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 ( I p ) 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 I p RU phase and maintaining H-mode in the I p RD as long as possible are instrumental to achieve low internal plasma inductance ( l i ) and to minimize flux consumption. In JET-ILW, at a given current rise rate similar variations in l i (0.7–0.9) 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 I p strong core radiation due to W influx led to hollow electron temperature ( T e ) profiles. In JET-ILW Z eff is significantly lower than in JET-C. W significantly disturbs the discharge evolution when the W concentration approaches 10 −4 ; this threshold is confirmed by predictive transport modelling using the CRONOS code. I p RD experiments in JET-ILW confirm the result of JET-C that sustained H-mode and elongation reduction are both instrumental in controlling l i .