Page 483 - Tunable Lasers Handbook

Tunable Free-Electron

Lasers

Stephen Vincent Benson
Accelel-mor- Division
Contiiziioiis Electroii Benin Accelerator Facili~
Neupol-t :Ve\ea.s, I'irginia

I. INTRODUCTION
1 .l Description of FEL Physics

The free-electron laser (FEL) uses a relativistic beam of electrons passing
through an undulating magnetic field (a wiggler) to produce stimulated emission
of electromagnetic radiation (Fig. 1). The quantum-mechanical description for
this device is based on stimulated emission of Bremsstrahlung [l]. The initial
and final states of the electron are continuum states so the emission wavelength
is not fixed by a transition between bound states. Although the initial description
by Madey was quantum mechanical, there was no dependence of the gain on
Planck's constant. This is a necessary but not sufficient condition for the exis-
tence of a classical theory for the laser. In fact, it was found that the device was
almost completely described by a classical theory [2].
The classical theory of FELs is an extension of the theory of the ubitron
developed by Phillips [5,4]. The ubitron is a nonrelativistic version of the FEL. It
was developed in a classified program between 1957 and 1964, It is a fast-wave
variant of the traveling-wave tube (TWT) amplifier and uses a transverse motion
of the electrons to couple a copropagating electromagnetic wave to the electron
beam. The classical formulation is therefore similar to the formulation €or a

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