Effect of magnetic field ripple on electron cyclotron current drive in Heliotron J

摘要

Electron cyclotron current drive (ECCD) experiments have been conducted in the helical heliotron device, Heliotron J. A wide configuration scan shows that the electron cyclotron (EC) driven current is strongly dependent on the magnetic ripple structure where the EC power is deposited. As the EC power is deposited on the deeper ripple bottom, the EC current flowing in the Fisch-Boozer direction decreases, and the reversal of directly measured EC driven current is observed. Measurement results using electron cyclotron emission and soft-x ray spectrum diagnostics imply that high-energy electrons are generated for ripple top heating while they are suppressed for ripple bottom heating, indicating that the generation and confinement of trapped electrons have an important role on ECCD. For ripple top heating, the typical ECCD efficiency is estimated as γ = n_eI_(EC)R/P_(EC) = 0.8 × 10~(17) AW~(-1) m~(-2) and ξ = e~3n_eI_(EC)R/ε_0~2P_(EC)T_e = 0.05, where n_e is in 10~(20) m~(-3), I_(EC) in A, R in m, P_(EC) in W and T_e in keV. The normalized ECCD efficiency is found to be independent of the absorbed EC power for both ripple top and bottom heating cases.