No abstract

Confinement quality as good as ELM-free H-mode at densities substantially above the Greenwald density limit (n e,0 ͞n GW 1.4) has been obtained in discharges with a radiative boundary under quasistationary conditions for 20 times the energy confinement time. This is achieved by optimizing the gas-fueling rate of RI-mode discharges which tailors their favorable energy confinement and leads to discharges with beta values just below the operational limit b n 2 of TEXTOR-94, thereby effectively avoiding confinement back transitions or disruptions. In addition, this high-density regime is favorable for helium removal and results in figures of merit t ‫ء‬ p,He ͞t E ഠ 10 15, relevant for a future fusion power reactor. A subject of intense investigations in present day fusion research is the development of an operational scenario, providing at the same time (i) confinement with at least H-mode quality and (ii) high densities around or above the empirical Greenwald density value n GW I P ͞pa 2 [1] [with I P the plasma current (MA), the minor plasma radius (m), and n GW in units 10 20 m 23 ]. Success in this direction has been obtained in the past years on limiter [2,3] and divertor tokamaks by pellet injection or adjusting deuterium fueling and pumping [4,5]. Recently, high energy confinement at a Greenwald numbern e,0 ͞n GW 1.4 (n e,0 being the central averaged chord density) was obtained at DIII-D by optimizing divertor pumping in gas puffed H-mode discharges with edge localized modes (ELM) [6,7]. This is in contrast to most H-mode discharges which undergo a confinement degradation when approaching this value [8]. In this paper we show that it is possible in addition to substantially overcome the Greenwald limit under quasistationary conditions with confinement of ELM-free H-mode quality, high b, q 3.4, and a regime favorable for heat and particle exhaust. This is obtained by optimizing the gas-fueling rate in radiatively improved (RI) mode plasmas on the limiter tokamak TEXTOR-94 in order to maintain a sufficiently low neutral pressure in the scrape off layer (SOL).The RI-mode on TEXTOR-94 is obtained with edge radiation cooling due to neon seeding under boronized wall conditions or sputtering of silicon under siliconized wall conditions. A careful adjustment of the edge and heating conditions is necessary to reach RI-mode confinement, proportional to the electron density, leading to high confinement at high density. The confinement improvement observed in the RI-mode has been interpreted as resulting from the suppression of ion thermal gradient (ITG) modes caused by the presence of impurities in the edge and by the resulting core density peaking [9]. TEXTOR-94 is a medium size tokamak (R 1.75 m, a 0.46 m) equipped with the toroidal belt limiter ALT-II which consists of eight pumped blades. Some blades are equipped with gas inlet apertures located at the surface of the limiter tiles. The gas flow through the ALT fueling apertures is smoother than it is for the standard TEXTOR gas feeders equipped with a fast feedback ...