Bernert, M.

Cavedon, M.

Lunt, T.

Pan, O.

Stroth, U.

Wischmeier, M.

English

MPG.PuRe

SOLPS-ITER simulations of the initiation of an X-point radiator in theASDEX Upgrade tokamakO. Pan1, M. Bernert1, T. Lunt1, M. Cavedon2, M. Wischmeier1, U. Stroth1,3,the ASDEX Upgrade teama and the EUROfusion MST1 teamb1 Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany2 Dipartimento di Fisica "G. Occhialini", Università di Milano-Bicocca, Milan, Italy3 Physik-Department E28, Technische Universität München, Garching, GermanyaSee author list of U. Stroth et al. 2022 Nucl. Fusion 62 042006bSee author list of B. Labit et al. 2019 Nucl. Fusion 59 086020The X-point radiator (XPR) [1, 2] is an attractive scenario that may contribute to solving thepower exhaust problem in future fusion devices, e.g. DEMO [3]. Recently, a series of experi-ments was carried out on the ASDEX Upgrade tokamak (AUG) to study the operational windowand access condition of the XPR. A reduced model [4] was derived to explain the physical mech-anisms for initiating a stable XPR. However, 2D numerical simulations are required to interpretFigure 1: Cross section of the line radiation den-sity in the SOLPS-ITER simulation with an X-pointradiator in AUG.the experimental observations not caught bythe reduced model, including the spatial dis-tribution of particle and power sources, cross-field transport and the high-field-side/low-field-side asymmetry of an XPR. In this work,the SOLPS-ITER transport code [5, 6, 7]was applied. The simulation [8] was able topresent an XPR phenomenon and reproducethe density and temperature profiles measuredby the newly installed divertor Thomson scat-tering [9]. Figure 1 shows the simulation re-sults of the line radiation density with an XPRwith nitrogen impurity seeding in AUG. Inthe confined region near the X-point, a coldXPR core with a temperature lower than 5 eVwas achieved, surrounded by a highly radia-tive band. Substantial volume processes, e.g. ionization and volumetric recombination, werefound in the cold XPR core.48th EPS Conference on Plasma Physics O3.106A simple sketch of an XPR in the SOLPS-ITER simulationBy analyzing the power, particle and momentum balances in the simulations, a simple sketchof an XPR can be depicted, as shown in Fig. 2. When the temperature in the divertor is low,impurity particles penetrate the confined region near the X-point and reduce the temperaturethere via line radiation. The parallel temperature gradient leads to a parallel conductive heatflux from the upstream position to the X-point in a narrow layer inside the separatrix. Thescrape-off layer and divertor receive less power and become colder, which decreases the opac-ity for neutrals. In addition to that, the large connection length and large magnetic flux ex-pansion in the vicinity of the X-point can facilitate the volume processes. As a result, moreneutrals are able to penetrate the confined region near the X-point and be ionized there. Plasmadensity and pressure increase in the highly ionizing zone, leading to a radial diffusive trans-port towards the inner confined region and a parallel convective particle flux from the ioniz-ing zone to the upstream position. The temperature at the X-point is reduced further by thedeuterium ionization losses, and the cold XPR core region of the XPR expands further intothe confined region. When the temperature at the X-point reaches around 1 eV, the volumet-ric recombination rate increases substantially. Plasma pressure drops in the cold XPR coreregion due to volumetric processes, momentum diffusion and radial transport, resulting in aparallel convective particle flux from the ionizing zone to the recombining zone. The particlerecycling from the divertor and main chamber wall decreases, while the total ionization ratekeeps almost constant, mainly supplied by neutrals generated by volumetric recombination.Figure 2: A simple sketch of an X-point radiator.With drifts, the cold and dense region ofthe XPR expands towards the upstream posi-tion on the high-field side. In the highly ion-izing region, a low plasma potential is found.Radial electric fields are generated by the po-tential gradients, pointing from the inner con-fined region to the highly ionizing region,resulting in Er×B drifts from the X-pointregion to the high-field-side upstream posi-tion. A potential hill is generated at the X-point, which corresponds to the poloidal cur-rent leaving the X-point to the divertor targetsand to the upstream position [10]. Given the48th EPS Conference on Plasma Physics O3.106temperature and its gradient as well as the electric conductivity near the X-point are alreadylow, the currents are mainly driven by the poloidal electric field. The poloidal electric fieldpointing away from the X-point leads to an Eθ ×B drift transporting particles clockwise fromthe high-field-side SOL to the low-field-side SOL above the X-point.Effect of neutrals and the magnetic connection length on the initiation of an XPRNeutrals penetrating from the adjoining cold divertor region and the large connection lengthnear the X-point play an important role in initiating an XPR. In the simulations, it was notpossible to achieve an XPR when an absorbing baffle was added artificially to hinder neutralspenetrating the confined region near the X-point, highlighting the role of neutrals.In Ref. [4], a reduced model indicates that an XPR is generated when the access parameter,XA ∼ n0,X ne,u fexpB2t,u/(T 2.5e,u B2θ ,u), (1)exceeds a certain value. In Eq. 1, n0,X is the neutral density at the X-point, ne,u and Te,u theelectron density and temperature at the outboard mid-plane (OMP), respectively, fexp the fluxexpansion between the OMP and the X-point, Bt,u and Bθ ,u the toroidal and poloidal magneticfields at the OMP, respectively.The access parameter of the reduced model was checked in the SOLPS-ITER simulations.It was found that an XPR with a temperature below 5 eV only occurred when the value of XAexceed a certain threshold. In addition to this, the threshold values were similar for the flux tubesat different radii. In order to check the effect of the magnetic pitch angle (sina ≈ Bθ ,u/Bt,u),similar scans of nitrogen seeding rate were carried out in simulations with a toroidal magneticfield of 1.8 T and 2.5 T. The results showed that the threshold of XA was robust with differenttoroidal field strengths. This indicates that the XPR access condition derived in the reducedmodel conclusively contains the main features related to the magnetic field.According to Eq. 1, an XPR can be achieved with a smaller magnetic pitch angle (i.e. largerconnection length) and a larger flux expansion at the X-point. Comparing the simulations withthe same input parameters but different toroidal magnetic fields, the case with a toroidal field of2.5 T (i.e. larger connection length) showed an XPR deeper inside the confined region comparedto the case with 1.8 T. In Ref. [11], it was also found that an XPR was achieved in the snowflakedivertor configuration with a much lower nitrogen seeding rate than that in the conventionalsingle-null configuration, benefiting from the dramatically larger connection length and fluxexpansion near the X-point in the snowflake configuration.48th EPS Conference on Plasma Physics O3.106SummaryThe SOLPS-ITER code package has been applied to study the X-point radiator phenomenonin the ASDEX Upgrade tokamak. The simulations showed qualitative agreement with the ex-perimental measurements and a reduced model for an XPR, filling in the gaps between theexperiments and theoretical analysis. The particle, power and momentum balances in the regionwith an XPR were analysed. In the flux tube close to the separatrix, a cold XPR core was foundwith recombining region at an electron temperature of ∼ 1 eV. The cold XPR core is surroundedby bands of ionizing and radiating regions with an electron temperature of ∼ 10 eV.The simulation results highlight the important role of neutrals and the magnetic connectionlength in the initiation of an XPR. By checking the dependence between the X-point temperatureand the access parameter derived in the reduced model, a threshold of the access parameterwas found in the simulations for the initiation of an XPR with a temperature below 5 eV. Thethreshold is robust for different flux tubes at various radii and for different toroidal magneticfield strengths, indicating that the reduced model already contains the most important physicsfor initiating an XPR.AcknowledgementsThis work has been carried out within the framework of the EUROfusion Consortium, fundedby the European Union via the Euratom Research and Training Programme (Grant AgreementNo 101052200–EUROfusion). Views and opinions expressed are however those of the author(s)only and do not necessarily reflect those of the European Union or the European Commission.Neither the European Union nor the European Commission can be held responsible for them.References[1] M. Bernert, et al., Nucl. Mat. Energy 12, 111 (2017)[2] M. Bernert, et al., Nucl. Fusion 61, 024001 (2020)[3] H. Zohm, et al., Nucl. Fusion 53, 073019 (2013)[4] U. Stroth, et al., Nucl. Fusion 62, 076008 (2022)[5] S. Wiesen, et al., J. Nucl. Mater. 463, 480 (2015)[6] F. Reimold, et al., J. Nucl. Mater. 463, 128 (2015)[7] I. Senichenkov, et al., Plasma Phys. Control. Fusion 63, 055011 (2021)[8] O.Pan, et al., to be submitted.[9] B. Kurzan, et al., Journal of Instrumentation 16.09 C09012 (2021).[10] V. Rozhansky, et al., Plasma Phys. Control. Fusion 63, 015012 (2021)[11] O. Pan, Technical University of Munich, PhD thesis (2020)48th EPS Conference on Plasma Physics O3.106

SOLPS-ITER simulations of the initiation of an X-point radiator in the ASDEX Upgrade tokamak

http://hdl.handle.net/21.11116/0000-000C-17A8-D

SOLPS-ITER simulations of the initiation of an X-point radiator in the ASDEX Upgrade tokamak

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