Gas balance and fuel retention in fusion devices

Abstract

The evaluation of hydrogenic retention in present tokamaks is of crucial importance to estimate the expected tritium (T) vessel inventory in ITER, limited from safety considerations to 350g. In the framework of the European Task Force on Plasma Wall Interaction (EU TF on PWI) efforts are underway to investigate gas balance and fuel retention during discharges, and to compare the data obtained with those from post-mortem analysis of in-vessel components exposed over whole experimental campaigns. This paper summarizes the principal findings from coordinated studies on gas balance and fuel retention from a number of European tokamaks, viz. ASDEX-Upgrade (AUG), JET, TEXTOR and Tore Supra (TS). For most devices, the long-term retention fraction deduced from integrated particle balance is ∼ 10-20 %. This is larger than the ~3-4% deduced from post mortem analysis of plasma facing components (PFCs). However, from the database available for tokamaks with their main PFCs made of carbon, the important conclusion is that the T inventory limit (set by the working guideline for operations) could be reached in ITER within fewer than 100 discharges. This, therefore, would seriously impact on operation of the device unless efficient T removal processes were developed.

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

“…A slight decrease indicate that the wall pumping is less at the end of the flat top. A similar behavior is found in gas balances for JET [24], but it is unclear, if it is possible to saturate a carbon wall completely. Typically 5 * 10 21 at/s are retained during a high density discharge.…”

Quantitative depth profiling of deuterium up to very large depths

“…Dots: Experimental data; Line: SIMNRA simulation; Arrows: Depth of origin of protons with the given energy. Bottom: Deuterium depth pro le including error bars using a tungsten density of 19.3 g/cm 3 . Applied doses of 3 He + : 690 keV 10 µC; 800 keV 5 µC; 2500 keV 5 µC; 4000 keV 5 µC.…”

“…Deuterium can penetrate in these materials up to depths of several tens to several hundreds of µm, where it is trapped at low concentrations [1,2]. It has been assumed that this deep penetration is an important process for the deuterium inventory in the tokamak Tore Supra [3], and it has been shown that deep penetration is dominating the remaining deuterium inventory in the all-tungsten tokamak ASDEX Upgrade [4]. In carbon materials this penetration is related to the porosity of the material, while in tungsten the di usion process is driven by the stress-eld induced by implanted deuterium [2].…”

The deuterium inventory in ASDEX Upgrade

“…It has been proposed that this di erence may be explained by deep di usion of D into carbon bre composite (CFC) material [32,33]. This deep di usion has been shown experimentally in laboratory experiments [34].…”