Long-pulse, high-power, large-orbit, coaxial gyrotron oscillator experiments

Long-pulse, large-orbit, coaxial gyrotrons are currently under investigation. The electron beam is generated by the Michigan Electron Long Beam Accelerator (MELBA) with parameters: V=-0.8 MV, I/sub anode//spl les//spl sim/4 kA, I/sub tube/=0.2-2 kA, and pulse length=0.5-1 /spl mu/s. Large-orbit, axis-encircling electron beams are generated by a magnetic cusp. Experimental gyrotron performance with coaxial cavities (unslotted and slotted) is compared to a noncoaxial cavity. The coaxial gyrotron demonstrated superior current transport and microwave production over the noncoaxial gyrotron. The coaxial rod apparently raises the limiting electron beam current in the diode, allowing higher currents to be extracted. The unslotted, coaxial gyrotron showed microwave power levels of 20-40 MW with pulse lengths of 10-40 ns, This coaxial gyrotron operated in two main modes: TE/sub 111/ and TE/sub 112/ with frequencies of 2.34 and 2.5 GHz, respectively. The gyrotron frequency is tunable between the respective modes by changing the magnetic field. The slotted, coaxial gyrotron showed the highest power of 60-90 MW and extremely short pulse lengths of 10-15 ns. For all three gyrotrons, the microwave pulse-shortening mechanisms of mode hopping and mode competition are definitively identified by time-frequency analysis of heterodyned microwave data.