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 a l., J. Nucl. Mater. 128, 71 (1984)]. By applying central electron cyclotron heating and current drive to negative central shear discharges, establishe d by neutral beam heating in the current ramp, the core electron temperatur es could be raised by more than a factor of 2. Despite the fivefold increas e of the central electron heat flux, the ion and electron energy and also a ngular momentum transport did not deteriorate. For neutral beam injection a lone and also with additional central electron cyclotron counter-current dr ive, a double tearing mode and the associated detrimental effect on the pla sma confinement is destabilized only transiently, when the minimum of the s afety factor (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. (C) 2000 American Institute of Physics. [S1070-664X (00)92105-0].