The electrical insulation of the DIII-D advanced divertor electrode

Abstract

The electrode for biasing experiments on the DIII-D tokamak was installed in the summer of 1990 and biasing experiments have shown positive results. For the electrode, electrical insulation had to provide voltage standoff in the DIII-D divertor environment of neutral pressures in the range of 10{sup {minus}8} to 5 {times} 10{sup {minus}2} torr, variable magnetic fields, and in the presence of ionizing radiation. The electrical insulation system was designed and tested in air and vacuum for voltages up to 3 kV. In this paper, we provide an update on our operating experience, problems encountered, and improvements to the system. Electrical breakdown of some components has occurred during tokamak operations and transient voltages, up to 5 kV, have been observed. The original concept for insulating the water and electrical feeds for the electrode, a thin layer of woven ceramic cloth insulation between the feeds and a ground plane to keep out stray plasma, was found to be prone to failure. A new scheme of rigid ceramic insulators surrounded by a ground plane was designed and is being implemented. Another problem was arcs from vessel potential surfaces to the electrode in several locations where vessel ground existed within 1 cm of the electrode. The arc traveled in a small crack between two insulators. Careful attention has been paid to closing this and other small gaps in the insulation. Coatings on the surface of plasma facing insulators have been found to be electrically conductive. Grooves are being machined into the insulators to give areas shadowed from the coating source. Tests are being done to demonstrate the design concepts in both vacuum and glow discharge environments. Plasma sprayed ceramic coatings were also tested to determine the voltage standoff capability in a glow plasma discharge. The results of these tests will be discussed. 2 refs., 4 figs., 1 tab