G. F. Brand

A 150–600 GHz step‐tunable gyrotron

An account of the operation of a step‐tunable, low‐power, continuous‐wave gyrotron, GYROTRON V, at the fundamental and second harmonic of the electron cyclotron frequency is presented. The design of this gyrotron was especially optimized for the second harmonic. The gyrotron operating frequency at the fundamental can be tuned from 150 to 300 GHz and extended by second‐harmonic operation to above 600 GHz. Under normal operating conditions where the accelerating voltage is 10 kV and the beam current is 50 mA, power levels in excess of 20 W are obtained at the fundamental and hundreds of milliwatts at the second harmonic. The paper includes observations of single mode operation at the second harmonic and mode competition between second harmonic and fundamental.

Development of a second cyclotron harmonic gyrotron operating at submillimeter wavelengths

The development of a high‐frequency, step‐tunable gyrotron operating at submillimeter wavelengths is described. The gyrotron design was optimized for operation at the second harmonic of the electron cyclotron frequency in the TE261 cavity mode, whose resonant frequency is 384 GHz. Experimental results show that second harmonic operation can occur without mode competition as long as the beam current is low (Ib ≲0.8 A), but as the current is increased, the fundamental TE231 cavity mode increases and eventually (Ib ≳1 A) suppresses the second harmonic. The competition between the two modes is studied in detail. The starting current for second harmonic operation is also studied experimentally and compared with calculated results. Other resonances have also been examined. With the present superconducting magnet, the maximum frequency achieved is 402 GHz (second harmonic operation in the TE551 cavity mode) at several kilowatts.

Mode competition in a high harmonic gyrotron

Abstract Non-linear theory is used to examine the limits on the maximum power that might be obtained from a high harmonic gyrotron. In particular, special attention is paid to the situation where the power is limited by mode competition from a nearby fundamental resonance. We find good qualitative agreement between theory and some observations made on a high harmonic gyrotron at Fukui University.

Operation of a tunable gyrotron at the second harmonic of the electron cyclotron frequency

The University of Sydneys GYROTRON IV is a single CW source of low-power millimetre-submillimetre radiation that can be tuned across a broad frequency range from below 100 GHz to above 500 GHz. The higher frequencies are obtained when the gyrotron operates at the second harmonic of the electron cyclotron frequency. The conditions for achieving second harmonic operation are discussed.

A tunable millimetre-submillimetre gyrotron

Abstract The tunable millimetre-submillimetre wave source, Gyrotron III, incorporates a split cavity which permits mechanical fine tuning so that any frequency in the range 100-325 GHz becomes accessible. This paper reports on the most recent results obtained with this gyrotron and discusses the cavity resonances that occur in this split cavity design.

Competition between fundamental and second‐harmonic operations in a submillimeter wave gyrotron

Competition between electron cyclotron fundamental (f=fc) and second‐harmonic (f=2fc) operation in a submillimeter wave gyrotron is described. Even when the magnetic field intensity is adjusted to the optimum value for second‐harmonic operation, we can get a pure mode only for small beam currents. As the beam current is increased, excitation of the fundamental appears and eventually suppresses the second harmonic. The observed competition between the fundamental and second harmonic is compared with a computer simulation.

Tuneable millimeter-wave gyrotrons

In the course of developing a low-power, tuneable millimeter-wave source, two gyrotrons have been constructed. Gyrotron I was a fixed-frequency device operating at 120 GHz while Gyrotron II produced more than 20 lines in the frequency range 130 to 260 GHz. The design of tuneable gyrotrons is discussed with reference to the Gyrotron II results.

Scattering from discrete Alfvén waves in a tokamak using a gyrotron radiation source

A tunable gyrotron is used as the radiation source in a far-forward scattering experiment to investigate the density fluctuations associated with the appearance of discrete Alfven wave (DAW) resonances and kinetic Alfven waves (KAWs) in the TORTUS tokamak plasma. A new arrangement of a quasioptical antenna and a Gaussian telescope focuses the gyrotron output into the plasma. Predictions of the scattered amplitude and phase take into account the diffraction and refraction of a Gaussian beam in the plasma. The scattering experiments enable DAW resonances and KAWs to be identified, and their characteristics are in broad agreement with the predictions.

Development and applications of frequency tunable, submillimeter wave gyrotrons

The development of single-frequency, high-power gyrotrons for the fusion community has always had a high profile. However, tunable, low-power gyrotrons are well suited as sources for plasma diagnostics and the spectroscopic study of materials. This paper reviews the work at the University of Sydney in Australia and Fukui University in Japan.

The strip grating as a circular polarizer

A conducting strip grating can convert a linearly polarized plane wave into one that is circularly polarized. The conversion of a normally incident wave is most efficient when the grating period is 0.874 times the wavelength, the width of the strips is half the grating period and the angle between the direction of polarization and the strips is 45°. This result is confirmed by a microwave experiment at 35 GHz.