A. H. Williams

Determination of laser intensity and hot‐electron temperature from fastest ion velocity measurement on laser‐produced plasma

Measurements of the velocity of the fastest ion emitted by a CO2‐laser‐produced plasma were made, and it was found that the relationship between velocity and laser flux was given by Vi =5.7×106 φ1/6l for single‐beam illumination. Comparison with computer simulation, ion velocity spectrum, and x‐ray data indicate that the hot‐electron temperature is given by Th =7.5×10−18 V2i. Measurements of emission from multiple‐beam experiments and from experiments at 1.06 μm wavelength are also described. Some simple models are used for discussion and correlation of the data.

SOME PROPERTIES OF A GRADED VACUUM SPARK GAP

A high power, low inductance vacuum spark gap combination (crowbar and main switch) is described which is capable of dc operation over a wide voltage range. The electrical properties are discussed in regard to shorting and multiple switch operation. The principal difficulty of vacuum spark gaps, the coating of the inner surface of the insulator with evaporated and sputtered electrode material, is absent in this design after conditioning. A mechanism to account for this, based on the establishment of a large number of nucleation centers on the insulating walls, is shown to be consistent with observation.

Stability of the Dense Plasma Focus

The influence of small dc axial magnetic fields upon the stability of the focused plasma column is shown in the light of soft x rays. The material damage to the center electrode and the intense x‐ray emission at the electrode face as a result of electron bombardment is considerably reduced. Although the final heating of the ions and electrons is inhibited as expected due to a trapped magnetic field, the spatial stability of the focused column offers a greater possibility for scaling to higher energies.

Spectra and angular distributions of electrons emitted from laser-produced plasmas

The spectra and angular distributions of high‐energy electrons emitted from laser‐produced plasmas have been measured. The plasmas were produced using short pulses from both CO2 and Nd : glass lasers to irradiate polyethylene slabs. For a CO2 laser intensity of approximately 3×1014 W/cm2 we find at least an order of magnitude greater energy conversion to high‐energy electrons than with a Nd : glass laser intensity of approximately 5×1015 W/cm2. The angular distribution of electron emission is strongly peaked along the target normal (i.e., density gradient) and fine angular structure is observed.

High‐Power Vacuum Spark Gap

The design and construction of a voltage graded vacuum spark gap is described. This gap is capable of switching currents as large as 106 amp at voltages up to 75 kv. The effect of the insulating walls of the gap is briefly discussed.

Inelastic neutron scattering from amorphous hydride of Zr2Pd

Abstract Time-of-flight inelastic neutron scattering data was obtained on hydrided Zr2Pd metallic glass using the Crystal Analyzer Spectrometer at the Los Alamos pulsed spallation neutron source. Energy transfers from about 40 meV to several hundred meV were obtained with sufficiently good statistics and signal to noise ratio to show the second harmonic as well as the fundamental hydrogen optic mode.

CO2 laser‐driven high‐density implosion experiments

Glass microballoons, coated with various thicknesses of plastic ablator and filled with deuterium‐tritium gas mixtures, were irradiated with the eight‐beam Helios CO2 laser. Neutron yield, fuel temperature, compressed fuel density, and implosion time were measured as functions of ablator thickness. Hydrodynamic efficiency and preheating of these targets were estimated. Transition from fuel conditions characteristic of earlier exploding pusher experiments to those characteristic of a more nearly isentropic mode of implosion is illustrated. Fuel densities above 2 g/cm3 were achieved.

Electron beam and dense plasma focus interaction heating experiments

Several attempts to transiently heat a dense plasma focus discharge using intense electron beams at voltages of 280 to 300 keV and 3.6 MeV are discussed. The results, although requiring further confirmation, suggest a positive interaction and indicate that low ν/γ beams are required for injection into the plasma focus column.

IMAGE CONVERTER OBSERVATIONS OF THE DEVELOPMENT OF THE DENSE PLASMA FOCUS DISCHARGE.

A multiplicity of radial striated light filaments is observed at the back plate of the dense plasma focus coaxial discharge during the early phases of breakdown and current sheath buildup. Later in time, the current sheath appears azimuthally symmetric.

Programmable data‐acquisition and stepping motor control system

The implementation of a compact, inexpensive, programmable stepping motor controller and data‐acquisition system based on the Zilog Z‐80 microprocessor is described, including its basic hardware and software configurations. While originally designed to facilitate data acquisition from scanning x‐ray diffractometers, the units have proven to be quite versatile and have been applied to a variety of instrumentation requiring automatic data acquisition and mechanical control.