This research program investigates high power microwave generration utilizing a microsecond electron beam accelerator to study means of eliminating microwave pulse-shortening. The particular device under study is the coaxial gyrotron oscillator in the S-band frequency range. Experiments have concentrated on three types of gyrotron cavities: 1) coaxial, unslotted, 2) coaxial, slotted, and 3) noncoaxial, unslotted. The first major result is that the coaxial rod raises the limiting current in the e-beam diode, permitting reliable, higher current extraction into the microwave tube. The second major finding is that the slotted cavity gives the highest peak powers (approx90 MW) but very short pulselengths (approx10-20 ns). The unslotted coaxial gyrotron emits power levels of 20-40 MW with longer pulselengths (up to 40 ns). The noncoaxial gyrotron radiates lowerpeak power levels (approx20 MW). All of the gyrotron types exhibited signs of the pulse shortening mechanisms of mode hopping and mode competition as diagnosed by time-frequency-analysis (TFA). TFA also shows that lower power microwave oscilation is maintained over some 300 ns, but the power level may be modulated due to e-beam voltage fluctuatons.
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