D. W. Swain

The characteristics of a medium current relativistic electron-beam diode

In a previous paper we reported observation of ion flow in the Nereus diode. We report here more detailed observations of the diode, employing a Thomson parabola analyzer to measure ion species and energies, a four‐pulse holographic interferometer to observe plasma blowoff from the electrodes, and an image‐converter streak camera to record total optical radiation and time‐resolved emission spectra. The large current on axis (the ’’pinch’’ current) of this diode appears to be caused by rapid local gap closure on axis. A model describing the behavior of this diode is discussed and compared with experimental data.

Observation of anode ions associated with pinching in a relativistic electron beam diode

Ions originating at the anode of a relativistic electron beam (REB) accelerator are observed at the cathode by a means of a Faraday cup. The dominant species is H+, probably from adsorbed impurities. These ions, having energies equal to the diode voltage, are not observed until 20–40 J/cm2 are deposited by the beam on the anode surface, and are closely associated with the formation of a pinch at the anode.

Ionization of a Background Gas by a Weak (ν/γ≪1) Relativistic Electron Beam

The injection of a relativistic electron beam into a neutral gas creates a background plasma. A simple model is presented for this process, which includes (a) direct ionization of the gas by the high‐energy electrons; (b) ionization of the gas caused by the acceleration of secondary electrons in the axial electric field of the beam. A self‐consistent calculation of the conductivity and the electric field in the beam is carried out. The predictions of the model are compared with an experiment on a 2‐MeV 4‐kA beam of 100‐nsec duration, injected into helium at 0.2–2.0 Torr. Results indicate that the theory agrees with experimental observations at lower pressures, but tends to deviate from experimental values at higher pressures.

Measurements of large ion currents in a pinched relativistic electron beam diode

We have observed ion currents of 10–20% of the total diode current in a diode exhibiting pinched electron flow. Measurements have been made utilizing two independent techniques—multiple Faraday cups, and neutron generation and counting. The results agree well with each other and are compared with both numerical simulations and an analytic model of ion flow. Agreement is found to within the accuracy of the experimental measurement, lending further credence to the theoretical models.

Influence of anode composition on the electrical properties of relativistic electron‐beam diodes

Current, voltage, x‐ray, and witness plate measurements of the diode characteristics of the Nereus accelerator (150 kV, 50 kA) have shown that the composition of the anode can have a profound effect on the properties of the electron beam. In particular, if the anode is heated to remove absorbed gases and volatile impurities, the diode impedance holds up longer than in the case of an anode having an unprepared surface or one that has been covered with an easily vaporized low‐Z substance (which leads to very rapid diode shorting). The fact that the anode surface can influence the electron current less than 10 ns into the beam pulse, before space‐charge‐limited electron flow has been established, indicates that ions, plasmas, or photons are generated at the anode and cross the diode gap before the electron beam has deposited more than 1 cal/cm2 into the anode.

Effect of induced axial electric field on a relativistic electron beam pulse propagating through a plasma

If a plasma exhibits anomalous conductivity, the shape of a high‐current beam pulse injected into the plasma may be strongly affected after propagating less than one‐pulse length.

Time-resolved measurement of an electron-beam distribution function

We have developed a technique for measuring the electron‐beam distribution function as it evolves in phase space. Data illustrating the successful use of this technique are presented. Transit time and other effects which can cause errors using this technique are discussed.

The use and limitations of an electrostatic analyzer for time resolving an electron beam distribution function

A technique is described for time resolving, to nanosecond precision, the velocity distribution of a periodically varying electron beam and for generating a direct plot of the entire phase plane distribution function. Calculations are presented to indicate the maximum time rate‐of‐change of the distribution function which may be accurately observed with this technique. The high‐frequency response obtainable with this technique makes it a powerful diagnostic tool for use in studying the interaction of an electron beam with plasmas and, in particular, the trapping phenomenon in beam‐plasma interactions.

Variable Output Coupling Device for Far Infrared Laser

A simple output coupler for far infrared lasers is described. The degree of coupling is continuously variable. The device is easily constructed, requires little precision machining, and is easily adjusted.

Doppler and Boltzmann temperatures of a cylindrical, nonuniform plasma

Abstract A numerical analysis of the Doppler widths and of the intensity distribution of the molecular rotational levels in a cylindrically symmetric plasma with radial gradients in emitter density and in temperature is presented. Local Maxwellian and Boltzmann distributions are assumed to be present in the plasma. Under conditions which are often found in the laboratory, the numerical results indicate that the measured Doppler and Boltzmann temperatures will deviate considerably from the average kinetic temperature.