STUDY OF CO-AXIAL FREE ELECTRON MASER BASED ON TWO-DIMENSIONAL DISTRIBUTED FEEDBACK

I.V. Konoplev ^a), P. McGrane ^a)_, W. He ^a), A.W. Cross ^a), A. D. R. Phelps ^a), CG Whyte ^a)_, K Ronald ^a), C.W. Robertson ^a), M. Thumm^b), N.S. Ginzburg^c)

N. Yu. Peskov ^c) and A.S. Sergeev ^b)

a) Department of Physics, University of Strathclyde, Glasgow, G40NG, UK

b) Forschungszentrum Karlsruhe, IHM and

Universitaet Karlsruhe, IHE, Karlsruhe, Germany, D-76021

c) Institute of Applied Physics, RAS, Nizhny Novgorod, Russia, 603950

First measurements of microwave radiation from a co-axial Free-Electron Maser (FEM) based on two-dimensional (2D) distributed feedback are presented. To drive the FEM a high current accelerator (HCA) based on a magnetically insulated explosive emission carbon cathode was used. A driving electron beam voltage (V_hsa) of 475 kV and pulse duration of ~200ns was applied to the HCA resulting in the generation of a thin (0.2cm) annular electron beam of current 500A and mean diameter of 7.0 cm. The electron beam was guided through a co-axial transmission line of length ~2m with the diameters of inner and outer conductors of 6 and 8 cm respectively. A guide solenoid of length 2.55m and diameter 30cm surrounded the co-axial electron gun, undulator and co-axial transmission line interaction region. A two-mirror cavity of length 81 cm (input 2D Bragg mirror 10.4 cm, output 2D Bragg mirror 5.6 cm both defined by ‘chessboard’ corrugations on the inner conductor) was located inside the transmission line along the uniform part of the undulator. The frequency of the output radiation of the FEM was measured to be 37.3GHz using a heterodyne frequency diagnostic. The mode pattern measured in the ‘hot’ experiments (when an electron beam was present) was compared with the radiation pattern measured in cold microwave experiments when the horn was excited by a TEM wave of frequency 37.3GHz. The output power of the FEM was measured by integrating the power densities over space and was calculated to be (15±1.5) MW (efficiency of 6%). Additional evidence of the high peak power was obtained from the observation of the illumination of a neon bulb panel located at a distance of 20cm from the output window. To reduce the amplitude of the RF field of the TEM wave on the surface of inner conductor a new 2D Bragg reflector has been constructed using copper electroforming technology, which has smooth sinusoidal corrugations on the inner surface of the outer conductor. Cold microwave measurements of the copper 2D Bragg reflector will be presented and results compared with theory and simulation.