Edge plasma physics issues for the Fusion Advanced Studies Torus (FAST) in reactor relevant conditions

Abstract

The issue of "First wall materials & compatibility with ITER /DEMO relevant plasmas" is among the R&D missions for possible new European plasma fusion devices that the FAST project will address. FAST can operate with ITER relevant values of P/R (up to 22 MW/m, against the ITER 24 MW/m, inclusive of the particles power), thanks to its compactness; thus it can investigate the physics of large heat loads on divertor plates. The FAST divertor will be made of bulk W tiles, for basic operations, but also fully toroidal divertor targets made of liquid lithium (L-Li) are foreseen. To have reliable predictions of the thermal loads on the divertor plates and of the core plasma purity a number of numerical self-consistent simulations have been made for the H-mode and steady-state scenario by using the code COREDIV. This code, already validated in the past on experimental data (namely JET, FTU, Textor), is able to describe self-consistently the core and edge plasma in a tokamak device by imposing the continuity of energy and particle fluxes and of particle densities and temperatures at the separatrix. In the present work the results of such calculations will be illustrated, including heat loads on the divertor. The overall picture shows that, marginally in the intermediate and, necessarily in the high density H-mode scenarios (<n e >=2 and 5·10 20 m -3 respectively), impurity seeding should be foreseen with W as target material: however, only a small amount of Ar (0.03% atomic concentration), not affecting the core purity, is sufficient to maintain the divertor peak loads below 18 MW/m 2 , that represents the safety limit for the W monoblock technology, presently accepted for the ITER divertor tiles. Li always needs additional impurities for decreasing divertor heat loads, the Z eff value being than 1.8. At low plasma densities (but 1.3·10 20 m -3 ), typical of steady state regimes, W by alone is effective in dissipating the input power by radiative losses, without excessive core contamination. Impurity seeding would lead to excessive W sputtering by Ar and too high Z eff . The impact of the ELMs on the divertor in the case of a good H-mode with low pedestal dimensionless collisionality will be discussed too.

Preliminary Comparison of the Conventional and Quasi‐Snowflake Divertor Configurations with the 2D Code EDGE2D/EIRENE in the FAST Tokamak

“…According to a previous work done with the 1D-2D coupled core-edge code COREDIV [11], we focused on the cases more critical from the power load point of view: the standard H-mode scenario at low density and high power. We tackled the difficulties of adapting a complex code as EDGE2D/EIRENE to run with the snowflake magnetic configuration.…”

“…These values were the same as in [11,12]. The transport coefficient assumed led to the following decay lengths on the outer mid-plane λ n = 2.0cm for the particle density, λ T = 1.0cm for both T e and T i , so we sampled the SOL with 25 flux rings for a width in the outer mid-plane of ∼ 2.4cm and ∼ 4cm for quasi-snowflake (QSF) and SN respectively.…”

Simulations of the SOL plasma for FAST, a proposed ITER satellite tokamak

“…They span the whole expected range in the reference H-mode scenario with a volume averaged density n e = 2 × 10 20 m −3 [1]. Indeed, usually n s,out is in the range n e = 2-3 × n s,out [4,9]. The highest density case n s,out = 1.2 × 10 20 m −3 has been added as a further indication for the achievement of a detached regime and as a first step towards the extension to the extreme H-mode scenario with n e = 5 × 10 20 m −3 [1].…”

“…Its value is ∼22, as in ITER or larger with x = 1, while with x = 1.5 it is ∼16, as in a reactor with R = 8 m and 350 MW of power loss (∼1.4 GW of neutron power). The additional possibility to control the radiated power fraction in a wide range (20-80%) through impurity injection [4], further broadens the FAST flexibility in studying the SOL physics and hence the PWI.…”

“…After a first general evaluation with the COREDIV code [4], a more detailed preliminary analysis is presented here with EDGE2D [5] coupled to the neutrals Monte Carlo code EIRENE. These first simulations, aimed mainly to optimize the divertor design and to identify the conditions for acceptable heat loads are illustrated in Section 2.…”