2024 – Development of standard QH-mode SVR Scenario

Development of standard QH-mode SVR Scenario

2024 Research Campaign, Thrust: Shape Rise Divertor

Purpose of Experiment

A fusion reactor requires and is predicted to operate at high high-density and high temperature, with low-collisionality, and high confinement. This must be achieved without Edge Localized Mode (ELM) transients, which typically limit edge pressures, because ELMs would exceed erosion and thermal stress limits on plasma facing surfaces. Fortunately, in future machines, we expect edge turbulent transport to limit edge pressures before ELMs occur. The goal of this experiment is to (1) leverage the DIII-D Shape and Volume Rise (SVR) divertor to develop very high density, ELM-free Super Quiescent H-Mode (QH) and Super Wide Pedestal QH-Mode (WPQH) regimes, and (2) exploit the expected reduced boundary electron temperature at high density to achieve plasma detachment for the first time in these regimes, while screening impurities. Detachment radiates exhaust power before it can reach surfaces, keeping them cool. The SVR with strong shaping opens a path to access higher density at low collisionality, which could enable us to integrate a future-relevant non-ELMing high performance operation with a dissipative divertor.

Experimental Approach

The stability limit on edge pressure is determined by large scale kink/peeling modes at high edge current density, and by smaller scale ballooning modes at high pressures. When the plasma cross-section is elongated and made more triangular at the top and bottom, a stable path between peeling and ballooning modes opens, which has been accessed by so-called Super H-modes. The SVR shape is more extreme and widens this stable path, allowing access to even higher pressures. Because QH-Modes operate on the kink/peeling mode stability boundary, they provide a natural, direct path into the stable channel at high pressure and density, avoiding ballooning instabilities. In fact, the first Super H-Modes were achieved using a Standard QH-Mode scenario. In addition to steady operation without ELMs, most recently, a variant of Standard QH-Mode regulated by turbulence shows turbulence-broadened divertor heat flux widths up to twice the empirical scaling which are matched by XGC gyrokinetic simulations. This experiment will further improve core-edge integration by injecting radiative impurities, including nitrogen, to detach the divertor. Because QH-Modes operate at low collisionality, which has generally resulted in lower density and higher boundary temperatures, detachment has not yet been achieved. The higher densities and lower edge temperatures achievable in the SVR shape should make detachment possible in QH-Modes for the first time. Recent impurity transport measurements in WPQH-Mode suggest that neoclassical inward convection of impurities, which increases with Z, dominates over turbulent transport for Z > 10. Without ELMs to flush high-Z impurities, neoclassical screening (or some other means) is essential to avoid unacceptable core impurity dilution and radiation. This is especially relevant to the DIII-D wall change decision and compatibility with metal walls. The higher density regimes we plan to access offer improved conditions for screening. The convection and diffusion of impurities will be directly measured in the core and pedestal using laser blowoff and gas puff perturbations.