Experimental mode priming of an overmoded gyrotron oscillator

Axial mode control in a gyrotron oscillator is accomplished by injection of an external signal into the oscillator. The mechanism by which the selection technique works is that of priming, where the injected signal is required only during the early stages of the oscillation buildup. Like phase priming, mode control can be obtained with an external signal power level nearly 40 dB below the oscillator power (with an external frequency close to that of the operating mode). It is found that the external signal can select a mode when the free gyrotron is in a region of 1) pulse-to-pulse mode skipping 2) stable single-mode operation or 3) bistable operation. Mode control is used as a diagnostic to study the coupling of an electromagnetic wave to the electron beam via electron cyclotron resonance absorption. It is found that the external electromagnetic signal couples best when launched as a right hand circularly polarized wave.