Development of frequency-tunable FEM-oscillator with Bragg resonator for testing high-gradient accelerating structures

This paper is devoted to progress in the JINR-IAP FEM experiment based on a 1 MeV/200 A/250 ns LINAC. This FEM generates 30 GHz pulses of 150 - 200 ns pulse duration and 20 - 30 MW power with an efficiency on the level of 20 $25%. The high efficiency together with high stability of single-mode single-frequency operation is provided by the use of a reversed guide magnetic field and a novel Bragg resonator with a step of phase of corrugation. At the present stage of the experiments precise tuning of the oscillation frequency was performed by mechanical variation of the value of the phase shift in the Bragg resonator. As a result, the FEM was tuned in the frequency range of 6% with a precision of about 0.1%. Both the spectrum width and the dependence of the frequency tuning value on the corrugation phase shift are in good agreement with the results of simulations. The project to use this high power FEM source for studying surface heating effects in 30 GHz accelerating structures is discussed. A special cavity has been designed to enhance magnetic fields at the cavity surface. The cavity will be studied with regard to 200 - 500°C surface pulsed heating stress of the cavity as a consequence of 10⁶ RF-pulses, which are planned to be produced at a repetition rate of 1 Hz.