Linewidth limits in free-electron lasers caused by sidebands

Journal Article · Thu Sep 01 00:00:00 EDT 1994 · Physics of Plasmas; (United States)

DOI:https://doi.org/10.1063/1.870499· OSTI ID:6655139

Riyopoulos, S ^[1]

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1. Science Application International Corporation, McLean, Virginia 22102 (United States)

Sideband excitation near the carrier determines the minimum spectral width for steady-state free-electron laser oscillators fed by continuous electron beams. A sideband separated by [delta][omega] from the carrier resonates with harmonics of the upshifted bounce frequency for trapped particles, [delta][omega]=[ital n]2[gamma][sup 2][sub [ital z]][Omega]. The analysis focuses on sidebands excited in the immediate vicinity of the carrier [delta][omega][r arrow]0, in resonance with particles trapped near the separatrix, [Omega][r arrow]0. For electrons distributed uniformly around their orbits, the growth tends to zero as [delta][omega], [Omega][r arrow]0, despite the infinite number of contributing harmonics. However, the distributions produced by injected electron beams are nonuniform around the trapped orbits, yielding finite growth rates [Gamma]. Stability depends on the nonlinear shift [delta][ital k][sub 0]([ital a][sub 0],[omega][sub 0]) of the carrier wave number from the empty cavity value, where the carrier amplitude [ital a][sub 0] and frequency [omega][sub 0] parametrize the free-electron laser (FEL) operation point. The curve [delta][ital k][sub 0]([ital a][sub 0],[omega][sub 0])=0 divides the FEL parameter space into areas stable and unstable to sidebands. If [delta][ital k][sub 0] is negative, near-the-carrier sidebands are stable, and the linewidth is limited only by quantum effects. If [delta][ital k][sub 0] is positive an unstable frequency band can emerge around the carrier, of width [Delta][omega][congruent]8[gamma][sup 2][sub [ital z]][ital v][sub 0] [delta][ital k][sub 0], and maximum growth rate [Gamma][sub max]/[ital k][sub 0][congruent](1/6)[2[pi][ital N]([delta][ital k][sub 0]/[ital k][sub 0])][sup 2], where [ital N] is the number of wiggler periods. The minimum linewidth is [Delta][omega] if the frequency separation between cavity modes is less than [Delta][omega].

• OSTI ID: 6655139
• Journal Information: Physics of Plasmas; (United States), Vol. 1:9; ISSN 1070-664X
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
FREE ELECTRON LASERS
LINE WIDTHS
ELECTRON BEAMS
HARMONICS
INSTABILITY GROWTH RATES
PHOTON-ELECTRON INTERACTIONS
TRAPPED ELECTRONS
TRAPPED-PARTICLE INSTABILITY
BEAMS
ELECTRONS
ELEMENTARY PARTICLES
FERMIONS
INSTABILITY
INTERACTIONS
LASERS
LEPTON BEAMS
LEPTONS
OSCILLATIONS
PARTICLE BEAMS
PARTICLE INTERACTIONS
PHOTON-LEPTON INTERACTIONS
PLASMA INSTABILITY
PLASMA MACROINSTABILITIES
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700340 - Plasma Waves
Oscillations
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