Development of a millimeter wave, frequency tunable gyrotron using a permanent magnet system

点击翻译按钮获取中文摘要

A novel, compact, high harmonic gyrotron with axis-encircling electron beam and permanent magnet system has been developed.It is based on a relatively new and less studied principle known as large-orbit gyrotron (LOG). Due to the greater mode separation and, hence, selectivity in such devices a single mode operation at higher harmonics can be obtained easier compared with the conventional gyrotrons. The device has been optimized for operation at the fourth harmonic of the cyclotron frequency and is suitable as a source of radiation in the millimeter wave spectroscopy. The use of a permanent magnet instead of superconducting one has several clear advantages such as easier and more economical operation and maintenance, smaller weight and dimensions, simple power supply. All these advantageous characteristics make the developed gyrotron applicable to a number of other fields where powerful portable sources of radiation with frequency around 100 GHz are needed. A theoretical investigations … More based on a computer simulations of both electron beam formation and beam-field interaction have been performed in order to study the operation of the gyrotron in a broad parameter space and to select the optimum conditions. The results were used to specify the requirements to the entire system taking into account the limitations which stem from the available power supply (accelerating voltages in the range 35-40 kV, beam currents 1-1.5 A) and attainable magnetic field (about 1 T).As a result of a computer-aided design (CAD) a high performance electron-optical system has been developed. It is based on a novel type of electron gun which uses a gradual (instead of abrupt) reversal of the magnetic field to form axis-encircling electron beams with appropriate parameters. The configuration of the electron-optical system has been optimized in an iterative process during which both the geometry of the electrodes and the magnetic field distribution were varied searching for a superior combination. The final design has been carried out using the measured magnetic field distribution inside the real permanent magnet.Although there is a noticeable deviation between calculated and measured field profiles the basic beam quality parameters (velocity ratio, velocity spread and beam ripple) are close to the required. In order to correct the field distribution produced by the real magnet we plan to include a set of additional coils for fine tuning in the final design which is near completion now.The magnetic circuit consists of a high-grade NdFeB permanent magnet. It includes axially (in the region of the resonant cavity) and radially (in both gun and collector regions) magnetized segments. The computer aided design of the magnetic system has been carried out using the ELF/MAGIC code. The permanent magnet has been produced and measured by Shin-Etsu Chemical Co. Ltd. The whole gyrotron tube will be fabricated by the Institute of Applied Physics of the Russian Academy of Sciences in N.Novgorod. Less