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 to kamak. Reliable mass measurements are obtained when a correction is ap plied to the frequency response of the crystal, owing to its temperatu re rise during the discharges. Particle fluxes in the scrape-off layer (SOL), perpendicular and parallel to the magnetic field lines along t he ion drift or electron drift directions were measured. As expected, a larger erosion rate was measured in the ion drift direction, while d eposition was observed on the quartz surfaces facing up and facing dow n but not facing the plasma (i.e. field lines at grazing incidence), R adial displacement of the set-up permitted the measurement of the radi al dependence of the erosion-deposition processes. An increase of the plasma current induces a slight increase of the deposit thickness. A d etachment of the plasma in the divertor doubles the thickness of the d eposit. The quartz microbalance showed that the deposition rate is str ongly increased for a few discharges following a disruption. This diag nostic method is also sensitive to the atomic number of injected impur ities.