Speaker
Description
Recent improvements of the EMC3-EIRENE code have allowed to assess for the first time the
detached divertor scenario foreseen for ITER during ELM suppression by resonant magnetic
perturbation (RMP) fields. This is a major breakthrough because ELM suppression is required
for ITER to maintain the integrity of the plasma wall interface. The ITER divertor has been
designed based on extensive 2D (axisymmetric) simulations, but whether the 3D (nonaxisymmetric)
boundary from RMP application remains compatible with divertor operation in a
dissipative, partially detached state remains unknown. New EMC3-EIRENE results show that
detachment transition with RMPs occurs at lower upstream density within the traditional strike
zone of the symmetric configuration (see figure 1). At the same time, however, nonaxisymmetric
strike locations with a magnetic connection into the bulk plasma appear further
outside, and those remain attached - even at higher upstream densities when the symmetric
configuration is already (partially) detached. Neon seeding can mitigate those non-axisymmetric
heat loads by about 30% for an average impurity concentration of 1% at the separatrix.
Fundamental for the extended application range of EMC3-EIRENE has been the numerical
stabilization of the iterative solver by linearization of the electron energy loss term [a].
Furthermore, volumetric electron-ion recombination is now activated in EMC3-EIRENE along
with neutral-neutral collisions (in BGK approximation) and molecular assisted recombination.
Simulations for the ITER Pre-Fusion Plasma Operation (30 MW) show that detachment
transition occurs at lower upstream density within the traditional strike zone of the symmetric
configuration while non-axisymmetric strike locations further outwards remain attached [b].
Figure 1 shows the correlation between the magnetic footprint (a) and the heat load pattern (b) on
the outer divertor target. A reference simulation with comparable upstream density of nup ≈ 1.7 ·
1019 m-3 (evaluated at the midplane position of the separatrix of the symmetric configuration) is
included in 1 (c). The same amount of power as in the symmetric reference configuration is now
distributed over perturbed field lines connecting the bulk plasma to the target (red areas in figure
1(a)). Not only is the upstream heat flux at strike point A reduced (green box in Fig. 1.c), more
energy is lost to neutral gas and dissipated through cross-field diffusion here compared to the
reference (magenta box). Evaluation of the upstream-downstream pressure balance confirms the
earlier onset of detachment here.
On the other hand, no pressure and power losses are found at strike point B (blue box) which
remains attached at temperatures well above 10 eV. Simulations with Ne seeding have been
performed to explore a possible mitigation strategy in anticipation of the Fusion Plasma
Operation phase at 100 MW. It can be seen in figure 1 (c) that Ne seeding can indeed mitigate
the non-axisymmetric heat loads, but it is more efficient within the traditional strike zone
(approximately 0 – 15 cm from the separatrix of the symmetric configuration). Nevertheless, a
heat load reduction of about 30 % can still be achieved at strike point B with impurity
concentrations of 1 % averaged along the former separatrix.
Plasma response is key for reliable predictions of
divertor heat and particle fluxes. The present
simulations are based on MARS-F results [c] within a
single fluid, linearized resistive magneto-hydrodynamic
model. The relative phasing between coil rows of the
externally applied perturbation field can be optimized
for ELM control based on the X-point displacement
caused by the edge-peeling component of the plasma
response, but this is found to be correlated with a
relatively large magnetic footprint on the divertor
targets. Despite screening of resonances in the bulk
plasma, field amplification near the separatrix is found
and this determines the magnetic footprint size. It is
possible for the magnetic footprint to extend beyond its
size in the vacuum perturbation field approximation,
and it can be seen in figure 2 that it can even extend
beyond the dedicated high heat flux region on the
divertor targets (dashed line) under certain assumptions
related to plasma rotation. This can bring high heat
loads to those locations, and it is found that Neon seeding is significantly less effective under
these conditions.
Acknowledgments: This work was supported by the U.S. Department of Energy under grants DE-SC0013911, DESC0020357
and DE-SC0020284, by the College of Engineering at the University of Wisconsin - Madison, and by
the ITER Scientist Fellow Network.
[a] H. Frerichs et al., Nuclear Materials and Energy 18 (2019) 62–66
[b] H. Frerichs et al., accepted in Physical Review Letters (2020)
[c] L. Li et al., Nuclear Fusion 59 (2019) 096038
| Affiliation | Department of Engineering Physics, University of Wisconsin - Madison, WI 53706, USA |
|---|---|
| Country or International Organization | United States |
