INTERNAL TRANSPORT BARRIERS IN ALCATOR C-MOD

Internal transport barriers (ITBs) marked by steep density and pressure profiles and reduction of core trans-port are obtained in Alcator C-Mod. Transient single barriers are observed at the back-transition from H- to L-mode and also when pellet injection is accompanied by ion cyclotron resonance frequency (ICRF) power. Double barriers are induced with injection of off-axis ICRF power deposition. These also arise spontaneously in ohmic H-mode plasmas when the H-mode lasts for several energy confinement times. C-Mod provides a unique platform for studying such discharges: The ions and electrons are tightly coupled by collisions with Ti /Te 1, and the plasma has no internal particle or momentum sources. ITB plasmas with average pressure greater than 1 atm have been obtained. To form an ITB, particle and thermal flux are reduced in the barrier re-gion, allowing the neoclassical pinch to peak the density while maintaining the central temperature. Gyrokinetic simulation suggests that long-wavelength drift wave tur-bulence in the core is marginally stable at the ITB onset, but steepening of the density profile destabilizes trapped electron modes (TEMs) inside the barrier. The TEM ul-timately drives sufficient outgoing particle flux to bal-ance the inward pinch and halt further density rise, which allows control of particle and impurity peaking