The foreseen tokamak fusion reactors exhibit considerable problems, not yet fully solved, concerning the exhaust of particle and power onto the divertor targets. There the ELMs should induce huge thermal and mechanical stresses. Also, in the inter-ELM phase the peak loads could easily overcome the technical limits due to the foreseen very narrow convection channel. The present ways out are still questioned to be fully compatible with the postulated high core performance. Conversely type III ELMs mode, with small and grassy ELMs would practically eliminate at the origin these problems, but the reduced core confinement mode would ask for increased reactor dimensions to preserve an acceptable overall performance. The unavoidably higher cost would partially be balanced by a simpler design, now rid of all the extra complications needed for mitigating the mentioned problems. The viability of this route is investigated with the code COREDIV that provides a self-consistent scenario, by coupling a simplified 2D treatment of the edge with a 1D core description. The SOL is then treated more accurately by the 2D code TECXY, which considers the real toroidal geometry.
Here we set τE (energy confinement time) =0.5 and 0.6 times the standard H-mode value,
as roughly expected for a grassy ELMy H regime. The major radius R is scanned from 9 m (EU DEMO value) up to 13 m. Aspect ratio A=3.1 and safety factor q95=3.5 are equal to EU DEMO, while auxiliary power Paux=100 MW is chosen higher due to the lower τE and larger dimensions. Some cases with Paux, DEMO =50 MW are also considered for reference. For each of these cases the toroidal magnetic field BT is set to 6, 7 and 8 T, with plasma current, Ip, scaling linearly with R from ≈ 18 to 26 MA. Disregarded are here problems of mechanical sustainability, as well of feasibility of the magnetic controls/ circuitry. The electron density is fixed at the Greenwald limit, nGW, which in turn scales as 1/R. Moderate peaking is postulated for its radial profile, ne0/ne,line≈ 1.35 (≈1.5 for the standard L-mode) while the temperature profile is fixed by the core transport coefficients, determined in turn by τE. Density at separatrix is set to ne,line/2. Tungsten is the divertor target material.
Following recent DEMO studies, we also assume that 71.6% of the alpha power heats electrons and the remaining 28.4% heats ions[1, 2], and that the ratio of particle to heat
diffusion coefficients in the core plasma is ζD/e=0.35 2. Lower ζ values can easily cause strong accumulation of helium ashes and heavy performances degradation, while they scarcely affect the present-day experiments, which are almost free from He ashes.
Additional impurities, Ar or Xe, are considered for mitigating the loads onto the targets, still preserving a self-consistent solution with a power crossing the separatrix, PSOL, safely above the threshold for the PH->L mode back transition, i.e. PSOL≥ 1.2P H->L as done in .
The required inputs to TECXY, namely PSOL, density and temperature at separatrix are provided by the COREDIV results. The cross-field transport coefficients are adjusted in order to give an e-folding decay length inside the power transport channel as done in [3, 1] `(q≈3 mm, for the present version of the EU DEMO), which roughly corresponds to the value of the ion poloidal gyroradius at the outer equator. Diffusion coefficients twice larger are considered also, as more appropriate to a 2 times reduction of τE, with q longer by about √2 times. Ar or Ne, are then puffed in addition to the quantity of Xe and He indicated by COREDIV until the peak loads are driven down to ≤15 MW/m2. Then consistency with COREDIV is checked.
The first results are briefly sketched in fig. 1 for COREDIV and fig. 2 for TECXY. They indicate an interesting working window around R=11 m and B=7 T, not negligibly higher than in past studies , where τE is instead enhanced by 1.2 times. Larger radii can cause an accumulation of He ashes in the plasma core that tends to depress the fusion reaction rate. Higher fields, in addition of being problematic at these large radii, can cause too high fusion rate and then require a higher level of core radiation in order to alleviate the power to be exhausted inside the SOL. This however increases the risk of instability since a small fractional variation of the core radiation may imply a very large increase of the power input into the SOL.
The full compatibility of TECXY with COREDIV may be at risk in some cases due to divertor loads significantly larger on the outer than on the inner target. Larger q, i.e. wider transport channels, clearly soften considerably this problem. Advanced configurations, as snow flake plus (SF+), X and Super X divertor (XD/SXD) help solving these issues because of their higher radiation losses and/or better load balance between the targets (here, again are not considered the issues of the integrability in the machine design, and of the magnetic controls feasibility).
This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This scientific work was partly supported by Polish Ministry of Science and Higher Education within the framework of the scientific financial resources in the years 2014-2019 allocated for the realization of the international co-financed project.
1 M. Siccinio et al., “Development of a plasma scenario for the EU-DEMO: current
activities and perspectives”, IAEA FEC 2018, Ahmedabad, India, Oct. 2018
2 I. Ivanova-Stanik et al., Fus. Eng. Des., V. 146 (2019) 2021–2025
 M. Siccinio et al., Nucl. Fusion, V. 58 (2018) 016032 (15pp)
 H. Zohm, Phil. Trans. R. oc. A 377: 20170437.
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