Pulse shapes for absolute and convective free-electron-laser instabilities

Abstract

This paper contains an analysis of pulse shapes produced by a delta-function disturbance of the equilibrium state of a relativistic electron beam propagating through a constantamplitude helical magnetic wiggler field. Pulse shapes are determined by using the relativistic pinch-point techniques developed by Bers, Ram, and Francis. Two pulses are produced corresponding to a convective upshifted pulse (representing the production of the high-frequency radiation desired in a free electron laser) and a downshifted pulse. The downshifted instability may be convective or absolute, depending upon the beam density and momentum spread. Parameter regimes in which the downshifted instability is convective are investigated. It is found that momentum spreads sufficiently large to suppress the absolute instability reduce the growth rate of the upshifted pulse to negligible values. Pulse shapes computed by using the Raman and Compton approximations are compared with exact pulse shapes. It is found that the Raman approximation should be applied to the downshifted regime for most systems of practical interest.

Pulse shapes for absolute and convective cyclotron-resonance-maser instabilities

“…; Danly et al 1988; Danly et al 1989). This paper also compliments an earlier paper on pulse shapes in the free electron laser (FEL) (Davies, Davidson & Johnston 1988).…”

“…In earlier work (Davies, Davidson & Johnston 1988), pulse shapes were analized for the free electron laser (FEL). A qualitative difference between FEL and CRM pulses is that (for typical FEL parameters) downshifted and upshifted FEL pulses are separated.…”

“…1981;Fliflet 1986;. Devices employing this instability are generally referred to as cyclotron resonance masers (CRMs).…”

“…The nonrelativistic, linear theory of the shapes of such pulses is given by Briggs (1964) and Bers (1983). The relativistic theory appears in Bers, Ram & Francis (1984), and a detailed summary of the latter work is given in Davies et al (1988). Consequently, we give only a brief, qualitative summary of the theory.…”