Microwave emission of large and small orbit rectangular gyrotron devices.

Abstract

Experiments were conducted on rectangular cross section (RCS) gyrotron devices. Rectangular interaction cavities with both small orbit and large orbit axis encircling electron beams were used in gyrotron mechanisms to generate high power microwaves. The Michigan Electron Long Beam Accelerator (MELBA) produced an annular electron beam with e-beam parameters: V = $-$0.7 to $-$1.0 MV, I$\sb{diode}$ = 1-10 kA, I$\sb{tube}$ = 0.1-3 kA, e-beam pulse length = 0.4-1.0 $\mu$S. The small orbit e-beam was spun up into an axis encircling e-beam by passing it through a magnetic cusp prior to entering the RCS interaction cavity. The issues under investigation included polarization control of the microwave emission as a function of the interaction cavity magnetic field, microwave power as a function of pulse length, and mode competition. Along with microwave power measurements, frequency analysis was conducted with the use of a heterodyne mixer. Measurements of the optical emission of plasma in the RCS interaction cavity beam dump have also been completed. Experimental results in the small orbit gyrotron demonstrated powers of up to 23 MW in the horizontal polarization with little power measured in the vertical fundamental mode, and hence, polarization control was not obtained. Pulse shortening was observed in the small orbit gyrotron and power efficiency was typically less than 1%. The large orbit gyrotron, operating at a lower current, produced much more successful results. Powers as high as 14 MW were measured in the fundamental TE$\sb{101}$ mode at a frequency of 2.18 GHz, and 11 MW in the horizontally polarized TE$\sb{011}$ mode at 2.85 GHz. The results showed a high degree of polarization(P(TE$\sb{101})$/P(TE$\sb{011})$ = 1000 or as low as 1/30) as a function of cavity B-fields. The megawatt microwave output shifts from the fundamental TE$\sb{101}$ mode to the TE$\sb{011}$ mode as the B-field is raised from 1.5 to 1.9 kG. Efficiencies in the large orbit gyrotron were found to be as high as 8%. MAGIC code simulations predicted the ability to shift the linearly polarized output from the fundamental TE$\sb{10}$ mode to the orthogonally polarized TE$\sb{01}$ mode. The highest power microwave pulses were on the order of 100 ns and demonstrated pulse shortening. Optical emission spectroscopy demonstrated the formation of hydrogen plasma in the cavity or output waveguide; these results imply that microwave pulse shortening could be due to this plasma produced by the e-beam dumping against the output waveguide walls.