A series of experiments, examining the confinement properties of ICRF heated H-mode plasmas, has been carried out on the C-Mod tokamak. C-Mod is a compact tokamak which operates at high particle, power, and current densities at toroidal fields up to 8T. Under these conditions the plasma is essentially thermal with very little contribution to the stored energy from energetic ions (typically no more than 5%) and with Ti~Te. Most of the data were taken with the machine in a single-null "closed" divertor configuration with the plasma facing components clad in molybdenum tiles. The data include those taken both before and after the first wall surfaces were coated with boron, with emphasis on the latter. H-modes obtained from plasmas run on boronized walls typically had lower impurity content and radiated power and attained higher stored energy than those run on bare molybdenum. Confinement enhancement, the energy confinement time normalized to L-mode scaling, for discharges with boronized walls, ranged from 1.6 to 2.4. The unique operating regime of the C-Mod device provided a means for extending the 1 tests of global scaling laws to parameter ranges not previously accessible. For example, the C-Mod ELMfree data was found to be 1.1-1.6 times the ITERH93 scaling and the ELMy data almost 2.0-2.8 times the ITERH92 ELMy scaling law, suggesting that the size scaling in both scalings may be too strong. While both ELMfree and ELMy discharges were produced, the ELM characteristics were not easily compared to observations on other devices. No large, low frequency ELMs were seen despite the very high edge pressure and temperature gradients that were attained. For all of our H-mode discharges, a clear linear relationship between the edge temperature pedestal and the temperature gradient in the core plasma was observed; the discharges with the "best" transport barriers also showing the greatest improvement in core confinement.

IntroductionRotation plays an important role in the transition from L-to H-mode [1][2][3][4].Poloidal rotation in the edge plasma region has been closely associated with the L-H transition [5-7], and toroidal momentum confinement is well correlated with energy confinement [8-11]. While there have been several diagnostic systems designed to measure impurity toroidal rotation in tokamak plasmas [8-33], most of the observations have been made in plasmas with an external momentum source, usually provided by neutral beams. Toroidal impurity rotation in ohmic plasmas (no net momentum input) is consistent with neoclassical predictions [24,33,34]; in ohmic L-mode discharges, impurities rotate in the direction opposite to the plasma current, so in general the assumption that the majority ions and impurities (which are most often measured) rotate in the same way might be questionable [34]. In neutral beam heated plasmas, which have substantial direct momentum input, the toroidal momentum confinement time is much shorter than the neoclassical predic- Experiment DescriptionThe observations presented here were obtained from the Alcator C- Mod [35] tokamak, a compact (major radius R = 67 cm, typical minor radius of 22 cm, and Observations of Toroidal Rotation and ScalingsShown in Fig clockwise, and the solid spectrum is blue-shifted, the argon is rotating clockwise, in same direction as the plasma current during the RF pulse. This is in the direction opposite to the rotation of impurities in ohmic L-mode plasmas [8,24,33,34]. The magnitude of the shift is -. 5 mA, which yields a toroidal rotation velocity of (.5 mA/3731.1 mA) x c = 4.0 x 106 cm/s, in the co-current direction. Since the major radius of Alcator C-Mod is 67 cm, this corresponds to an angular rotation speed of 60 kRad/sec.The toroidal rotation may also be determined from the heliumlike argon forbid-4 den line. Shown in Fig.3a and 3b are spectra recorded from a spectrometer viewing the plasma mid-plane, at an angle of 60 from a major radius, in a slight counter clockwise view. The spectra of Fig.3a were obtained from a 1.1 MA discharge (clockwise current), and there is a blue-shift of .13±.01 ml during the H-mode phase (solid spectrum), when the plasma stored energy increased by .12 MJ. This corresponds to a toroidal rotation velocity increase of (.13 mA/3994.3 mX)/sin(6*)x c = 1.0 x 107 cm/s (150 kRad/sec), in the co-current direction. The spectra of Fig.3b were obtained with the same view from a 1.0 MA discharge with the plasma current in the counter clockwise direction, and there is a red-shift of .07t.01 mA during the 2.7 MW ICRF pulse, indicating that the argon is rotating in the counter clockwise direction, again co-current. This L-mode discharge had a stored energy increase of 45 kJ, when the magnitude of the toroidal rotation velocity increased by 5.3 x 106 cm/s, and still opposite to the rotation direction in ohmic discharges.The phasing of the RF antennas for this case was the same as in Fig.3a. Shown in Fig.3c are spectra recorded from a 6* clockwise ...