q-dependent, H-mode-like phenomena in TFTR

A new phenomenon has been observed in TFTR discharges with neutral beam injection and slowly ramped plasma current. When q/sub /psi// (a) reaches certain values close to low-order rational numbers, the D/sub ..cap alpha../ emission decreases and the electron density profile broadens and increases, similar to observations during the H-mode in other tokamaks. This ''q-mode'' appears to have a neutral beam power threshold of /approximately/ 6 MW. During co-injection, the plasma toroidal rotation decreases by /approximately/ 50% at the transition. Very slowly toroidally rotating coherent adge modes, which slow and lock at the transition, were identified from the measured perturbations in /tilde B//sub theta// with mn /approximately/ q/sub /psi//(a). Calculations based on a tearing-mode model indicate that large islands, consistent with these measurements, could be present in the plasma edge and alter the plasma limiter interaction. At the transition Z/sub eff/ increases typically from 3 to 3.5. This increase and the electron density increase are caused largely by an increase in the carbon density. These increases raise the drag on fast ions, rapidly thermalizing some of the energy stored by beam ions, causing a transient rise in the plasma thermal energy. In addition, the global energy confinement time increases transiently.more » 26 refs., 7 figs., 1 tab.« less