Session KI1 - Transport Barriers and High Performance Regimes.
INVITED session, Wednesday morning, October 31
Ballroom B,
Double transport barrier modes (core and edge barrier) have been observed with intense, off-axis ICRF heating in Alcator C-Mod. An internal transport barrier (ITB) is routinely produced in enhanced D_\alpha H-mode, 4.5 T, sawtoothing discharges with the minority resonance layer r/a\sim -0.5 to the high field side of the magnetic axis during current flat top. The measured density and calculated \chi_eff (from TRANSP) profiles suggest the central particle and thermal barriers are formed less than one energy confinement time after the H-mode develops. The density, radiation and \chi_eff profiles indicate that the foot of the barrier is r/a\sim 0.5. Furthermore, the thermal and particle confinement are improved across the entire region inside the barrier. Interestingly, the central toroidal rotation reverses from co-current direction, typical of H-mode plasmas, to the counter-current direction as the density profile becomes more peaked. Typically, increased core impurity radiation, presumably due to improved particle confinement, leads to a barrier collapse after \sim 10 energy confinement times. A B_T scan showed that the double barrier mode was accessed for B_T=4.1-4.5 T with the foot of the ITB remaining at r/a\sim 0.5. Importantly, experiments with additional central ICRF heating maintained the double barrier mode for as long as the ICRF was applied (\sim 6 confinement times). With the application of central heating, the central rotation reversed back to the co-current direction. In addition, the density peaking and impurity accumulation were arrested with the application of the central heating. Thus, the additional central heating appears to provide a means for controlling this mode.