Cavedon, M.

Dux, R.

Kallenbach, A.

McDermott, R.

Vogel, G.

English

MPG.PuRe

Influence of CX-reactions on the radiation in the pedestal region atASDEX UpgradeR. Dux, M. Cavedon, A. Kallenbach, R. M. McDermott, G. Vogel and ASDEX Upgrade teamMax-Planck-Institut für Plasmaphysik, Garching, GermanyThe radiation loss in the pedestal region of H-mode plasmas is an important ingredient to tunethe power flux across the separatrix in a fusion reactor. The radiation critically depends on theionisation balance of the radiating impurity in this region. The abundance of each ion stage isdetermined by the rates for ionisation and recombination and the radial fluxes. It can be calcu-lated with an impurity transport code. The charge exchange (CX) reaction of neutral hydrogenwith an impurity ionAz++H0 → A(z−1)++H+ (1)is a very effective recombination process in addition to the electronic channels: radiative anddi-electronic recombination. The CX cross sections scale linearly with the impurity charge,10 100 1000 10000Te=Ti=Tn [eV]10-3310-3210-3110-30Prad/(ne n z) [Wm3 ]NeArKrXeNeArKrXenD/ne=0nD/ne=4x10-5Fig.1: Cooling factors of Ne, Ar, Kr, and Xe usingthe ionisation equilibrium without transport. Solidlines ar without CX-reactions and dashed line withCX for nD/ne=4×10−5.are very weakly dependent on the collisionenergy [1] and even a low fraction of neu-tral hydrogen can lead to a reduction of theaverage charge of the radiating species. Im-purities in lower charge stages can usuallyradiate more efficiently than higher chargestages leading to a higher radiation loss fora given impurity concentration. Fig.1 com-pares the radiative power parameter of no-ble gases with (dashed lines) and withoutCX-reactions (solid lines) using the ionisationequilibrium without transport. The CX ratesare included using a fixed fraction of neutralsnD/ne=4×10−5. The yellow region indicatesthe relevant temperature range in the H-modepedestal. For Ne, the radiation increase is verystrong. The effect becomes small for elementswith high nuclear charge as can be seen forXe. For temperatures lower than 100 eV, theincrease due to CX is again small. Thus, weexpect the largest effect in the pedestal regionwhere neutral densities can still be sufficiently large. Experiments in ASDEX Upgrade havebeen performed to measure the ionisation balance of neon in the pedestal region of H-modeplasmas and to assess the importance of CX recombination on the radiated power emitted byneon and argon. Details about these measurement and the modelling are given in Ref.[2] andare summarised below.47th EPS Conference on Plasma Physics O3.J501Measurement and modelling of the Ne ionisation balanceA steady neon puff was applied during the flat top phase of a type-I ELMy H-mode plasmawith plasma current Ip=0.8 MA, toroidal field Bt=2.5 T, heating power P= 7.7 MW, and lineaveraged density of 7.5×1019 m−3. The radial density profiles of Ne10+ and Ne8+ were mea-sured from the centre up to the separatrix using the core and edge systems of the charge ex-change recombination spectroscopy (CXRS). For Ne10+, the n =11-10 transition of NeX at524.9nm was used while Ne8+ was measured at the n =10-9 transition of NeVIII at 606.9nm,which is accompanied by the n =13-11 transition of NeVIII at 606.4nm. It is difficult to dis-tinguish with CXRS between the He-like Ne8+ ion and the fully ionised oxygen O8+ sincethe emission of NeVIII and OVIII are at the same wavelength. Therefore, the discharges wereperformed directly after a fresh boronisation of the vessel walls in order to have as low oxygen0 0.2   0.8 1 05ρpol[1017 m-3]densityNe 10+Ne8+CX on CX off 0150[kWm-3]radiated power #37062CX onCX offFig.2: Radial profiles of the Ne radiation and thedensities of Ne8+ and Ne10+. Black lines are fromexperiment, red/blue lines from the model with CXon/off.concentration as possible. Furthermore, theneon puff was started about 0.6 s after the startof the flat top to allow for a measurement ofthe density profile of O8+.The ionisation balance of neon has beenmodelled with the 1.5D-impurity transportcode STRAHL. It solves the radial transportequations of all ion stages of an impuritywhere neighbouring ion stages are coupledvia ionisation and recombination reactions.The neutral deuterium density is not con-stant on a flux surface in the confined plasma.However, the CX-reactions affect the ionisa-tion balance on the whole flux surface. Thus,it is sufficient to take the flux surface aver-aged CX-rates into account and a 1D trans-port model is a good approximation. Only alarge neutral density extending up to a few cmabove the X-point will cause a local changeof the ionisation equilibrium due to the longconnection length of the field lines in this re-gion. These neutrals are not included in our flux surface average and a calculation of the ioni-sation equilibrium in this area would require a 2D model.The lower graph of Fig.2 shows the experimental (black lines) and modelled density pro-files of Ne8+ and Ne10+ for #37062 t=7.2-8.5 s. A simultaneous fit of both densities can not beachieved when CX-reactions are not considered. When fitting the density of Ne10+, the densityof Ne8+ is too low by a factor of 15-28 for ρpol=0.9-0.98 (blue lines). The measured profilescan only be described when adding the CX-reactions (red lines). The fit yields for the neutraldensity at the separatrix nD,sep=1.9×1016 m−3 and nD=4.1×1015 m−3 at the maximum posi-47th EPS Conference on Plasma Physics O3.J501tion of the He-like neon density. These densities lead to a strong increase of the recombinationrates and the neutrals cause more recombination reactions than the electrons for ρpol >0.85,where nD/ne=2.8×10−6. The ratio of the two recombination channels reaches a value of 25at ρpol =0.98 with nD/ne=2.7×10−4 and a bit further out it has a maximum value of 50 withnD/ne=7.2×10−4.The much higher density of partially ionised neon inside of the pedestal top leads to anincrease of the total neon radiation, which is much larger than expected without CX-reactions,since line radiation from the measured He-like stage, but also from H-like and Li-like ions canradiate much more compared to the bremsstrahlung and recombination radiation invoked bythe fully ionised stage. The upper graph of Fig.2 shows the radiated power density of Ne forthe two cases. In the case with CX, it is larger by up to a factor of 12 than without CX. Thetotal radiated power emitted by neon inside the separatrix Prad,sep(Ne) increases by a factor of5 from 220 kW to 1.1 MW. The main drivers for this increase are the line radiation from Ne9+and Ne8+ whose contribution to Prad,sep is a factor of 9.6 and 16 larger than without CX. Themodelled radiation with CX explains the change of the bolometric measurements with the Nepuff much better than the model without CX.Effect of CX reactions on argon radiation in the pedestalIn the type-I ELMy H-mode discharge #36448 a strong argon puff was applied. The detectedradiances by bolometers, soft X-ray cameras and by the SPRED spectrometer measuring inthe vacuum ultraviolet (VUV) range are strongly dominated by argon radiation, such that thisplasma is a good test case to study the effect of CX-recombination on the pedestal radiationof argon. The argon density profiles could be inferred from the soft X-ray radiation and fromCXRS of Ar16+, which measured the n =15-14-transition of ArXVI at 541.2nm. However,without CX-recombination, the modelled radiances of the total argon radiation are about a factorof 2.2 lower than measured by the bolometer on lines-of-sights observing the main plasma. Thereason for this discrepancy is that in the pedestal and in the near SOL, the He-like ion stage,which can not radiate so efficiently, has the highest fractional abundance when CX is not takeninto account. This is strongly changed when CX is included. Here, the stages with Z = 8−15have higher abundances than the He-like stage and can contribute to the total emission of Ar inthis part of the plasma.The SPRED spectrometer, which has a radial line-of-sight at the plasma equator, measures inthe VUV region from 12 to 90 nm. Many VUV-multiplets emitted by argon from charge stagesbetween Z = 6 (Mg-like) and Z = 15 (Li-like) could be identified. The radiances of the multi-plets that could be well fitted account for about half of the total line radiation emitted by theseion stages. Fortunately, new atomic data sets from Bluteau [3] exist for all ion stages of argon,which enabled the calculation of the radiances of these multiplets for the case with and withoutCX as shown in Fig.3. With the exception of one outlier (ArXV at λ=26.5 nm), which is a factorof 7.4 too small, there is rather good agreement with the measurement when CX is on. Here, themean deviation between model and measurement is 24% and the maximum deviation is -44%.When CX is off, the emission from Z = 5 and 6 and from Z =15 are not much lower, but espe-47th EPS Conference on Plasma Physics O3.J501cially the lines from Z = 11 and 12, which come from the outer pedestal close to the separatrix,are much too low. The higher radiances with CX are not due to a shift of the emitting shell into a1018101910201021L [Ph m-2sr-1s-1 ]58.6670.5716.8915.319.321.91116.216.318.520.724.41218.619.925.31322.126.51437.115λ[nm]=Z=expwith cxwithout cx#36448 t=4.20-4.40sFig.3: The radiances of 17 multiplets emitted by argon ions with chargeZ = 6− 15 in the VUV range along a radial line-of-sight at the plasmaequator and the corresponding model values without (blue) and with (red)CX recombination. The main difference between the models with andwithout CX is for lines emitted in the confined plasma close to the sepa-ratrix (Z=11-13).region with higher ne andthus higher excitation andemission rates, but due toa higher fractional abun-dance of the emitting ion.For the Mg-like ion stage,which has its maximumin the SOL, the densitiesand the radiation are al-most equal. Even thoughthe CX recombination ismuch larger than the elec-tronic recombination forthis stage, it does not in-fluence the density distri-bution much. Rather it isgoverned by a balance ofthe ionisation rate and theradial transport, i.e. ionswhich are transported in-ward get ionised to higherstages, while most ionswhich move outwards do not recombine to lower ion stages during their travel time towardsthe plasma facing components.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] R.K. Janev and J.J. Smith (1993) Atomic and Plasma Material Interaction Data for Fu-sion (Atomic and Plasma Material Interaction Data for Fusion no 4) (Vienna: INTERNA-TIONAL ATOMIC ENERGY AGENCY)[2] R. Dux, M. Cavedon, A. Kallenbach, R.M. McDermott, G. Vogel et al, Nucl. Fus.60 (2020)126039.[3] M.M Bluteau (2019) Fundamental atomic data and prototype techniques for a generalisedcollisional-radiative model of medium-weight elements in fusion and astrophysical plas-mas, PhD thesis University of Strathclyde.47th EPS Conference on Plasma Physics O3.J501

Influence of CX-reactions on the radiation in the pedestal region at ASDEX Upgrade

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Influence of CX-reactions on the radiation in the pedestal region at ASDEX Upgrade

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