7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Experimental study and interpretative modelling of the power exhaust in
configurations with multiple divertor X-points in TCV

S. Gorno1, O. Février1, C. Theiler1, F. Bagnato2, C. Colandrea1, B. P. Duval1, F. Felici1, T. Lunt3, L.
Martinelli1, A. Merle1, A. Perek1, H. Reimerdes1, L. Safar1, L. Simons1, M. Wischmeier3 and theTCV teama1
École Polytechnique Fédérale de Lausanne (EPFL), Swiss Plasma Center (SPC), Lausanne,
Switzerland; 2 ITER Organization, Route de Vinon-sur-Verdon, CS 90 046, 13067 Saint Paul Lez
Durance Cedex, France; 3 Max-Planck-Institut für Plasmaphysik, Garching bei München, Germany;
aSee the author list of H. Reimerdes et al. 2022 Nucl. Fusion 62 042018.
e-mail (speaker): sophie.gorno@epfl.chPower exhaust remains a major challenge in magnetic
confinement fusion, as the unmitigated heat flux reaching
the wall in future reactors is predicted to greatly exceed
material limits. Alternative divertor configurations arebeing developed to reduce target heat loads, should thestandard Single Null (SN) not scale to reactors such as
DEMO [1]. Additional divertor X-points, such as in theSnowflake-minus (SF-) configuration [2], can increase theplasma wetted area by diverting the scrape-off layer(SOL) towards multiple divertor targets, and can greatly
increase the parallel connection length (L||) by enlarging
the region of traversed low poloidal field.
SF- experiments in TCV have demonstrated a reduction ofthe peak parallel target heat flux by up to 66% comparedto the SN, while being able to control the heat flux
reaching each active target by varying the distance
between the two X-points [3]. Interpretative modelling ofthese plasmas with the coupled fluid plasma and kinetic
neutral code EMC3-EIRENE suggests that theredistribution of power to the far-SOL strike-point (andcorresponding reduction of peak target heat flux) is due toincreased cross-field transport. Simple analytical models
propose that divertor radiative cooling capability
increases with L|| [4]. However, these low density SFexperiments and simulations do not indicate any
significant increase in radiated power compared to the SN
configuration. An increased core effective charge in theSF- compared to the SN indicates a degradation in divertor
impurity retention, which would reduce the impurity
radiation. In an effort to increase SOL-core plasma
screening, further SF- experiments were performed at
higher core density, for which a small increase in divertor
radiated power is observed in the SF-.
To accentuate the effect of additional X-points andenhanced connection length on power exhaust, an extreme
divertor shape was developed in TCV, named the Jellyfish(JF). The JF features three divertor X-points and 4x longer
L|| than in the SN, bringing strong target heat flux
reductions in regions of enhanced L|| with respect to the
SN and SF- geometries. Remarkably, the JF shows noincrease in divertor radiated power with respect to the SN,
despite the extreme connection length enhancement. A
reduced 1D SOL model will be used to study the effect ofconnection length on divertor power exhaust in isolationfrom effects of impurities and other geometrical aspects.
References[1] H. Reimerdes et al. 2021 Nucl. Fusion 61, 024002[2] D. Ryutov, V. Soukhanovskii 2015 Phys. Plasmas 22,110901[3] S. Gorno et al. 2023 Plasma Phys. Control. Fusion
65, 035004[4] D. Post et al. 1995 Phys. Plasmas 2, 2328