Generation of high power microwaves in a large orbit coaxial gyrotron

The goals of this thesis were: (a) to assess whether coaxial structures could enhance the performance of large orbit gyrotrons by increasing the limiting current and improving mode selection, and (b) to study the physics of microwave pulse shortening utilizing a long (0.5–1.0 μs) pulse electron beam. The electron beam is generated by MELBA with parameters: V = –0.8 MV, I_diode = 1–10 kA, I_tube = 0.3–1.9 kA, pulselength = 0.5–1 μs. Power and frequency of microwave emission were examined for the coaxial non-slotted, coaxial slotted and the non-coaxial gyrotron. The unslotted, coaxial gyrotron gave power levels of 20–40 MW with pulselengths of 10–40 ns. This coaxial gyrotron operates in two main modes: TE₁₁₁ and TE₁₁₂ with frequencies of 2.34 and 2.5 GHz respectively. The gyrotron frequency is tunable between modes by the magnetic field. The slotted coaxial gyrotron showed the highest power of 60–90 MW and extremely short pulselengths of 10–15 ns. The slotted coaxial cavity operated mainly in the TE₁₁₂ mode. The non-coaxial gyrotron tended to produce lower microwave power with broader resonance in the magnetic field tuning conditions.; The current transport out of the diode was more stable for the coaxial gyrotron. The presence of the center rod near the anode aperture produces a change in the electric fields in the critical diode region which apparently raises the limiting current of the diode. The limiting current was calculated to be about 3 kA for no center rod. Apertured anode currents with the coaxial center rod reached 3.6 kA, which exceeds the upper aperture current limit observed in the non-coaxial gyrotron. The e-beam was never lost in the diode for the coaxial gyrotrons.; In order to generate a large-orbit electron beam, an annular e beam is injected in to a cusp magnetic field. The dynamics of a single electron are investigated using an original code written in C to numerical integrate the relativistic equations of motion. The results of these simulations are compared to experimental diagnostics of measured e-beam velocity ratio, α (V _perp/V_parallel). The e-beam dynamics are further investigated by comparing experimental, radiation-darkened glass plate diagnostic results to analytical theories previously derived and extended for use on magnetic cusps by this author.; These microwave gyrotrons exhibit pulse shortening of the microwave power output. This phenomenon is not completely understood and is mainly observed at high power levels of operation. Heterodyne microwave data are analyzed by time-frequency-analysis to investigate possible reasons for the pulse shortening of the microwave output, including: (1) e-beam voltage fluctuations, (2) mode hopping, (3) mode competition, and (4) plasma effects.