Cavedon, M.

Dunne, M.G.

Dux, R.

Fischer, R.

Viezzer, Eleonora

English

Dunne, M. G.

idUS. Depósito de Investigación Universidad de Sevilla

Assessment of the current density evolution during an ELM cycle usingbeam emission polarimetry at ASDEX UpgradeR. Dux1, E. Viezzer2, M. Cavedon1, M. G. Dunne1, R. Fischer1,and the ASDEX Upgrade team1Max-Planck-Institut für Plasmaphysik, Garching, Germany2Dept. of Atomic, Molecular and Nuclear Physics, University of Seville, SpainThe knowledge of the current density in the pedestal region of H-mode plasmas is importantto understand the stability of the pedestal with respect to edge localised modes (ELMs). Thecurrent density is connected to the poloidal magnetic field Bp via Ampères law, which can bedetermined by an accurate measurement of the field line angle γ = arctan(Bp/Bt) where Btis the toroidal magnetic field. In an H-mode discharge with low frequency type-I ELMs, theevolution of the γ-profiles in the pedestal region has been measured with the beam emissionpolarimetry (BEP) diagnostic at ASDEX Ugrade.Diagnostic SetupThe Dα line of fast neutral beam atoms is split due to an electric field in the rest frameof the atoms (motional Stark effect). This electric field ~F is given by the velocity of the fastatoms ~v and the magnetic field ~B: ~F =~v× ~B and might also have a small contribution fromthe radial electric field in the H-mode edge. The multiplet contains in the centre σ -lines, whichare polarised perpendicular to the electric field, and π-lines, which are polarised parallel tothe electric field. The BEP diagnostic [1] determines their polarisation direction at 5 spatiallocations at the plasma edge covering ≈8 cm. Three independent observations of each spatiallocation are used, where each optical head includes a polariser, which is roughly oriented at 0o,45o, and 90o with respect to the electric field. The full multiplet is measured with a spectrometerwith 3 ms time resolution. The 0o/90o channel serves as a reference for the strength of the π-/σ -lines, while the 45o channel yields via radiance variations of the π- and σ -lines informationabout changes of the electric field direction, which are connected to changes in the direction ofthe magnetic field. The line radiance of the transition i of Dα emitted by beam neutrals (of onespecific energy) with a polarisation angle α between ~F and the electric field of the light waveis given by an integral over the line-of-sight (LOS), which for σ - and π-lines just differs by anangle factor, which is here denoted with the value of the pure Stark effect.Lσ ,i =14π∫LOSn(l)r(l) fi sin2 α dl Lπ,i =14π∫LOSn(l)r(l) fi cos2 α dl (1)Here, n(l) is the number density of the unattenuated beam neutrals, r(l) is the total rate ofemitted Dα photons per injected beam atom, fi is the relative strength of transition i, whichusually differs from the known values obtained with thermally equilibrated occupation of theupper levels [2]. The deviation of fi from the thermal equilibrium is described by 3 non-thermaloccupation numbers, which deliver fi for all 9 lines. The line radiance yields with the sensitivitysLOS a count rate ċi = sLOSLi at the CCD camera of the spectrometer. Since the variation of the46th EPS Conference on Plasma Physics P1.1002signal with the polariser orientation is measured on individual LOS, all factors that can influenceċi besides the change of α need to be taken into account.The sensitivity of each LOS is determined by measuring the count rates when illuminatingthe optical heads with a calibrated light source. The relative signals of the individual LOS are        σρ=1.026        σρ=1.004        σρ=0.982        σρ=0.960     012 σρ=0.937       σ+π       σ+π       σ+π       σ+π     01 σ+π 658  660    π 658  660    π 658  660    π 658  660    π 658  660  01π#34436 t=3.431-3.434swavelength [nm]spectral radiance [1018  Ph/m2 /sr/s/nm]Fig.1: The measured spectra (black) and the fit (red) for all radialpositions (columns) and the three polariser orientations (rows). Theupper row with α≈90o shows mainly σ -lines, the middle row withα≈45o π- and σ -lines and the lower row with α≈0o just π-lines.then measured by injecting thebeam into the gas filled toruswithout any magnetic field.Here, the excitation of thebeam atoms is in an equi-librium and the photon rater is a constant. There is nosplitting into Stark compo-nents and the count rates are:ċ = sLOSr/(4π)∫n(l)dl, where∫n(l)dl is calculated from abeam model which takes thebeam source geometry, thevertical and horizontal focallength and the angular spreadof the beam into account. Themodel delivers a small vari-ation of∫n(l)dl around thevalue for the central LOS, which reaches up to ±4% for the outer LOS. The measured Dαdistribution shows some scatter around the results of the beam model with a maximum devia-tion of 5%. It is used to refine the sensitivity values for each LOS.In H-mode plasma discharges, the photon rate r strongly increases within the edge transportbarrier (roughly with the main ion density) and becomes much flatter after an ELM. A numericalmodel of the beam emission in that region showed, that r is sufficiently well described whenusing a dependence on the flux surface coordinate only. Furthermore, the model revealed that astrong r gradient introduces radiance variations of the 0o- and 90o-channel with respect to thecorresponding 45o-channel of up to ±5%. Thus, the actual r-profile has to be determined withinthe fit procedure.Fit procedureThe fit procedure of the obtained spectra is performed in three steps. In the first step, the spec-tra of three LOS which observe the same radial location through different polarisers are fitted to-gether. For the Stark multiplets of the full, half and third energy component of the beam, the totalDα line radiance is an individual fit parameter. The non-thermal occupation of the upper levelis taken to be equal for the three LOS and also the field line angle, which determine the relativeintensities of the individual lines. Furthermore, the spectral radiance of the continuum emis-sion, the red wing of the passive Dα-emission around 657 nm and the radiance of the CII dou-46th EPS Conference on Plasma Physics P1.1002 360370380390400410Bp [mT]BEPEQH 0.941.00ρ pol4.2 4.4 4.6 4.8 5.0t [s]0.91.01.1I p [MA]#34536 t=4.000-5.098sFig.2: The temporal change of Bpfrom BEP and the equilibrium re-construction agree well on the inner-most BEP channel for a dischargewith a ±2% modulation of Ip.blet at 657.8 nm and 658.3 nm are individual fit parametersfor each LOS. In the next step, signal combinations, whichare insensitive to the exact values of α and fi are formed,i.e. L45 = ∑Li,45 and L0+90 = ∑(Li,0 +Li,90), and are fittedusing a common profile r(Ψ). In the third step, the spectra ofthree LOS are fitted again but now the total Dα line radiancecomes from the line integral using r(Ψ). The non thermaloccupation numbers, the radiance of CII, the passive Dα andthe continuum background are taken from the old fit. Here,the field line angle is effectively the only free remaining fitparameter. In all fits, the small influence of Er is taken intoaccount using CXRS-measurements. The quality of the fit isvery good as is shown in Fig. 1 for a single exposure duringthe inter-ELM phase of discharge #34336.Experimental ResultsIn an H-mode discharge with a pre-programmed modulation of Ip of ±20 kA around the meanvalue of 1 MA (lowest graph in Fig.2), the evolution of Bp on the innermost channel of the BEP 234567 ρ=0.96ρ=1r[104 Ph/s/at] 340360380400ρ=0.96ρ=1Bp[mT] 010203040∆ρ=1-0.96∆Bp[mT]-5 0 5 10 15 20∆tELM [ms]05101520I SOL[kA]#34436 t=3.0-6.5sFig.3: The evolution of r and Bpfrom BEP at ρpol=0.96 and 1 andof ∆Bp=Bp(1)-Bp(0.96) are shownfor 3 ms wide time bins. The redstars depict the results from pressureconstrained equilibrium reconstruc-tions.diagnostic at a normalised poloidal flux label ρpol≈0.96(middle graph of Fig. 2) reproduces well the modulationfrom the equilibrium reconstruction (upper graph in Fig.2).In the type-I ELMy H-mode discharge #34436 with con-stant Ip=1 MA, Bt=-2.5 T, PNBI=5 MW and PECRH=1.2 MWthe evolution of the Bp profile in the pedestal region duringan ELM cycle could be measured. The discharge had a ra-dial sweep of the plasma column by 1 cm to obtain a denseradial grid of BEP measurements. The data measured from3 to 6.5 s were considered, where there were 124 ELMs,but only the 67 most similar ELM periods were selected forfurther analysis. The selection was based on a correlationanalysis of the divertor shunt current ISOL evolution. Theselected ELMs cause an energy loss of ∆WELM=50 kJ andthe mean period is 28 ms. ISOL is shown in the lowest graphof Fig. 3. The BEP data were grouped in 1 ms wide timebins using the time difference of the measurement time t tothe ELM onset time ∆tELM= t-tonset, where t is the centreof the 3 ms long integration time of the CCD camera. Theprofiles of r and Bp within each bin were fitted to obtain themean profile and the temporal evolution of the mean valuesat ρpol=0.96 (pedestal top) and 1. These mean values are shown in Fig. 3. With the ELM, theprofile of the photon rate, which is roughly proportional to the main ion density profile, and46th EPS Conference on Plasma Physics P1.1002of Bp flatten and subsequently recover to the pre-ELM value. The evolution of Bp during bothphases is much slower than of r. 05101520[kPa]p 0.00.20.40.60.81.0j tor [MAm-2] PSBS+CD+OHMBSDIA0.94 0.96 0.98 1.00ρpol0.00.51.01.5j tor [MAm-2] total350360370380390400[mT]Bp#34436 t=3-6.5s before ELMEQBNEOFig.4: Profiles of pressure, toroidalcurrent densities and Bp at the lo-cation of the BEP diagnostic fromEQB and a neoclassical code calcu-lation.A fit to the data of r and Bp with 4 ms-averages of a simplefunction, which changes linearly in time from the pre-ELMvalue at ∆tELM=0 to a peak value and then exponentially de-cays back, yields the solid lines in the graphs. The poloidalfield difference ∆Bp between ρpol=1 and 0.96 is shown in thethird graph of Fig.3. The solid line is just the difference ofthe two fit functions for Bp. ∆Bp is about 25 mT at the end ofthe recovery. This is much more than calculated from stan-dard equilibrium reconstruction. Fitting pedestal profiles ofne, Te and Ti profiles after an ELM and after the recoveryallowed to calculate equilibria which are constrained by theedge pressure profiles and the SOL currents [3]. The ∆Bpvalues for these equilibria (EQB) are in good agreementwith the measured values and are shown as red squares inFig. 3. We ascribe a 20% uncertainty to the equilibrium val-ues due to the uncertainties of the pressure profile, since thetoroidal current is mainly driven by the pressure gradient as is shown below. For the pre-ELMequilibrium reconstruction, the Bp profile at the location of the BEP measurements (outboardside, 30 cm below the magnetic axis) is shown in the lower panel of Fig. 4. The absolute Bpvalues from EQB are 48 mT below the BEP result and the Bp traces in Fig.3 have already beencorrected using this offset. The reason for the shift is under investigation. Fig. 4 shows also theprofiles of pressure and toroidal current densities from EQB at this location (solid lines). Thestrongest contribution to jtor delivers the Pfirsch-Schlüter (PS) current, then comes the sum ofbootstrap (BS), ohmic and externally driven current, and finally the low diamagnetic current.In addition, BS-, PS- and diamagnetic current from a neo-classical calculation with a similarpressure profile is depicted in Fig.4 using lines with crosses. By comparison of the profiles, itcan be concluded that the ohmic and current drive contribution is about 50% of the BS-part.The local bootstrap current at this outboard side location amounts only to about 22% of thetotal toroidal current density and the BEP measurements are not very sensitive to test models ofthe bootstrap current in this region.AcknowledgementThis work has been carried out within the framework of the EUROfusion Consortium and has received fundingfrom the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053.The views and opinions expressed herein do not necessarily reflect those of the European Commission.References[1] E. Viezzer, R. Dux et al, Rev. Sci. Instrum.87 (2016) 11E528.[2] O. Marchuk et al, J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 011002.[3] M. G. Dunne et a Nucl. Fusion 52 (2012) 123014.46th EPS Conference on Plasma Physics P1.1002

Assessment of the current density evolution during an ELM cycle using beam emission polarimetry at ASDEX Upgrade

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Assessment of the current density evolution during an ELM cycle using beam emission polarimetry at ASDEX Upgrade

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idUS. Depósito de Investigación Universidad de Sevilla