Confinement studies of neutral beam heated discharges in TFTR

The TFTR tokamak has reached its original machine design specifications (I/sub p/ = 2.5 MA and B/sub T/ = 5.2T). Recently, the D0 neutral beam heating power has been increased to 6.3 MW. By operating at low plasma current (I/sub p/ approx. = 0.8 MA) and low density anti n/sub e/ approx. = 1 x 1019m-3), high ion temperatures (9 +- keV) and rotation speeds (7 x 105 m/s) have been achieved during injection. At the opposite extreme, pellet injection into high current plasmas has been used to increase the line-average density to 8 x 1019m-3 and the central density to 1.6 x 1020m-3/ This wide range of operating conditions has enabled us to conduct scaling studies of the global energy confinement time in both ohmically and beam heated discharges as well as more detailed transport studies of the profile dependence. In ohmic discharges, the energy confinement time is observed to scale linearly with density only up to anti n/sub e/ approx. 4.5 x 1019m-3 and then to increase more gradually, achieving a maximum value of approx. 0.45 s. In beam heated discharges, the energy confinement time is observed to decrease with beam power and to increase with plasma current. With P/sub b/ = 5.6 MW, anti n/sub e/ = 4.7 x 1019m-3, I/sub p/ = 2.2 MA and B/sub T = 4.7T, the gross energy confinement time is 0.22 s and T/sub i/(0) = 4.8 keV. Despite shallow penetration of D0 beams (at the beam energy less than or equal to 80 keV with low species yield), tau/sub E/(a) values are as large as those for H0 injection, but central confinement times are substantially greater. This is a consequence of the insensitivity of the temperature and safety factor profile shapes to the heating profile. The radial variation of tau/sub E/ is even more pronounced with D0 injection into high density pellet-injected plasmas. 25 refs