Understanding and Controlling Tungsten Accumulation in JET Plasmas with the ITER-Like Wall

The development of H-mode scenarios in JET with ITER-like wall (ILW) has indicated the need of understanding and controlling W accumulation, which hinders the attainment of good confinement for long durations. This paper presents experimental results, interpretative modeling and theory predictions of W transport in JET ILW standard H-mode (STH) and Hybrid plasmas, including first results of heavy impurity Laser Blow-offs (LBO). A key element for the understanding has been the availability of theory models for W neoclassical and turbulent transport (NEO [3] and GKW respectively) that include the effects of rotation on transport coefficients and poloidal asymmetries. The typical pattern towards W accumulation has been identified in a Hybrid discharge as due to progressive peaking of the density and reversal of neoclassical convection from outward to inward, but is in fact common to both scenarios, as can be seen from the time traces shown in Figure 1. Both discharges feature NBI heating only and show similar time evolution of the density profiles and SXR emission (Figure 1 d and e). In the STH however the more regular and frequent sawtooth activity (Figure 2) limits the central W peaking, allowing attainment of a stationary discharge, which in Hybrids is not possible (Figure 1b). The sawtooth activity seems a better candidate than ELMs to explain the difference in stationarity in STHs vs Hybrids, because ELMs, although more frequent in STH (Figure 2), regulate the W influx from the edge rather than its spatial profile. So the presence of regular ELMs is a necessary condition in both scenarios to reduce the W influx and avoid high W contamination, but it is only a central expulsion mechanism like MHD crashes that can counteract the unfavourable transport that causes central accumulation.
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