Effect of ‘Baseline’ and ‘Hybrid’ Operational Parameters on Plasma Confinement and Stability in JET with a Be/W ITER-Like Wall
Database studies on JET with the Carbon wall (JET-C) and JET-ILW suggest that the transition in confinement properties between the scenarios so-called 'baseline' (aiming at demonstrating/ studying plasmas suitable for ITER baseline with q95~3, H98(y,2)~1, bN~1.8) and 'hybrid' (for Q = 5 long ITER pulse with q95~4, but also possibly for Q = 10, requiring H98(y,2) > 1, bN 2.5) is of a continuous nature. The comparison gains relevance as in the first JET-ILW campaigns, 'baseline' plasmas showed a reduced confinement by ~20­30% (bN~1.4, H98(y,2)~0.7­0.8) compared to similar plasmas in JET-C with possible impact on ITER's predicted performance of Q = 10 with H98(y,2) = 1 assumed. In contrast, the 'hybrid' scenario performed equally well with bN~3, H98(y,2)~1.2 in both JET-C and JETILW. In order to understand whether the difference between scenarios is due to the different operational space, pedestal physics and/or turbulent transport in the core plasma, an experiment was conducted where the input power (hence bN) and q95 were varied in ranges overlapping those typical of hybrid and baseline plasmas. Only low triangularity plasmas were used, and no N2 seeding. To minimise the effect of neutrals on confinement, the same low amount of D2 was injected during the main heating. Note these experiments focused on reproducing the engineering parameters in both types of plasmas, to ensure valid comparisons, and not on optimising the plasma performance.