Plasma wave wigglers for free-electron lasers

We explore the possibility of using a relativistic plasma density wave as a wiggler for producing free-electron laser radiation. Such a wiggler is a purely electric wiggler with frequency ω
p
(plasma frequency) and wavenumber k
p
. If an electron beam is injected parallel to the plasma wave wavefront, it is wiggled transversely with an apparent wiggler wavelength
\lambda_{w} = 2\pi c/\omega_{p}
. Using plasma densities in the 10
¹⁷
(cm
^-3
) range, λ
w
of order 100 μm may be obtained, thereby permitting generation of short wavelength radiation with modest energy beams. The effective wiggler strength
a_{w} = eA/mc^{2} \sim 0.5
can be extremely large. We discuss the excitation methods for such wigglers and examine the constraints imposed by the plasma medium on FEL gain in this scheme.