Anda, G.

Biedermann, C. 

Buzas, A.

Cseh, G.

Csillag, B.

Dunai, D.

Kocsis, G.

Krause, M.

Nagy, D.

Otte, M. 

Refy, D.

Szepesi, T.

Vecsei, M. 

Zoletnik, S.

English

MPG.PuRe

Cross-diagnostic analysis of plasma filaments on the Wendelstein 7-XstellaratorA. Buzás1,2, G. Anda1, C. Biedermann3, G. Cseh1, B. Csillag1, D. Dunai1, G. Kocsis1, M.Krause3, D. Nagy1, M. Otte3, D. Réfy1, T. Szepesi1, M. Vécsei3, S. Zoletnik1, and the W7-X Team * 1 HUN-REN Centre for Energy Research, Budapest, Hungary2 Institute of Nuclear Techniques, BME, Budapest, Hungary3 Max-Planck-Institute for Plasma Physics, Greifswald, GermanyPlasma filaments are a common feature of the edge plasma in toroidal fusion devices. Theyare field-aligned density perturbations linked to the interchange instability caused by the steepedge profiles, propagated by ExB drift [1]. Unlike in tokamaks, recent studies [2, 3] argue thattheir contribution to cross-field transport across the SOL is less significant on W7-X, insteadtheir importance lies in setting the density profiles of the magnetic islands [4]. In the complexmagnetic  geometry  of  W7-X,  the  usual  2D description  of  filaments  is  insufficient.  Theirbehavior also varies depending on the magnetic configuration [4, 5]. Fast cameras provide anexcellent tool for studying that complex, 3D structure, however, they are limited by the widthof the radiation belt. This shortcoming can be compensated by including other diagnostics inthe analysis. Alkali beam emission spectroscopy (ABES), for example, has excellent radialresolution across the SOL, mid-plane island, and edge. In this study, a joint analysis of fastcamera and ABES data is presented. By correlating the two diagnostics, the toroidal structure,and  poloidal  and  radial  movement  of  the  filaments  are  observed  in  the  W7-X  standardmagnetic configuration.To investigate filament activity, the W7-X fast camera system [6] and the ABES diagnosticswere used. ABES is based on injecting a neutral sodium beam into the plasma. As the neutralatoms collide with plasma electrons, they get excited and emit light while returning to theirground state. This light is roughly proportional to the local plasma density and is measured byAPD cameras. [3] In standard configuration, the beam crosses the o-point of the island at theequatorial plane. Figure 1 shows the radial positions of the observation points of the ABESchannels. Figure 2 shows the projections of the field lines connecting the camera view with theABES channels in both toroidal directions. For analyzing visible emissions, a Photron fastcamera  was  used.  It  is  located  at  port  AEA21.  While  the  camera  view covers  the  entire* See Pedersen et al 2021 (https://doi.org/10.1088/1741-4326/ac2cf5) for the W7-X Team.50th EPS Conference on Contr. Fusion and Plasma Phys, 8-12 July 2024 A. Buzás et al. : P1.057 (2024)poloidal cross-section of the plasma, its radial observations are limited to a visible radiationbelt a few cm wide. For this analysis, the camera was equipped with a 465 nm interferencefilter which is  the C III  emission line.  C2+ ions are major  contributors to the light of theradiation belt. For more detail, please refer to [5].In this contribution,  the results  from shot 20230323.044 are presented.  It is  a standardconfiguration shot recorded at 40 kHz. The images were binned (averaged over 3x3 grids toreduce  noise)  and  filtered  between  2  and  11  kHz,  which  is  typical  for  filament  activity.Pixelwise  correlations  are  then  calculated.  Reference  points  are  usually  selected  on  theprojection of a flux surface at  a given toroidal angle so they can be matched to points ofknown 3D coordinates. For more detail, see [5]. Figure 3 shows the maximal correlation ofeach pixel to the reference point indicated by a green star. Figure 4 shows the correspondingtime  delay.  Here,  the  succession  of  colors  (from  blue  to  red)  indicates  the  direction  ofmovement.  In  this  image,  the  movement  of  filaments  is  clockwise,  which  in  this  viewcorresponds to a negative Er, which is typical in the edge for similar shots. [7] Their poloidalvelocity is estimated to be around 1.2 km/s, which agrees well with earlier results. [2, 5] Basedon the correlation patterns, it is apparent that filaments loop around the device toroidally atleast once. The multiple correlated lines in these images are caused by separate events, eachlooping around, projected over the same pixel.The APDCAM was operating at 2 MHz, and the beam was chopped out of the plasma at200 kHz. Averaging over the segments when the beam was on and subtracting the backgroundgives a signal of 100 kHz, which is filtered between 2 and 10 kHz. Correlations between theFigure 2: Projection of field lines connecting the cameraview with the ABES channels in both toroidal directions(solid line forward, dotted backward).Figure 1: Radial positions of the ABES observationpoints. Blue ones correlate with the fast camera images.Flux surfaces of standard configuration are drawn forvisual reference.50th EPS Conference on Contr. Fusion and Plasma Phys, 8-12 July 2024 A. Buzás et al. : P1.057 (2024)channels provide insight into the radial movements of filaments. For this shot, meaningfulinformation can only be gained from the SOL. In the edge, the signal is dominated by theshadows of SOL filaments caused by beam loss due to ionization (see the negative correlationwith 0 time lag inside the LCFS in both figure 5 and 6). After the LCFS, the correlated area isabout 1.8 cm wide (figure 5). There is no time delay for the correlation maxima, so if there isradial movement, it must be faster than 500 m/s. There is anti-correlation on both sides of thisregion, meaning that it is affected but not dominated by the shadow of filaments further out.Closer to the o-point (figure 6), there are clear signs of radial motion of about 200 m/s. In thefar SOL, there is no correlation between the channels.There is a strong correlation between the two diagnostics over multiple channels on bothsides of the LCFS. Figure 7 shows the time delay of the maximal correlation with a channel 1cm outside the LCFS, while figure 8 shows a channel 1 cm inside. The reversed succession ofcolors between the two figures indicates poloidal movement in the opposite directions, whichFigure 3: Max correlation to a reference point (greenstar). The red curve is the projected flux surface fromwhich the reference point is chosen. Black lines are fieldlines whose projections cross the reference point.Figure 4: Time delays belonging to figure 5. Colorsindicate toroidal turnaround of filaments and clockwisemovement. Separate but not connected events projectedover the same region (solid and dashed lines).Figure 5: Correlation between ABES channels.Reference 1 cm outside the LCFS (vertical lines).Figure 6: Here, the reference is close to the o-point.The tilted line is fitted to the correlation maxima.50th EPS Conference on Contr. Fusion and Plasma Phys, 8-12 July 2024 A. Buzás et al. : P1.057 (2024)is consistent with the expected sign change of Er around the LCFS. [7] The lack of positivetime delay in figure 7 can be interpreted by either the observed structures leaving the radiationbelt or decorrelating right after crossing the beam. The time lag along their path is consistentwith the typical lifetime of filaments [5]. Interestingly, correlation appears not only over thefield lines that connect the ABES channels with the camera view (solid lines in figure 7-8), butover  others  too  (dashed  lines).  Theory  suggests  that  alongside  filaments,  reduced  densitystructures (holes) form as well, moving radially inwards [1]. If such structures appear on thecamera images, they may correlate positively with the shadows of filaments radially furtherout. However, this is only an assumption yet and requires further investigation. Based on theradial distance of the correlated channels and the width of the correlation between them, arough estimate of about 5 cm can be given for the width of the radiation belt.*References[1] D’Ippolito D. A. et al Phys. Plasmas 18, 060501 (2011)[2] Killer C. et al Plasma Phys. Control. Fusion 62, 085003 (2020)[3] Zoletnik S. et al Plasma Phys. Control. Fusion 62, 014017 (2020)[4] Csillag B. et al Nucl. Fusion 64, 016017, (2020)[5] Buzas A. et al Nucl. Fusion 64, 066012, (2020)[6] Kocsis G. et al Fus. Eng. Des. 96-97, 808, (2015)[7] Carralero D. et al Nucl. Fusion 60, 106019, (2020)* This  work  has  been  carried  out  within  the  framework  of  the  EUROfusion  Consortium,  funded  by  theEuropean Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 —EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarilyreflect  those of the European Union or the European Commission. Neither  the European Union nor theEuropean Commission can be held responsible for them.Figure 7: Time delay of max correlation between thenear SOL and the camera images. Solid lines connectthe reference channel and the camera view. The dashedline is a different field line on the LCFS.Figure 8: Time delay of the correlation between theedge and the camera. The solid line connects thereference channel with the camera view. The dashedlines are different field lines.50th EPS Conference on Contr. Fusion and Plasma Phys, 8-12 July 2024 A. Buzás et al. : P1.057 (2024)

Cross-diagnostic analysis of plasma filaments on the Wendelstein 7-X stellarator

http://hdl.handle.net/21.11116/0000-0010-324E-F

Cross-diagnostic analysis of plasma filaments on the Wendelstein 7-X stellarator

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