Impact of Localized Gas Injection on ICRF Coupling and SOL Parameters in JET-ILW H-mode Plasmas
Recent JET-ILW experiments reiterated the importance of tuning the plasma fuelling in order to optimize ion cyclotron resonance frequency (ICRF) heating in high power H-mode discharges. By fuelling the plasma from gas injection modules (GIMs) located in the mid-plane and on the top of the machine instead of adopting the more standardly used divertor GIMs, a considerable increase of the ICRF antenna coupling resistances was achieved with moderate gas injection rates (<1.5¥1022e/s). This effect is explained by an increase of the scrape-off-layer density in front of the antennas when mid-plane and top fuelling is used, making the evanescence region of the launched fast wave shorter. By distributing the gas injection to optimize the coupling of all ICRF antenna arrays simultaneously, a substantial increase in the ICRF power capability and reliability was attained. Although similar fuelling efficiencies were observed for the different injection cases - leading to similar core / pedestal plasma properties - the experiments indicate that the RF-induced impurity sources are reduced when switching from divertor to main chamber gas injection.