Photoconductive semiconductor switches

Optically activated GaAs switches operated in their high-gain mode are being used or tested for pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high impedance, low-current Pockels cell or Q-switch drivers for lasers; high-voltage drivers for laser diode arrays; high-voltage, high-current, compact accelerators; and pulsers for ground penetrating radar. This paper will describe the properties of high-gain photoconductive semiconductor switches (PCSS), and how they are used in a variety of pulsed power applications. For firing sets, we have switched up to 7 kA in a very compact package. For driving Q switches, the load is the small (30 pF) capacitance of the Q switch which is charged to 6 kV. We have demonstrated that we can modulate a laser beam with a subnanosecond rise time. Using PCSS, we have demonstrated gain switching a series-connected laser diode array, obtaining an optical output with a peak power of 50 kW and a pulse duration of 100 ps. For accelerators, we are using PCSS to switch a 260 kV, 60 kA Blumlein. A pulser suitable for use in ground-penetrating radar has been demonstrated at 100 kV, 1.3 kA. This paper will describe the specific project requirements and switch parameters in all of these applications, and emphasize the switch research and development that is being pursued to address the important issues     

Longevity of optically activated, high gain GaAs photoconductivesemiconductor switches

The longevity of high gain GaAs photoconductive semiconductor switches (PCSSs) has been extended to well over ten million pulses by reducing the density of carriers at the semiconductor to metal interface. This was achieved by reducing the density in the vertical and lateral directions. The latter was achieved by varying the spatial distribution of the trigger light thereby widening the current filaments that are characteristic of the high gain switches. We reduced the carrier density in the vertical direction by using ion implantation. These results were obtained for currents of about 10 A, current duration of 3.5 ns, and switched voltage of ~2 kV. At currents of ~70 A, the switches last for 0.6 million pulses. In order to improve the performance at high currents, new processes such as deep diffusion and epitaxial growth of contacts are being pursued. To guide this effort we recorded open shutter, infra-red images, and time-resolved Schlieren images of the current filaments, which form during high gain switching. We measured, under varying conditions, a carrier (electrons or holes) density that ranges from 3×10¹⁷ cm^-3 to 6×10¹⁸ cm^-3     

A technique for orbital exponent optimization in ab initio HF?SCF?LCAO?MO calculations

A technique for Slater orbital exponent optimization in an HF? SCF? LCAO? MO calculation is proposed in which orbital exponent variation is incorporated into the SCF scheme. This is accomplished by rewriting Slater's rules so that the shielding terms depend on the molecular charge distribution through the elements of the population matrix. The SCF scheme then includes a calculation of a new set of orbital exponents from the coefficients of self-consistent molecular orbitals obtained from the previous set of exponents. The process is iterated until the energy attains its lowest value. The technique is illustrated by minimal basis calculations on LiH, BH, and HF. Near optimization is obtained with considerably less effort than is necessary for other reported techniques. Aside from interesting properties, the technique can be important for extended basis calculations where exponent optimization is a difficult task.     

Photoionization resonance in Cr(CO)6: implications for bonding of adsorbed CO

The partial photoionization cross sections of the valence levels of Cr(CO)₆ are reported in the photon energy range of 30 to 100 eV. The 4_o derived level shows a resonance at ≈ 18 eV kinetic energy. This resonance is well understood and has been seen in gas phase CO at 12 eV kinetic energy and in CO adsorbed on Ni(100) at 20 eV kinetic energy. The Cr(CO)₆ data show that the large shift in the kinetic energy of the resonance in adsorbed CO is due to potential changes upon bonding, not changes in the C to O distance. This resonance is not observed in the Cr(CO)₆ energy levels derived from the CO 5_o level. Calculations of the initial and final wavefunctions are presented to explain the sensitivity of the resonance in the 5_o level to bonding.     

Photoelectron spectra of chromium tetranitrosyl

The ultraviolet and X-ray-excited photoelectron spectra of Cr(NO)₄ are reported. The electron binding energies, shake-up spectra, relative peak intensities and the Auger peak kinetic energies have been measured, and are compared to the spectra from gas-phase NO, from NO adsorbed on a transition metal surface, and to SCF-MS calculations. The theoretical calculations are utilized to obtain ionization energies and charge distributions, and consequently to discuss the bonding in nitrosyl complexes.