Use of a quartz microbalance for plasma-wall inter-action studies

This paper reports on the first use of quartz crystal microbalances as a useful diagnostic for plasma-wall interaction studies in tokamaks. Two quartz microbalance systems were installed recently in the TdeV tokamak. Reliable mass measurements are obtained when a correction is applied to the frequency response of the crystal, owing to its temperature rise during the discharges. Particle fluxes in the scrape-off layer (SOL), perpendicular and parallel to the magnetic field lines along the ion drift or electron drift directions were measured. As expected, a larger erosion rate was measured in the ion drift direction, while deposition was observed on the quartz surfaces facing up and facing down but not facing the plasma (i.e. field lines at grazing incidence), Radial displacement of the set-up permitted the measurement of the radial dependence of the erosion-deposition processes. An increase of the plasma current induces a slight increase of the deposit thickness. A detachment of the plasma in the divertor doubles the thickness of the deposit. The quartz microbalance showed that the deposition rate is strongly increased for a few discharges following a disruption. This diagnostic method is also sensitive to the atomic number of injected impurities.