Overview of Recent Progress in the GAMMA 10 Tandem Mirror

• Published in: Fusion science and technology 2007-02, Vol.51 (2T), p.11-16
• Main Authors: Cho, T., Higaki, H., Hirata, M., Hojo, H., Ichimura, M., Ishii, K., Islam, K., Itakura, A., Katanuma, I., Kohagura, J., Minami, R., Nakashima, Y., Numakura, T., Saito, T., Tatematsu, Y., Yoshikawa, M., Watanabe, O., Kubota, Y., Kobayashi, T., Yamaguchi, Y., Saimaru, H., Higashizono, Y., Miyata, Y., Kiminami, S., Shimizu, K., Itou, M., Ikuno, T., Mase, A., Yasaka, Y., Sakamoto, K., Yoshida, M., Kojima, A., Ogura, K., Nishino, N., Horton, W., Kariya, T., Imai, T., Pastukhov, V.P., Miyoshi, S.
• Format: Article
• Language: English
• ISSN: 1536-1055; 1943-7641
• DOI: 10.13182/FST07-A1305

(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 (dE r /d r ) 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 Er r × B 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 i[perpendicular]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.