R. E. Pechacek

Channel cooling by turbulent convective mixing

Results from a series of experiments are described which show that hot, reduced‐density channels in the atmosphere usually cool by a process of turbulent convective mixing. Five different types of channels were created: (a) by the interaction of a pulsed CO2 laser with aerosols in the atmosphere, (b) by electric discharges in the atmosphere, (c) by laser‐guided electric discharges in the atmosphere, and (d) and (e) by the absorption of CO2 laser radiation in nitrogen doped with sulfur hexafluoride. For channels in which the energy deposition was almost cylindrically symmetric and axially uniform, (e), the rate of cooling, after reaching pressure equilibrium, was within an order of magnitude of thermal conduction. But for channels in which the energy deposition was asymmetric and/or axially nonuniform, the rate of cooling was typically one thousand times faster than thermal conduction (for channels whose radius at pressure equilibrium was ∼1 cm). These channels were seen to be turbulent and to cool by mixi...

Electron beam tracking in a preformed density channel

High‐current charged particle beams can be guided by reduced density channels. Such guiding occurs when the distribution of plasma currents in the density channel causes a net attractive force to be exerted on the beam. In particular, a relativistic electron beam (REB) injected parallel to a spatially offset, reduced density channel is pulled toward the channel. The force exerted on the beam is predicted to increase as the beam current increases and as the offset between the beam and the channel increases out to offsets equal to the channel radius. An experiment with a 1 MV, ≊10 kA beam was performed that demonstrates this effect.

A segmented concentric Faraday cup for measurement of time‐dependent relativistic electron beam profiles

A multi‐element segmented concentric Faraday collector has been developed for measuring the time evolution of the beam half‐current radius (a1/2) of an intense relativistic electron beam. Each collector segment measures the total current within its radius. The data analysis procedure fits the data from all five segments at a given time to a prescribed beam profile and calculates a1/2 from the parameters of the fitted curves. The effect of beam centroid offsets on the data analysis was investigated numerically. Beam centroid offsets as large as half the beam radius produce only a 10% error in the experimental measurement of a1/2. The use of a thin graphite overlayer followed by range‐thick stainless steel reduces scattering from one collector element to the next. The instrument has been used extensively on the SuperIBEX relativistic electron beam accelerator for measurement of the half‐current radius as a function of time. Radius variations in excess of 4:1 have been measured over the duration of the beam ...

Fast passive integrator

We describe a passive integrator capable of correctly giving outputs with rise times as short as ∼250 ps. The integrator can drive a 50‐Ω load and is intended to exploit the capabilities of the 7A29 preamp in the Tektronix 7104 oscilloscope (tr≤380ps). The decay time for the integrator is ∼250 ns.

Plasma production by staged laser irradiation of mm‐size deuterium pellets

A fully ionized plasma consisting of 2×1019 D+ ions with an average energy of 50 eV is produced by irradiating a free‐falling solid D2 pellet with a sequence of two laser pulses. An Nd–glass laser pulse first evaporates the pellet and a CO2 laser pulse ionizes and heats the resulting D2 cloud. This staged laser process couples about 2/3 of the energy of the 70 ns CO2 laser spike into the plasma. The techniques for making, dropping, and irradiating the pellet are discussed in detail. The properties of the resulting plasma are described, and a brief account is given of the cusp containment experiment for which this plasma production method was developed.

A compact, four‐way image splitter

A compact, four‐way, optical image splitter is described which maintains equal intensity and equal optical path length in each arm of the apparatus. The four output beams are parallel with the input light beam.

Use of varistors to produce single high‐current pulses

A current pulse closely resembling a half‐sine wave may be produced by an LRC circuit in which the resistance is a voltage‐dependent resistor (varistor). A simple analysis of this circuit is given, and curves are presented to aid in the design of such circuits. A comparison is given between predictions of the analysis and a circuit used to produce the 350‐kA pulse that drives the magnetic field coils of a plasma cusp containment experiment.