Overview of Recent Progress in the GAMMA 10 Tandem Mirror

(1) Four-time progress in ion-confining potentials _c to 3.0 kV in comparison to _c attained 1992-2002 is achieved in the hot-ion mode (T_i=several keV). A scaling of _c, which favorably increases with plug electron-cyclotron heating (ECH) powers (P_PECH), is obtained. (2) The advance in _c leads to a finding of remarkable effects of radially sheared electric fields (dE_r/d_r) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in _c with increasing n_c to ~10¹⁹ m^-3 with the recovery of _c with increasing P_PECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential _c, generates a strong E_r shear and peaked vorticity with the direction reversal of Er_r × B sheared flow near the _c peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in T_e and T_i rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises T_e0 from 70 to 300 eV together with T_i[perpendicular]0 from 4.5 to 6.1 keV, and T_i//0 from 0.5 to 1.2 keV with _p0=95 ms for _c (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of _p0/_E0 is observed to be 1.7 for _c trapped ions (consistent with Pastukhov's theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (_ICH~0.3; nl_c=4.5×10¹⁷ m^-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (T_e=600 eV and T_i=6.6 keV) with azimuthal E_r×B sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.