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

Abstract

Transport calculations illustrate that the lower hybrid current drive(LHCD) and off-axis electron cyclotron current drive (ECCD) are theonly preheating methods that can create a wide, deeply reversedq-profile, i.e. large negative magnetic shear, on the JETtokamak. Off-axis neutral beam injection (NBI) and off-axision cyclotron resonance heating (ICRH) preheating yields a weaklyreversed q-profile (small negative magnetic shear), whereas NBI andICRH on-axis heating as well as ohmic preheating produce a monotonicq-profile in the preheating phase. Here, on-axis power depositionand current drive refers to heating and current drive at or close tomagnetic axis and correspondingly, off-axis refers to heating andcurrent drive deposited typically around the half minor radius(r/a = 0.3-0.6). The results on LHCD, ICRH and ohmic preheating havebeen verified in the recent JET experiments. The current driveefficiency scan shows that in the case of LHCD, ECCD and off-axis NBI,the driven current is absolutely crucial to obtain a reversedq-profile and to modify the current profile evolution drasticallyin the preheating phase. Taking into account only the direct electronheating effect, LHCD does not create a reversed q-profile. Thetiming scans indicate that the radial location of qmin at theend of the preheating phase is generally quite insensitive to thestart time of the preheating, once started 0-2 s after the plasmainitiation if the method relies upon the driven current. On the otherhand, methods relying only upon electron heating are very sensitive tothat. In both cases, the magnitude of the negative magnetic shear,however, seems to be very sensitive to the start time of the preheating.