ICRF and Enhanced Confinement Modes in Alcator C-Mod

Introduction Understanding divertor detachment is essential to predicting the operation of future diverted tokamaks. The current plan for ITER operation assumes that the plasma is detached from the divertor, that f e be near the Greenwald limit [1] and that the main plasma is not diluted by impurities. However, we do not know whether these conditions can be achieved simultaneously. In this set of experiments the core plasma parameters have been varied systematically in order to determine when divertor detachment starts, how the radial extent of detachment grows and how the detachment process can be controlled. Various divertor geometries have been studied in Alcator C-Mod, with most emphasis on equilibria with strike points on the vertical plates[2,3] as shown in Figure 1. In general, the threshold density for divertor detachment of Alcator C-Mod plasmas has been found to be linearly dependent on the input power for ohmic plasmas [4]. Te at the divertor plate is-5 eV for flux surfaces about to detach. Under ohmic conditions the core density at the divertor detachment threshold is fie-0.25 x nGreenwald. Other tokamaks, with more open, flat-plate divertor geometries, more typically find that the detachment threshold is closer to the Greenwald density limit (0.5-0.9 x B nGreenwald)[5]. More detailed characteristics of divertor detachment and the available diagnostics are found elsewhere [2,4,6]. Figure 1: The Alcator C-Mod divertor configuration. A representative equilibrium with outer strike point on the vertical section of the plate is shown. Density dependence of detachment The detachment of plasma on a particular flux surface from the outer divertor plate has been determined by comparing the plasma pressure profiles at the target to upstream values in the SOL outside the divertor. Each flux surface is labeled by its distance outside the separatrix on the midplane (p in mm). The absolute uncertainty of this mapping is ± 1-2 mm but the relative uncertainty, shot to shot, is-1 mm. Figure 2 shows the growth of the radial extent of detachment for typical ohmic 0.8 MA (5.3 Tesla) plasmas. Included are data from discharges with outer 1 divertor strike point on the vertical section (figure 1; region B-C) and with the strike point on the floor (figure 1; region A-B)-the so called 'slot-divertor' configuration. From this data it is clear that the radial extent of detachment is a strong function of fl e until the divertor pressure deficit reaches the 'nose' of the outer divertor …