Mode Locking of Free Electron Sources of Microwave and Millimeter Radiation

摘要

This report gives the results of theoretical and experimental work on modelocking gyrotron oscillators. The theoretical work is outlined in the first three sections of the report. Experimental work is given in the final section. In Section 1, a quasilinear theory is outlined which predicts mode locking in the closed cavity gyrotron configuration. Here the dispersive properties of the electron beam are used to compensate for the electromagnetic dispersion of the cylindrical cavity. An rf modulation of the electron beam axial velocity is used to mode lock eight longitudinal TE(11n) modes. It is shown that sub-nanosecond radiation pulses can be generated with substantial power. Section II covers an improved cavity geometry which does not require electron beam compensation for mode locking. This tapered cavity is shown to generate short, mode locked radiation pulses via a nonlinear, large-signal theory. Optimum mode locking is found when the electron beam is strongly modulated. At beam currents several times the start oscillation value, the character of the mode locking is found to change. A particle-in-call code simulation of the tapered cavity locked oscillator is presented in Section III. The code MAGIC is used to verify cold test results as well as predict oscillator performance with a modulated electron beam. It is found that short radiation pulses (trains of over 50 pulses) can be generated with moderate levels of rf modulation. The best results are found by using modulation of the electron density rather than the axial velocity. Section W includes the design and experimental cold test of the oscillator cavity, and the design of the prebunching cavity. Cold test results are compared with both a moving phase front theory and, indirectly, with MAGIC calculations. The prebunching cavity is simulated with MAGIC to determine resonant frequency and tuning range. Res.