Lingertat, J.

Otte, M.

English

MPG.PuRe

Initial Results of Magnetic Surface Mapping in the WEGA StellaratorM. Otte, J. LingertatMax-Planck-Institut für Plasmaphysik, Euratom Association, TI Greifswald,Wendelsteinstraße 1, 17491 Greifswald / GermanyAbstractThe vacuum configuration of magnetic flux surfaces in the WEGA stellarator has been studied usingthe fluorescence method. In the experiments the rotational transform ι was varied between 1/9 and 1for a toroidal magnetic field strength of Bt=87.5mT. We have shown that the magnetic coil system ofthe WEGA generates good nested magnetic surfaces especially at low ι values. However, in theexperiments non-natural islands have been observed caused by field perturbations which are stillunder investigation. The experimental results are compared with calculations obtained from theGourdon- and Werner-code.1. IntroductionThe WEGA (Wendelstein Experiment in Greifswald für Ausbildung) is a classical stellaratorand was established as an EURATOM collaboration between the Max-Planck-Institut fürPlasmaphysik/Garching, the Institute C.E.N./Grenoble and the Institute ERM/Bruxelles [1].The machine was originally built in the ́70s in Grenoble/France as a tokamak with 40 fieldcoils. Later the machine was converted to a stellarator by changing the torus (now R=0.72mand a=0.19m) and adding 4 helical coil packets forming a l=2 and n=5 configuration. As aunique feature the WEGA torus can be easily separated into two halves without changes ofthe helix conductors. This is achieved by the introduction of so - called theta bridges, whichat the ends of both half-tori return the current of each helical packet into two neighbouringpackets.After successful operation in Grenoble the WEGA was transferred via the Institut fürPlasmaforschung, Stuttgart to the Max-Planck-Institut für Plasmaphysik, TI Greifswald in2000 and rebuilt in a modernised version. The machine is used mainly for educationaltraining, testing of new diagnostic equipment and for basic research in plasma physics. InJuly 2001 the first stellarator discharge was carried out in the new WEGA device. One of thefirst tasks performed on the WEGA device has been the evaluation of the magnetic fieldstructure. The results are essential for the interpretation of data obtained by diagnosticequipment located at different toroidal and poloidal positions.2. Experimental Set -UpFor the experimental mapping of the magnetic flux surfaces the well established fluorescentmethod was chosen [2, 3, 4]. A small electron gun (diameter = 6.5mm) emits a directed29th EPS Conference on Plasma Phys. and Contr. Fusion Montreux, 17-21 June 2002 ECA Vol. 26B, P-5.036  (2002)electron beam with an intensity between 5 and 200µA and an energy between 20 and 300eV.Typically, a beam current of ≤30µA at 230eV was used. The diameter of the beam wasrestricted to ∼1.5mm at gun exit. The electron gun was positioned at the toroidal positionϕ=252° and was moveable by a vacuum manipulator in radial direction (Fig.1). For thevisualisation of the magnetic flux surfaces a wire-ellipse was inserted at the toroidalpositions ϕ=72° and ϕ=144°, respectively. The wire-ellipse could be moved in radialdirection scanning nearly the whole cross section of the torus within 15s. The wire with adiameter of 1.0mm was coated with fluorescent powder (ZnO:Zn). The fluorescent lightgenerated by electrons striking the wire at the crossing points with the electron beam wasrecorded by a long-time integrating CCD-camera (Starlight-Xpress HX916) with a resolutionof 1300x1030 pixel and a dynamic range of 16bit.Fig.  1   Top view of experimental setup. The images were taken through an endoscope with a nearly perpendicular view of thepoloidal plane to be mapped as can be seen in Fig. 1. For each position of  the electron gunwe obtained a single magnetic surface. Due to the small diameter of the electron beam andthe high intensity of the fluorescent signal up to 20 flux surfaces could be superimposedusing a step width of 0.5cm of the electron gun.3. ResultsThe experiments have been carried out scanning a wide ι range from 1/9 ≤ ι ≤ 1 by changingthe current in the helical field coils at a constant toroidal field Bt = 87.5mT. For someconfigurations up to 40 single striking points each representing one toroidal turn of theϕ = 0°ϕ = 180°fluorescent wire-ellipse at ϕ = 72° and ϕ = 144°electron gun at ϕ = 252°ccd-camera ccd-camera 29th EPS 2002; M.Otte et al. : Initial Results of Magnetic Surface Mapping in the WEGA Stellarator 2 of 4electron beam could be distinguished experimentally. The experiments show a goodagreement with the calculations in the existence of closed nested magnetic field surfaces aswell as the position of the magnetic axis which is shifted inwards by approximately 1.5cm.Also, as can be seen from Fig. 2a and Fig. 3a, the magnetic surfaces at ι = 1/7 and ι = 1/3 aredisturbed by non-natural islands caused by unwanted error fields. In fact all islands forι = (n = 1) / m with m = 7, ..., 3 have been distinguished experimentally.(a) (b)Fig. 2   Measured (a) and computed (b) magnetic surfaces for ι = 1/7 at ϕ = 72°.(a) (b)Fig. 3   Measured (a) and computed (b) magnetic surfaces for ι = 1/3 at ϕ = 72°.29th EPS 2002; M.Otte et al. : Initial Results of Magnetic Surface Mapping in the WEGA Stellarator 3 of 4The observed error fields could be caused by a small displacement of the helix windingsrelative to the toroidal field [2] or an inaccurate position of the helix conductors in the thetabridges. In Fig. 2b and 3b the results of calculations using the Gourdon- [5, 6] and theWerner-code [7] adapted to the configuration of the WEGA are shown. In order to obtain asimilar shape and position of the experimentally observed islands an artificial horizontalhomogeneous magnetic field of Bx = 0.2mT and By = -0.2mT was superimposed and thehelical field current was lowered to approximately 95.5%. The same result is obtained byallowing a horizontal shift of approximately 1mm between the axis of the toroidal andhelical field coils.SummaryThe recent experimental investigations of the magnetic flux surfaces of the WEGAstellarator have shown the existence of closed nested flux surfaces in a wide ι range.However, the magnetic surfaces are disturbed by an error field of the order n=1. Theexperimental results are in good agreement with calculations based on the Gourdon- andWerner-code if additional error-fields are assumed. With the knowledge of the size andshape of the islands for the various configurations we are now in a position to investigatetransport properties of magnetic islands.References1. R. Fritsch et al., Technological aspects of the WEGA stellarator, Proc. 9th SymposiumFusion Technology, Pergamon Press 1976.2. H. Hailer et al., Max-Planck-Institut für Plasmaphysik, Garching, Annual Report (1987),191-193.3. R. Jaenicke et al., Nuclear Fusion 33 (1993) 687.4. E. Ascasibar et al., J. Plasma Fusion Res. Series, 1 (1998), 183.5. C. Gourdon, Programme optimise de calcul numerique dans le configurationsmagnetique toroidales, DPH-PFC-EUR-CEA-FC 449, Janvier 1968.6. K. Schwörer, Institut für Plasmaforschung, Stuttgart, private communication.7.   A. Werner,  Max-Planck-Institut für Plasmaphysik, private communication.29th EPS 2002; M.Otte et al. : Initial Results of Magnetic Surface Mapping in the WEGA Stellarator 4 of 4

Initial results of magnetic surface mapping in the WEGA Stellarator

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Initial results of magnetic surface mapping in the WEGA Stellarator

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