Mass changes in NSTX surface layers with Li conditioning as measured by quartz microbalances

Dynamic retention of deuterium, lithium deposition, and the stability of thick deposited layers were measured by three quartz crystal microbalances (QMB) deployed in plasma shadowed areas at the upper and lower divertor and outboard midplane in the National Spherical Torus Experiment (NSTX) that was equipped with a lithium evaporator. Deposition of 185 μg/cm2 over 3 months in 2007 was measured by a QMB at the lower divertor while a QMB on the upper divertor, that was shadowed from the evaporator, received an order of magnitude less deposition. During helium glow discharge conditioning both neutral gas collisions and the ionization of lithium and subsequent drift of the ion interrupted the lithium deposition on the lower divertor. We present calculations of the relevant mean free paths. Occasionally strong variations in the QMB frequency of thick lithium films were observed suggesting relaxation of mechanical stress and/or flaking or peeling of the deposited layers.