H-mode Density Limit and Detachment in JT-60U
A.V.Chankin, K.Itami and N.Asakura
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment, 801-1 Muko-yama, Naka-machi, Naka-gun, Ibaraki-ken, 311-0193, Japan
Abstract.
ρtor abcμs Prediction for the maximum plasma density achievable in the ELMy H-mode in the tokamak-reactor requires knowledge of physics of the H-mode density limit (DL). A number of mechanisms have been experimentally observed in high density H-modes and proposed as an explanation for the DL. They broadly fall into two main categories: increased edge transport (see e.g. [1]) and detachment of the plasma from the target [2]. Certain degree of detachment is always observed in the DL experiments, in particular, in between ELMs. The problem to be addressed is whether the detachment itself can impose the limit on the H-mode plasma density, or other mechanisms, such as deterioration of edge confinement, or back H-L transition, set in first. This issue is critical for deriving a physics-based scaling for the H-mode density limit and projections onto the next step machine.
In most of JT-60U discharges with good confinement, the density is limited by the increase in the ELM frequency, H-L or type I to type III ELM transitions, in the absence of any significant divertor detachment. In high density ELMy H-modes with detachment, analysis of inter-ELM periods reveals that the edge density rises in between ELMs, despite the ongoing detachment. In search of discharge conditions close to the full detachment, but where the plasma would still remain in the H-mode, "marginal" H-modes with X-point MARFE in JT-60U have been found, where alternate L-H-L transitions were observed in the MARFE state. In these discharges, stronger detachment corresponded to H-mode phases, apparently due to the reduced power to the SOL. Both core and edge densities where rising in these phases. In contrast, in the L-mode phases the densities were falling, in spite of weaker detachment. This example, alongside other experimental evidence from JT-60U, suggests that the detachment itself has no direct impact on the density limit, and the mechanism for the H-mode DL is in all cases transport-related (H-L transition, poor confinement or ELMs).
[1] Saibene G et al 1997 Proc. 24th Eur. Conf. On Control. Fusion and Plasma Phys. (Berchtesgaden 1997) vol 21A, part I (Geneva: European Physical Society) p 49.
[2] Borrass K, Lingertat J and Schneider R, Contrib. Plasma Phys. 38 (1998) 130.