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

Abstract

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 e I EC R/P EC = 0.8 × 1017 A W−1 m−2 and , where n e is in 1020 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.

Simulation Study of ECCD in Helical Plasmas

“…In order to study the ECCD physics, a number of ECCD experiments have been performed in many devices [3]. Especially in Heliotron J, which is the helical-axis heliotron device, EC currents were measured experimentally and ECCD dependence on the magnetic configuration and the density were obtained [4,5]. It was found that the EC current decreased as electron density was increased and that the current changed depending on magnetic configurations.…”

Physics of Heliotron J Confinement

“…ECCD experiments were performed, focusing on the effects of the magnetic field ripple [30,31]. The EC-driven current decreased as power was deposited at the deeper ripple bottom.…”

Direct measurement of refracted trajectory of transmitting electron cyclotron beam through plasma on the Large Helical Device

“…Electron cyclotron resonance heating (ECRH) has been used for the control of local plasma parameters such as a radial electric field, particle/heat flux [3], and toroidal current [4][5][6], and for stabilization of neoclassical tearing mode [7][8][9]. ECRH is also expected as an essential heating and a current drive source in advanced toroidal devices such as ITER [10,11] and Wendelstein 7-X [12,13].…”