Plasma fueling by sputtering and gas desorption during high power RF operation on the Phaedrus-T tokamak

Summary form only given. The particle confinement time in tokamaks, /spl tau//sub p/, is at least an order of magnitude less than the length of the discharge. In order to maintain a constant density during the discharge, it is necessary to provide an external hydrogen fueling source. On Phaedrus-T, approximately 4/spl times/10/sup 17/ hydrogen atoms per millisecond must be puffed into the vessel to maintain a constant density. Ion impact onto materials within the vacuum vessel causes sputtering and gas desorption, and these processes provide an additional uncontrolled fueling source. On the Phaedrus-T tokamak, high power RF (100-400 kW, 4-7 MHz) operation increases the sputtering and gas desorption rates. This increased gas influx, called RF fueling, can lead to disruptions or create ambiguity in interpreting the nature of the RF physics. Antenna designs have played a critical role in determining the amount of RF fueling that occurs. On Phaedrus-T, insulating boron nitride (BN) protection limiters intercept the plasma flow to the antenna straps. This is different from conventional tokamak antenna designs which surround their antenna with a conducting shield.