INVESTIGATION OF PHYSICAL PROCESSES LIMITING PLASMA-DENSITY IN HIGH CONFINEMENT MODE DISCHARGES ON DIII-D

A series of experiments was conducted on the DIII-D tokamak [J. L., Lu xon and L. G, Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mo de) discharges. The typical H-mode to low confinement mode (L-mode) tr ansition limit at high density near the empirical Greenwald density li mit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevent ed formation of a high density, intense radiation zone (MARFE) near th e X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit a nd increased nonlinearly with the plasma current. magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power leve ls, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was less than or equal to 3 MW. Formation of MARFEs on closed field lines was avoided with low safety factor (q) operation but was observed at high q, qualitatively consis tent with theory. By using pellet fueling and optimizing discharge par ameters to avoid each of these limits, an operational space was access ed in which density similar to 1.5 x Greenwald limit was achieved for 600 ms, and good H-mode confinement was maintained for 300 ms of the d ensity flat-top. More significantly, the density was successfully incr eased to the limit where a central radiative collapse was observed, th e most Fundamental density limit in tokamaks. (C) 1997 American Instit ute of Physics.