<title>Nonlinear study of mode locking in a quasi-optical gyrotron</title>

Abstract

Nonlinear study of mode locking in a quasi-optical gyrotronHao Wu and Alan H. McCurdyUniversity of Southern California, Department ofElectrical Engineering/ ElectrophysicsLos Angeles, California 90089-0271ABSTRACTNonlinear, time-dependent multimode calculafions have been carried out to study mode locking in quasi-opticalgyrotron oscillators. The calculations are based on the rate equation model of modal growth and saturation. The slow-timeformalism is used for particle motion and both the time varying electric and magnetic fields are included. It is found thatradiation pulses of width 400 ps can be generated in nonlinear regime. The gyroiron features an open resonator of length 100cm formed by a pair of spherical mirrors and a single pencil electron beam guided by external magnetic field in transversedirection to the axis of symmetry of the cavity. The strong cuirent modulation is provided at frequency of 300 MHz, thenominal modal spacing between two odd modes in such a cavity. Eight odd modes are found to be locked to generateextremely short radiation pulses. Application for short pulse radiation in miffimeter and submillimeter wavelength rangeinclude radar, plasma diagnosis, time domain metrology and communication systems. Parametric dependencies investigatedinclude static magnetic field, beam current and beam voltage, as well as the drive signal amplitudes and frequencies. The workis geared towards support of a proof of principle experiment to generate high power radiation pulses of short duration viasynchronous mode locking.Keywords: quasi-optical gyrotron, mode locking, short pulse, millimeter wave radiation1. INTRODUCTIONOperation of high power microwave oscillators at high frequencies typically involves the competition among manyelectromagnetic modes. This problem is seen in devices such as free electron lasers, gyrolrons, plasma based devices as wellas the conventional slow wave devices such as traveling wave tubes. The physical mechanism for this mode competition liesin the nonlinear character of the electron beam from which the modes derives their energy. Modes compete with each otherfor energy. In solving the mode competition problem, much research has focused on suppression of unwanted modes forapplications requiring extremely narrow spectral widths. However for some applications sharpness in the frequency spectrumis not important, it may be desired that the signal be short-pulse or frequency modulated. For these applications it may bepossible to utilize the microwave oscillator in the overmoded state. In general, the output of the overmoded oscillator is most