Plasma density behavior with new graphite limiters in the Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao , Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-q(a) was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Z(eff)(1/2)f(GW)=0.9 similar to 1.2, where f(GW)=<overline>n</overline>(e)/n(GW). (Here <overline>n</overline>(e) is the maximum line average electron density and n(GW) is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter. (c) 2005 American Institute of Physics.