Studies of MHD Effects on Fast Ions: towards Burning Plasma with ITER-like Wall on JET

Full text: Fast ion studies on JET aiming at assessing the peak values of fast-ion losses caused by plasma disruptions, TAE, and NTM are performed. This work is carried out as continuation of fast ion research on JET due to its significance for the future JET operation with ITER-like Be wall. Study of the MHD induced peak losses of fast ions play a crucial role for the PFC selection for DEMO. Gamma-ray diagnostics, neutron spectrometry, NPA, Faraday Cups and Scintillator Probe (SP) were used for simultaneous measurements of various species of confined and lost fast ions in the MeV energy range. The high time resolution of diagnostics allowed studying both resonant and non-resonant MHD effects on redistribution and losses of energetic ions. The fast ion populations were generated in fusion reactions and were also produced by NBI and by accelerating with ICRH. Significant fast ion losses preceding disruptions were often detected by SP in discharges with high βN . It was found that the losses are caused by the kink mode and these losses typically occur at the same time as the thermal quench before the current quench that follows. Bursts of metal impurities influx following the losses were observed. A set of experiments were carried out where interactions of core-localised TAE modes with fast ions in the MeV energy range were studied in plasmas with monster sawteeth. Simultaneous measurements of confined and lost ions allowed distinguishing the energy ranges of fast ions and their spatial redistribution during the TAE activity preceding monster sawtooth crashes. Energy and pitch angle resolved SP measurements of MeV-ions ejected from the plasma due to the non-resonant fishbone oscillations driven by NBI-ions are also studied. The lost ions are identified as fast protons accelerated by ICRH. Numerical simulations were performed with HAGIS, MISHKA and SELFO codes. Experiments with EFCC stabilising ELMs have exhibited a strong effect of EFCC on NTM and on NTM-induced losses of NBI ions. It was found that amplitude of NTMs decreases significantly during EFCC while the frequency of NTM sweeps down. The decrease in the NTM amplitude has a profound effect on losses. Work partly funded by UK EPSRC and EURATOM, and carried out under EFDA. (author)