Development of electron cyclotron wall conditioning with argon gas in the spherical tokamak QUEST for hydrogen removal from metal walls

Abstract Electron cyclotron wall conditioning with argon gas (Ar-ECWC) has been performed on the normal conducting spherical tokamak QUEST with metal walls for the purpose of applying this technique to future superconducting fusion devices with metal walls as an inter-shot wall conditioning tool. An EC wave with a frequency of 8.2 GHz with a mixed ordinary and extraordinary mode with an injection power of 12 kW is applied with a poloidal magnetic configuration of trapped particle configuration. The Ar-ECWC removes hydrogen from the wall without significant argon retention, resulting in a reduction of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> intensities or hydrogen recycling during the following tokamak discharge. This leads to an increase in the plasma current and a decrease in the line-integrated electron density without a significant increase in the Ar I and Ar II intensities measured by a visible spectrometer. The Ar-ECWC also recovers the wall pumping of the subsequent tokamak discharge. However, defects such as voids, bubbles and dislocation loops are formed in tungsten samples exposed to the Ar-ECWC plasma.