Power requirements for superior H-mode confinement on Alcator C-Mod: experiments in support of ITER

Abstract

Power requirements for maintaining sufficiently high confinement (i.e., normalized energy confinement time H 98 ≥1) in H-mode and its relation to H-mode threshold power scaling, P th , are of critical importance to ITER. In order to better characterize these power requirements, recent experiments on the Alcator C-Mod tokamak have investigated H-mode properties, including the edge pedestal and global confinement, over a range of input powers near and above P th . In addition, we have examined the compatibility of impurity seeding with high performance operation, and the influence of plasma radiation and its spatial distribution on performance. Experiments were performed at 5.4T at ITER relevant densities, utilizing bulk metal plasma facing surfaces and ion cyclotron range of frequencies waves for auxiliary heating. Input power was scanned both in stationary enhanced D α (EDA) H-modes with no large ELMs and in ELMy H-modes in order to relate the resulting pedestal and confinement to the amount of power flowing into the scrape-off layer, P net , and also to the divertor targets. In both EDA and ELMy H-mode, energy confinement is generally good, with H 98 near unity. As P net is reduced to levels approaching that in L-mode, pedestal temperature diminishes significantly and normalized confinement time drops. By seeding with low-Z impurities such as Ne and N 2 , high total radiated power fractions are possible, along with substantial reductions in divertor heat flux (>4x), all while maintaining H 98 ~ 1. When the power radiated from the confined vs. unconfined plasma is examined, pedestal and confinement properties are clearly seen to be an increasing function of P net , helping to unify the results with those from unseeded H-modes. This provides increased confidence that the power flow across the separatrix is the correct physics basis for ITER extrapolation. The experiments show that P net /P th of one or greater is likely to lead to H 98~1 operation, and also that such a condition can be made compatible with a low-Z radiative impurity solution for reducing divertor heat loads to levels acceptable for ITER.

Characterization of the pedestal in Alcator C-Mod ELMing H-modes and comparison with the EPED model

“…A scan in elongation over 1.45 < κ < 1.6 was also achieved in an altered equilibrium conducive to ELMing operation [17] with lower elongation and upper triangularity than standard C-Mod equilibria, as shown in figure 1. All ELMing H-modes were ICRF heated, using H minority resonance for the 3.5T and 5.4T discharges, and 3 He minority heating for the 8T plasmas.…”

20 years of research on the Alcator C-Mod tokamak

“…13. 83,84 Interestingly, the impurity seeding also improved ICRF coupling. 85 That effect is not understood but believed to be caused by changes in the edge plasma profiles or fluctuations.…”

Observation of Flat Electron Temperature Profiles in the Lithium Tokamak Experiment

“…NSTX, EAST [19], and DIII-D [18] also saw modification and suppression of edge-localized modes (ELMs), explained in NSTX by the change in recycling that modified and stabilized pedestal profiles [17,20]. Methods to reduce edge neutrals without Li can also cause beneficial changes to the pedestal, reducing pedestal density and collisionality and changing the bootstrap current and density pedestal structure in C-Mod, JET, and AUG [21][22][23]. In both NSTX and the devices without Li, peeling-ballooning stability improved, allowing a higher temperature and pressure pedestal and improving confinement [24][25][26].…”