H. P. Freund

Three‐dimensional simulation of the Raman free‐electron laser

The nonlinear evolution of the free‐electron laser amplifier is investigated numerically in the collective Raman regime for a configuration in which a relativistic electron beam propagates through a loss‐free cylindrical waveguide in the presence of a helical wiggler and an axial guide magnetic field. A set of coupled nonlinear differential equations is derived that governs the evolution of the TE waveguide modes, the beam space‐charge mode, and the trajectories of an ensemble of electrons. Comparison with experiment shows good agreement for cases in which the intersection between the vacuum waveguide mode and the beam resonance line are near ‘‘grazing’’ (i.e., when the intersections are sufficiently close together to result in one broad gain bandwidth). For interactions in which two distinct gain bands occur, the numerical procedure tends to underestimate the beam–plasma frequency and results in a 15%–20% discrepancy with experiment.

Study of gain, bandwidth, and tunability of a millimeter‐wave free‐electron laser operating in the collective regime

Frequency-resolved measurements of the emission of a collective free-electron laser operating at millimeter wavelengths have shown emission spectra that agree with theoretical predictions for the collective free-electron laser instability. Broad tunability, moderate emission linewidths, and high single frequency gain have been observed. In addition, adjusting the axial field in the end region of the interaction has been found in some cases to cause a large increase in measured power and efficiency.

Radiation Growth in a Millimeter-Wave Free-Electron Laser Operating in the Collective Regime.

Frequency-resolved measurements of radiation growth have been performed on a millimeter-wave free-electron laser using an intense relativistic electron beam. These measurements have shown large radiation growth rates (approx. 2 dB/cm) over a broad instantaneous bandwidth (66-90 GHz), in good agreement with predictions of theory for operation in the collective regime. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of greater than or equal to 75 MW centered at 75 GHz with 6% experimental efficiency.

Oscillating two-stream and parametric decay instabilities in a weakly magnetized plasma

The effects of a weak ambient magnetic field on the oscillating two‐stream and parametric decay instabilities, with particular emphasis on the dependence of the angular variation of the instability thresholds and growth rates on the magnetic field, are considered. A dispersion relation is derived in the limit in which ωe≳≳Ωe (where ωe and Ωe are the electron plasma and cyclotron frequencies, respectively), and is solved for dipole and monochromatic pump spectra. The analysis shows that the presence of a magnetic field can substantially enhance thresholds and reduce growth rates for waves propagating at an oblique angle with respect to the ambient magnetic field and k2λ2e∼Ω2e/ω2e (where k is the wave vector of the perturbation and λe is the Debye length). Thus, the magnetic field has a stabilizing influence on the off‐parallel propagating modes.

Strongly turbulent stabilization of electron beam‐plasma interactions

The stabilization of electron beam interactions due to strongly turbulent nonlinearities is studied analytically and numerically for a wide range of plasma parameters. A fluid mode coupling code is described in which the effects of electron and ion Landau damping and linear growth due to the energetic electron beam are included in a phenomenological manner. Stabilization of the instability is found to occur when the amplitudes of the unstable modes exceed the threshold of the oscillating two‐stream instability. The coordinate space structure of the turbulent spectrum which results clearly shows that soliton‐like structures are formed by this process. Phenomenological models of both the initial stabilization and the asymptotic states are developed. Scaling laws between the beam‐plasma growth rate and the fluctuations in the fields and plasma density are found in both cases, and shown to be in good agreement with the results of the simulation.

Spontaneous emission of radiation from localized Langmuir perturbation

The radiation at the first and second harmonics of the electron plasma frequency (ωe) from localized Langmuir oscillations is computed for a field‐free plasma. The localized perturbations are assumed to have cylindrical symmetry about the direction of propagation of the localized perturbation. It is shown that when the wavelengths of the excited modes are much greater than the scale lengths of the Langmuir perturbation, a quadrupole radiation pattern is recovered for emission at 2ωe. An application to type III solar radio bursts is discussed briefly.

Radiation from a localized Langmuir oscillation in a uniformly magnetized plasma

The radiation at the first and second harmonics of the electron plasma frequency from a localized Langmuir perturbation is computed for the case of a uniformly magnetized plasma. It is assumed that the localized perturbations in both the electrostatic field and plasma density have cylindrical symmetry about the direction of the ambient magnetic field. The analysis is initially performed for such an arbitrary localized perturbation, and then applied to treat the case of a one-dimensional Langmuir soliton propagating in the direction of the magnetic field. An extensive numerical study of the angular dependence of the radiation spectrum as a function of the ratio of the electron plasma and cyclotron frequencies is described.

Wave excitation by inhomogeneous suprathermal electron beams

Wave excitation by an inhomogeneous suprathermal electron beam in a homogeneous magnetized plasma is studied. Not only is the beam density nonuniform, but the beam electrons possess a sheared bulk velocity. The general dispersion equation encompassing both electrostatic and electromagnetic effects is derived. Particular attention is given to the whistler mode. It is established that the density-gradient and velocity-shear effects are important for waves with frequencies close to the lower-hybrid resonance frequency.

Studies Of Radiation Growth And Emission Spectrum Of A Millimeter-Wave Free-Electron Laser Operating In The Collective Regime

Frequency-resolved measurements of emission from a superradiant millimeter-wave free-electron laser using an intense relativistic electron beam have demonstrated high power, good efficiency, high gain, and broad gain bandwidth, and broad tunability, all in good agreement with theoretical predictions for operation in the collective regime.

Reply to comments of Bekefi and Fajans

Both the free‐electron laser (FEL) and cyclotron‐maser (CM) instabilities can play a role in wave generation laser experiments employing both an axial guide magnetic field and a transverse wiggler magnetic field. The experimental distinction between these two mechanisms can only be made by comparing measured radiation characteristics with the predictions of either model. For a recent intense beam experiment [Phys. Fluids 26, 2683 (1983)], most of the data are in good agreement with the FEL mechanism, as previously concluded, rather than with the CM instability, and the data taken far from gyroresonance are of unambiguous FEL origin.