In JET discharges where lower hybrid heating and current drive (LHCD)is applied early during the current ramp, a region of the plasma withzero current density is formed near the axis. At the boundary of thisregion the current density is large and Bθ increases rapidlyover a small distance. In the central region the safety factor, q, iseffectively infinite, but this falls steeply in the boundaryregion. Outside the boundary region q reaches a minimum, where themagnetic shear s≡r/q (dq/dr) becomes zero.The formation of this region of zero current is dependent on both theheating and the current drive effects of the LHCD. When LHCD isswitched off the current profile begins to relax towards the resistivepeaked current distribution of fully inductive tokamak operation. IfLHCD is not used in the current rise then these current profiles arenot established. Although the physical mechanism exists to drive the central plasmacurrent below zero, in most cases it appears to be prevented fromgoing negative. At least one MHD mechanism has been identified whichcould be responsible for this. The presence of the zero central current is closely linked to theperiodic relaxation events seen in these discharges. In these discharges, internal transport barriers have been observed with additional heating powerssubstantially below the values requiredto obtain barriers in monotonic q profile cases.
Impact of different heating and current drive methods on the earlyshape q-profile evolution in JET