A High Harmonic Gyrotron With an Axis-Encircling Electron Beam and a Permanent Magnet

Abstract

A gyrotron with an axis-encircling electron beam is capable of high-frequency operation, because the high-beam efficiency is kept even at high harmonics of the electron cyclotron frequency. We have designed and constructed such a gyrotron with a permanent magnet. The gyrotron has already operated successfully at the third, fourth, and fifth harmonics. The frequencies are 89.3, 112.7, and 138 GHz, respectively, and the corresponding cavity modes are TE 311 , TE 411 , and TE 511. The permanent magnet system is quite novel and consists of many magnet elements made of NbFeB and additional coils for controlling the field intensities in the cavity and electron gun regions. The magnetic field in the cavity region can be varied from 0.97 to 1.18 T. At the magnetic field intensities, the output powers at the third and the fourth harmonics are 1.7 and 0.5 kW, respectively. The gyrotron is pulsed, the pulse length is 1 ms and the repetition frequency is 1 Hz. The beam energy is 40 kV and the beam current is 1.2-1.3 A. Beam efficiencies and emission patterns have also been measured. In this paper, the experimental results of the gyrotron are described and compared with computer simulations. Index Terms-Axis-encircling electron beam, gyrotron, high harmonic, permanent magnet. I. INTRODUCTION T HE development of gyrotrons is being advanced in two ways. One is the development of high-power, millimeter-wave gyrotrons. At the present time, a gyrotron with a diamond window has achieved 2 MW output power at 170 GHz in a pulse several seconds long [1]. Such high-power gyrotrons are being used for the electron cyclotron heating of fusion plasmas. The other is the development of high-frequency, medium-power gyrotrons as millimeter-to-submillimeter-wave radiation sources for a broad range of applications.

Beam-Wave Coupling Strength Analysis in a Gyrotron Traveling-Wave Amplifier

“…7, indicates that the coupling impedance of the lower order harmonic operation is one order stronger than that of the close higher order harmonic operation. More extensive calculation also reveals that such a ten-times relation is also valid in the gyrotron system with large electron beam orbit (r c =0) operation [1,11,12] Fig. 6 The comparisons of the linear growth rate and coupling impedance between (a) unloaded empty waveguide (nonlinear stage), and (b) lossy dielectric-lined waveguide (linear stage).…”

Numerical Simulation of a Gyro-BWO with a Helically Corrugated Interaction Region, Cusp Electron Gun and Depressed Collector

“…This type of gun is ideal for gyrotrons operating at the fundamental waveguide mode but operation with a harmonic mode is prone to parasitic oscillations. Many high-frequency gyro-devices operate at harmonics (Cooke et al, 1996;Idehara et al, 2004;Wang et al, 2000; to allow for the use of a larger cavity diameter and to decrease magnetic field strength by a factor of s, the harmonic number. An axis-encircling electron beam is ideal for harmonic gyro-devices due to its good mode selectivity as the beam-wave coupling requires that the azimuthal index of the waveguide mode, m to be equal to s (Chu, 1978).…”

Design and simulation of a ∼390 GHz seventh harmonic gyrotron using a large orbit electron beam

“…As the output frequency increases, both larger magnetic fields and reduced interaction region size are needed when operating at the fundamental cyclotron frequency. An alternative approach to generate CW high frequency radiation is to work at higher cyclotron harmonics [8,9,10,11] which allow the use of larger cavity sizes and smaller magnetic fields by a factor of s, where s is the harmonic number. A CW high harmonic gyrotron operating at a frequency of~390 GHz at relatively lower B-field was designed and is presented in this paper.…”