EFDA-JET-PR(13)60
ITER-Like Current Ramps in JET with ILW: Experiments, Modelling and Consequences for ITER
Since the ITER-like wall in JET (JET-ILW) came into operation, dedicated ITER-like plasma current (Ip) ramp-up (RU) and ramp-down (RD) experiments have been performed and matched to similar discharges with the carbon wall (JET-C). The experiments show that access to H-mode early in the Ip RU phase and maintaining H-mode in the Ip RD as long as possible are instrumental to achieve low internal plasma inductance (li) and to minimize flux consumption. The measured H-mode power threshold during Ip RU is similar to that obtained during flat top, and lower than in JET-C. In JET-ILW, at a given current rise rate similar variations in li (0.7­0.9) are obtained as in JET-C. In most discharges no strong W accumulation is observed. However, in some low density cases during the early phase of the Ip RU (ne/neGw ~ 0.2) strong core radiation due to W influx led to hollow electron temperature (Te) profiles. In JET-ILW Zeff is significantly lower than in JET C. W significantly disturbs the discharge evolution when the W concentration approaches 10­4; this threshold is confirmed by predictive transport modelling using the CRONOS code. Similar modelling for the ITER Ip RU showed a comparable critical W concentration, however with opposite effect: in ITER a too high Wconcentration leads to a very peaked Te profile and too high li. Ip RD experiments in JET-ILW confirm the result of JET-C that sustained H-mode and elongation reduction are both instrumental in controlling li.