Study of global wall saturation mechanisms in long-pulse ELMy H-mode discharges on JT-60U

Variation of particle absorption at the first wall has been investigated in long-pulse (similar to 30 s) ELMy H-rnode discharges on JT-60U. Quantitative analysis of particle balance indicated that particle inventory at the first wall was globally saturated with a time scale of 10-15 s after several long-pulse discharges. To understand mechanisms of the global wall saturation, distribution of a local wall saturation time on the first wall was calculated from the ion and neutral particle fluxes to the first wall evaluated using a Monte-Carlo neutral particle transport code. The local wall saturation time was estimated to be shorter than 1 s at the divertor plates and the divertor dome, similar to 10 s at the lower half of the baffle plates and similar to 100 s at the main chamber wall, respectively. This result suggested that the divertor plates, the divertor dome and the lower half of the baffle plates were saturated in a single discharge. On the other hand, the main chamber wall was not saturated in a single discharge. Based on the above result, a model of the global wall saturation was proposed, where dynamic and static inventory regions are defined depending on the wall temperature.