EFFECT OF LITHIUM COATING ON THE IMPURITIES AND SHIELDING EFFECT OF PLASMA ON THE RESONANT MAGNETIC PERTURBATIONS FIELD IN THE STOR-M TOKAMAK PLASMA

Abstract

Effects of lithium coating of the chamber wall on the impurities in the STOR-M tokamak plasma were studied in this thesis work. Impurities have been identified as one of the major concerns since the beginning of tokamak plasma research, as they enhance the radiation losses and prevent plasma from being heated to a desired high temperature. The radiation losses are primarily due to line radiation from incomplete stripped impurity ions. Impurities are introduced into the plasma from the walls of the tokamak due to plasma-wall interactions, and the type of impurities observed in a tokamak is partially determined by the kind of material used for the tokamak chamber wall and the gases absorbed in the wall. In the STOR-M tokamak, inner surface walls are made of bare stainless steel, and the major impurities observed are from carbon and oxygen. The emission lines from these impurities are in the visible range of the electromagnetic spectrum. They are CIII which is observable at 464.74 nm, CVI at 529.05 nm, and OV at 650.02 nm. Before the chamber was coated with lithium, the intensities of the impurity lines were measured and then compared to the intensities after the lithiumization of the chamber wall. The intensities of the impurity lines were recorded during the stable period of plasma before and after the lithium coating using a spectrometer and an intensified charge-coupled device (ICCD) camera. It was observed that the intensities of the impurities reduced during the discharges immediately after the lithium coating. Further experimental analysis revealed that the freshly coated lithium caused plasma density to decrease, and increase after 300 plasma discharge shots. It was also found that after 600 and 900 plasma discharge shots, lithium coating does not appear to play any role in the reduction of the impurity intensities, but repetitive plasma discharge cleaning may be responsible for the decrease in the impurity intensities. In another experiment, an internal radial magnetic probe array was used to investigate effects of plasma and tokamak chamber wall on resonant magnetic perturbation (RMP) field applied externally to plasma. An internal magnetic probe array was used to measure the magnetic field at four radial locations at plasma edge after the application of RMP current. The plasma response magnetic field measured was subtracted from the vacuum field measured when RMP current was fired without plasma. The time delay caused by the plasma and tokamak chamber wall to the RMP field was also studied by calculating the difference between the RMP current waveform peak time and the magnetic field waveforms peak times in plasma. It was observed that RMP field in vacuum was 50% larger than the RMP field in plasma, and the penetration time of the RMP fields decreased as they penetrate through the vacuum wall into the plasma. The RMP field was found to travel faster in plasma than in vacuum.