Response of internal transport barriers to central electron heating and current drive on ASDEX Upgrade

Internal transport barriers with the central electron temperature as large as the central ion temperature both in excess of 10 keV have been achieved in the Axi-symmetric divertor experiment (ASDEX Upgrade) [H. Vernickel et al., J. Nucl. Mater. 128, 71 (1984)]. By applying central electron cyclotron heating and current drive to negative central shear discharges, established by neutral beam heating in the current ramp, the core electron temperatures could be raised by more than a factor of 2. Despite the fivefold increase of the central electron heat flux, the ion and electron energy and also angular momentum transport did not deteriorate. For neutral beam injection alone and also with additional central electron cyclotron counter-current drive, a double tearing mode and the associated detrimental effect on the plasma confinement is destabilized only transiently, when the minimum of the safety factor ${\mathrm{(q}}_{\min })$ passes through 2. For co-current drive, however, the confinement does not recover after $q_{\min }$ has dropped below 2, as the (2,1) mode persists due to the influence of the current profile modification in the very plasma center.