Garcia-Muñoz, M.

Jacobsen, A.

Jensen, T.

Kappatou, A.

Korsholm, S.

Nielsen, S.

Rasmussen, J.

Salewski, M.

Vallar, M.

Verdier, T.

English

MPG.PuRe

Fast-ion dynamics at ITER-relevant densities in ASDEX Upgrademeasured with collective Thomson scatteringJ. Rasmussen1, T. Verdier1, S. K. Nielsen1, A. S. Jacobsen1, A. Kappatou2, T. Jensen1,S. B. Korsholm1, M. Salewski1, M. Garcia-Muñoz3,4, M. Vallar5, the ASDEX Upgrade Team61 Technical University of Denmark, Dept. of Physics, Lyngby, Denmark2 Max-Planck-Institut für Plasmaphysik, Garching, Germany3 Centro Nacional de Aceleradores, University of Seville (CSIC), Seville, Spain4 Dept. of Atomic, Molecular and Nuclear Physics, University of Seville, Seville, Spain5 Ecole Polytech. Fédérale de Lausanne (EPFL), Swiss Plasma Center, Lausanne, Switzerland6 See author list of U. Stroth et al. 2022 Nucl. Fusion 62 0420061. Introduction: Energy transfer from confined fast ions will play a key role in maintainingthe core plasma at fusion-relevant conditions in future fusion devices such as ITER. The dy-namics of such ions is experimentally well characterized in low-density plasmas, owing to goodfast-ion coverage with diagnostics such as fast-ion D-α spectroscopy (FIDA), neutron/γ-rayspectrometry, and collective Thomson scattering. In contrast, this applies much less to high-density discharges, in which the relevant diagnostic performance is more limited by the shorterslowing-down time of energetic particles, the reduced neutral beam (NBI) penetration, and thesignificantly increased Bremsstrahlung emission. Collective Thomson scattering (CTS) is lesshampered by the latter of these limitations and will be the main diagnostic for measuring con-fined fast ions in ITER across their full energy range [1, 2, 3]. Although ITER will achieve acombination of high density n and low core collisionality ν ∝ nT−3/2 that cannot be accessedin present devices, it remains relevant to experimentally test predictions of energetic-ion dy-namics at high density as a step in preparing for ITER operation. Here we present the first CTSmeasurements of fast-ion dynamics in ASDEX Upgrade (AUG) performed at a local density ofne ≈ 9×1019 m−3, i.e., similar to projections for the high-density ITER baseline scenario [4].2. Measurements and analysis: Our measurements are based on AUG discharge #39648,with timetrace shown in Figure 1. This H-mode discharge involved 5 MW of NBI heating and3.2 MW of electron cyclotron resonance heating (ECRH). The discharge was divided into anintermediate– and a high-density phase, with the central line-integrated density increasing to(7.0± 0.2)× 1019 m−2 at t > 4.0 s. In both phases, NBI source Q3 was active throughout toallow FIDA measurements, while Q8 (on-axis deposition) and Q6 (off-axis) were interspersedin order to vary the fast-ion distribution function.48th EPS Conference on Plasma Physics O4.1121 2 3 4 5 6 700.51 AUG #39648 (a)Ip [MA]WMHD [MJ]Prad [10 MW]1 2 3 4 5 6 702468(b)H-1 [1019 m-2]ne,CTS [1019 m-3]Te,CTS [keV]1 2 3 4 5 6 7t [s]0123P [MW](c)ECRHCTS Gyr6NBI Q3Q6Q8Q5111122221 1.5 2 2.5R [m]-0.8-0.6-0.4-0.200.20.40.60.8z [m](d)ProbeReceiverH-1Figure 1: (a) Timetrace of AUG discharge #39648 (plasma current Ip, stored energy WMHD, radiatedpower Prad). (b) Line-integrated density "H-1" along the sightline shown in panel (d) and ne and Te in theCTS measurement volume. Shaded areas mark the TRANSP fast-ion output times discussed in Section 3.(c) Auxiliary heating power and power of the 105 GHz CTS probe gyrotron Gyr 6. (d) CTS geometry att = 4.7 s, showing the CTS probe and receiver beams and the resulting scattering volume (ellipsoid).CTS measurements were taken from t = 2.0–6.0 s, using both "active" and "passive" receiverviews [5]. The measurement geometry derived from raytracing is illustrated in Figure 1(d). Herewe focus on results obtained during the high-density phase at t = 4.0–6.0 s, in which the av-erage electron density and temperature in the CTS measurement volume at ρt = 0.11± 0.02was ne = (9.0± 0.3)× 1019 m−3 and Te = 3.9± 0.2 keV, respectively, with a measurementangle relative to the local magnetic field of φ = 108±1◦. To aid the interpretation of the mea-surements, we performed forward modelling of the acquired CTS spectra based on the CTSscattering geometry in Figure 1(d), along with ne, Te, and ion temperature in the CTS volumefrom integrated data analysis [6] where relevant, and the fast-ion distribution function predictedby TRANSP/NUBEAM v. 20.3 [7]. TRANSP was here run without anomalous ion diffusion,given the absence of clearly identifiable MHD modes at the times of interest.3. Results and comparison to modelling: Resulting CTS spectra obtained during phases withNBI Q3+Q8 and Q3+Q6 heating are shown in Figure 2, together with the corresponding for-ward models. The data represent results from the CTS filterbank alone and from averaging thefilterbank and fast digitizer [8] data. Both data sets reveal reasonable overall agreement withthe TRANSP-based forward model, whereas a model without fast ions cannot reproduce theobserved spectra. In particular, the CTS spectrum is observed to narrow from t = 4.7–5.4 s in48th EPS Conference on Plasma Physics O4.112Figure 2: CTS spectra and corresponding forward models based on TRANSP simulations during thehigh-density (a) NBI Q3 + Q8 phase at t ≈ 4.7 s and (b) Q3 + Q6 phase at t ≈ 5.4 s. Solid magenta linesrepresent a model using the fast-ion concentration nfast/ne predicted by TRANSP in the CTS measure-ment volume, while dashed blue lines show a model with no fast ions.response to the reduced fast-ion concentration predicted by the model. Nevertheless, there areindications of deviations from the model predictions, notably at f ≈ 106.0–106.5 GHz in Fig-ure 2(a). This feature is not related to fast ions, which generate a monotonic spectrum for thisgeometry, but is possibly associated with parametric decay of the probe beam [9].Inversions of both spectra to infer the underlying fast-ion distribution functions are shownin Figure 3(a). The results broadly agree with the TRANSP neoclassical predictions, althoughminor discrepancies are seen close to the thermal bulk. As a further comparison, we show inFigure 3(b) the FIDA signal in AUG #39648 normalized by the corresponding beam emission,a proxy for fast-ion density [10]. Although the density step at t ≈ 3.7–4.0 s reduces the FIDAsignal-to-noise ratio (whereas that of CTS is largely unaffected), useful FIDA data are still ob-tained at high ne. The FIDA profiles would suggest a drop in fast-ion content in the CTS volumefrom the Q3+Q8 to the Q3+Q6 phase by a factor ≈ 2.0, consistent with the factor ≈ 2.1 inferredoutside the thermal bulk from both CTS and TRANSP in Figure 3(a).4. Conclusions and outlook: To our knowledge, these results represent the first CTS measure-ments at reactor-relevant densities (∼ 1×1020 m−3 [4]) in any tokamak. Forward modelling ofthe acquired CTS spectra in this high-density discharge demonstrates that a measurable fast-ionpopulation is present in the plasma core (ρt ≈ 0.11) at a level of 3–5% of ne. Comparison toTRANSP simulations suggests core fast-ion dynamics consistent with neoclassical transport inboth real space and velocity space at high density in ASDEX Upgrade. Our measurements thusprovide experimental validation of the ability of CTS to characterize fast-ion dynamics under48th EPS Conference on Plasma Physics O4.112Figure 3: (a) 1D fast-ion distribution functions g(u) outside the thermal bulk versus projected ion ve-locity u along the CTS measurement angle. (b) FIDA signal normalized by beam emission (BES) atthe times of interest, integrated over wavelengths corresponding to fast-ion energies of 34.0–53.7 keV.Shaded region marks the location of the CTS volume at the relevant times.conditions that may be challenging for certain other diagnostics and at fast-ion levels of justa few % of ne, as will be relevant for CTS at ITER [3]. Full inference of the underlying 1Dion velocity distribution from the measured CTS spectra across the discharge will be presentedelsewhere [Verdier et al., in prep.] and will allow detailed comparison of our results to TRANSPpredictions with and without anomalous diffusion.Acknowledgements 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] M. Salewski et al., Nucl. Fusion 58, 096019 (2018)[2] S. B. Korsholm et al., EPJ Web of Conf. 203, 03002 (2019) and Rev. Sci. Instrum. submitted (2022)[3] J. Rasmussen et al., Nucl. Fusion 59, 096051 (2019)[4] A. C. C. Sips et al., Nucl. Fusion 58, 126010 (2018)[5] S. K. Nielsen et al., Plasma Phys. Control. Fusion 57, 035009 (2015)[6] R. Fischer et al., Fusion Sci. Technol. 58 675 (2010)[7] J. Breslau et al., "TRANSP." Computer software. June 27, 2018. https://doi.org/10.11578/dc.20180627.4[8] M. Stejner et al., Rev. Sci. Instrum. 85, 093504 (2014)[9] S. K. Hansen et al., Nucl. Fusion 60 106008 (2020)[10] B. Geiger et al., Rev. Sci. Instrum. 84 113502 (2013)48th EPS Conference on Plasma Physics O4.112

Fast-ion dynamics at ITER-relevant densities in ASDEX Upgrade measured with collective Thomson scattering

http://hdl.handle.net/21.11116/0000-000C-174C-6

Fast-ion dynamics at ITER-relevant densities in ASDEX Upgrade measured with collective Thomson scattering

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