Role of the implosion kinetic energy in determining the kilovolt x-ray emission from aluminum-wire-array implosions

Abstract

Aluminium wire arrays have been imploded on the 6-TW, 4-MA Double-EAGLE generator. An initial diameter of 12.5 mm and mass loading of 164 \ensuremath{\mu}g/cm have been identified as the optimum parameters to maximize the aluminum kilovolt K-shell yield. The implosion time of 90 ns and the initial radius of 0.625 cm correspond to an implosion kinetic energy of 19.2\ifmmode\pm\else\textpm\fi{}3.6 keV/ion. Larger-diameter arrays have higher kinetic energies per ion but lower masses, and they produce lower kilovolt K-shell x-ray yields. Smaller-diameter arrays have kinetic energies per ion less than the minimum 12 keV required to ionize aluminum to its K shell; so these arrays produce lower K-shell yields. Time-resolved x-ray pinhole photography and time-resolved spectroscopy are utilized to determine the K-shell emitting plasmas' sizes, temperatures, and densities as functions of time during the pinched phase. It is found that both the percentage of the mass radiating K-shell x rays and the electron temperature increase during the first 20 ns of emission. For the optimum implosion parameters, it is found that only a small percentage of the mass is radiating in the K shell and, furthermore, that the conversion of kinetic energy to kilovolt emissions alone does not account for the measured x-ray yields. The data analysis suggests that the problem of maximizing aluminum K-shell emission breaks down into two parts. On the one hand, K-shell emission is enhanced when, during the implosion phase, sufficient kinetic energy is generated to drive the aluminum plasma into the K shell on thermalization. This is in agreement with pure-kinetic-energy calculations. On the other hand, there appears to be an additional anomalous heating mechanism that scales with mass and adds to the K-shell emission during the current confinement on axis. As a result, the optimum x-ray production occurs in these experiments by maximizing the mass on axis while both achieving enough kinetic energy per ion and imploding at peak current.