First test results with COMPASS ECRH system

The first phase of the ECRH system for COMPASS consists of five 60GHz 200kW gyrotrons for pre-ionisation and start-up. Prior to installation on COMPASS, which is now in the final stages of construction, the equipment has been connected to the DITE tokamak. This has enabled us to gain experience in the operation of such gyrotrons and the long transmission lines, windows and antennae required to convey power from the source to the plasma. Three units have been used for this purpose with a fourth also in operation and available for the high power testing of componemts omtp an nechoic chamber. The performance of these systems is described, in particular a comparison of inside launch antennae using windows both inside and outside the resonance zone close to the tokamak. Some difficulty has been experienced with arc breakdown in the long transmission lines and much effort has been put into analysing the spurious mode content using a k-spectrometer designed and supplied by IPF Stuttgart. The best of these lines has transmitted powers up to 190kW to a point 50m from the source with pulse lengths up to 200ms. In addition to enchancement of transmission line performance other improvements have been made to the power supplies and control systems. The requirements for the second phase of the COMPASS ECRH system is currently being installed and will give, when complete, 2MW for ECRH, current drive and profile control. (author). 4 refs.; 5 figs.; 2 tabs