New results in development of mw output power gyrotrons for fusion systems

Abstract

Ciyrotrons are widely used in electron-cyclotron-wave systems of fusion installations. Modem and fiitnre installations require microwave sourees with power at least 1 MW There are two main lines in development of megawatt gyrotrons at IAP/GYCOM. The first line is an enhancement of parameters for conventional gyrotrons. The main efforts are aplicd now to implementing of CVD diamond windows into the gyrotrons and providing Frotron operation in CW (or very long pulses of tens seconds) regime. At megawatt power level practically all g?Totron subassemblies have to operate at thermal loads close to their limits and this fact defines design features ol" the gyrolrons. Now hvo grotrons with frequencies 140 GHz and 170 GHz eqnippcd with diamond widows are under tests. The gyrotrons operate with vel). high order modcs: TEZZs and TEBIO which allow efficient cooling of thcir cavity walls. The calculations show the possibilih of 1 M w microwave generation in the cavity in CW regime. 170 GHz gyrotron is equipped with a depressed collector which provide power load on the collector surface essentially (up to two times) lower than without electron energy recoveW. Higkeficiencp quasi-optical mode converters based on synthesized reflectors are used to transform the gyrotron operating modes into Gaussian wavebeams. Gaussian mode content of the output wave beams exceeds 96% for the gyrotrons. Due to difiaction losses some amount of radiation is lost in the srotrons. This radiation is very dangerous because of possible overheating gyrotron body, and. especially, insulators in the electron guns and depress collectors. Additional (so called relier) ccramics windows arc applicd in the protrons in order to diminish power of stray radiation in the hibes. The arotrons are designed for the operation in moderate-size cvo-magnets with a bore hole of I60 mm diameter. The magnets are relatively cheap and convenient in use. The qvrotron tests are now in progess. After successhl preliminq testing the gyrotrons with output ceramics (BN) windows in the regime of 1-2 seconds operation the windows were replaced by the CVD diamond ones. The use of the diamond windows allowed arotron operation with much longer pulses up to 80 seconds (see Table 1). Further increase of power and pulse duration is limited by thc capabilih of microwave load and some limitations of the main gyrotron power supply. Nearest plans (2003 year) include the pulse extension up to 300 seconds. Table 1. Test results of the gyrotrons with diamond windows 140 GHz 0.7 MW I O sec gyrotron 0.85 MW 3.5 sec 170 GHz 0.5 MW 80 sec gyrotron 3.5 sec 0-7803-7699-4/03/$17.00 02003 IEEE 2a