Cold Relativistic Electron Beam Research Articles

Free-Electron Laser with a Strong Axial Magnetic Field

A small-signal theory is given for gain in a free electron laser comprising a cold relativistic electron beam in a helical periodic transverse, and a strong uniform axial, magnetic field. Exact finite-amplitude steady-state helical orbits are included. If perturbed, these orbits oscillate about equilibrium, so that substantial gain enhancement can occur if the electromagnetic perturbations resonate with these oscillations. This gain enhancement need not be at the cost of frequency up-shift.

Two-Stream Instability of Plasma Waves Trapped in a Relativistic Electron Beam of Finite Perpendicular Extent

Effect of finite perpendicular extent of a cold relativistic electron beam on the two-stream instability excited in a warm plasma is investigated. It is shown that if the beam density profile is sufficiently slowly-varying the fastest growing modes are deeply trapped in the beam region and have growth rates comparable to the maximum growth rate of the usual two-stream instability by a uniform beam. The trapping is due to the frequency downshift of the space-charge wave below the Bohm-Gross frequency in the beam region. Both the dispersion relation analysis based on the WKB approximation and the eigenvalue analysis are presented.

Parametric excitation of ion density fluctuations in the relativistic beam-plasma interaction

When a weak and cold relativistic electron beam interacts with a dense plasma, it generates a nearly monochromatic wave of high intensity which triggers the growth of parametric instabilities. The present work investigates this process by using a hybrid technique in which the fully relativistic motion of the beam particles is treated numerically, while the behavior of the background plasma and the parametrically excited waves is treated analytically through appropriate linear dielectric functions. It is found that the self-consistent excitation of large ion density fluctuations leads to the decoupling of the beam from the background plasma, hence limiting the amount of energy that can be extracted from the beam. By adjusting the various parameters of the problem it is found that up to 23% of the initial beam energy can be transferred to the electrostatic waves. The large ion fluctuations responsible for decoupling the beam correspond to purely growing modes which exhibit secular growth in time.

On the injection of a cold relativistic electron beam into a cold plasma

In a one-dimensional model the initial phase of the injection of a relativistic electron beam into a plasma is studied, both analytically and computationally. We find that in a time of a few beam plasma periods the beam obtains a “temperature” which is much higher than expected from the analytic treatment.

Stimulated collective scattering from a magnetized relativistic electron beam

We analyze stimulated scattering of a right-circularly-polarized electromagnetic pump wave, of frequency ${\ensuremath{\omega}}_{0}$, from a cold relativistic magnetized electron beam. The pump wave is chosen to satisfy the dispersion relation associated with the magnetized beam. The scattered waves consist of collective plasma oscillations as well as right- and left-polarized electromagnetic waves, traveling parallel and antiparallel to the beam. The frequency of the forward-scattered electromagnetic wave is Doppler shifted up twice and is of order ${{\ensuremath{\gamma}}_{0z}}^{2}{\ensuremath{\omega}}_{0}$, where ${\ensuremath{\gamma}}_{0z}={(1\ensuremath{-}\frac{{v}_{0}^{2}}{{c}^{2}})}^{\ensuremath{-}\frac{1}{2}}$ and ${v}_{0}$ is the beam's speed. Enhanced stimulated scattering results if the frequency of the pump, in the beam frame, is approximately equal to the electron's cyclotron frequency. Enhanced growth rates for the scattering process are obtained for the scattered radiation. This theory describes a mechanism which is a plausible explanation of a recent experiment in which \ensuremath{\sim} 100 kW of submillimeter radiation was observed. In view of the large up-conversion in frequency and enhanced growth rates of the scattered radiation, a new high-frequency generative device based on this mechanism seems possible.

Linear theory of a cold relativistic beam propagating along an external magnetic field

The linear theory of a cold relativistic electron beam propagating parallel to an external magnetic field and through a cold, homogeneous plasma is investigated. The electromagnetic dispersion relation is solved numerically and compared with analytical predictions based on the electrostatic approximation. It is found that electromagnetic effects are important for determining the entire unstable spectrum. However, except for the strong magnetic field regime, the maximum growth rates and corresponding frequencies are in agreement with those predicted by the electrostatic approximation. In the strong magnetic field regime the two−stream spectrum is found to be much narrower in angle than predicted by the electrostatic approximation. In the moderate and strong magnetic field regime the growth rate of waves propagating at large angles with respect to the beam are independent of beam energy.