Experimental study of a high power free electron maser based on a

co-axial two-dimensional Bragg cavity

C. G. Whyte ^a)_, K Ronald ^a), A. D. R. Phelps ^a) , I. V. Konoplev ^a), P. McGrane ^a)_, A. W. Cross ^a), W. He ^a),

C.W. Robertson ^a), N. S. Ginzburg ^b), N. Yu. Peskov ^b), A. S. Sergeev ^b) and M Thumm ^c)

a) Department of Physics, University of Strathclyde, Glasgow, G4 0NG, Scotland, UK

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

c) Universität Karlsruhe, Institut für Höchstfrequenztechnik und Elektronik, Karlsruhe, D-7612, Germany

and Forschungszentrum Karlsruhe, IHM, Karlsruhe, D-7602, Germany

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The use of two-dimensional (2D) Bragg structures has been suggested for application in microwave electronics to synchronise radiation from different parts of an oversized electron beam and to improve mode selection inside the interaction space [1][2]. The results of studies of RF field evolution inside 2D Bragg structures of co-axial geometry and electron beam formation experiments will be presented. The properties of the structures and their use in oversized two-mirror cavities will be discussed.

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FIGURE 1 Photograph of 2D Bragg co-axial FEM

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The 2D Bragg structures were used to form the cavity of a high power co-axial Free Electron Maser (Figure 1). The 2D Bragg FEM has been constructed from a number of components. This includes (a) pulsed wiggler and capacitor bank power supply, (b) output window, (c) support table containing ten 250mF, 20kV capacitors used to generate 0.5T from a pulsed solenoid, (d) X-ray shielded enclosure, (e) large diameter (30cm) pulsed solenoid of length 2.5m, (f) co-axial cavity and electron beam diagnostics, (g) diode tank containing electron gun and transmission line output spark gap and (h) low impedance (10W) 1MV, de-ionised water-filled transmission line resonantly charged using a 1.5MV Marx bank generator.

The high voltage accelerator was connected to the electron gun and triggered to coincide with the firing of the pulsed solenoid. A high current (1.6kA), high voltage (up to 500kV) large diameter (mean diameter is 7 cm) annular electron beam was generated. A witness plate beam diagnostic was used to obtain the transverse position of the electron beam, with a beam current of 1.6kA measured using an in-line Rogowski coil beam diagnostic. The 2D Bragg cavity has been designed, constructed and cold tested. An electron beam of power 750MW was measured. An FEM output efficiency of up to 15% has been predicted. Progress made towards the generation of microwave radiation at a frequency of 37.5GHz will be presented.

[1] Ginzburg N. S., Sergeev A. S., Peskov N. Yu., Robb G. R. M., Phelps A. D. R., IEEE Trans. Plasma Sci., 24, p.770, (1996).

[2] Cross A. W., Konoplev I. V., Ronald K., Phelps A. D. R., He W., Whyte C. G., Ginzburg N. S., Peskov N. Yu.,. Sergeev A. S, Appl. Phys. Lett., 80, p.1517, (2002).