Optical emission spectroscopy and effects of plasma in high power microwave pulse shortening experiments

Mechanisms of microwave pulse shortening are investigated with a multi-megawatt, large-orbit, coaxial gyrotron. Experiments are primarily concerned with plasma production inside the microwave cavity and e-beam. collector. This gyrotron operates in the S-band at 10 to 40 MW and is driven by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V = −800 kV, I _cathode = 6 kA, I_tube = 0.8 kA, and pulselengths of 0.5–1.5 μs. Plasma H-alpha line radiation is measured inside the microwave cavity and e-beam collector via fiber optics/monochromator and correlated with output microwave power. The temporal correlation between reduced output microwave power and growing H-alpha optical emission is measured.; Experimental results show a roughly linear relationship between premature microwave power cutoff and growth of H-alpha optical emission. Heterodyne mixer data show that the e-beam/microwave cavity are still oscillating and producing microwaves beyond their apparent cutoff, yet only low levels of microwave power are radiated from the experiment. These observations suggests that the plasma is reducing the output microwave power as the plasma reaches critical density (∼8 × 1010 cm−3).; RF plasma cleaning is examined on the coaxial cavity and e-beam collector as a technique for mitigating microwave pulse shortening of this gyrotron device. Improvements in the microwave energy output of this device ranged from 15% to 245%. The mechanism for this improvement is believed to be sputtering of excess water vapor from the cavity/waveguide structure by ions produced in the nitrogen RF plasma discharge and subsequent removal of the contaminant by vacuum pumps. Therefore, the total quantity of H₂O molecules available to contribute to the plasma is reduced. This is supported by the reduction of H-alpha optical emission measured during some RF plasma cleaning cases examined in this experiment.; Gas backfilling of the experiment with sulfur hexaflouride (SF₆ ) is also investigated. Preliminary data show improvement of the microwave energy output of this device. However, this technique requires further investigation as a means of mitigating microwave pulse shortening effects.