the Z generator

Abstract

Al:Mg alloy wire arrays of mass loads 1.3–3.6 mg/cm have been imploded with peak currents of 19 MA on the 60 TW Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The large mass loads have resulted in the highest K-shell x-ray line optical depths (∼103) produced to date in Z-pinches. Analysis of the time-resolved spectrum of a 2.1 mg/cm shot near the time of peak compression has yielded a temperature–density profile of the pinch that approximately reproduces all features of the x-ray data except the continuum above 5 keV, which is underpredicted. The Ly α/He α ratio for Al is shown to be enhanced relative to that of Mg by two mechanisms: photopumped ladder ionization and absorption of the Al He-like line in a cool outer halo. This analysis and comparisons to some Ti shots demonstrates that the K-shell yield of Al is significantly reduced by line and continuum self-absorption, but that of Ti is not.

Efficient argon K-shell radiation from a Z pinch at currents >15 MA

“…1,2 In a Z pinch, a cylindrical mass is accelerated radially inward by the JϫB interaction of the pinch current and its self-generated magnetic field. When the mass stagnates on axis, a hot plasma is created that converts some of the implosion energy to radiation.…”

Analysis of radially resolved spectra and potential for lasing in Mo wire array Z pinches

“…Power measurements in the previous work, as well as fundamental atomic physics considerations [2], confirm that L-shell emitters as pulsed power loads have smaller x-ray yields than K-shell emitters with similar photon energies [3,4]. A separate potential point of interest is whether L-shell Z pinch plasmas can achieve conditions relevant to x-ray laser studies: in 1987, Mo plasmas ionized to the L-shell by laser irradiation were established as high-gain (~ 4 cm -1 ) media for collisionally-pumped x-ray lasers [5].…”

“…In contrast, disagreement in un-optimized features serves as a warning that further plasma effects or atomic processes should be considered. In particular, the disagreement between the 0-1 mm experimental data and the model probably points to the importance of opacity effects in the dense central portion of the pinch [3], where optical depths for the strong 3C and 3D Ne-like lines can reach 100 over a ~3 mm pinch diameter. The most accurate modeling of a plasma with such optical depths would include radiative transfer effects, however, the optically thin modeling presented here gives useful first estimates of plasma conditions, indicating a plasma with sufficient density and extent to support significant gain in lasing transitions if population inversions exist.…”

The physics of radiation transport in dense plasmas

“…A 3 mm diam Al plasma is assumed to have a core electron temperature of 600 eV and an outer surface temperature of 200 eV. Its calculated K-shell spectrum is plotted for assumed ͑uniform͒ ion densities of 3ϫ10 19 and 10 22 cm Ϫ3 . At the lower of these densities, the lines are strongly in emission, but at the higher density, in absorption.…”