Effenberg, F.

Jakubowski, M.

König, R.

Niemann, H.

Pedersen, T.

Wurden, G.

Zhang, D.

English

MPG.PuRe

Power loads in the limiter phase of Wendelstein 7-XH. Niemann1, M. Jakubowski1, T. Sunn Pedersen1, R. König1, G. A. Wurden2, F. Effenberg3,D. Zhang1 and W7-X Team11 Max-Planck-Institut für Plasmaphysik, Greifswald, Germany2 Los Alamos National Laboratory, Los Alamos, USA3 University of Wisconsin , Madison, USAWendelstein 7-X (W7-X), an advanced stellarator with five-fold symmetry, started its firstplasma operation phase (OP 1.1) in December 2015. Electron cyclotron resonance heating(ECRH) power between 4 MW (1 sec) to 0.6 MW (6 sec) was used. The plasma duration wasrestricted admistratively to a 4 MJ energy limit. The wall was protected in the initial campaignof W7-X with 5 uncooled fine grain graphite limiters installed on the inboard side of the plasmavessel [1]. In this paper the first results of the power load onto these limiters are presented.The surface temperature on the limiters is investigated with two infrared (IR) cameras: amicrobolometric camera (8-14 µm, spatial resolution of order of 5 mm) and a high resolutionIR camera (3-5 µm, spatial resolution of order of 1 mm). The microbolometric camera observesthe left side of the limiter in modulel 5 from the top (Fig. 1(b)) and the high resolution IRcamera observes five of the nine tiles above the midplane of limiter 3. With a framerate up to50 Hz, the microbolometer camera can resolve a half limiter with a time resultion of 20msec,while the high resolution camera can run with a framerate up to 400 Hz (for a cropped image)to measure fast temperature changes during short events. The heat flux density is evaluatedwith the THEODOR (THermal Energy Onto DivertOR) code [4] from evolution of the surfacetemperature data by solving a linear heat diffusion equation for a bulk of tile (2D) [5]. The codetakes a 1D profile of the surface temperature and the thickness of the material as input.The magnetic field configuration for the initial campaign has been chosen to avoid stochasticregions and large magnetic islands at the plasma boundary. This assures that the limiters effi-ciently intercept > 99% of the convective plasma heat load at the plasma edge. (For 4 MW ofECR heating each limiter is expected to absorb 400 kJ at heat fluxes of up to 10 MWm−2[2]).Simulations [3] indicate that the scrape-off layer consists of magnetic field lines with limiter-to-limiterl connection lengths of the order of a few tens of meters. Three separate helical magneticflux bundles of different connection length of 36 m, 43 m and 79 m form a 3-D structure of mag-netic footprints on the limiter surface (Fig. 1(a)). This results in heterogenous power depositionpatterns over the limiter surface. In particular, two heat stripes are predicted and seen by the IRcameras, running the length of the limiter, whenever the plasma contacts the limiter (Fig. 1(b)).43rd EPS Conference on Plasma Physics P4.005For plasmas that don’t contact the limiter (for example high density cold collapsing plasmas),no heat stripes are seen. As predicted by EMC3-Eirene modeling the maximum is located in theregion of the flux tube with a connection length of 79 m [3] (Fig. 1(a)).(a) (b) (c)Figure 1: (a) Modeled connection length footprint on the limiter (b) view of the microbolometriccamera. Three slices (yellow lines) are used to evaluate limiter power fluxes (c) timetrace of a 6 sdischarge, top: ECRH power, middle: max heat flux density for three different profiles, bottom:line integrated power density for different tiles(a) (b) (c)Figure 2: the 3 different profiles on tile 2,3 and 8 for: (a) 0.9 s, (b) 2s , (c) 5.9 sFor a first evaluation three different profiles are taken (yellow slices in Fig. 1(b) and Fig.3). They are choosen to investigate the local power through 3 different connection length fluxtubes. These profiles are evaluated for a 6 sec low power, high energy discharge. The dischargestarts with 1 MW ECRH for 1 sec with a step down to 600 kW ECRH for the next 5 sec (Fig.1(c)). The maximum temperature increase in this shot is found on tile 3 (Fig. 1(b)), where the43rd EPS Conference on Plasma Physics P4.005(a) (b)Figure 3: heat flux density over time t and surface position s for different connection length: (a)36 m (b) 43 m (c) 79 m79 m long flux tube strikes the limiter. This results also in a higher maximum heat flux densitymeasureed in the profile taken on tile 3 (Fig. 1(c)). The time evolution of the heat flux densityon all three tiles follow the time behavior of the ECRH with a slight delay of about 40 to 60msec. This initial delay can be explained by the plasma growing during the start up. At first themaximum heat flux density rises up to 1.8MW/m2 on tile 3 (Fig. 1(c), 2(a)). The profiles on theother two tiles show at the same time a similar behavior but a 30% lower maximum heat fluxdensity (Fig. 1(c),2(b),2(c)). The profile width is nearly the same for all three profiles (Fig. 2(a))during the first second, which ends also in same line integrated power density for the two tileswith the shorter connection lengths. When the heating power is reduced the heat flux density onall tiles decreases with again a delay of 40-60 msec. While the ECRH power decreases about40% in less than 1 ms, the heat flux density shows a decay of about 40% on a much longertimescale of ∼ 240 ms. This longer decay time is related to the plasma energy confinementtime. Over the 5 sec the heat flux density shows a slighty decay over time to 1MW/m2 for theprofile on tile 3 and down to 0.6MW/m2 for the other two profiles. The reason for this is notfully clear at the moment. The three different profiles with the different connection length showa very similar time behavior and profile width but different maximum values. This ends up in30% lower maximum heat flux and also integrated power density for the two profiles in theregion of the flux tubes with shorter connection length. These results shows that the connectionlengths have an impact onto the heat flux density on the limiter in Wendelstein 7-X.Looking at shots with different ECRH power profiles and line integrated densities allows usto observe the maximum heat flux density on tile 3 as a function of ECRH power (Fig. 3(a)).The scaling shows a linear characteristic. The two outliers near 4 MW ECRH power (Fig. 3(a))43rd EPS Conference on Plasma Physics P4.005are from discharges with a heavy gas puff (a high line integrated density (> 3∗1019m−2)). Themaximum heat flux goes up to 5.6MW/m2 for 4 MW ECRH power. This maximum is belowthe estimated maximum of 10MW/m2 for the designed graphite limiter. This lower heat fluxdensity is not inconsitent with higher fraction of radiated energy.By taking several profiles on the limiter (every 5 mm) and integrating over all, we can estimatethe deposited power onto the limiter. To make statements about the total power fraction thecalculated power on the half limiter is multiplied by 10 under the assumption of the five foldstellarator symmetrie (Fig. 3(b)). This shows a similar scaling to the previous maxmimum heatflux density estimate, with the same two outliers for high line integrated densities. At the lowECR heating power of 600 kW, the fraction of deposited power goes up to 50%. For high heat-ing powers up to 4 MW the fraction of limiter deposited power drops to ∼ 30%. The cleanestshots were usually the first few discharges of the day (following helium glow discharge clean-ing). Later in the day, deteriorating wall conditions would actually prevent the achievement oflong discharges. So for now, we can not completely untangle the energy balance[6] differencesbetween high ECRH power (short discharges) and long pulse (low power) ones.AcknowledgmentThis 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 grantagreement No 633053. The views and opinions expressed herein do not necessarily reflect thoseof the European Commission.References[1] T. Sunn Pedersen et. al and W7-X Team, Nuclear Fusion 55, 55 (2015)[2] S.A. Bozhenkov et a, 41st EPS Conf. on Plasma Physics 38F, P-1.080 (2014)[3] F. Effenberg, Y. Feng, et. al and W7-X Team, submitted to Nuclear Fusion, (2016)[4] A. Hermann et. al and ASDEX Upgrade Team , EPS Vol 25A, 25A (2001)[5] B. Sieglin, M. Faitsch, A. Hermann, et. al, Rev. Sci. Instrum. 86, 113502 (2015)[6] S. A. Bozhenkov et. al, talk this conference, 201643rd EPS Conference on Plasma Physics P4.005

Power loads in the limiter phase of Wendelstein 7-X

http://hdl.handle.net/21.11116/0000-0008-80FA-D

Power loads in the limiter phase of Wendelstein 7-X

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