PRODUCTION OF HIGH POLOIDAL BETA-PLASMA BY BIDIRECTIONAL LOWER HYBRID CURRENT DRIVE IN THE WT-3 TOKAMAK

Lower hybrid waves propagating both parallel and anti-parallel to the toroidal field (bidirectional waves) produce suprathermal electron tails in both directions. The contribution of these tails is additive for the pressure but subtractive for the plasma current I(p). Therefore, the poloidal beta due to tail electrons, beta-p(tail) becomes much larger than that in the usual case of unidirectional current drive, where beta-p(tail) exceeds unity only when I(p) is smaller than the Alfven critical current I(A). In the WT-3 tokamak, lower hybrid current drive plasmas produced by bidirectional and unidirectional injection have been compared in terms of beta-p. The experimental results show that a high beta-p plasma is generated with I(p) larger than I(A) in the bidirectional case.