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

Abstract

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 ͑HT-7͒ ͓X. Gao et al., Phys. Plasmas 7, 2933 ͑2000͔͒ for long pulse operation. The extension of operational region and density behavior with graphite ͑C͒ 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 ͑Mo͒ limiter. The critical density of multifaceted asymmetric radiation from the edge ͑MARFE͒ onset is observed in the region of Z ef f 1/2 f GW = 0.9ϳ 1.2, where f GW = n e / n GW . ͑Here n 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.

Study of confinement improvement in presence of a MARFE in the HT-7 superconducting tokamak

“…The two poloidal water-cooling limiters and one toroidal water-cooling belt limiter at high field side were developed in 2002 [21]. A new set of actively cooled toroidal double-ring graphite limiters [22], at bottom and top of the vacuum vessel was developed for long pulse operation, and up to 240 s of long pulse plasma has been achieved in the HT-7 in 2004 as shown in Fig. 1.…”

“…In HT-7 with graphite limiters [22], the critical density of MARFE onset is observed in the ohmic discharges (Z e f f = 2∼8 and 80% ∼ 100% of the n GW ). The best correlation has been found between the total input ohmic power and the product of the line average density, measured at the outermost interferometer channel at r = 20 cm (r/a ∼ =0.7, the minor radius a = 27∼28 cm), and Z e f f .…”

“…The Hefei Tokamak-7 (HT-7) is a superconducting tokamak [7][8][21][22][23][24][25][26], and it was reconstructed from the original Russian T-7 tokamak in 1994. It has a major radius of R = 1.22 m, minor radius of a = 0.27 m in the circular cross section.…”

“…Very stable and reproducible discharges were obtained with significantly increased density and a large increase in the available pulse length, as com- pared to Mo limiters. Up to 240 s of long pulse discharge has been achieved by lower hybrid current drive (LHCD) with new graphite limiters [22] in the HT-7 in 2004.…”

Analysis of disruptive discharges during density limit experiments on the HT-7 Tokamak

“…Recently, a moveable graphite limiter [3], has been developed in order to investigate the influence of the horizontal plasma position on the density limit in the HT-7 tokamak. The purpose of this work is to optimize the configuration for synergy of high power LHW and IBW heating at high density in the HT-7 long pulse discharges [4]. Further investigations relating to this effect were not possible, due to magneto-hydrodynamic (MHD) instabilities have been observed when the density limit is approached at values of the edge safety factor 3 < q(a) < 3.5, with a moveable graphite limiter [3], on the HT-7 tokamak.…”