Comparisons of small ELM H-Mode regimes on the Alcator C-Mod and JFT-2M tokamaks

Comparisons of H-mode regimes were carried out on the Alcator C-Mod and JFT-2M tokamaks. Shapes were matched apart from aspect ratio, which is lower on C-Mod. The High Recycling Steady (HRS) H-mode on JFT-2M and Enhanced Dα (EDA) regime on C-Mod, which both feature very small or no ELMs, are found to have similar access conditions in q95-ν* space, occurring for pedestal collisionality ν* > 1. Differences in edge fluctuations were found, with lower frequencies but higher mode numbers on C-Mod. In both tokamaks an attractive regime with small ELMs on top of an enhanced Dα baseline was obtained at moderate ν* and higher pressure. The JFT-2M shape favoured the appearance of ELMs on C-Mod, and also resulted in the appearance of a lower frequency component of the quasicoherent mode during EDA. 1.0 Introduction The high confinement or “H-mode” regime, characterized by an edge transport barrier, requires some form of edge relaxation mechanism increasing particle transport, so as to control impurities and maintain steady density while maintaining good energy confinement. This is often provided by Edge Localized Modes (ELMs); the Type I ELM regime is obtained on many tokamaks and is the reference scenario for ITER. However, there are concerns about the heat pulses resulting from such large ELMs and their possible impact on divertor erosion. This motivates the exploration of high confinement regimes with smaller or no ELMs. A number of such regimes have been discovered on various tokamaks, as recently reviewed by Oyama. However, understanding of the physics of, the relations between and the access conditions for these regimes is incomplete. This paper reports on a series of recent experiments aimed at comparing two such regimes, the Enhanced D-alpha (EDA) H-mode regime first observed on the Alcator C-Mod tokamak in 1996 and the High Recycling Steady H-mode seen on the JFT-2M tokamak in 2002. The EDA regime is characterized by good steady-state energy confinement, with normalized confinement HITER89-P up to 1.9, steady density and low, steady radiation levels. No periodic ELMs are typically seen. Instead, the particles are found to be regulated by a continuous ‘quasicoherent (QC) mode’ located in the density barrier. This mode, observed by several diagnostics, has f~100 kHz and poloidal wavenumber kθ ~2 cm near the outer midplane. At high pressure, βN > 1.7, the QC mode broadens and is largely replaced by a series of small ELMs appearing on top of the generally high Dα level. The HRS H-mode shares many of the global characteristics of the EDA regime, with HITER89-P < 1.6. Studies of edge fluctuations have found both high frequency (HF) components, with f~200-350 kHz, n~8, and low frequency (LF), broadband fluctuations with f~ 50 kHz, which have n=1 and m~4. Under some conditions, a ‘mixed’ regime with some small ELMs occurs. Both the EDA and HRS regimes are generally favoured by higher target density or neutral pressure and by increased triangularity δ and safety factor q95. It is thus natural to ask whether they in fact represent the same physical phenomenon, and if so, how this would extrapolate to other devices. 2.0 Description of Experiments H-mode regimes, stability and fluctuations are known to depend sensitively on shape. To reduce these effects in the intermachine comparison, shapes on JFT-2M and C-Mod were matched in poloidal cross-section, as shown in Figure 1. This also shows that the JFT-2M shape is quite different from the more typical C-Mod shapes in which prior H-mode studies have been done, notably lower κ (1.49) and smaller X-point radius. However, it was not practical to match the aspect ratio, which is higher on JFT-2M (R/a = 1.29/0.26m=4.9) than on C-Mod (R/a = 0.68/0.21m=3.2); this means that the plasma dimensionless parameters and safety factor profile q(r) cannot all be matched exactly. BT was 5.4 T for C-Mod and 1.6-2.2 on JFT-2M. Experiments were first carried out on JFT-2M, using variation in target gas pressure in a boronized vessel to scan density for several Ip/BT combinations, giving q95 of 2.9, 3.5 and 4.8. Heating was provided by 1.4 MW of balanced NBI. Details of the resulting Hmode regimes, pedestal parameters and fluctuations were reported by Kamiya. C-Mod then conducted experiments in the matched shape on several run days in the 2004 and 2005 campaigns, varying both target density and ICRH power (0-5.2 MW) to vary pedestal parameters at each of the above q95 values. The vessel was boronized in most experiments, with some variation in coating thickness. In both tokamaks, boronization was found to be necessary to achieve steady H-modes and to access the EDA or HRS regimes. 3.0 Results of Comparisons 3.1 H-Mode regimes and access conditions The H-mode regimes obtained span the range of those observed in prior JFT-2M and C-Mod experiments. The pedestal parameter ranges accessed are summarized in Figure 2. Figure 2a shows typical trajectories of JFT-2M discharges at q95=3.5. At the lowest target density, shown by the L-H transition curve, large ELMs were seen with a baseline Dα level below that of Lmode. These appear similar to Type I ELMs, though this has not been definitively established. There is a clear pressure limit, corresponding to toroidal beta at the pedestal βe,ped~ 0.2%. At the highest L-mode ne, a steady HRS regime is found with very small ELMs and a Dα level more than twice L-mode. A reduced pressure limit, βe,ped~ 0.14%, is found in this regime. The intermediate regime labelled ‘mixed’ has more distinct ELMs but still a high Dα level and βe,ped~ 0.17%. All three regimes lead to H-modes with fairly steady density and temperature. On C-Mod, the greatest range of pedestal parameters, and regimes, was achieved in the scans at q95=3.5 (figure 2c). Because of the variable heating power, a wider range of pressures was accessed in the C-Mod experiments. Pedestal Ti, where measured spectroscopically, was close to that of Te so total β is about twice the βe,ped shown. Steady EDA discharges without any discrete ELMs occurred at high densities and modest pressures. At similar densities but higher pressures, the small ELM regime with enhanced Dα, labeled ‘EDA+ELMs,’ was obtained as described in Figure 1: CMod shape used in comparison experiments (red) is a good match to scaled JFT-2M shape (blue, dashed) and differs from more typical CMod shapes (eg, green).