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

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the "upstream" pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H-and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality-strong evidence that critical-gradient transport physics plays a key role in setting the power exhaust channel.

Abstract. Non-local heat transport experiments were performed in Alcator C-Mod Ohmic L-mode plasmas by inducing edge cooling with laser blow-off impurity (CaF 2 ) injection. The non-local effect, a cooling of the edge electron temperature with a rapid rise of the central electron temperature, which contradicts the assumption of "local" transport, was observed in low collisionality linear Ohmic confinement (LOC) regime plasmas. Transport analysis shows this phenomenon can be explained either by a fast drop of the core diffusivity, or the sudden appearance of a heat pinch. In high collisionality saturated Ohmic confinement (SOC) regime plasmas, the thermal transport becomes local: the central electron temperature drops on the energy confinement time scale in response to the edge cooling. Measurements from a high resolution imaging x-ray spectrometer show that the ion temperature has a similar behavior as the electron temperature in response to edge cooling, and that the transition density of non-locality correlates with the rotation reversal critical density. This connection may indicate the possible connection between thermal and momentum transport, which is also linked to a transition in turbulence dominance between trapped electron modes (TEMs) and ion temperature gradient (ITG) modes. Experiments with repetitive cold pulses in one discharge were also performed to allow Fourier analysis and to provide details of cold front propagation. These modulation experiments showed in LOC plasmas that the electron thermal transport is not purely diffusive, while in SOC the electron thermal transport is more diffusive like. Linear gyrokinetic simulations suggest the turbulence outside r/a=0.75 changes from TEM dominance in LOC plasmas to ITG mode dominance in SOC plasmas. Non-local Heat Transport in Alcator C-Mod Ohmic L-Mode Plasmas2