Negative triangularity and diverted operation of LTX-β

LTX-β, the upgrade to the Lithium Tokamak Experiment, is a high-field side limited, low aspect ratio tokamak (with R/a nominally 1.6). We have recently been exploring the range of equilibria accessible in LTX-β. These modifications include the use of a pair of high field side coils, which were installed to compensate for vacuum vessel eddy currents during breakdown, as divertor coils (with positive triangularity). We have also explored the use of a pair of low field side poloidal field coils, which are internal to the vacuum vessel, but external to the lithium-coated liner system, to generate tokamak discharges with negative triangularity. Diverted, positive triangularity equilibria offer a closer comparison to NSTX-U diverted discharges. Negative triangularity discharges offer the possibility of reduced drive for trapped electron modes, which in combination with flat temperature profiles, may further reduce transport in a tokamak. While the collisionless scrape-off layer (SOL) in conventional, positive triangularity discharges (with lithium walls) is expected to have a large, mirror trapped population, the SOL in negative triangularity discharges should have no trapped population. Here we discuss both positive and negative triangularity equilibria which can be achieved with the LTX-β coilset. We will also briefly discuss the possible reactor implications of low recycling, negative triangularity tokamaks, with a hot, collisionless SOL.