Hydrogen Recycling in the RFX Reversed Field Pinch

摘要

In most of the circumstances the density behaviour in RFX strongly depends on recycling at the wall, entirely made of graphite tiles. Boronisation increases by approximately ten times the wall reservoir with respect to the unconditioned tiles and allows a better density control in the entire range of plasma current so far experimented. Wall modes locking concentrates the dissipation of the relatively high power (20-80 MW) on to a narrow region of the wall and strongly affects recycling. Localised wall surface heating in high power discharges entails blooming processes with strong density build-up, wall saturation and also fast annihilation of boronisation through strong carbon redeposition. Dragging around toroidally the locked modes by introducing rotating perturbation modifies the topology of the plasma wall interaction, smears the power dissipation onto a larger surface, prevents the occurrence of blooming and improves the density control capability. Considering that the energy confinement time in RFX scales positively with density, one may deduce that since high densities are associated to high recycling at the wall, the latter is not detrimental per se as to the plasma performance. Impurity concentration in general is not a severe issue especially in high density regimes, apparently due to the strong impurity screening at the edge. It is however believed that a reduction of the wall recycling combined with other means of plasma refuelling such as pellet injection would beneficially affect the overall confinement. Pellet injection experiments have produced plasmas with the best dilution ever reached in RFX, with values close to one. In addition, the reduction of wall recycling and therefore of the neutral particle density at the edge should mitigate the effects of the related viscosity . In particular it should beneficially influence both topology and magnitude of the (EXB)/B~2 flow shear at the edge, which appears to be important for improving transport through turbulence stabilisation. In this perspective, an experiment to actively control the wall recycling has been envisaged: a vented pump limiter will be soon experimented on RFX to verify its pumping efficiency in the Reversed Field Pinch configuration.