Overview of Recent Progress in the GAMMA 10 Tandem Mirror

(1) Four-time progress in ion-confining potentials Φ c to 3.0 kV in comparison to Φ c attained 1992-2002 is achieved in the hot-ion mode (T i =several keV). A scaling of Φ c , which favorably increases with plug electron-cyclotron heating (ECH) powers (P PECH ), is obtained. (2) The advance in Φ c leads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φ c with increasing n c to ∼10 19 m -3 with the recovery of Φ 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 Φ c , generates a strong E r shear and peaked vorticity with the direction reversal of E r ×8 sheared flow near the Φ 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 ι⊥0 from 4.5 to 6.1 keV, and T i//0 from 0.5 to 1.2 keV with τ P0 =95 ms for Φ c (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of τ P0 /τ E0 is observed to be 1.7 for Φ 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 (η ICH ∼0.3; nl c =4.5×10 17 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.