Kinetic modifications to parallel transport in a low recycling scrape-off layer: A one dimensional gyrokinetic study

Abstract A liquid lithium boundary offers a route to improved energy confinement via reduced recycling, but the resulting low-density, high-temperature edge can become sufficiently collisionless that standard fluid models are no longer applicable. Experimental measurements of scrape-off-layer (SOL) parameters, including density decay length estimates presented here, can be used for simple analytic estimates of kinetic effects, but more complete models are needed. We present 1-D, two-velocity-space gyrokinetic simulations of low and higher-recycling LTX-beta SOL conditions using Gkeyll, an open-source continuum gyrokinetic code. The simulations follow a single open field line from the low-field-side midplane to the high-field-side limiter, evolving kinetic ion and electron distribution functions, electrostatic potential, collisions, sources, and sheath losses with experimentally constrained mid-plane parameters. The low-recycling case develops a hotter SOL, a larger electrostatic potential variation along the field line, and a density response not recovered with an adiabatic Boltzmann-electron closure. This indicates that kinetic-electron dynamics and the associated ambipolar potential are important for maintaining quasi-neutrality and reducing electron end loss. Collisionality scans show that as the ion neoclassical collisionality drops below unity, the ion distribution develops a stronger non-Maxwellian structure. The corresponding SOL particle and energy confinement times increase at low collisionality, especially for low recycling, because ions become mirror trapped and electron transport is suppressed by the ambipolar potential. These results suggest that low recycling lithium walls in LTX-beta access a SOL regime where mirror trapping and electrostatic modifications alter field-line residence times relative to fluid expectations, effects expected to amplify in larger, hotter tokamaks.