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Radiation power profiles and density limit with a divertor in the W7-AS stellarator

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Giannone,  L.
Experimental Plasma Physics 3 (E3), Max Planck Institute for Plasma Physics, Max Planck Society;
Experimental Plasma Physics 1 (E1), Max Planck Institute for Plasma Physics, Max Planck Society;
W7-AS, Max Planck Institute for Plasma Physics, Max Planck Society;

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Burhenn,  R.
VINETA, Max Planck Institute for Plasma Physics, Max Planck Society;
W7-AS, Max Planck Institute for Plasma Physics, Max Planck Society;

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McCormick,  K.
Stellarator Scenario Development (E5), Max Planck Institute for Plasma Physics, Max Planck Society;

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Brakel,  R.
Experimental Plasma Physics 3 (E3), Max Planck Institute for Plasma Physics, Max Planck Society;

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Feng,  Y.
Experimental Plasma Physics 3 (E3), Max Planck Institute for Plasma Physics, Max Planck Society;
W7-AS, Max Planck Institute for Plasma Physics, Max Planck Society;

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Grigull,  P.
W7-X: Physics (PH), Max Planck Institute for Plasma Physics, Max Planck Society;

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Igitkhanov,  Y.
Experimental Plasma Physics 3 (E3), Max Planck Institute for Plasma Physics, Max Planck Society;
Stellarator System Studies, Max Planck Institute for Plasma Physics, Max Planck Society;
W7-AS, Max Planck Institute for Plasma Physics, Max Planck Society;

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Citation

Giannone, L., Burhenn, R., McCormick, K., Brakel, R., Feng, Y., Grigull, P., et al. (2002). Radiation power profiles and density limit with a divertor in the W7-AS stellarator. Plasma Physics and Controlled Fusion, 44(10), 2149-2165.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0027-41B9-4
Abstract
The addition of a divertor into the W7-AS stellarator has allowed access to a high density regime where the radiation profiles reach a steady state. In earlier limiter discharges, the plasma suffered a radiative collapse at high densities. In contrast to limiter experiments, where the impurity confinement time measured by Al laser blow-off increased with increasing line integrated density, in divertor discharges, above a density threshold, the impurity confinement time decreased with increasing line integrated density. The observation that the divertor plasma radiates mainly at the plasma edge rather than the plasma centre is a further indication that changes to the impurity transport coefficients at these high densities are the basis for the achievement of steady state discharges in the divertor configuration of W7-AS. The maximum line integrated density reached with a divertor is compared to that reached with a limiter. The previously derived scaling law for the density limit with a limiter shows that the achieved densities do not exceed those predicted when the higher deposited power is taken into account. In a divertor the radiated power is located at the plasma edge and increasing the density, cooling the plasma edge and radiating sufficient power to cause plasma detachment determines the density limit.