Birkenmeier, G.

Cano-Megias, P.

Doyle, S.

Gage, K.

Galdon-Quiroga, J.

Gonzalez-Martin, J.

Sanchis, L.

Suarez Lopez, G.

Tal, B.

Willensdorfer, M.

English

MPG.PuRe

Fast-ion first orbit displacement induced by externally applied magneticperturbations in the ASDEX Upgrade tokamakJ.Galdon-Quiroga1,2, L.Sanchis3, K. Gage4, G.Birkenmeier2, P.Cano-Megias1,S.Doyle1, J.Gonzalez-Martin1,4, G.Suarez Lopez2, B.Tal2, M.Willensdorfer2,and the ASDEX Upgrade Team21 University of Seville, Seville, Spain2 Max Planck Institute for Plasma Physics, Garching, Germany3 Aalto University, Espoo, Finland4 University of California, Irvine, United StatesIntroductionThe so called H-mode of operation is considered as a possible candidate for future fusionreactors. Inherent to the H-mode, instabilities known as edge localized modes (ELM) appearin a cyclic fashion expelling energy and particles from the confined region to the walls of themachine. These instabilities can cause excessive heat loads to the plasma facing components,and therefore must be avoided. One of the most explored avoidance techniques is the ELMcontrol using externally applied magnetic perturbations (MP) [1]. However, it has been observedboth experimentally and numerically, that these 3D applied MPs can have a deleterious impacton fast-ion confinement [2, 3]. Therefore, a good understanding of the mechanisms responsiblefor the associated fast-ion transport is required to develop optimized MP configurations and tovalidate our numerical tools for reliable predictions. In this work we present an experimentalstudy at the ASDEX Upgrade tokamak (AUG) of fast-ion first orbit displacement induced byexternally applied MPs as a function of the applied spectrum by means of the so-called light ionbeam probe technique (LIBP) [4]. Experimentally we control the applied MP spectrum via thephase between upper and lower set of MP coils (∆Φul) installed at AUG. The LIBP techniqueallows to infer the fast-ion orbit displacement from the modulation of the fast-ion loss detector(FILD) signal. In the case of experiments dealing with 3D perturbations, a generalization of theLIBP formula [4] is needed:∞∑n=1ξFIn · cos(n ·φ +βn) = Lne(~rbirth) ·[F−〈F〉〈F〉]−∞∑n=1ξpln · cos(n ·φ +αn) (1)where φ is the toroidal coordinate, n is the toroidal mode number of the applied perturbation,ξ FI is the fast-ion orbit displacement induced by the MPs, ξ pl is the plasma boundary displace-ment, Lne is the density scale length evaluated at the birth position of the probe ions measured47th EPS Conference on Plasma Physics P5.1024by FILD, F is the FILD signal and 〈F〉 is the FILD signal averaged over a full MP cycle. Here,the term in the left hand side is what we want to infer, while the terms in the right hand side areobtained from experimental measurements.Experimental measurements   (n=2) mm02468101214Figure 1: (a) Plasma boundary dis-placement as measured by the Li-BES diagnostic. (b) Fast-ion orbitdisplacement inferred by the appli-cation of the LIBP technique. Themeasurement associated to the fast-ion population of NBI source Q7 isplotted in red. In blue, the same forQ8.These experiments were carried out in ELM mitigated H-mode plasmas at Bt = −1.8 T and Ip = 0.8 MA, in lowersingle null configuration with low triangularity, character-ized with an edge density of ∼ 2− 2.7 · 1019 m−3, a plasmaβ ∼ 2− 2.5, q95 = 3.85 and Ze f f ∼ 1.4. All the shots fol-lowed the same structure: 2.7 MW of ECRH power wereused, together with 2.5 MW of NBI source Q3, in the back-ground, for diagnostic purposes. On top of this, phases of∼ 2s of 2.5 MW of the corresponding probe beam wereapplied. A rigid rotation at 1 Hz of an n=2 MP configu-ration was applied with a fixed ∆Φul , which was modifiedon a shot-to-shot basis. For each case the plasma boundarydisplacement (PBD) at the outer midplane is measured us-ing the lithium beam emission spectroscopy diagnostic (Li-BES), following the same procedure as in [5]. The correctiondue to the plasma control system was taken into account. Thefast-ion orbit displacement is inferred using the FILD signaland applying Eq.1. The correction due to the effect of the 3Dplasma boundary displacement on the FILD signal is takeninto account.The results are shown in Fig.1. In (a) the PBD is plottedagainst ∆Φul . In (b) the inferred fast-ion orbit displacement is plotted against ∆Φul , for NBIsource Q7 (in red) and Q8 (in blue). It can be seen that the PBD ranges from 2-10 mm, whilethe fast-ion orbit displacement ranges from 3-20 mm. Within experimental errorbars, there isno difference between NBI sources Q7 and Q8. Interestingly, there is a shift between bothcurves: the minimum of the PBD appears at ∆Φul ∼ 0o, while the minimum of the fast-ion orbitdisplacement appears at ∆Φul ∼ 50o. This shift suggests that the effect of MPs on ELM controlcould be decoupled from the MP induced fast-ion transport. A similar experiment was carriedout where the MP coil current IMP was scanned from 0 to 1.2 kA, also at a fixed MP spectrum.Two cases were studied: an n=2 perturbation at ∆Φul = −120o and 45o, corresponding to the47th EPS Conference on Plasma Physics P5.1024values of maximum and minimum fast-ion orbit displacement found on Fig.1(b). Again, bothNBI sources Q7 and Q8 were used as probing beams. The results are shown in Fig.2, whereonly the case corresponding to ∆Φul = −120o is shown, for NBI source Q8 in (a) and Q7 in(b). In both cases, it can be seen that the intensity of the FILD signal scales almost linearly withthe applied current. In the case of NBI source Q7, a clear threshold of the onset of the fast-ionlosses is obsvered at a coil current of ∼ 0.3 kA. Such a clear threshold is not observed for NBIsource Q8. There is also no clear evidence of a threshold for the saturation of the FILD signal,as was observed in KSTAR experiments [6].FILD1 (a.u.)1.00.80.60.40.2IBl5 (kA)0 0.2 0.4 0.6 0.8 1AUG #38013FILD1 (a.u.)1.00.80.60.40.2 =-120ºNBI Q8NBI Q7(a)(b)Figure 2: (a) Evolution of theFILD signal as a function of theapplied MP coil current in AUGshot 38013 for NBI source Q8(a) and Q7 (b).ModellingModelling of these experiments has been carried out with theorbit following code ASCOT. In the first place, the impact of theperturbed 3D kinetic profiles on the FILD signal was evaluated,independent from the MP induced fast-ion orbit displacement,by carrying out simulations in a 2D magnetic equilibrium andshifting the kinetic profiles accordingly. It was found that for aplasma boundary displacement of ±1 cm, consistent with theexperimental observations, a modulation of the FILD signal ofup to ∆F〈F〉 ∼ 0.5 can be obtained. This justifies the need to in-clude a correction due to the plasma boundary displacement tothe LIBP formula in Eq.1. In a second step, the first-orbit fast-ion displacement induced by the MPs has been evaluated. Thesimulations are done by using a magnetic configuration witha fixed ∆Φul and following markers with fixed initial position(R,Z) and pitch angle (λ =v‖v ), and scanning the initial toroidalcoordinate φ from 0− 2π . The markers are followed during∼ 30µs, corresponding to a couple of poloidal transits, and the radial displacement is evalu-ated at the outer midplane. An example is illustrated in Fig.3. In (a) the FILD signal for AUGshot 37620 is shown, corresponding to ∆Φul = 120o and beam source Q8. In (b) the results froman ASCOT simulation using ∆Φul = 120o (including the plasma response) and an initial pitchangle λ = 0.40 (consistent with the pitch angle range measured by FILD) is shown. In (c) thefourier decomposition of both signals is plotted. In both cases it can be seen that the n=2 com-ponent is clearly dominant. However, it is worth to note that other components such as the n=4and n=6 also show up, both in the experimental measurement and in the ASCOT simulation.This suggests that, in addition to the main toroidal mode number of the applied MP, including47th EPS Conference on Plasma Physics P5.1024the additional sidebands might be important for reliable predictions towards future experiments.The toroidal phases corresponding to the maximum and minimum fast-ion orbit displacementspredicted by ASCOT seem to be consistent with the experimental trend. =120ºAUG #37620FILD1 [a.u.]24680(b)(a)(c)5.0 5.5 6.04.54.0ASCOTR max [m]2.202.21R ini=2.175 mPitch=0.40; PRFILDASCOT0 2 4 6 8-8 -6 -4 -2Toroidal Mode NumberFFT Amp [a.u.]1.00.60.2Time [s]Figure 3: (a) Experimental modula-tion of FILD signal during AUG shot37620 for beam source Q8. (b) Simu-lated modulation of FILD signal withASCOT. (c) Fourier transform of thesignals shown in (a) and (b).Summary and outlookIn this work the first-orbit fast-ion displacement in-duced by externally applied MPs has been measured fortwo different NBI sources as a function of ∆Φul by meansof the LIBP technique in the AUG tokamak. The mea-sured displacements range between 3-20 mm, being theminimum at ∆Φul ∼ 50o. No difference is observed be-tween NBI sources Q7 and Q8, within experimental er-rorbars. This is compared to the plasma boundary dis-placement at the outer midplane, inferred from Li-BESmeasurements. The PBD ranges from 2-10 mm, being theminimum at ∆Φul = 0o, which is consistent with previ-ous observations [7]. The shift between the minimum inthe fast-ion orbit displacement and the PBD at the outermidplane suggests that the MP induced fast-ion transportcould be decoupled from the ELM control, thus openingthe possibility for the optimization of the applied MP configuration. A scan of the fast-ion lossesas a function of the intensity of the applied MP reveals a threshold for the onset of fast-ion lossesfor NBI source Q7, similar to what was reported in KSTAR [6]. The FILD signal scales roughlylinearly with the applied MP coil current. No clear evidence of saturation of fast-ion losses isobserved.Acknowledgements: This work has been partially supported by the Helmholtz Associationunder grant no. VH-NG-1350 and by the Spanish Ministry of Science and Innovation undergrant no. FJC2019-04192-IReferences[1] T.Evans et al, Nature Physics 2, 419-423 (2006)[2] M.Garcia-Munoz et al, Plasma Physics and Controlled Fusion 55, 124014 (2013)[3] L.Sanchis-Sanchez et al., Plasma Physics and Controlled Fusion 61, 014038 (2019)[4] X.Chen et al., Review of Scientific Instruments 85, 11E701 (2014)[5] M.Willensdorfer et al., Plasma Physics and Controlled Fusion 58, 114004 (2016)[6] K.Kim et al., Physics of Plasmas 25, 122511 (2018)[7] M.Willensdorfer et al., Plasma Physics and Controlled Fusion 61, 014019 (2019)47th EPS Conference on Plasma Physics P5.1024

Fast-ion first orbit displacement induced by externally applied magnetic perturbations in the ASDEX Upgrade tokamak

http://hdl.handle.net/21.11116/0000-0009-ACB1-D

Fast-ion first orbit displacement induced by externally applied magnetic perturbations in the ASDEX Upgrade tokamak

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