E. J. Lauer

Measurements of hose instability of a relativistic electron beam

The observed disruption of a self‐focused, relativistic electron beam propagating through a gas is shown to result from the growth of m=1 (’’kink’’ or ’’hose’’) perturbations. Measurements of the frequency dependence of the spatial amplification rate are presented. An upper cutoff to the frequency range for hose amplification is observed, in agreement with a theoretical model that includes the damping effects of a spread in the particle betatron frequency.

Particle Behavior in Static, Axially Symmetric, Magnetic Mirror and Cusp Geometries

A mean containment time τ of many seconds has been observed for a significant fraction of the energetic (≤ 2.2‐MeV) positrons emitted from Ne19 isotropically and uniformly in a mirror geometry with mirror ratio R in the range 1.1–3.7. Long‐time containment has been observed for particles that escape through a mirror at radial distances for which the particle orbits do not encircle the axis, as well as for those that escape on or near the axis. The dependence of τ on the Z of the scattering gas, the gas pressure, the positron energy, the coil current, and the diameter and separation of the coils has been investigated. Most of the data were obtained with the two mirror coils separated one coil mean diameter. For sufficiently small values of λ (the ratio of the maximum orbit diameter on axis to the coil mean diameter), τ is independent of λ and its dependence upon the above parameters agrees with the predictions of scattering theory. Also, the fraction of particles trapped as a function of the mirror ratio a...

Radial expansion of self-focused, relativistic electron beams

Measurements of the radial expansion from gas scattering of a low ν/γ relativistic electron beam are presented. The measured current density profiles approach a Bennett shape, as predicted theoretically, and the rate of expansion of the beam radius with distance is in good agreement with the predictions of an envelope equation. locus has been determined. It terminates at zero temperature at

Guiding Center Motion and Plasma Behavior in the Bumpy Torus

Drift surfaces of guiding centers in the Bumpy Torus static magnetic field have been located and analyzed for various torus parameters by making use of the transverse adiabatic invariant μ = p⊥2/B and the longitudinal adiabatic invariant J = ∮ p∥ dl. In addition, the drift equations (for which μ is an invariant) have been solved numerically to investigate limitations on the application of the invariance of J. In general, J is an invariant for the drift equations under adiabatic conditions; however, some special guiding‐center trajectories are presented for which the drift per longitudinal period is sufficiently large that the assumption of the invariance of J is not valid. Closed surfaces of constant U = ∮ B−1 dl have been found. These are also surfaces of constant plasma pressure under static equilibrium conditions at low β for a hydromagnetic model. The stability of the equilibrium is discussed.

Beam-target interaction experiments for multipulse bremsstrahlung converters applications

As part of the Dual Axis Radiography Hydrotest Facility, Phase II (DARHT II) Multipulse Bremsstrahlung Target effort, we have been performing an investigation of (1) the possible adverse effects of backstreaming ion emission from the bremsstrahlung converter target and (2) the hydrodynamic behavior of the target after the electron beam interaction. Theory predictions show that the first effect would primarily be manifested in the static focusing system as a rapidly varying X-ray spot. From experiments performed on ETA-II, we have shown that the first effect is not strongly present when the beam initially interacts with the target. Electron beam pulses delivered to the target after formation of a plasma are strongly affected, however. Secondly, we have performed measurements of the time varying target density after disassembly was initiated by the electron beam. The measurements presented show that the target density as a function of time compares favorably with our LASNEX models.

Search for backstreaming ion defocusing during a single pulse of a 2 kA relativistic electron beam

Desorption and subsequent ionization of the monolayers from the vacuum wall of an accelerator system can have a detrimental effect on the performance of the beam transport system. Ions extracted from the resultant plasma neutralize the spacecharge and dynamically perturb the net focusing forces within the beam. To study the effect, a transparent first foil, presumably with contaminants on the surface, intercepts the beam. Placing an imaging foil tens of centimeters downstream from the first foil allows observation of minor fluxuations in the envelope. Using conducting foil targets, we see no effect unless the beam radius is small enough to damage the foil. Non-conducting foils produce a strong effect.

Improved geometry of the vacuum-insulator-anode triple junction

We propose a modification of a vacuum-insulator-electrode geometry that is commonly found in particle accelerators to reduce the electric field near the anode triple junction and reduce dielectric flashover.

High Intensity Beam and X-Ray Converter Target Interactions and Mitigation

Ions extracted from a solid surface or plasma by impact of an high intensity and high current electron beam can partially neutralize the beam space charge and change the focusing system. We have investigated ion emission computationally and experimentally. By matching PIC simulation results with available experimental data, our finding suggests that if a mix of ion species is available at the emitting surface, protons dominate the backstreaming ion effects, and that, unless there is surface flashover, ion emission is source limited. We have also investigated mitigation, such as e‐beam cleaning, laser cleaning and ion trapping with a foil barrier. The temporal behavior of beam spot size with a foil barrier and a focusing scheme to improve foil barrier performance are discussed.

Electron avalanche on a dielectric-vacuum interface

We present an improved numerical model for anode initiated electron avalanche breakdown of a dielectric-vacuum interface. An initiating event occurs in the high field region near the anode triple junction to start the first avalanche. The first avalanche grows and then stops, and a new one starts closer to the cathode. The breakdown process is modeled as a sequence of avalanches progressing from the anode to the cathode.