Helicon-Wave-Excited Helium Plasma Performance and Wall-Conditioning Study on EAST

Helium helicon wave plasma (He-HWP) wall conditioning was performed with a specially designed antenna system on the Experimental Advanced Superconducting Tokamak (EAST) under strong magnetic field (0.5–1 T). The maximum effective RF power $P_{\mathrm {eff}}$ was 30 kW at 27 MHz, and the He pressure was fixed at ~0.27 Pa. Charge-coupled device (CCD) images indicated that toroidal homogeneous but poloidal inhomogeneous He-HWPs were obtained. An obvious desorption of H2 and HD was found during He-HWP operation. Deuterium was desorbed mainly in the form of HD, with a removal rate of ~1021–1022D atoms/h. Additionally, the residual gas analyzer (RGA) results also showed a steep rise in H/D desorption during mode transition (inductively coupled plasma (ICP) $\to $ HWP) with increased $P_{\mathrm {eff}}$ from 5 to 15 kW, then a slight decline in the HWP mode with relatively higher $P_{\mathrm {eff}}$ (>15 kW), which was further verified by the spectral data of hydrogen Balmer series ( $\text{H}_{\alpha }$ and $\text{D}_{\alpha }$ ).