Sliding contact tests at high R.F. current under vacuum

On each of the Tore Supra ICRH antennas [ 1 ], 4 variable vacuum capacitors supplied by COMET * are used to match the antenna feed point impedance to the generator output impedance. On such a system, the whole transmission line is working near matched conditions, and high RF voltages are limited to the front part of the antenna. The quality, performance and reliability of the COMET capacitors, fixed or variable type, have been proved by thousands of units in operation in high power oscillators and amplifier circuits. The variable type (10 to 150 pF) fitted in Tore Supra antennas has allowed remarkable results. A record radiated power density of 1.6 kW/cm 2 , has been achieved, and RF peak voltages in excess of 40 kV are commonly used during plasma shots. However, by construction of the capacitor, a water cooled bellows on the RF current path limits the velocity of the capacitance variation. Reliable RF experiments during transients phenomena like pellet injection, monsters sawteeth or plasma displacements require the matching system to follow rapid variations of loads. Therefore, a new concept of variable vacuum capacitor allowing faster capacitance adjustment, is studied in collaboration with COMET. In this concept, the RF current flows through a sliding contact strip from PANTECHNIK + working in the sealed vacuum. The RF current path is therefore dissociated from the vacuum barrier. Sliding contacts, both in vacuum conditions and handling high RF current are not in common use in the RF field and have to be tested. This paper describes the existing variable capacitor, the new capacitor concept, the RF contact test device as well as the experimental results.