Study on wall recycling behaviour in CPD spherical tokamak

Experiments to study wall recycling behaviour have been performed in the small spherical tokamak compact plasma–wall interaction experimental device (CPD) from the viewpoint of global as well as local plasma wall interaction condition. Electron cyclotron resonance (ECR) plasma of typically ∼50 to 400 ms duration is produced using ∼40 to 80 kW RF power. In order to study the global wall recycling behaviour, pressure measurements are carried out just before and after the ECR plasma in the absence of any external pumping. The recycling behaviour is found to change from release to pumping beyond a certain level of pressure value which is again found to be a function of shot history. The real-time local wall behaviour is studied in similar RF plasma using a rotating tungsten limiter, actively coated with lithium. Measurement of Hα light intensity in front of the rotating surface has indicated a clear reduction (∼10%) in the steady-state hydrogen recycling with continuous Li gettering of several minutes.