Electrostatically pumped cross-field free electron laser

We report experimental and theoretical studies of a novel type of cross-field microwave generator. The cathode is smooth while the anode is rippled (Fig.1) to produce an azimuthally periodic modulation of the radial electric field (analogous to an electrostatic wiggler). The system is operated in the inverted geometry,1 where electrons are field-emitted from the inside of the smooth outer conductor. The depth of the grooves is adjusted so as to favor FEL interactions and minimize the usual slow-wave (magnetron) interactions.2 Approximate analytic single-particle orbits in the external crossed uniform electric and magnetic fields and the radial electric wiggler field of an idealized planar device are obtained. These orbits provide a basis for uniform and nonuniform kinetic and fluid equilibrium distribution functions. These equilibria in turn provide a basis for linear stability analyses of the device, which yield appropriate dispersion relations. Experimentally, we observe for our parameters (V ∼ 300 kV, I ∼ 1 kA, B‖ ∼ 5 kG, N = 0,8,12,24 wiggler periods) a high power microwave signal in the Ka-band. Removal of the wiggler (smooth-bore magnetron configuration) results in a reduction of the signal by ∼ 20 dB. These results suggest an emission mechanism analogous to that of the rippled-field magnetron.3