RFX-mod experiment is a circular section Reversed Field Pinch (RFP) device with major/minor radius 2.0/0.46 m, maximum plasma current 2 MA and first wall entirely covered by graphite tiles. Due to the high recycling related to the graphite wall, plasma-wall interaction (PWI) is an issue in RFX-mod for the operation at plasma current over 1 MA. At the highest plasma currents (1.5-2 MA) PWI influences the performance affecting both Zeff and density and temperature control. In particular the improved single-helical-axis states (SHAx), spontaneously developing at high plasma current (Ip>1.2 MA), disappear when the density is increased to n/nG>=0.2. Following tokamak experience, in order to improve density and impurity control He glow discharge cleaning, high current He discharges, wall boronization and baking have been applied. All such techniques were effective in improving the operation reliability but none of them provided a strong improvement in term of plasma performance. As a further step ahead, based on good Tokamak and Stellarator results, we recently tested the effect of wall conditioning by Lithium. As a first lithization method to deposit on the wall a controllable amount of Lithium we have used a room temperature pellet injector (max pellet diameter of 1.5 mm and max length of 6 mm). Coating deposition was optimized by adjusting plasma discharges used as target for lithium pellets, obtaining the best results with short 1 MA Helium discharges. Lithium coatings with a nominal thickness of about 10 nm were applied both directly to the graphite tiles and over a fresh boronization. The technique proved to be effective in maintaining Hydrogen wall influx very low Good indication on the lithization potential benefits have been obtained at plasma edge, where a lower density, higher temperature and an improved particle confinement time were observed. Yet such improvements are limited in amplitude and last only a small number of discharges. Graphite samples (and wall tiles) have been exposed to lithization and plasma discharges; the surface analysis indicated that after hundreds of plasma discharges lithium is still in place on the wall but it loses the capability to improve plasma-wall interaction. Experiments with a Liquid Lithium Limiter (LLL) with a capillary porous system have been also started, in order to improve the lithization efficiency by producing a thicker lithium coating on the wall and providing a preferred path for plasma-wall interaction to a hot Lithium limiter. For this experiment a LLL on loan from FTU experiment of ENEA laboratories in Frascati has been used. The particular RFP feature of an edge magnetic field essentially poloidal has to be considered, as it makes difficult a toroidally uniform deposition. The LLL has been used both as limiter than as evaporator. The use as evaporator has been followed by He low current plasma discharges to spread Lithim. Till now the evaporator only has provided an evidence of the wall conditioning on plasma discharges: after conditioning by evaporation Hydrogen discharges showed a remarkably high adsorption capacity of the first wall. As a preliminary result, though on a single shot, a sensible reduction of the resistive loop voltage at n/nG > 0.15 was observed. In addition, a Quasi Single Helical State associated to the formation of an Internal Transport Barrier appeared at 1.2 MA, whereas both usually develop at higher plasma current and lower density.
Lithization on RFX-mod reversed field pinch experiment
P Innocente;
2011
Abstract
RFX-mod experiment is a circular section Reversed Field Pinch (RFP) device with major/minor radius 2.0/0.46 m, maximum plasma current 2 MA and first wall entirely covered by graphite tiles. Due to the high recycling related to the graphite wall, plasma-wall interaction (PWI) is an issue in RFX-mod for the operation at plasma current over 1 MA. At the highest plasma currents (1.5-2 MA) PWI influences the performance affecting both Zeff and density and temperature control. In particular the improved single-helical-axis states (SHAx), spontaneously developing at high plasma current (Ip>1.2 MA), disappear when the density is increased to n/nG>=0.2. Following tokamak experience, in order to improve density and impurity control He glow discharge cleaning, high current He discharges, wall boronization and baking have been applied. All such techniques were effective in improving the operation reliability but none of them provided a strong improvement in term of plasma performance. As a further step ahead, based on good Tokamak and Stellarator results, we recently tested the effect of wall conditioning by Lithium. As a first lithization method to deposit on the wall a controllable amount of Lithium we have used a room temperature pellet injector (max pellet diameter of 1.5 mm and max length of 6 mm). Coating deposition was optimized by adjusting plasma discharges used as target for lithium pellets, obtaining the best results with short 1 MA Helium discharges. Lithium coatings with a nominal thickness of about 10 nm were applied both directly to the graphite tiles and over a fresh boronization. The technique proved to be effective in maintaining Hydrogen wall influx very low Good indication on the lithization potential benefits have been obtained at plasma edge, where a lower density, higher temperature and an improved particle confinement time were observed. Yet such improvements are limited in amplitude and last only a small number of discharges. Graphite samples (and wall tiles) have been exposed to lithization and plasma discharges; the surface analysis indicated that after hundreds of plasma discharges lithium is still in place on the wall but it loses the capability to improve plasma-wall interaction. Experiments with a Liquid Lithium Limiter (LLL) with a capillary porous system have been also started, in order to improve the lithization efficiency by producing a thicker lithium coating on the wall and providing a preferred path for plasma-wall interaction to a hot Lithium limiter. For this experiment a LLL on loan from FTU experiment of ENEA laboratories in Frascati has been used. The particular RFP feature of an edge magnetic field essentially poloidal has to be considered, as it makes difficult a toroidally uniform deposition. The LLL has been used both as limiter than as evaporator. The use as evaporator has been followed by He low current plasma discharges to spread Lithim. Till now the evaporator only has provided an evidence of the wall conditioning on plasma discharges: after conditioning by evaporation Hydrogen discharges showed a remarkably high adsorption capacity of the first wall. As a preliminary result, though on a single shot, a sensible reduction of the resistive loop voltage at n/nG > 0.15 was observed. In addition, a Quasi Single Helical State associated to the formation of an Internal Transport Barrier appeared at 1.2 MA, whereas both usually develop at higher plasma current and lower density.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
