Non-boronized compared with boronized operation of ASDEX Upgrade with full-tungsten plasma facing components

Abstract

Abstract. After completion of the tungsten coating of all plasma facing components, ASDEX Upgrade has been operated without boronization for 1 1/2 experimental campaigns. This has allowed the study of fuel retention under conditions of relatively low D co-deposition with low-Z impurities as well as the operational space of a full-tungsten device for the unfavourable condition of a relatively high intrinsic impurity level. Restrictions in operation were caused by central accumulation of tungsten in combination with density peaking, resulting in H-L backtransitions induced by too low separatrix power flux. Most important control parameters have been found to be the central heating power, as delivered predominantly by ECRH, and the ELM frequency, most easily controlled by gas puffing. Generally, ELMs exhibit a positive impact, with the effect of impurity flushing out of the pedestal region overbalancing the ELM induced W source. The restrictions of plasma operation in the unboronized W machine occured predominantly under low or medium power conditions. Under medium-high power conditions, stable operation with virtually no difference between boronized and unboronized discharges was achieved. Due to the reduced intrinsic radiation with boronization and the limited power handling capability of VPS coated divertor tiles ( 10 MW/m 2 ), boronized operation at high heating powers was possible only with radiative cooling. To enable this, a previously developed feedback system using (thermo-)electric current measurements as approximate sensor for the divertor power flux was introduced into the standard AUG operation. To avoid the problems with reduced ELM frequency due to core plasma radiation, nitrogen was selected as radiating species since its radiative characteristic peaks at lower electron temperatures in comparison to Ne and Ar, favouring SOL and divertor radiative losses. Nitrogen seeding resulted not only in the desired divertor power load reduction, but also in improved energy confinement, as well as in smaller ELMs.

Dynamic and static deuterium inventory in ASDEX Upgrade with tungsten first wall

“…Comparing the carbon and tungsten device the carbon deposition at the inner divertor drops by a factor of 15. One would expect the carbon content in the SOL to behave similar, but only a reduction by a factor of 2 was found [14].…”

Influence of nitrogen pre-implantation on deuterium retention in tungsten

“…Recent experiments have shown that nitrogen (N) seeding can not only efficiently cool the edge plasma but also improve the overall plasma performance [1][2][3]. But so far, little is known about the plasma-surface interaction of deuterium (D) with N-implanted W surfaces.…”

Ten years of W programme in ASDEX Upgrade—challenges and conclusions

“…They are either the response to specific questions arising from ITER or they The almost complete suppression of low-Z divertor radiation, which was achieved after boronisation, provided valuable information on the requirements of radiative cooling by artificially introduced impurities [17]. ITER will be in a similar situation after having replaced the CFC at the strike point position by tungsten PFCs: Be is an inefficient radiator and W, although having a high radiation loss parameter, will not provide enough edge and divertor radiation, because of its low concentration allowed in ITER.…”

“…The main ingredients for successful W operation are supplying enough central heating and using a gas puff in the range of several 10 21 s −1 . More quantitative details on the operational space can be found in [17]. To widen the operational window for investigation with collisionalities in the centre closer to those of ITER, boronisation has been performed in the 2008 campaign.…”

“…This allows to operate at lower edge densities and higher edge temperatures. After boronisation, the C and O concentrations were reduced to ≈ 0.1 % and below, respectively, consequently leading to very low divertor radiation [17]. In the course of these investigations the maximum available power was used, which in turn led to power loads above 10 MWm −2 and to the damage of the divertor coatings (see Sec.…”