9^th IAEA Technical Meeting on H–mode Physics and Transport Barriers - FRC Contributions:

A G Lynn†, P E Phillips†, A E Hubbard‡, S J Wukitch‡, and M H Redi§
† Fusion Research Center, University of Texas at Austin, Austin, TX
‡ Plasma Science & Fusion Center, MIT, Cambridge, MA
§ Princeton Plasma Physics Laboratory, Princeton, NJ

In the Alcator C-Mod tokamak, a high-resolution heterodyne ECE radiometer has been used to measure electron temperature in plasma discharges with internal transport barriers(ITBs). ITBs are formed by application of off-axis (r/a ~ 0.5) ICRF power. Strong density peaking indicates the formation of the ITB. When the ITB forms, the ECE radiometer detects a small amplitude mode localized near the magnetic axis. Surprisingly, as this mode amplitude grows a dip in the temperature profile is clearly observed at the same location. If sawteeth are present, the mode amplitude appears to be suppressed by the sawtooth crash and no dip in the temperate profiles is observed. TORAY, a ray-tracing code, has been used to investigate the possible refractive effects of the steep density gradients in the ITB and its effects on the ECE observations. The results show that refractive effects can explain the observed local changes in temperature. Recent predictive simulations of nonlinear gyrokinetic ITG-TEM drift modes at the trigger time of off-axis RF H-mode core plasmas finds a high frequency mode which appears to be a geodesic acoustic mode GAM. The frequency of the mode (77 kHz) is near to that observed by ECE (80 kHz) in the core of the ITB plasmas. Observations of broadband fluctuations are also described.

Control of internal transport barriers on Alcator C-Mod

C.L. Fiore,  P. T. Bonoli, D. R. Ernst, A. E. Hubbard, M. J. Greenwald, A. Lynn,^a E. S. Marmar, P. Phillips,^a  M. H. Redi,^b  J. E. Rice, S. M. Wolfe, S. J. Wukitch, K. Zhurovich
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139; ^a Fusion Research Center, University of Texas at Austin, Austin, TX; ^bPrinceton Plasma Physics Laboratory, Princeton, NJ

Abstract

Recent studies of internal transport and double transport barrier regimes in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas 1, 1511 (1994)] have explored the limits for forming, maintaining, and controlling these plasmas. C-Mod provides a unique platform for studying such discharges: the ions and electrons are tightly coupled by collisions  and the plasma has no internal particle or momentum sources.  The double-barrier mode comprised of an edge barrier with an internal transport barrier (ITB) can be induced at will using off-axis ion cyclotron range of frequency (ICRF) injection on either the low or high field side of the plasma with either of the available ICRF frequencies (70 or 80 MHz).  When enhanced D_alpha high confinement mode (EDA H-mode) is accessed in Ohmic plasmas, the double barrier ITB forms spontaneously if the H-mode is sustained for ~2 energy confinement times.  The ITBs formed in both Ohmic and ICRF heated plasmas are quite similar, regardless of the trigger method. They are characterized by strong central peaking of the electron density, and reduction of the core particle and energy transport.   Control of impurity influx and heating of the core plasma in the presence of the ITB have been achieved with the addition of central ICRF power in both Ohmic H-mode and ICRF induced ITBs. The radial location of the particle transport barrier is dependent on the toroidal magnetic field but not on the location of the ICRF resonance.  A narrow region of decreased electron thermal transport, as determined by sawtooth heat pulse analysis, is found in these plasmas as well. Transport analysis indicates that reduction of the particle diffusivity in the barrier region allows the neoclassical pinch to drive the density and impurity accumulation in the plasma center.  Examination of the gyrokinetic stability at the trigger time for the ITB suggests that the density and temperature profiles are inherently stable to ion temperature gradient (ITG) and trapped electron (TEM) modes in the core inside of the ITB location.