Impact of different heating and current drive methods on the early q-profile evolution in JET

Transport calculations illustrate that the lower hybrid current drive (LHCD) and off-axis electron cyclotron current drive (ECCD) are the only preheating methods that can create a wide, deeply reversed q-profile, i.e. large negative magnetic shear, on the JET tokamak. Off-axis neutral beam injection (NBI) and off-axis ion cyclotron resonance heating (ICRH) preheating yields a weakly reversed q-profile (small negative magnetic shear), whereas NBI and ICRH on-axis heating as well as ohmic preheating produce a monotonic q-profile in the preheating phase. Here, on-axis power deposition and current drive refers to heating and current drive at or close to magnetic axis and correspondingly, off-axis refers to heating and current drive deposited typically around the half minor radius (r/a = 0.3-0.6). The results on LHCD, ICRH and ohmic preheating have been verified in the recent JET experiments. The current drive efficiency scan shows that in the case of LHCD, ECCD and off-axis NBI, the driven current is absolutely crucial to obtain a reversed q-profile and to modify the current profile evolution drastically in the preheating phase. Taking into account only the direct electron heating effect, LHCD does not create a reversed q-profile. The timing scans indicate that the radial location of q(min) at the end of the preheating phase is generally quite insensitive to the start time of the preheating, once started 0-2 s after the plasma initiation if the method relies upon the driven current. On the other hand, methods relying only upon electron heating are very sensitive to that. In both cases, the magnitude of the negative magnetic shear, however, seems to be very sensitive to the start time of the preheating.