Angioni, C.

Artaud, J.

Bourdelle, C.

Colas, L.

Devynck, P.

Dumont, R.

Fajardo, D.

Fedorczak, N.

Goniche, M.

Hillairet, J.

Huynh, P.

Maget, P.

Manas, P.

Morales, J.

Ostuni, V.

Vézinet, D.

English

MPG.PuRe

Understanding tungsten accumulation during ICRH operation on WESTP. Maget, P. Manas, R. Dumont, C. Angioni1, J-F Artaud, C. Bourdelle, L. Colas,P. Devynck, D. Fajardo1, N. Fedorczak, M. Goniche, J. Hillairet, Ph. Huynh,J. Morales, V. Ostuni, D. Vézinet and the WEST team∗CEA, IRFM, F-13108 Saint Paul-lez-Durance, France. 1 Max-Planck-Institut fürPlasmaphysik, D-85748 Garching, Germany. ∗ see http://west.cea.fr/WESTteamSummary The transport of impurities in a tokamak with metallic walls can lead to accumulation eventsand radiative collapses, and this is particularly true when neoclassical transport is enhanced by sourcesof impurity poloidal asymmetry [1, 2]. On WEST, where the RF heating scheme (torque-free withoutcore particle sources) is relevant for ITER operation, these events are rare, but still radiative collapsesare sometimes observed [3, 4]. The physics of collisional tungsten transport in this case involves 3 mainchannels: i) a direct channel by which the hydrogen temperature anisotropy driven by ICRH drives apoloidal asymmetry of the electrostatic potential, ii) a non-linear channel by which the tungsten accu-mulation mitigates the ICRH drive by increasing the parallel temperature of the fast ions and reducingthe temperature anisotropy in a self-limited process, and iii) an indirect channel related with the ICRHinduced toroidal rotation. The role of Finite Orbit Width (FOW) effects is found to be instrumental : itstrongly reduces the hydrogen temperature anisotropy, the favorable Hydrogen Temperature Screening(HTS) effect, and the electron heat source. The tungsten peaking as a function of toroidal rotation andICRH power shows 2 possible branches consistent with bolometry inversion, one with a peaking drivenby ICRH and one with a peaking driven by rotation. The condition for a radiative collapse in the corebrings an additional constraint, and indicates that the electron heat source is probably much lower thancomputed, suggesting important losses of fast ions in the ripple. The radiative collapse would then bedue to a limited electron heat source and a tungsten peaking driven by the induced rotation. This peakingis initially moderate, and it is reinforced during the collapse as the temperature screening effect reverses.Observation of a radiative collapse during ICRH operation, and modelling tools Anexample of radiative collapse during ICRH operation is shown in fig. 1. The plasma current is0.5 MA and the magnetic field 3.7 T in this experiment, operated in L-mode at a Greenwaldfraction around 60%. As the power is ramped up, the radiative power in the core region slowlyincreases and as the power exceeds 4.3 MW the collapse takes place. The tungsten peakingnW (0)/nW (0.3) reconstructed by bolometry inversion shows a moderate value around 1.5 beforethe collapse, but it rapidly increases to about 4 during the fall of the core temperature. Althoughwe have no core rotation measurement for this experiment, the acceleration of a magnetic islandlocated near the plasma edge at q = 4 shows that plasma rotation is modified by ICRH. We48th EPS Conference on Plasma Physics O4.114Figure 1: Radiative collapse in ICRH oper-ation: ICRH and radiative power, radiativelosses in central and peripheral regions, andcore electron temperature.Figure 2: Electron heat source (top) andnormalized temperature anisotropy (bottom)with / without FOW effects (EVE/AQL).analyze here this experiment with the collisional transport codes FACIT [5, 6] and NEO [7], thesuite of codes EVE [8] and AQL [9] for the ICRH heat deposition and temperature anisotropy,and the METIS integrated simulation tool for the current profile evolution [10].ICRH deposition : Finite Orbit Width (FOW) effects The hydrogen temperature anisotropythat forms the main specific drive for tungsten peaking during ICRH is very sensitive to FiniteOrbit Width (FOW) effects as the perpendicular temperature becomes large. A surrogate modelfor FOW effects has been implemented in EVE, correcting the Zero Orbit Width (ZOW) compu-tation. We find that both the electron power density and the temperature anisotropy are stronglyreduced in the core region (fig. 2), thus reducing at the same time the margins for compensatingcore radiation and the drive for tungsten peaking through the electrostatic force. Another conse-quence is that the Hydrogen Temperature Screening effect, that produces an outward tungstenpinch for a sufficiently large radial gradient of the perpendicular hydrogen temperature [11], isstrongly reduced in the core. This is verified with NEO [7]: when FOW effects are neglected, ahollow tungsten profile is expected (fig. 3), in contradiction with experimental observations.Tungsten peaking: the 3 channels There are 3 main channels by which the tungsten profilein the plasma core is influenced by ICRH. The first one is direct, and is due to the poloidal asym-metry of the electrostatic potential. It generates an increased W concentration at the High FieldSide, which is higher with the temperature anisotropy of the hydrogen minority. The secondone is non-linear and has not been considered in previous studies. It is related with the increase48th EPS Conference on Plasma Physics O4.114Figure 3: Normalized tungsten gradient com-puted with NEO without (top) and with (bottom)FOW effects, and with / without HTS effect.Figure 4: Electron heat source (top) and par-allel hydrogen temperature (bottom) com-puted by EVE/AQL for Ze f f = 1.4 and 2.6,and theory T‖ ∝ Z2/3e f f ("th.").of the parallel hydrogen temperature by pitch angle scattering on impurities [12], and givesT‖ ∝ Z2/3e f f , while the perpendicular temperature and electron heat source remain unchanged, asverified with EVE/AQL (fig. 4). The accumulation of tungsten in the core tends therefore toreduce the temperature anisotropy T⊥/T‖ (direct channel), and leads to a self-regulating mech-anism. Finally, the third channel is indirect, and is related with the ICRH induced rotation.The steady-state tunsgten profile in the core is determined using the collisional transport codeFACIT [5, 6], and a map of the tungsten peaking as a function of toroidal rotation and ICRHpower is computed using EVE/AQL results at t = 5.6s, just before the radiative collapse, witha temperature anisotropy assumed to be proportional to the ICRH power (fig. 5). This diagramshows two regions of large tungsten peaking : one at low rotation where the peaking is due toICRH (direct channel), and one at large rotation where it is due to the centrifugal force (indirectchannel in the absence of external torque). The two regions are separated by a valley where thetungsten profile is nearly flat.Understanding the collapse The experimental situation can be evaluated by using the in-verted bolometry signals, that give at t = 5.6s a power radiated inside r/a = 0.3 of about 0.4MW. Mapping this constraint in the diagram of figure 5 gives 2 branches of solutions, on bothsides of the valley (stars in the bottom plot of fig. 5). If we add the criterion pe/pmaxrad that shouldapproach unity at the time when the collapse starts, we find that the low rotation branch is dis-qualified, with a ratio that is below 40%. The collapse criterion can be met only on the largerrotation branch, but then the effective ICRH power should be significantly lower than computed.48th EPS Conference on Plasma Physics O4.114Figure 5: Tungsten peaking and ratiopmaxrad /pe as a function of toroidal rotationand ICRH power.Figure 6: ICRH power (reduced by 60%), coreelectron temperature, predicted and bolometryinverted maximum radiated power density, andelectron heat source (pe).In such case, the linear relation that has been assumed between the electron heat source and thetemperature anisotropy is no longer valid, and the analysis can only provide a rough estimateof the real situation. If we reduce the ICRH electron heat source and temperature anisotropy by60%, we do cross the collapse criterion at the right time, for a plasma rotating at about 50 km/s(fig. 6). The tungsten peaking is then driven by the induced rotation, and rises strongly duringthe collapse as the temperature screening effect reverses.Aknowledgments This work has been carried out within the framework of the EUROfusion Consor-tium, funded by the European Union via the Euratom Research and Training Programme (Grant Agree-ment No 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. Neitherthe European Union nor the European Commission can be held responsible for them.References[1] C. Angioni and P. Helander, Plasma Physics and Controlled Fusion 56 (2014) 124001.[2] C. Angioni, Plasma Physics and Controlled Fusion 63 (2021) 073001.[3] M. Goniche, submitted to Nuclear Fusion (2022).[4] V. Ostuni, This conference O4.62 (2022).[5] P. Maget et al., Plasma Physics and Controlled Fusion 62 (2020) 105001.[6] D. Farajo et al., Plasma Physics and Controlled Fusion 64 (2022) 055017 .[7] E.A. Belli and J. Candy, Plasma Physics and Controlled Fusion 50 (2008) 095010.[8] R. Dumont, Nuclear Fusion 49 (2009) 075033.[9] R. Dumont and D. Zarzoso, Nuclear Fusion 53 (2012) 013002.[10] J. Artaud et al., Nuclear Fusion 58 (2018) 105001.[11] F. Casson et al., Nuclear Fusion 60 (2020) 066029.[12] T. Stix, Nuclear Fusion 15 (1975) 737.48th EPS Conference on Plasma Physics O4.114

Understanding tungsten accumulation during ICRH operation on WEST

http://hdl.handle.net/21.11116/0000-000C-1747-B

Understanding tungsten accumulation during ICRH operation on WEST

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