Recent results from the naval research laboratory experimental FEL program

Abstract

The Naval Research Laboratory has been very active in the development of high-power, millimeter-centimeter wavelength FELs. These devices have been operated as amplifiers, superradiant amplifiers, and as an oscillator. The amplifier experiments demonstrated power gains of 50 dB with > 3% efficiency (with no enhancement), and superradiant experiments produced 75 MW pulses at λ = 4 mm with 6% efficiency. These experiments were performed using a 1 2 MeV pulse line accelerator having a 50 ns pulse duration. Most recently, an induction linac having a 2 μs pulse duration has been employed for oscillator experiments. This work has demonstrated steady-state operation for 1 μs at power levels up to 10 MW. The initial power gain per pass is as large as 10 4, so that turn-on occurs rapidly. This device has operated with various mirror configurations, including a Bragg resonator. The experimental results agree well with recent theoretical work, which predicts efficiencies > 20% in a tapered wiggler oscillator. These results are also directly applicable to the operation of a two-stage FEL experiment.