Performance Test of CW 300 GHz Gyrotron FU CW I

Abstract

A 300 GHz CW gyrotron FU CW I has been developed and installed in the Research Center for Development of Far-Infrared Region, University of Fukui as a power source of a high frequency material processing system. Its performance was tested and the maximum power of 1.75 kW / CW has been attained at the beam voltage of 15 kVand the beam current of 1A. The maximum window power efficiency of 15.5% has been obtained at the cathode voltage slightly lower than 15 kV. This gyrotron is designed to deliver a Gaussian beam after mode conversion from the oscillation mode TE 22,8 in the cavity with a complex of an internal radiator and beam shaping mirrors. The detailed measurement with an infrared camera has confirmed that a Gaussian beam is radiated when the magnetic field strength B c at the cavity is adjusted at a proper value. However, within a range of B c , the output power is emerged into multiple directions, which suggests simultaneous oscillation of competing cavity modes.

Simulation of Mode Interaction in the Gyrotron FU CW I

“…This, in principle, makes possible the occurrence of diverse multimoding phenomena [11][12][13] such as nonlinear excitation of parasitic modes [13], mode competition and cooperation [14][15][16], mode suppression, mode jumping [17] (also known as mode shift [18]), mode locking, simultaneous oscillation of several modes [19,20] etc. The experimental investigation of the oscillation characteristics of FU CW I and especially the study of its radiation patterns [7,8] have indicated that this tube exhibits a complex behavior which could be explained namely by mode interaction. In order to check some of the working hypothesis and get better physical insight in the underlying processes a series of numerical experiments (presented in the preceding section) have been carried out.…”

“…It has already demonstrated high performance and output parameters corresponding to the design target values. In order to study in detail its operational characteristics a set of experiments have been carried out [6,7]. In these experiments several interesting phenomena have been observed.…”

“…The next series of experiments has been performed aiming more detailed understanding of the oscillation characteristics of FU CW I [7]. The experimental set up used to study the radiation patterns is shown schematically on Fig.…”

“…Therefore it is quite straightforward to suppose that these two phenomena are connected and that the saturation of the output power could be attributed to the mode competition. First preliminary analysis of the conditions for excitation of different modes has been done in the framework of the linear theory calculating the starting currents of different candidates [7].…”

Long Pulse Operation of the THz Gyrotron with a Pulse Magnet

“…The cavity mode is TE 22,8 at the fundamental operation. [20,21] The second gyrotron, Gyrotron FU CW II [14][15][16] was constructed and the operation test has been carried out. The principal designed cavity mode is TE 2,6 at the frequency of 394.6 GHz.…”