Impact of the Carbon and Tungsten Wall Materials on Deuterium Recycling and Neutral Fuelling in JET using EDGE2D/EIRENE

To investigate the impact of full metal walls on the operational boundaries of next step devices, the former, all-carbon plasma-facing components (PFCs) of JET (JETC) were replaced with a full metal ITER-like wall (JET-ILW), with beryllium main chamber limiters and tungsten divertor. This refurbishment is anticipated to have a strong impact on the fuel recycling processes, where deuterium particles impacting material surfaces are either reflected as atoms, with a considerable fraction of the impact energy, or implanted in the material. These implanted particles can be further thermally re-emitted as molecules into the plasma. In steady state conditions, particles will be implanted and reemitted with equal rates, thereby, leading to particle balance in the plasma. The reflection process depends on the impact angle, energy, and on the mass ratio between the projectile and substrate. Tungsten with its high mass is predicted to lead to higher particle and energy reflection for recycling species, compared to carbon surfaces. Hence, one of the fundamental consequences expected, is the increase of atomic versus molecular fraction in the recycling fluxes, which is expected to impact the divertor conditions, plasma fuelling, core charge exchange (CX) losses, and the pedestal width and height. The beryllium surfaces used in the main chamber may also have different fuel recycling properties compared to the carbon surfaces, since the reflection coefficients vary strongly between mass ratios 4.5 for beryllium and 6 for carbon, assuming deuterium fuel particles. The surface chemistry differences between beryllium and carbon might also play a role. In JET-C, molecules were observed to dominate the recycling flux with a fraction of about 70­ 90%. Similar observations have been reported on deuterium recycling on graphite surfaces in TEXTOR [5]. In JET-ILW, on the other hand, a larger contribution of atoms versus molecules in the divertor plasmas were measured. In this study, the multi-fluid code EDGE2D/EIRENE was employed to investigate numerically the impact of the PFC materials on the edge neutral sources in JET for unseeded L-mode plasmas (2.5T/2.5MA/~3MW) in V5/Stack-C and vertical target (VT) configurations [9] as well as for the inter-ELM phase of unseeded H-modes (2.7T/2.5MA/~16MW) in HT3R configuration(Fig. 1a). In the simulations the poweracross the core boundary was obtained from the core power balance: 2.2MW (L-mode), 10MW (steady-state inter-ELM H-mode). By neglecting impurities, the simulations are conducted using exactly the same input parameters for both JET-C and JET-ILW. In the Hmode simulations, such an approach, however, neglects the strong reduction in the unseeded H-mode pedestal confinement in JET-ILW compared to JET-C [11]. Therefore, while the majority of the H-mode simulations are conducted assuming JET-C like pedestal based on [12], a few simulations in the JET-ILW are run with reduced pedestal temperatures corresponding to the actual measurements by increasing the pedestal conductivity by a factor of 2.
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