Laser light backscattering off an electron beam-plasma system

The case of stimulated backscattering Raman instability that is combined with a two-stream Buneman instability has been analyzed. This problem bears on fabricating a rapidly tunable free electron laser (FEL) for IR countermeasures in the
8-10 \mu
m wavelength range. The analysis is based on a weakly nonlinear theory and assumed a cold nonrelativistic electron beam and an equilibrium plasma to be described with a two-fluid model. The physical mechanism is based on the coherent bunching of the beam-plasma system due to the nonlinear ponderomotive force and the slow beam wave having negative energy. This paper analyzes a novel scattering configuration in which the IR (CO
2
) laser at 10.6 μm collides head-on against a nonrelativistic
e
-beam firing through a puff of laser produced plasma of
n_{p} \lsim 10^{16}
cm
^-3
, which has been previously produced by the CO
2
laser itself (e.g., producing the critical density 10
¹⁹
cm
^-3
and the reduced quiver velocity
\upsilon_{0s}/c = 0.1-0.01
at pulsed 10
¹⁴
-10
¹²
W/cm
²
) and becomes underdense subsequently. The upshifted radiation is tunable at an aribtrary Ω
1
determined by the Doppler frequency relationship
\Omega_{1} = \Omega_{0}(1 + \beta_{0})/ (1 - \beta_{0})
, where Ω
0
is the IR pump frequency and
\beta_{0} = V_{0}/c
(≈0.14 for the 8 μm wavelength) is the
e
-beam parameter of 5.11 keV and A/cm
²
such that
n_{b}/n_{p} \approx 10^{-6}
. The synergic backscattering growth rate is the sum of both instabilities, e.g.,
[(\sqrt{3}/2) (n_{b}/2n_{p})^{1/3} + (\upsilon_{0s}/ 4c) (\Omega_{0}/\omega_{0p})^{1/2}] \omega_{0p}
, and may be referred to as the plasmon avalanche instability. Assuming the wide-interval-pulsed (WIP)
e
-beam can be piecewisely flushed outside the interaction chamber after streaming through a laser produced plasma on each pass, then a useful free electron laser operation will result. Such a device is appropriately called the quasi-free electron laser (QFEL). The effect of axial magnetic field within the interaction chamber has also been included in the analysis. The advantages of QFEL result from 1) efficient use of the nonrelativistic
e
-beam generation and two-stream bunching, 2) use of proven CO2 laser technology, and 3) its rapid tunability.