Y. Iwata

Hysteresis-like effects in gyrotron oscillators

Special experiments devoted to studying hysteresis in gyrotron oscillators have been performed for the first time. Clear hysteresis-like effects with respect to variation of the cathode voltage have been observed in the mode competition scenario of the Forschungszentrum Karlsruhe coaxial gyrotron [B. Piosczyk et al., IEEE Trans. Plasma Sci. 30, 818 (2002)] and with respect to variation of the magnetic field and voltage in a single-mode operation of the Fukui IV gyrotron [T. Idehara et al., Int. J. Infrared Millim. Waves 19, 793 (1998)]. The observed phenomena are explained theoretically.

High quality operation of high frequency gyrotron

High frequency, medium power gyrotrons (Gyrotron FU series) have been developed at Fukui University as radiation sources covering a broad frequency band from millimeter to submillimeter wave region. Conversion from the gyrotron output of circular waveguide mode into a Gaussian-like beam is useful for applications of the gyrotron to a wide field. A high mode purity output is necessary for an effective conversion. In order to improve mode purity, a nonlinear cavity which is designed using a scattering matrix code and fabricated, is installed in the Gyrotron FU VA. The measurement of radiation patterns confirmed a high purity mode operation.

Development of a large orbit gyrotron (LOG) operating at higher harmonics

Large orbit gyrotrons (LOGs) are being developed by use of a permanent magnet system. The operation test shows that the LOG radiates at high harmonic frequency up to fifth. Computer simulation results are compared with the experimental results.

Observation of Frequency Down-Shift in a Submillimeter Wave Gyrotron

Frequency down-shift in the initial stage of CW or long pulse operation of a submillimeter wave gyrotron, Gyrotron FU IV, is observed. The shift occurs in a few minutes after turning on the operation and the amount of the shift attains even 0.1 GHz. The observation results are analyzed on the basis of a simple model for heat conduction in the region of a resonant cavity. The frequency shift is explained consistently by expansion of the cavity.

Development of a large orbit gyrotron with a permanent magnet system

The development of a high harmonic gyrotron using a permanent magnet system is presented. On the basis of the design for a high harmonic gyrotron, using an encircling large-orbit electron beam and a permanent magnet system (large orbit gyrotron, LOG), a gyrotron tube has been constructed. The construction of a permanent magnet system has been also completed. The maximum field intensity is a little bit higher than 1 T. This work is being carried out as a collaboration with the Shin-etsu Kagaku Company, Institute of Applied Physics, Russian Academy of Science (IAP-RAS) and Institute of Electronics, and Bulgarian Academy of Science (IE-BAS). The gyrotron has already achieved 3rd, 4th and 5th harmonic operations with relatively high efficiency. Corresponding frequencies are 89 GHz, 113 GHz and 138 GHz, respectively. The output power at the 3rd harmonic operation was 2.5 kW and the efficiency 6.3%.

Hysteresis in gyrotrons

We present a general theory of hysteresis in gyrotrons. The theory is illustrated by numerical examples referring to two specific gyrotrons: the FZK TE/sub 31.17/ coaxial gyrotron and the Fukui FU IV gyrotron. Corresponding experimental observations are described.

Operation results of a high harmonic gyrotron with axis-encircling electron beam and permanent magnet

Summary form only given, as follows. Summary form only given. A gyrotron with an axis-encircling electron beam is useful for high frequency operation, because the higher beam efficiency is kept even at high harmonic of electron cyclotron resonance. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already succeeded in the operations at the third harmonic and the fourth harmonic resonance. The frequencies are 84.88 GHz and 112.8 GHz, respectively. Operation cavity modes are TE/sub 311/ and TE /sub 411/. The magnet system consists of the main permanent magnet and additional coils for controlling the field intensities at cavity and electron gun regions. The magnetic field at the cavity region can be varied from 0.97 T to 1.18 T. At the optimum condition of the magnetic field intensity, the output powers at the third harmonic operation is 1.2 kW. The operation is pulsed, the pulse length is 1 ins and the repetition frequency is 1 Hz. Starting current, beam efficiency and emission pattern also have been measured. In this paper, the operation results of the gyrotron and comparison with the computer simulation results are described.