High-power, submillimeter free electron lasers based on intense relativistic electron beams

The physical process in free electron lasers at submillimeter wavelengths may be viewed as stimulated Raman scattering in which a powerful pump-wave at relatively long wavelength λo is scattered into a plasma oscillation in a relativistic electron beam plus an exponentially-growing, backscattered e.m. wave at submillimeter wavelength λs ≈ λo/2γ². Strong submillimeter radiation which was observed to emanate from an intense relativistic electron beam in the presence of a powerful microwave signal ^1,2 was attributed to this process. During the past year, a joint experiment³ between Columbia University and NRL has used a more ‘conventional’ free electron laser configuration (i.e. a quasi-optical laser cavity, and a quasi-pump-wave consisting of a spatially periodic magnetostatic field). This experiment produced 1 megawatt at λs = 400 µm, and evidenced significantly improved coherence with linewidth of the submillimeter emission narrowed to Δλ/λ = 2%. Efficiency of subillimeter-wave generation was only ∼10⁻⁴, whereas theoretical calculations predict saturated efficiency 1–10%. We believe this apparent discrepancy occurred in part cause the limited gain of the process together with the limited e-beam pulse wavelength did not allow wave-growth to approach saturation. We have now designed a new experiment with a much stronger pump wave (Bo = 2.5 kG) and a higher quality electron beam (ΔE/Eo ≲ 3%) which we expect will be a test of the nonlinear theory. Producing a 2 MeV, 2 kA electron beam we expect to produce submillimeter radiation with efficiency for an output power of 200 megawatts.