Suppression of Tearing Modes by RF Current Condensation

Currents driven by rf (radio frequency) waves in the interior of magnetic islands can be used to stabilize deleterious tearing modes in tokamaks. Present analyses of stabilization strategies assume that the local deposition of rf power and the local rf acceleration of electrons are unaffected by the presence of an island, implying that the current required to stabilize the island increases as its width increases. It is shown here, however, that there is a threshold island width above which this assumption is significantly violated. There is a current condensation in the island, with a threshold width above which there is a decrease in the required current, and a second threshold at which the condensation effect is dramatically enhanced, allowing the stabilization of larger islands with more efficient use of the driven current. There is a hysteresis effect above the second threshold that shrinks the island to smaller width. The condensation effect also reduces the difficulty of aiming the rf current deposition for accurate radial alignment with the center of the island by concentrating the current deposition near the island center.