Role of geometry and drifts on particle exhaust and detachment in DIII-D

Experiments demonstrate the impact of divertor geometry and cross-field drifts on particle exhaust and detachment with cryopumping: the exhaust rate in attached plasmas is higher with the ion B×∇B drift out of the divertor whereas the exhaust rate in detached plasmas is higher with the opposite drift direction. The upper divertor on DIII-D has the flexibility of flat, vertical and tight-private baffled configurations that provide trade-offs in neutral compression, detachment onset and pumping rate. In cases where the divertor is driven deep into detachment, neutral compression is maintained indicating divertor baffling can effectively trap neutrals, albeit with decreasing core confinement. Divertor conditions are modified by the B×∇B direction (change in compression, pumping rate, resulting pedestal density), but ultimately result in similar confinement, offering some flexibility. The drift direction can lead to a strong inner/outer exhaust imbalance in certain conditions: the inner cryopump can exhaust up to 10X more particles than the outer pump in single null configurations with the B drift direction into the divertor when both targets are in a high recycling regime.