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

Abstract

Abstract:Dynamic retention, 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).Deposition of 185 µg/cm 2 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 and subsequent drift of Li + interrupted the lithium deposition on the lower divertor. We present calculations of the relevant mean free paths. Occasionally strong variations in the QMB frequency were observed of thick lithium films suggesting relaxation of mechanical stress and/or flaking or peeling of the deposited layers.

Simulation of Diffusive Lithium Evaporation Onto the NSTX Vessel Walls

“…Observations of this effect were reported previously in conjunction with evaporation during helium glow discharge cleaning [5]. The mean free path of the lithium atoms scales inversely with the helium pressure, so lower pressures coat the bottom of the vessel most effectively and higher pressures lead to thicker coatings closer to the injectors at the top of the vessel.…”

“…3.1. The differences between the mean free paths for the two processes, given in [5], are smaller than the uncertainties in either. If we assume, in the interest of simplicity, that they are the same, we can treat the two background species (He and D 2 ) as one by virtue of their equal masses and temperatures.…”

“…The first component of the simulation model is a 3-D description of the NSTX vacuum vessel, including the two LITER evaporators used in this run campaign, as well as a surface representing the quartz micro-balance (QMB) [5] that provides the deposition data with which the model is compared.…”

“…The operation of the QMB monitors is described in [9], [10] and [5]. The raw data from the monitors is a frequency that is directly proportional to deposited mass once the effects of temperature changes have been taken into account.…”

“…A related effect is that nonuniformities in the deposits can lead to mechanical stresses and consequent changes in the observed QMB frequency [5]. Since such effects would enter intermittently or gradually over intervals longer than those considered here, we neglect them.…”