Wall conditioning and density control in the Reversed Field Pinch RFX- mod

In the Reversed Field Pinch RFX-mod at the highest plasma current of 2MA, when error fields are not effectively feedback controlled, localized thermal loads up to tens of MW/m 2 can be produced. The graphite tiles withstand such high power loads, but their high H r etention makes the density control extremely diffic ult. Several wall conditioning techniques have been opti mized in the last campaigns, including helium glow discharge cleaning, with dedicated experiments to i dentify the conditions of better uniformity and eff ectiveness, and wall boronization by diborane glow discharges o f different duration with samples exposed and analy zed to study the uniformity, thickness, erosion and redepo sition of Boron. More recently, Lithium conditionin g has been applied for the first time in a Reversed Field Pinch. The main results are discussed in this cont ribution. Lithization leads to important operative advantages : a significant improvement of the density control is obtained. Densities up to n/n G ? 0.5 can be produced in a controlled way. At the sa me value of input power, plasmas at higher densities can be sustained. However, due to the short particle confinement time, such densities are reached with high rates of gas puffing. The resulti ng profiles at high density are edge peaked. A Lit hium multipellet injector, aiming at a more uniform depo sition, has been also tested.