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

Long-pulse, large-orbit, coaxial gyrotrons are currently under investigatio n. The electron beam is generated by the Michigan Electron Long Beam Accele rator (MELBA) with parameters: V = -0.8 MV, I-anode less than or equal to s imilar to4 kA, I-tube = 0.2-2 kA, and pulse length = 0.5-1 mus, Large-orbit , axis-encircling electron beams are generated by a magnetic cusp. Experime ntal 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, T he coaxial rod apparently raises the limiting electron beam current in the diode, allowing higher currents to be extracted. The unslotted, coaxial gyr otron showed microwave. power levels of 20-40 MW with pulse lengths of 10-4 0 ns, This coaxial gyrotron operated in two main modes: TE111 and TE112 wit h frequencies of 2.34 and 2.5 GHz, respectively. The gyrotron frequency is tunable between the respective modes by changing the magnetic held, The slo tted, coaxial gyrotron showed the highest power of 60-90 MW and extremely s hort pulse lengths of 10-15 ns, For all three gyrotrons, the microwave puls e-shortening mechanisms of mode hopping and mode competition are definitive ly identified by time-frequency analysis of heterodyned microwave data.