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, ß_p^tai1, becomes much larger than that in the usual case of unidirectional current drive, where ß_p^tai1 exceeds unity only when I_p is smaller than the Alfvèn 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 ß_p. The experimental results show that a high ß_p plasma is generated with I_p larger than I_A in the bidirectional case.