Lithisation effects on density control and plasma performance in RFX-mod experiment

Plasma-wall interaction is one of the most important issues that present magnetic confinement devices have to face. In the RFX-mod experiment density profile control has recently become even a more crucial point due to the observation that single-helical-axis states (SHAx), and their consequent improved confinement regime obtained at high plasma current (Ip > 1.2 MA) and low density (n/nG?0.15), spontaneously disappear when operating at medium plasma density (n/nG>=0.25). It is believed that reducing hydrogen influx from the wall and controlling density profile will allow operating at higher density improving in such way the energy confinement time. Different techniques have been tested to reduce hydrogen wall influx: He glow discharges cleaning, He discharges at high plasma current, wall boronization and baking. All such techniques were able to improve the situation but none allowed a complete and stationary hydrogen influx reduction. At high plasma current their effect lasts only few discharges, characterized by a lower hydrogen influx and low density equilibrium with a recycling factor still equal to one; furthermore, the wall sometimes responds in an uncontrollable way providing very high influxes during the discharges. Given the good results obtained on Tokamak and Stellarator experiments, in order to improve this situation we tested the effect of wall conditioning by Lithium. As a first lithization method to deposit on the wall a controllable amount of Lithium we used a room temperature pellet injector (max pellet diameter of 1.8 mm and max length of 5). Lithization was applied both directly to the graphite tiles and over a fresh boronization. The technique of depositing Lithium by pellet injection has been found effective in maintaining Hydrogen wall recycling very low. A good effect has benn observed also on edge density providing more peaked density profiles. Preliminary tests have been performed also by a Liquid Lithium Limiter with a capillary porous system on loan from FTU experiment of ENEA laboratories in Frascati.