Generation of short pulse radiation in gyrotrons

It is demonstrated that high peak power, extremely short-pulse radiation can be obtained in gyrotron oscillators by using the method of synchronous mode locking. These sources of coherent microwave radiation can operate in a highly overmoded regime. Both closed and quasi-optical gyrotron oscillators are analyzed extensively using particle-in-cell (PIC) simulation and fully nonlinear time-dependent theory. It is shown that a closed-cavity gyrotron oscillator composed of a radially tapered section and a straight section of waveguide can obtain an equidistant frequency spectrum. Details of such a cavity design are analyzed using the WKB theory and the moving phase front (MPF) method. Excellent agreement between experimental cold-test measurements and MAGIC PIC simulation results are found. A train of narrow pulses (FWHM {dollar}sim{dollar} 1 ns) at a 280 MHz repetition rate is observed in simulations using a 5% sinusoidal current density modulation. Though the gyrotron does not appear to be mode locked, uniform reproducible pulse trains of 30-50 pulses are obtained. A nonlinear time-dependent calculation is carried out to study generation of short pulse radiation in a quasi-optical gyrotron oscillator. The calculation is based on the rate equation model of modal growth and saturation. A vectorized time-dependent numerical code is written based on the nonlinear slow-time scale multi-mode theory. The code is extensively benchmarked against known analytical results. It is shown in simulation that radiation pulses of width 400 ps can be generated using one single modulated beam in a quasi-optical gyrotron oscillator. The strong pulsed current modulation is provided at 300 MHz, the nominal frequency spacing between two odd numbered modes in such a cavity. Eight odd numbered modes are locked and generate extremely short radiation pulses. The investigation is extended to a cavity where the mirror axis is tilted relative to the axis of the magnetic field by a small angle. Fifteen modes are locked and generate short radiation pulses. Single pulse propagation in the cavity is achieved.