The electrical insulation of the DIII-D advanced divertor electrode

For the electrode for biasing experiments on the DIII-D tokamak, electrical insulation had to provide voltage standoff in the DIII-D divertor environment of neutral pressures in the range of 10^-8 to 5×10^-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. The operating experience, problems encountered, and improvements to the system are described. Electrical breakdown of some components and transient voltages, up to 5 kV, have been observed. The original concept for insulating the water and electrical feeds for the electrode was found to be prone to failure. A novel 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