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A “snow-flake” divertor as a possible approach to reducing divertor heat loads in tokamaks

Author: Dmitri D. Ryutov
Requested Type: Poster Only
Submitted: 2006-12-17 16:41:00

Co-authors: R.H. Cohen

Contact Info:
Lawrence Livermore National Laboratory
7000 East Avenue
Livermore, CA   94551

Abstract Text:
Using a simple set of poloidal field coils, one can reach the situation where the null of the poloidal magnetic field in the divertor region is second order, not first order as in the usual X-point divertor. Then, the separatrix in the vicinity of the null-point splits the poloidal plane not into four sectors, but into six sectors, making the whole structure looking like a snow-flake (whence the name, [1]). This arrangement allows one to spread the heat load over much broader area than in the case of a standard divertor. A disadvantage of this configuration is in that it is topologically unstable, and, with the current in the plasma varying with time, it would switch either to the standard X-point mode, or to a mode with two X-points close to each other. To avoid this problem, we suggest having the current in the divertor coils roughly 5% higher than in an “optimum” regime (the one where a snow-flake separatrix is formed). In this mode, the configuration becomes stable and can be controlled by varying the current in the divertor coils in concert with the plasma current. The configuration thus obtained has formally one X-point, but maintains the basic features of an ideal snow-flake configuration in the divertor region. We analyze geometrical properties of such a divertor in the simple case of a large aspect ratio tokamak with a circular cross section. We study the flux-expansion parameter for various positions and tilts of the target plates and find that the flux expansion can be made much higher (i.e., the heat flux much lower) than in the standard divertor. We demonstrate also that the magnetic shearing of the flux tubes in the vicinity of the null-point is substantially stronger than in the standard divertor – a factor beneficial from the view-point of controlling the divertor plasma without affecting the main SOL (e.g., [2]). Finally we demonstrate that, for a plasma minor radius of 2 – 2.5 m, the divertor coils can be placed outside the radiation shield; the total current in the coils is ~ 30- 40 % of the plasma current. Work performed for the U.S. DoE by UC LLNL under contract # W-7405-Eng-48.

[1] D.D. Ryutov, R.H. Cohen. “Enhancing the cross-field transport in divertors.” DoE TCC Remote Seminar, November 21, 2006; UCRL-PRES-226224.
[2] R.H. Cohen, D.D. Ryutov. “Plasma convection induced by toroidal asymmetries of the divertor plates and gas puffing.” Nucl. Fusion, v. 37, p. 621 (1997).

Characterization: A4,E1


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