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Transport of trace, non-recycling, injected impurities has been studied on the Alcator C tokamak. Changes of impurity confinement times with varying plasma density, current, toroidal field, majority ion species mass, impurity charge and mass, Z e ff, and major and minor radius have been delineated. An empirical scaling is developed from these results and compared with the results of similar transport studies undertaken on other tokamak devices. The agreement is reasonable. A computer model simulating the transport is utilized to compare several models with the empricial results. With the possible exception of low-density, high-Z e ff discharges, the transport is not consisten with the predictions of neoclassical theory, but can be well described by simple spreading diffusion with a diffusion coefficient ranging from 1 to 5 X 10 3 cm'-s" 1 , depending on plasma parameters. This model yields good agreement both with the time histories of single-chord measurements of various ionization states, and with radial soft-X-ray emission profiles. Increased impurity transport with the onset of strong MHD oscillations has also been observed, with the effective diffusion coefficient scaling approximately as (AB) 4 .

A system for' producing 300 us bursts ofi lo1' metal atoms. with 3 'eV .average. energy is described. It is shown that.'-th'i's. system can be successfully used to introduce impurities'into '... .. . : : CTR oriented tokamaks for transport. and con£ inement studi'es .-. .

The major results and accomplishments of the MIT tokamak program are surveyed. These are considered to be 1) discovery of an ohmic-heating confinement law in which TE -naR 2 ; 2) reduction of anomalous ion conduction to the neoclassical value by use of pellet fueling; 3) formulation of an empirical model for confinement of impurities in ohmically-heated tokamaks; 4) seminal experiments on current drive by lower hybrid waves and production of quasi-stationary driven current discharges with n -1020 m-3 ; and 5) heating of electrons by Landau damping of lower hybrid waves withATe -I keV. The advance of nOTE is also traced from values of -1018 sec-m-3 which were typical of tokamaks at the beginning of the Alcator program to values achieved on Alcator C in excess of 6 x 1019 sec-m-3 , which is required for thermalized energy breakeven at higher temperature.

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Poloidally asymmetric impurity-ion emission has been observed in high-density discharges of the Alcator Tokamak. The sense of the asymmetry reverses when the direction of the toroidal field is reversed, and all observed dependences of the strength of the asymmetry upon proton density and safety factor are consistent with the explanation that the gradient and curvature drifts of the highly coUisional impurity ions cause the asymmetry. Thus the asymmetry implies that the drift is a major impurity transport process at high densitieSo Basic to all tokamak and stellarator plasmaconfinement systems are helical magnetic field lines which reduce to a diffusion process (neoclassical diffusion) the particle drifts due to the gradient and curvature inherent in the confining field (hereafter, VB drifts). This Letter reports the first experimental evidence that, in Alcator^ plasmas, the V5-drift distance of the highly coUisional impurity ions (OVI, OV, and Nv) near the plasma edge is sufficiently large at line-average electron densities >5xlO^^ cm"^ that a large number of impurity ions strike the torus wall or limiter before making a full poioidal excursion and thus break the poioidal symmetry. Since this effect is not specific to Alcator discharges, but generally significant for highly collisional ions, a simple formalism which estimates the magnitude of the effect as a function of basic plasma parameters is presented. The observations make it clear that the V5 drift must now be included explicitly as a major impurity transport process in tokamaks, and previous poloidally symmetric models of impurity penetration and transport must be reviewed, Up-down scans of resonance emission from impurity ions were performed with a 0.4-m normalincidence monochromator. The field of view at the plasma was 1 cm (vertical) by 3 cm (horizontal), and the line of sight was scanned on a shot-toshot basis by tilting the system about a fixed point in the beam line. The measured quantity was volume emission rate integrated along a chord of the plasma cross section. This quantity is proportional to the number of impurity ions (in a particular ionization state) along the line of sight since the excitation rate should be poloidally symmetric.

Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1 (1994) 1511.] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems.Measured time-resolved brightness profiles of helium-and lithiumlike transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjuction with an atomic physics package for individual line emission. During L-mode plasmas, the transport can be characterized by pure diffusion, with coefficients ~ 5000 cm 2 /sec, reflecting the ~ 20 ms decay in the x-ray and VUV line brightnesses. During H-modes, the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/sec) near the plasma edge, with greatly reduced diffusion (of order 100 cm 2 /sec), also in the region of the edge transport barrier. These edge values of the transport coefficients during H-mode are qualitatively similar to the neo-classical values. Nitrogen has also been injected, and after the H-to L-mode transition, the inner shell satellite lines of lithiumlike nitrogen dominate in intensity the resonance line of heliumlike NS+ in a thin shell near the plasma edge.

New observations of the formation and dynamics of long-lived impurity-induced helical "snake" modes in tokamak plasmas have recently been carried out on Alcator C-Mod. The snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced density to a large crescent-shaped helical structure inside q<1, with a regularly sawtoothing core. The observations show that the conditions for the formation and persistence of a snake cannot be explained by plasma pressure alone. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.