Estimates of global recycling coefficients for LTX-β discharges

Abstract

We report the first observation of global recycling coefficient R near 0.5 in the Lithium Tokamak eXperiment-β (LTX-β), significantly below the minimum R previously reported in other devices. In a series of experiments with varied Li wall conditioning, estimates of the recycling coefficient have been made using a Lyman-α array and DEGAS2 modeling. A progressive reduction in Lyman-α emission with increased lithium and an increase in edge electron temperature are observed. It is also observed that with increasing Li coating thickness, the effective particle confinement time τp* is reduced and approaches TRANSP calculated energy confinement time (τE), with τp* near τE,TRANSP for the lowest recycling coefficients. Edge temperatures approaching core plasma temperatures, first reported in LTX, can now be directly connected to estimates of the recycling coefficient and qualitatively agree with previous UEDGE simulations. The particle flux to the limiting surfaces appears to be significantly reduced in comparison with fluid scrape-off layer (SOL) models, indicating that a large fraction of the SOL ions are mirror trapped. SOL 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.

Investigating the role of edge neutrals in exciting tearing mode activity and achieving flat temperature profiles in LTX-β

“…Flat temperature profiles were observed in-spite of that, indicating some reduction in recycling. Further, analyses comparing discharges with partially passivated Li and fresh Li PFCs indicate a relative reduction of ∼70%-80% in recycling coefficient from passivated to fresh Li PFCs [39].…”

“…Later the entire set of experiments are repeated with fresh lithium coating too, but the threshold for the tearing modes to appear seems to be robust at ∼1 × 10 19 m −3 . With fresh lithium, the wall recycling is expected to be lower [39] and it requires much more gas puffing or SGI pulses to achieve the comparable n e ave . Once the density level for exciting the tearing mode is determined, three specific n e ave cases are selected for further analysis.…”