Experimental Studies of Microwave Discharge Induced by Gyrotron

This chapter comprehensively reviews experimental investigations on gyrotron-induced microwave discharges and associated theories, comparing over-critical and under-critical conditions. Simple calculation models of the ionization threshold that separates over-critical and under-critical conditions are explained, and specific calculation examples are given. Filamentary structures of the induced plasma in over-critical and under-critical conditions are discussed with high-speed camera images. The study underscores the need for experiments on the ionization front’s propagation velocity in various incident beam frequencies for assessing microwave rocket’s thrust performance. Investigations also reveal limitations imposed by the transition from nonequilibrium to equilibrium discharge in the focusing optics of the incident beams and abnormal ignitions due to residual plasma.