Aguiam, D.

Bobkov, V.

Dunne, M.

Faugel, H.

Fünfgelder, H.

Noterdaeme, J.

Ochoukov, R.

Strumberger, E.

Suarez Lopez, G.

Suttrop, W.

Willensdorfer, M.

Zohm, H.

English

MPG.PuRe

ICRF antenna coupling in ASDEX Upgrade 3D plasmasG. Suárez López1,2, R. Ochoukov1, M. Willensdorfer1, H. Zohm1,2, D. Aguiam3, V. Bobkov1,M. Dunne1, H. Faugel1, H. Fünfgelder1, J.-M. Noterdaeme1,4, E. Strumberger1, W. Suttrop1,the ASDEX Upgrade Team∗, and the EUROfusion MST1 Team.†1 Max Planck Institute for Plasma physics, Garching b. Munchen, Germany2 Ludwig-Maximilians-University of Munich, Munich, Germany.3 Instituto de Plasmas e Fusao Nuclear, Universidade de Lisboa, Lisboa, Portugal4 Applied Physics Department, University of Ghent, Ghent, BelgiumIntroduction: Ion cyclotron range of frequencies (ICRF) heating is a widespread method forplasma heating in experimental fusion reactors. It relies on the excitation of the fast mode of amagnetized plasma at a frequency matching the cyclotron frequency of a given ionic species inthe plasma core, such that resonant damping can occur. The fast wave, however, is evanescentfrom where it is excited due to the low-density conditions in the plasma edge, and the givenantenna k‖ spectrum. The cold plasma dispersion relation can be approximated as [6]:n2⊥ =(R−n2‖)(L−n2‖)S−n2‖(1)Where R, L and S are the elements of the cold plasma dielectric tensor and n‖,⊥ = ck‖,⊥/ω isthe parallel/perpendicular refractive index. The evanescent region is defined as the volume fromthe antenna to the R-cutoff where R = n2‖ such that n2⊥ = 0. The minimization of this volumeis beneficial for the ICRF system, as it improves the accessibility of the fast wave to the coreplasma, and thus relaxes the voltages in the feeding lines. Undesirably, plasma instabilities suchas MHD modes and edge localized modes (ELMs) modify the edge density and thus changeICRF coupling [3] . A controlled way to study the effect of non-axisymmetric MHD modes onICRF coupling is through the application of magnetic perturbations (MPs) from in-vessel saddlecoils. At the same time, the effect of MPs themselves on ICRF coupling can be quantified in thismanner [5]. In this paper, we describe a set of coupling experiments conducted on the ASDEXUpgrade tokamak in which MPs were applied on discharges with ICRF heating.ASDEX Upgrade experiments: ICRF coupling can be influenced by MPs in two ways. Onthe one hand, the known pump-out effect tends to improve ICRF coupling as it produces arelaxation of the density profiles in the Scrape-Off Layer (SOL) [4]. On the other hand, the∗For a complete list of authors, see A. Kallenbach et al, Nucl Fus 57 (2017) 102015†For a complete list of authors, see H. Meyer et al, Nucl Fus 57 (2017) 10201445th EPS Conference on Plasma Physics P2.1072plasma response to the MPs results in field-aligned kink displacements that compress or expandthe density profiles [7]. In this study, we focus on the last effect.PICRF   10 19  10 **abcd21MProtationFigure 1: On the left, (a,b,c,d) time traces of the discharges: Safety factor at 95%poloidal flux q95, line-averaged density 〈ne〉, total deuterium puff Dpufftot and totalpower Ptot. On the right, ICRF embedded reflectometry lines of sight and ICRF an-tennas. An axisymmetric and an exaggerated perturbed separatrix are shown.A set of four H-modedischarges plus a ref-erence one were per-formed in order tostudy the effect ofMPs on ICRF cou-pling. The plasma pa-rameters were kept con-stant with Bt =−2.5T,Ip = 0.8MA, resultingin an edge safety factorq95 ∼ 5.3, with centralelectron density ne ∼ 5∗1019 m−3. A total heating power of Ptot ∼ 12MW was applied, out ofwhich PICRF ∼ 2.7MW was delivered simultaneously by 4 ICRF antennas in dipole phasing, re-sulting in a low pedestal top collisionality ν∗e < 0.3. The fraction of PICRF/Ptot ∼ 23% was keptlow as to reduce the impact of the changing coupling conditions on plasma global parameters.The rest of the heating power was supplied by NBI (∼ 6.8MW) and ECRH (∼ 2.5MW). TheMPs were switched on at 2 seconds, during the flat top of the discharge and were supplied withIcoil = 5kA× turns. Two differential phasings ∆ϕUL between the upper and lower row of coilswere applied per discharge in n=2 configuration and rotated with a frequency of ν = 3Hz for di-agnostic purposes. The last two discharges ended in prompt disruption, so only one ∆ϕUL phasewas acquired for them, resulting in a set with ∆ϕUL = {−145◦,−45◦,0◦,+45◦,+90◦,180◦}.For the discharge with ∆ϕUL = {−45◦,+45◦} a tungsten event produced a density spike from1.5-3s, hence this time slice is rejected in the analysis. ICRF coupling conditions were assessedthrough measurements of the loading resistance. A voltage probe in the voltage anti-node of thetransmission lines provides Vmax and a directional coupler behind the matching system providesPcoupled, such that:RL =2PcoupledZ20Vmax2(2)Where Z0 is the characteristic impedance of the line. The non-axisymmetric density profile infront of one of the three-strap antennas was obtained with embedded reflectometry [1] withthree active channels, labeled as {1,4,8} in figure 1. The R-cutoff position is obtained fromeq. 1 with the assumption of a H-D mixture of 5%-95% in the plasma, and using the CLISTE45th EPS Conference on Plasma Physics P2.1072axisymmetric magnetic induction field calculation. This way, the total tokamak field is includedin the analysis, although a small error is made in considering a pure axisymmetric field. For thethree-strap antennas, a value of k‖ = 11m−1 is usually representative for dipole phasing, as thepower balance between the central and outer straps was kept at 2:1 [2], however, the R-cutoffwas positioned beyond the density measurements for this k‖, thus two values of k‖ = 6m−1(for Ref. 8) and 8m−1 (for Ref.1 and Ref.4), the latter being the main parallel wavenumberfor the two-strap antennas spectrum, are used to illustrate the cutoff behavior instead. Since thereflectometer uses X-mode, the maximum resolvable depth differs among channels dependingon their radial location inside the vessel. Density and coupling measurements are presented infigure 2 for the ∆ϕUL = {0◦,180◦} discharge. All considered time traces were analyzed withELM-filtered data. The density profiles are plotted along their line of sight distance dLOS.abcdeI [A]R  [Ω]LdLOSFigure 2: On the left, (a,b,c) density measurements at the different reflectometer channels Ref1, Ref4, Ref8plotted along lines of sight, (d) Loading resistance measured at the feeders of the three-strap antenna, (e) appliedcurrents to the MPs corrected for the PSL attenuation. The arrow marks the direction of rotation of the kinkdisplacement along the antenna. On the right plot, the poloidal cross section of ASDEX Upgrade with reflectometerchannels.A clear density oscillation is observed in front of the ICRF antenna when the MPs are rotated.This corresponds to excited stable kink modes being rotated in front of the antenna. The R-cutoff position oscillates with the density profile, and the amplitude of the oscillation directlydepends on the k‖ value, with larger oscillations the larger k‖ is. Coherent coupling oscillationsare registered as measured in the two feeding lines of the same antenna. It is worth noting thatwhen the MPs change from in-phase (∆ϕUL = 0◦) to out of phase (∆ϕUL = 180◦), the plasmaresponse changes, decreasing the displacements in front of the antenna and thus the couplingoscillations. It is well-known that the plasma kink displacement varies as a function of theapplied poloidal spectra from the MPs, ∆ϕUL. By performing a least square sinusoidal fit to theloading resistances for every one of the applied ∆ϕUL phasings, the coupling amplitude variationcan be obtained. This is displayed in figure 3, where ∆RfeedL is the amplitude of the sinusoidal45th EPS Conference on Plasma Physics P2.1072fit and 〈RL〉 is the loading resistance time average of the considered window. The noise levelof the fit is calculated by performing the same sinusoidal fit to the loading resistances for thedischarge with no MPs, and then taking the average. The error bars represent the uncertainty ofthe fit alone. It is to be noted that induced currents on the passive stabilization loop (PSL) dueto the MP rotation, lag and attenuate the strength of the MP field with respect to its DC value.Therefore, the resultant field is slightly out of phase with respect to the applied coil currents.[h!]2 strap antennas3 strap antennasFigure 3: Loading resistance percentage change for the (upper) 2-strap anten-nas and (lower) 3-strap antennas. Left and right straps from the 2-strap anten-nas, as seen from the antenna reference frame towards the plasma. Outer strapsin each three-strap antenna are connected to the same feeder.A clear dependence of theloading resistance oscilla-tion amplitude on the ap-plied ∆ϕUL is observed wellabove the noise level. Thiscorroborates the picture ofthe coupling change beingdirectly correlated to theplasma kink displacements.A maximum amplitude of10% is reached, with a sim-ilar behavior between thetwo-strap antennas and thethree-strap antennas. Nevertheless, larger coupling changes are expected, well correlated withlarger plasma displacements, if the MP field attenuation becomes smaller, for instance, withslower MP rotation frequencies or larger coil currents.Conclusions: The influence of MP-induced plasma displacements on ICRF coupling has beencharacterized for H-mode discharges in ASDEX Upgrade. Density variations due to the ex-cited kink modes produce accompanying displacements of the fast wave R-cutoff, which in turncauses antenna coupling changes. Rotation of the kink modes in front of the antennas producesperiodic coupling variations up to 10%, coherent with the MP field rotation.Acknowledgments: This work has been carried out within the framework of the EUROfusion Consortium andhas received funding from the Euratom research and training programme 2014-2018 under grant agreement No633053. The views and opinions expressed herein do not necessarily reflect those of the European CommissionReferences[1] D. E. Aguiam et al. In: Review of Scientific Instru-ments 87.11 (2016).[2] V. Bobkov et al. In: Nuclear Fusion 56.8 (2016).[3] V. Bobkov et al. In: Nuclear Fusion 46.7 (2006).[4] V. Bobkov et al. In: AIP Conference Proceedings1580.2014 (2014).[5] G. S. López et al. In: EPJ Web of Conferences 157(2017).[6] T.H.Stix. AIP, New York, 1992.[7] M. Willensdorfer et al. In: Nuclear Fusion 57.11(2017).45th EPS Conference on Plasma Physics P2.1072

ICRF antenna coupling in ASDEX Upgrade 3D plasmas

http://hdl.handle.net/21.11116/0000-0008-7874-F

ICRF antenna coupling in ASDEX Upgrade 3D plasmas

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