Low recycling, low collisionality and high performance with lithium conditioning in LTX-β

We report the first observation of access to global recycling (Rw) near 0.5 from the plasma-facing components in the Lithium Tokamak eXperiment Beta (LTX-??), significantly below the minimum Rw ~ 0.85 reported in other devices using Li conditioning. We also observe that with increasing Li coating thickness, the effective particle confinement time ??p* is reduced while energy confinement ??E (at fixed density) increases, with ??E ~ ??p* at the lowest recycling coefficients. Flat Te profiles, first reported in LTX, have been sustained for multiple ??E – another clear signature of access to the low recycling regime. LTX-?? operates with near complete coverage of lithium on its all-metal PFCs, and has achieved LTX-record central Te,i at high plasma currents due to an ohmic power supply upgrade. In a series of experiments with varied Li wall conditioning, estimates of the recycling coefficient have been made using a Lyman-?? array and DEGAS2 modeling. We observe a progressive reduction in Lyman-?? emission with increased lithiumization and an increase in edge Te. The particle flux to the limiting surfaces appears to be significantly reduced in comparison to fluid SOL models, indicating that a large fraction of the SOL ions are mirror trapped. TRANSP calculated collisionality drops more than an order of magnitude below the banana regime boundary, indicating the importance of kinetic effects. Full-f 1x2v gyrokinetic simulations of SOL field lines with the GKEYLL code indicate that the fraction of ions trapped along field lines increases as collisionality drops, as a result of increased lithium evaporation. Predictions of the impact of high trapped ion fraction from GKEYLL are being evaluated with available experimental data. Finally neutral beam heating was implemented in LTX-?? to supplement the ohmic heating, and increases in Te have been measured.