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 ...

Detailed measurements and transport analysis of divertor conditions inAlcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are presented for a range of line-averaged densities, 0.7 < fie < 2.2 x 1020 M-3 . Three parallel heat transport regimes are evident in the scrape-off layer: sheath-limited conduction, high-recycling divertor and detached divertor, which can coexist in the same discharge. Local cross-field pressure gradients are found to scale simply with local electron temperature. This scaling is consistent with classical electron parallel conduction being balanced by anomalous cross-field transport (XI -0.1 m 2 s-1) proportional to the local pressure gradient. 60 to 80% of divertor power is radiated in attached discharges, approaching 100% in detached discharges. Detachment occurs when the heat flux to the plate is low and the plasma pressure is high (Te -5 eV). High neutral pressures in the divertor are nearly always present (1 -20 mTorr), sufficient to remove parallel momentum via ion-neutral collisions.

Ohmic H modes have been achieved in ALCATOR C-MOD with toroidal fields up to BT ≈ 5.25 T and line averaged densities up to ne ≈ 1.4 × 1020 m-3 with less than 1 MW of input power. In ELM-free cases at lower toroidal fields, the energy confinement increases by at least 60% and the particle confinement increases by more than a factor of two. The ELM-free H modes appear to fit the threshold power scaling observed on other tokamaks, P/S (MW/m2)=4.4 × 10-3 neBT (1019 m-3.T), up to power densities of about 0.15 MW/m2. At higher toroidal fields, ELMy H modes were observed after the injection of lithium pellets at about half of the scaled threshold power density. All of these results were obtained with molybdenum plasma facing components and the ELM-free H modes were obtained without boronization or other wall coatings