Analysis of ICRF-heated transport barrier experiments in Alcator C-Mod

Abstract

An internal transport barrier (ITB) mode has been produced in the Alcator C-Mod tokamak via the application of 1–2 MW of off-axis ICRF heating power at 4.5 T and 80 MHz. This mode is characterized by strong peaking of the electron density profile with ne(0) increasing to 6–8×1020 m−3 while the plasma remains H-mode like outside the barrier foot (r/a≃0.5) with ne≃1.5−2.0×1020 m−3. Formation of the density barrier is typically accompanied by a decrease in the toroidal plasma rotation velocity from co-current to zero. Transport analysis of these discharges using the TRANSP code indicates the formation of a transport barrier in χeff(r) at r/a≲0.5. Heating and control of the transport barrier was successfully demonstrated by simultaneously injecting 0.6–1.0 MW of on-axis ICRF heating power at 70 MHz.