Changeover from Deuterium to Helium with Ion Cyclotron Wall Conditioning and diverted plasmas in ASDEX Upgrade

In its non-nuclear operation phase, ITER foresees the first H-mode operation in Helium, due to the limited additional heating power and the expected high L-H threshold in Hydrogen. Operating a W divertor with Helium plasmas may lead however to specific Plasma Wall Interaction issues which must be investigated. While retention and release of hydrogenic fuel has been extensively studied in metallic Plasma facing Components (PFCs), in particular in ASDEX-Upgrade [1] and in JET-ILW [2], and compared with the carbon cases, there is much less detailed data available during Helium operations. This paper reports on changeover from D to He operation in ASDEX Upgrade with W PFCs. For the initial changeover, a series of twenty Helium Ion Cyclotron Wall Conditioning (ICWC) [4] discharges were executed, at a toroidal field of 2T using its two three-straps antennas in monopole phasing at 30MHz. The He content, measured before and after ICWC with a Neutral Particle Analyzer in two reference He plasma pulses (ne = 9×1019 m -3 , Ip=0.8MA, BT=-2.5T, PNBI=2.2MW, PICRF=4.0MW, PECRH=2MW), was found to rise from 30% to 80% with respect to the total density before and after ICWC respectively. Twenty five nearly identical diverted Helium plasmas were operated afterwards [4]. Six tungsten samples, three of them loaded ex-situ with D, were exposed to ICWC discharges using the midplane manipulator of ASDEX-Upgrade. Nuclear reaction analysis (3He NRA) and elastic recoil detection analysis (ERDA 2 MeV 28Si+ ) reveal He uptake in all cases, as well as of D in pristine samples. These are correlated with discharge parameters, in particular the ion flux, measured close to the probe location. The amount of D atoms removed and He retained, is assessed on a shot to shot basis from mass spectrometry. Though detailed analysis is pending, the data show that the D content in ICWC discharges decreases by a factor 4, within the 200 sec. cumulated operation time. A similar decay is also seen on the D? Balmer line intensity, even in the divertor area. In diverted plasmas, mass spectrometry and D? line emission in the divertor, in particular at the outer strike-point, indicate that the plasma D content is only halved within 165 sec. cumulated flat top duration.