Michael A. Mostrom

Simple criteria for the absence of the beam‐Weibel instability

Rigorously sufficient and approximately necessary conditions for the absence of the beam‐Weibel instability are derived. These conditions include previously known stability criteria and resolve the seeming contradiction that these modes can be stabilized by beam temperature when the plasma is cold, but they cannot be stabilized by beam temperature when the plasma has infinitesimally small temperature.

Analytical and numerical studies of foilless diodes

The generation of intense annular beams by foilless diodes is studied through analytic equilibrium models and particle‐in‐cell simulation. In the high‐voltage regime, the foilless diode operates below the space‐charge limit and the impedance is nearly independent of the voltage. The current density is proportional to the current and to the square of the external magnetic field. From a kinetic theory equilibrium model, the beam scattering angle is found to be inversely proportional to the diode voltage, external magnetic field strength, and cathode radius. The predicted scaling is in good agreement with the simulation results. Criteria for adiabatic expansion and cooling of the beam, supported by simulation results, have also been considered.

Shear‐driven instabilities of annular relativistic electron beams in vacuum

The study of instabilities driven by azimuthal shear (diocotron) and axial shear is extended to annular relativistic beams. The analysis is done nonrelativistically in the beam frame where the instability is assumed to be electrostatic, and the dispersion relation is then transformed back to the laboratory frame where magnetic perturbations are non‐negligible. This requires keeping finite axial wavenumber kz in the analysis. The axial and azimuthal shears are related through the self‐consistent equilibrium, assuming emission from an equipotential cathode. Axial shear destabilizes the diocotron modes at higher azimuthal wavenumbers l. It also produces a set of modes including l=1 modes, that are unstable as the result of wave–particle interactions. The annular configuration introduces an important second pole in the differential equation.

Proton Beam Transit-Time Oscillator (TTO) For Producing High Power Microwaves

Self-consistent, 2D, electromagnetic particle-in-cell computer simulations agree with the linear and nonlin-ear theoretical model presented in a companion paper and confirm the importance of space-charge effects. Simulations under idealized conditions demonstrate 18% conversion efficiency of ion beam power into 100 GW of extracted microwaves. Internal fields in the cavity reach 13 MV/cm. Typical constraints on the beam parameters are Vb > 3 MeV to avoid resonance shifts due to gap closure, Ib > 100 kA to maximize the growth rate and minimize the interaction Q, and Tb > 30 ns to keep the cavity gap d > 1 cm. This excludes small scale experiments from any proof-of-principal demonstration. Further theoretical research is needed for the necessary external magnetic field in cylindrical cavities and for plasma formation.

Magnetically insulated plasma sheath in coaxial transmission lines with an external magnetic field

A relativistic Brillouin flow equilibrium model is assumed for the plasma sheath and a cylindrical analytic solution including a uniform external magnetic field is obtained. A free parameter appearing in all previous magnetic insulation theories is shown to be accurately determined by maximizing the transmitted power flow. The theory is supported by particle‐in‐cell simulations.

The diocotron instability in annular relativistic electron beams

A dispersion relation which includes finite axial wavevectors, k, is derived in the rest frame of the beam where the instability is assumed to be electrostatic. The dispersion relation is obtained in the lab frame by a Lorentz transformation. The instability is found to have a finite k bandwidth with the most unstable mode occurring at k≠0. The growth rate for the most unstable mode is found to be reduced by approximately 1/γ2 from the nonrelativistic result while the real frequency of this mode remains virtually unchanged. The effects of finite vz shear, which are left out of the analysis, are estimated and discussed.

Space‐time evolution of the beam‐plasma instability

Particle‐in‐cell simulations of the beam‐plasma instability confirm that the behavior of the interaction can be described as a wave packet that continually grows in both space and time. A consequence is that the energy deposition length of the instability becomes shorter in time, offering increased potential for this interaction to be used as an inertial fusion driver.

Comparison of axial and radial electron-beam-breakup transit-time oscillators

Comaprison of two configurations of a novel high-power microwave generator is presented in this article. Coupling the beam-breakup instability with the transit-time effect of the electron beam in the cavity, rapid energy exchange between the electrons and cavity modes can occur. The dominant cavity modes in the axial and radial configurations are different but their growth rates are comparable. We found that the radial configuration can have a beam impedance of less than 10 (Omega) and are therefore more suitable for low-voltage and high power operation. Good agreements have been obtained between linear theory and simulation for both configurations.

Linear And Nonlinear Theory Of The Proton Beam Transit-Time Oscillator (TTO)

The Proton Maser (PM) introduced by D. Ensely falls within the general class of devices known as transit-time oscillators (TM). A theory for both the small and large amplitude behavior of the intense-beam-driven TTO is summarized. It includes the effects of the beam self-fields and space-charge effects and it has been used to develop expressions which are compared with the results of particle simulations, which are presented in a companion paper.