Efficiency improvement by application of horizontal magnetic fields for second- and fundamental harmonic EC wall conditioning in JT-60U

By application of a horizontal magnetic field, BH, with 0.3% against a toroidal magnetic field, BT, electron cyclotron wall conditioning (ECWC) plasmas produced by 2nd-harmonic X-mode (X2-mode) EC waves expand radially toward the inner wall, which is to be cleaned for plasma start-up, but BH shrinks the plasma vertically. Excessive BH, BH/BT> 1.1%, produces vertically limited pan-cake shaped plasmas, leading to a reduction of D2 removal efficiency. Efficiency of D2 removal by fundamental harmonic X-mode (X1-mode) ECWC is higher than that by X2-mode one at BH/BT ∼ 0.3%. The amount of D2 gas removed by the X1-mode ECWC for 1s is one order of magnitude lower than that by Taylor discharge cleaning (TDC) for 7min× 2 times after the plasma disruption with similar stored energies (3MJ). The X1-mode ECWC successfully produces the following tokamak plasma. Keywords: Electron cyclotron wall conditioning, Second-harmonic electron cyclotron, Wall conditioning, JT-60