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.

Direction reversals of intrinsic toroidal rotation have been observed in Alcator CMod Ohmic L-mode plasmas following modest electron density or toroidal magnetic field ramps. The reversal process occurs in the plasma interior, inside of the q = 3/2 surface. For low density plasmas, the rotation is in the co-current direction, and can reverse to the counter-current direction following an increase in the electron density above a certain threshold. Reversals from the co-to counter-current direction are correlated with a sharp decrease in density fluctuations with k R ≥2 cm −1 and with frequencies above 70 kHz. The density at which the rotation reverses increases linearly with plasma current, and decreases with increasing magnetic field. There is a strong correlation between the reversal density and the density at which the global Ohmic L-mode energy confinement changes from the linear to the saturated regime.

I IntroductionAlcator C-MOD', the third high-field compact tokamak in the Alcator line, has been operating tokamak plasmas since May 1993. Its design capability includes toroidal field, BT = 9 T, plasma current I, up to 3 MA, in plasmas with major radius R = 0.67 m, minor radius a = 0.21 m, with elongation up to n = 1.8. Divertor operation can be either into its closed, baffled, divertor chamber or to open flat plates. The magnetic configuration is rather similar to that presently envisaged for the International Thermonuclear Experimental Reactor, ITER, except that it is about a factor of ten smaller.The high particle-, current-and power-densities characteristic of such compact tokamaks lead to edge conditions that are in many respects comparable to those expected in ITER, and offer the opportunity to investigate so-called dissipative divertor operation, in which the power scraped off into the divertor is exhausted through a combination of neutral and radiative processes rather than through plasma conduction direct to the divertor plates.Alcator C-MOD offers excellent port access to the plasma for diagnostic and heating purposes. Its present complement of diagnostics includes full magnetics for equilibrium reconstruction, electron temperature profiles from electron cyclotron emission (ECE), density profiles from a ten-channel CO 2 laser interferometer, ion temperature profiles from high-resolution x-ray doppler measurements, neutron emission, and fast neutral particle analysis, various spectroscopic measurements such as visible bremsstrahlung, H. arrays, and vacuum ultraviolet impurity measurements, bolometer arrays, and x-ray and UV tomography. In addition, detailed edge, scrape-off-layer and divertor diagnosis based on probes and spectroscopy is available.The primary auxiliary heating method in the short term is ICRF, and two transmitters are available, providing a total 4 MW at 80 MHz. Thus far, experiments have concentrated on plasma coupling studies using a movable monopole antenna. Good power coupling into high density plasmas has been obtained, with loading resistance in the range of 5 to 15 Q, 2 in reasonable agreement with the theoretical calculations.So far the magnetic field has been limited to about 5.3 T awaiting power systems upgrades that will enable full-field operation next year. Even so, plasma currents up to 1 MA have been obtained, and durations over 1 second. Peak electron densities up to 9 x 1020 m-3, and temperatures up to T = 2.6, Ti = 1.6 keV have been achieved. Energy confinement is observed to exceed Neo-Alcator scaling.In section II we review some MHD and operational characteristics of the plasma.Section III discusses divertor experiments, section IV the confinement results, and section V the first ICRF coupling studies. II MHD and OperationA unique feature of the design of Alcator C-MOD is its thick stainless-steel vacuum vessel and structure. For reasons of mechanical strength, these have no insulating breaks and thus constitute 'shorted turns' on the ohmic transformer and the eddy ...

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D α ) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V φ ∼ 50 km/s, and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge source of toroidal torque (which disappears at the H-to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼ 0.07 m 2 /s during EDA H-mode and ∼ 0.2 m 2 /s in L-mode. These values are large compared to the calculated neoclassical momentum diffusivities, which are of order 0.003 m 2 /s. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V φ (0) exceeding 100 km/s in some cases), which suggests the workings of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼ 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V φ (0) ∼ −60 km/s in some cases), which may be related to the observed higher Hmode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to stop (V φ ≤5 km/s). 1

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behaviour has been seen during ohmic H modes, which suggests that the rotation, generated in the absence of an external momentum source, is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, the central toroidal rotation significantly decreased together with the formation of an internal transport barrier (ITB). During the ITB phase, electrons and impurities peaked continuously for |r/a| ⩽ 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some ohmic H mode discharges.

High resolution measurements on the Alcator C-Mod tokamak [I.H. Hutchinson et al,Phys. Plasmas 1, 1551 (1994)] of the transport barrier in the "Enhanced D α " (EDA) regime, which has increased particle transport without large edge localized modes, show steep density and temperature gradients over a region of 2-5 mm, with peak pressure gradients up to 12 MPa/m. Evolution of the pedestal at the LH transition is consistent with a large, rapid drop in thermal conductivity across the barrier. A quasi-coherent fluctuation in density, potential and B pol , with f o~5 0-150 kHz and k θ~ 4 cm -1 , always appears in the barrier during EDA, and drives a large particle flux. Conditions to access the steady-state EDA regime in deuterium include δ=> 0.35, q 95 > 3.5 and L-mode target density e n > 1.2 x 10 20 m -3 . A reduced q 95 limit is found for hydrogen discharges.2

The steady-state H-mode regime found at moderate to high density in Alcator C-Mod, known as enhanced D α (EDA) H-mode, appears to be maintained by a continuous quasi-coherent (QC) mode in the steep edge gradient region. Large amplitude density and magnetic fluctuations with typical frequencies of about 100 kHz are driven by the QC mode. These fluctuations are measured in the steep edge gradient region by inserting a fast-scanning probe containing two poloidally separated Langmuir probes and a poloidal field pick-up coil. As the probe approaches the plasma edge, clear magnetic fluctuations were measured within about 2 cm of the last-closed flux surface (LCFS). The mode amplitude falls off rapidly with distance from the plasma centre with an exponential decay length of k r ≈ 1.5 cm −1 , measured 10 cm above the outboard midplane. The root-mean-square amplitude of the fluctuation extrapolated to the LCFS was Bθ ≈ 5 G. The density fluctuations, on the other hand, were visible on the Langmuir probe only when it was within a few millimetres of the LCFS. The potential and density fluctuations were sufficiently in phase to enhance particle transport at the QC mode frequency. These results show that the QC signature of the EDA H-mode is an electromagnetic mode that appears to be responsible for the enhanced particle transport in the plasma edge.

The time evolution of toroidal rotation velocity profiles has been measured 2