First experimental tests of a new small angle slot divertor on DIII-D

Abstract

A new small angle slot (SAS) divertor concept has been developed to enhance neutral cooling across the divertor target by coupling a closed slot structure with appropriate target shaping. Initial tests on DIII-D find a strong interplay between such anticipated ‘SAS’ effects and cross-field drifts, favouring operation with the ion B × ∇B drift away from the X-point, as currently employed for advanced tokamaks. This offers the following key improvements relative to DIII-D’s open lower divertor or partially-closed upper divertor: (i) SAS allows for transition to low temperature moderately detached divertor conditions with Te ≲ 10 eV at very low main plasma densities, lower than are usually attainable at all in DIII-D high confinement (H-mode) plasmas as used in these tests; (ii) Pedestal performance and core confinement are significantly improved with SAS. The final confinement collapse associated with the onset of X-point MARFE (multifaceted asymmetric radiation from the edge) following deep detachment occurs at significantly higher pedestal densities, thus widening the window of H-mode operation compatible with a dissipative divertor. For operation with the ion B × ∇B drift toward the X-point, the divertor plasma transitions to a bifurcative detached state at much higher densities, similar to other divertor configurations in DIII-D. These results highlight the strong interplay between divertor closure and drifts, and point to an interesting divertor optimization path to explore that offers potential for future fusion reactors.

Time-dependent probability density function analysis of H-mode transitions

“…2. These plasmas were part of a dedicated L-H transition investigation on DIII-D at plasma current, I p = 0.98 MA and toroidal magnetic field, B T = 2.01 T. Both transitions took place in the initial NBI-only auxiliary heating phase, with shot 185461 operating with the small angle slot (SAS) divertor configuration [20], and 185498 with the conventional "partially open" divertor configuration.…”

Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas

“…The closed divertor geometry generally facilitates the dissipation of heat and particle fluxes due to inherently better neutral compression [35,36,37,38,39,40].…”

Design and Testing of DiMES Carbon Ablation Rods in the DIII-D Tokamak

“…The third geometric design is a concaveshaped rod, which allows for exploring the possibility of plasma detachment due to neutrals trapped in the cavity. Such phenomena have been studied in DIII-D using the Small Angle Slot divertor [12] and have potential impact on the ablation rate. With a concave geometric feature facing the flow within the tokamak, we would like to investigate if the cavity "traps" a pocket of less energetic neutrals that serve to protect the main body from high heat flux.…”