Internal transport barriers on alcator C-Mod

The formation of internal transport barriers (ITBs) has been observed in th e core region of Alcator C-Mod [I. H. Hutchinson , Phys. Plasmas 1, 1511 (1 994)] under a variety of conditions. The improvement in core confinement fo llowing pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB pheno mena have been observed which require no externally applied particle or mom entum input. Short lived ITBs form spontaneously following the high confine ment to low confinement mode transition and are characterized by a large in crease in the global neutron production (enhanced neutron or EN modes). Exp eriments with ion cyclotron range of frequencies power injection to the pla sma off-axis on the high field side results in the central density rising a bruptly and becoming peaked. The ITB formed at this time lasts for ten ener gy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been obser ved in ohmically heated H-mode plasmas. All of these ITB events have strong ly peaked density profiles with a minimum in the density scale length occur ring near r/a=0.5 and have improved confinement parameters in the core regi on of the plasma. (C) 2001 American Institute of Physics.