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resulting low-density, high-temperature edge can become sufficiently collisionless such that standard fluid models are no longer applicable. Experimental measurements of scrape-off-layer (SOL) parameters, including density decay length ($λ_n$) estimates presented here can be used for simple analytic estimates of kinetic effects, but need more complete kinetic treatment. We present gyrokinetic simulations, with one configuration-space dimension and two velocity-space dimensions, of low and higher-recycling LTX-$β$ SOL conditions using Gkeyll, an open-source continuum gyrokinetic code. The simulations follow a single open field line, self-consistently evolving kinetic ion and electron distribution functions, electrostatic potential, collisions, sources, and sheath losses constrained by experimental midplane measurements. The hotter low-recycling case develops a larger parallel potential variation, and a density response not captured by an adiabatic Boltzmann-electron closure of the field equations, demonstrating the importance of kinetic electrons and the ambipolar potential in reducing electron end losses. Collisionality scans show that, as ion neoclassical collisionality falls below unity, the ion distribution develops pronounced non-Maxwellian features. The corresponding SOL particle and energy confinement times increase at low collisionality, particularly for low recycling, because ions become mirror-trapped and electron losses are throttled by the ambipolar potential. These results indicate that low-recycling lithium boundaries in LTX-$β$ access a regime where mirror trapping and electrostatic confinement fundamentally alter parallel residence times relative to conventional fluid expectations, an effect expected to be more pronounced in larger devices.