Bogar, K.

Cano-Megias, P.

Cavedon, M.

Cruz-Zabala, D.

Dux, R.

Garcia-Munoz, M.

Jansen van Vuuren, A.

Kappatou, A.

Nishizawa, T.

Schneider, P.

Viezzer, E.

English

MPG.PuRe

Determination of the background neutral density from passive Balmer alpha CXemission at the ASDEX Upgrade tokamakA. Jansen van Vuuren1, K. Bogar2, D.J. Cruz-Zabala1, M. Cavedon3, P. Cano-Megias4, A. Kappatou3,R. Dux3, P.A. Schneider3, E. Viezzer1, T. Nishizawa3, M. Garcia-Munoz1 and the ASDEX Upgrade Team31 Dept. of Atomic, Molecular and Nuclear Physics, University of Seville, Spain2 Institute of Plasma Physics of the CAS, CZ-18200 Prague, Czech Republic3 Max-Plank-Institut für Plasmaphysik, D-85748 Garching, Germany4 Dept. of Energy Engineering, University of Seville, SpainIntroductionKnowledge of the background neutral density is useful for detailed plasma transport studies. This is be-cause background neutrals act both as a particle source via ionisation processes and a sink via charge ex-change (CX) reactions with plasma particles. Recently developed techniques have demonstrated that 1Dneutral deuterium density profiles can be inferred by analysing the contribution of CX reactions to the mea-sured neon emission [1] as well as directly from neutral particle analyser (NPA) measurements [2]. However,additional techniques that rely on independent measurements can help to further constrain the inferred neutraldensity.In this work, the passive Balmer alpha emission is used to obtain information on the density and distribu-tion of background neutrals. This is possible as information on the neutral density along a given line of sight(LOS) is contained in the measured intensity. Furthermore, the spatial information on the distribution of theneutrals along a LOS is contained in the spectral shape of the emission. This is due to the Doppler shift pro-duced by neutrals that have undergone CX reactions and therefore have velocity distributions determined bythe local ion temperature. Given information on the plasma kinetic profiles, 1D neutral density profiles canbe inferred by inversion techniques as have been demonstrated on fusion devices such as the LHD stellarator[3].Experimental setupMeasurements of the passive Balmer alpha emission were performed with two dedicated spectroscopicsystems recently installed on the ASDEX Upgrade tokamak [4]. The systems were connected to in vesseloptics that view the high-field-side (HFS) [5] and low-field-side (LFS) midplane regions, respectively. Figs.1a and b illustrate the viewing geometry of two LOS from the respective optical heads as well as that of thepassive NPA diagnostic. The contour maps indicate the expected birth distribution, in logarithmic scale, ofneutrals resulting from CX reactions that emit Balmer emission measured by either LOS. The distributionshave been calculated using the FIDASIM code [6] using a 1D neutral density profile calculated by the 1Dneutral transport code KN1D [7]. As can be seen in fig. 1a and b the CX neutrals are able to travel a consider-able distance before crossing a LOS and emitting a photon. However, due to the large density of backgroundneutrals in the scrape-off layer (SOL) the majority of emitting neutrals originate near the separatrix as canbe seen in fig. 1c.47th EPS Conference on Plasma Physics P3.1012Figure 1: (a) Poloidal projection and (b) top down view of the FIDASIM predicted spatial distribution ofneutrals that have undergone CX reactions and emit Balmer photons collected by one of the two LOS. (c)The radial distribution of the neutrals observed by both views as well as the NPA diagnostic.Analysis methodologyA forward modelling approach is followed to determine the 1D neutral density profiles that produce syn-thetic spectra that best agree with measurements. Here, the model developed in [8, 9] is used as it has theadvantage of being computationally inexpensive which allows for direct fitting of the measured spectra. Im-portantly, the model considers that neutrals may have originated from regions not directly in view of a givenLOS. A brief overview of the model is presented here, closely following the description in [9]. It is importantto note that the model assumes the emission and hence, neutral density is toroidally and poloidally symmet-ric. Furthermore, the emission is assumed to result from two populations of excited background neutrals.The first population being atomic neutrals formed during the dissociation of molecular deuterium whichhave energies of a few eV, here referred to as cold neutrals. The second population is formed by neutrals thatresult from CX reactions with thermal plasma ions and hence have energies determined by the plasma iontemperature.The cold emission contribution is modelled by combining ten Gaussians with temperatures Tcold = 1 to10 eV:Icold(λ ) =10∑i=1ci exp[−∆λ2w2i],wi = λ0√2kTcold,iMc2(1)Where λ0 =656.1 nm, M is the mass of deuterium and c is the speed of light. Here, the individual intensity ciis used as the fit parameter. The CX emission contribution is modelled by dividing the plasma minor radius rin N (up to 20) sections and summing the emission from each section, labelled ri (of length ∆L(ri)). Similarto the cold contribution, the spectral shape in terms of wavelength (λ ) is determined by a Gaussian, however47th EPS Conference on Plasma Physics P3.1012in this case with a width determined by the local ion temperature TD+(ri):ICX(λ ) =N∑i=1ne(ri)n0(ri)∆L(ri)PEC3→2×1√πwCX(ri)exp[−(∆λwCX(ri))2],wCX(r) = λ0√2kTD+(r)Mc2(2)Where PEC3→2 is the Balmer alpha photon emission coefficient obtained from the ADAS open database.This formulation allows n0(r) to be treated as a parameter given that the plasma density ne(r) and ion tem-perature TD+(r) are known and can be determined directly by fitting ICX to the measured spectra. However,the problem is ill-posed and regularisation is required to obtain physically reasonable n0 profiles. In thiswork a well-behaved parametric function for n0 is used instead. The function expresses n0 in terms of thenormalised radius ρ using 5 parameters as described in [2].Figure 2: (a) Example input plasma profiles. (b) Fit to measured spectra with the initial guess in grey and thevarious contributions to ICX plotted with dashed lines. (c) Initial guess and output neutral density profiles.In order to forward model the Balmer emission for a given discharge and time point input profiles includingthe electron density, temperature and ion temperature are prepared as illustrated in figure 2a. Here it isassumed that the deuterium ion temperature is equal to the impurity ion temperature. The profiles are dividedin N (in this example 6) sections indicated by the coloured blocks above the radial axis, to calculate thecontribution of neutrals originating from these regions. Note, the sections are smallest at the separatrix wherethe profile gradients are steepest and increase in size towards the core as determined by the expressionρi = 1.2× (1− i/N)3. A fit routine is used to vary the parameters of the n0(ρ) function and obtain a bestfit of Icold+ ICX to the measured spectrum as shown in fig 2b. Here Icold+ ICX is plotted in cyan, while theindividual contributions to ICX from each region are plotted with dashed lines in the corresponding colour.Fig. 2c shows the n0 profiles corresponding to the initial guess of fit parameters in grey and the resulting bestfit parameters in red.Results and discussionThe analyses method has been applied to obtain neutral densities from measurements of the HFS andLFS Balmer emission for discharge #39015 (see fig. 3). The fit to the HFS spectrum is shown in fig. 3aand is compared to a FIDASIM calculated spectrum using the fitted neutral density (in magenta) and aKN1D calculated neutral density profile (in blue). Good agreement between the spectrum fit and FIDASIM47th EPS Conference on Plasma Physics P3.1012Figure 3: (a) HFS and (b) LFS measured spectra (black) including fit and FIDASIM forward modelled results.(c) Calculated neutral density profiles.is observed suggesting that the analysis method models well the effect of distant neutrals reaching the LOS.Fig. 3b shows the fit to the LFS measured spectra. Here again a good agreement between the fit and FIDASIMcalculated spectrum is found, however it should be noted that the fit does not yet produce a good match tothe cold emission and improvements to the fit are currently ongoing. Finally, fig. 3c compares the neutraldensity profiles obtained from the fits to the spectra. These profiles are compared to a KN1D calculatedneutral density profile as well as the neutral density values obtained from NPA measurements. As can beseen the neutral density is calculated to be larger in the HFS SOL compared to the LFS, while the reverseis observed in the steep gradient region. The HFS profile agrees well with the KN1D modelled density inthe SOL and high gradient regions however a large difference is observed further inside. However, sincethe measured emission largely originates from the region around the separatrix the uncertainty in the fittedneutral density profiles in the core region is expected to be large. Lastly, comparing the neutral density valuescalculated from NPA measurements show a reasonably good agreement with the HFS density profile, whilethe LFS neutral density is calculated to be larger. This may be explained by a strong toroidal asymmetry inthe background neutral density as the NPA and LFS LOS view toroidally separate regions.AcknowledgementsThis work has been carried out within the framework of the EUROfusion Consortium and has receivedfunding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agree-ment No 633053. The views and opinions expressed herein do not necessarily reflect those of the EuropeanCommission.References[1] R. Dux et al, Nucl. Fusion 60, 126039 (2020)[2] K. Bogar et al, Nucl. Fusion 61, 036001 (2021)[3] M. Goto et al, Nucl. Fusion 51, 023005 (2011)[4] A. Jansen van Vuuren et al, Rev. Sci. Instrum. 90, 103501(2019)[5] D.J. Cruz-Zabala et al, JINST 14 C11006 (2019)[6] B. Geiger et al, Plasma Phys. Control. Fusion 62 105008(2020)[7] B. LaBombard, Massachusetts Institute of Technology,Plasma Science and Fusion Center Research Report PSFC-RR-01-3 (2001)[8] K. Fujii et al, Rev. Sci. Instrum. 85, 023502 (2011)[9] K. Fujii et al, Nucl. Fusion 55, 063029 (2015)47th EPS Conference on Plasma Physics P3.1012

Determination of the background neutral density from passive Balmer alpha CX emission at the ASDEX Upgrade tokamak

http://hdl.handle.net/21.11116/0000-0009-A9E1-A

Determination of the background neutral density from passive Balmer alpha CX emission at the ASDEX Upgrade tokamak

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