Numerical assessment of the new V-shape small-angle slot divertor on DIII-D

Abstract

The small-angle slot (SAS) divertor of the DIII-D tokamak, and its upcoming upgrade, the V-shape small-angle slot (SAS-V) divertor, are numerically investigated using the SOLPS-ITER code package, including the effect of particle drifts, for a range of plasma density, heating power, strike point position in the slot, and for both magnetic field directions. The simulations show that the electron temperature near the strike point is reduced in SAS-V compared to SAS, for both magnetic field directions, such that SAS-V achieves divertor detachment at a lower value of the outboard mid-plane separatrix electron density. The detachment threshold is lower because the V-shape focuses recycling neutrals on the V-end, densifying and cooling the plasma in the slot. At sufficiently high density, the V-shape also reduces the radial gradient of the temperature profile at the target, which in turns reduces the radial electric field and the E × B drift velocities, further densifying and cooling the plasma in the slot and leading to detachment. The V-shape effect, however, is reduced for higher heating power. With more heating power, the detachment density increases, reducing the ionization mean free path of recycled neutrals, which therefore become less sensitive to target shape changes. This suggests that in a fusion reactor, where the heating power is high, optimization of the divertor target shape needs to be combined with other strategies to lower the detachment density, such as in-slot injection of low-Z impurities.

Predicting tungsten erosion and leakage properties for the new V-shaped small angle slot divertor in DIII-D

“…Modifications are then made to the wall geometry on the inside of the slot to create a mesh to simulate SAS-V (figure 2(c)), which places the OSP at the slot vertex. SOLPS-ITER input parameters are specified based on previous modeling of the DIII-D tokamak [15,16,[23][24][25]. Select DIII-D experimental data from previous discharges was also used to further constrain the simulation.…”

“…All charge states of D and C are tracked in SOLPS-ITER; no other impurities are included. A list of reactions simulated by EIRENE can be found in [25]. While charge-exchange reactions between D and C are simulated, chemical interactions (such as the formation and dissociation of C x D y ) are not included in SOLPS-ITER.…”

“…The primary independent variable is the core ion density, which corresponds to the D + ion density value at the innermost radial ring (n i,core ). More information on the input parameters used can be found in [25], where a similar grid is used for detailed detachment studies in SAS and SAS-V.…”

“…Due to increased densities at the slot vertex in SAS-V, T e remains lower throughout the outboard side of the slot than in SAS, only becoming similar in magnitude at the slot entrance. More detailed studies on the differences in closure between the SAS and SAS-V divertors and its impact on detachment physics and particle transport can be found in [25].…”

“…The narrower slot geometry in SAS-V is expected to place more challenging demands on strike point control. SOLPS-ITER simulations of SAS-V in [25] predicted that keeping the OSP at the slot vertex will yield the plasma solution with the lowest target temperatures. Moving the OSP either inboard or outboard from the slot vertex reduced neutral densities and increased T e [25].…”