A "Snowflake" Divertor and its Properties

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Handling the power and particle exhaust in fusion reactors based on tokamaks is a challenging problem [1,2]. To bring the energy flux to the divertor plates to an acceptable level (< 10 MW/m2), it is desirable to significantly increase poloidal flux expansion in the divertor area. Some recent ideas include that of a so-called X divertor [3] and a 'snowflake' divertor [4]. We use an acronym SF to designate the latter. In this paper we concentrate on the SF divertor. The general idea behind this configuration is that, by a proper selection of divertor (poloidal field) coils, one can make ... continued below

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6 p. (0.5 MB)

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Ryutov, D. June 21, 2007.

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Handling the power and particle exhaust in fusion reactors based on tokamaks is a challenging problem [1,2]. To bring the energy flux to the divertor plates to an acceptable level (< 10 MW/m2), it is desirable to significantly increase poloidal flux expansion in the divertor area. Some recent ideas include that of a so-called X divertor [3] and a 'snowflake' divertor [4]. We use an acronym SF to designate the latter. In this paper we concentrate on the SF divertor. The general idea behind this configuration is that, by a proper selection of divertor (poloidal field) coils, one can make the null point of the second, not of the first order as in the standard divertor. The separatrix in the vicinity of the X point then acquires a characteristic hexapole structure (Fig. 1), reminiscent of a snowflake, whence the name. The fact that the field has a second-order null, leads to a significant increase of the flux expansion. It was noted in Ref. [4] that the SF configuration is topologically unstable: if the current in the divertor coils is somewhat higher than the one that provides the SF configuration, it becomes a single-null X-point configuration. Conversely, if the coil current becomes somewhat lower, there appear two separate X-points. To solve this problem, one can operate the divertor at the current by roughly 5% higher than the value needed to create the second-order null. Then, configuration becomes robust enough and the shape of the separatrix does not change significantly if the coil current varies by 2-3%. At the same time, the flux expansion still remained by a factor of {approx}3 larger compared to a 'canonical' divertor. Following Ref. [4], we call this configuration a 'SF-plus' configuration. Specific examples in Ref. [4] were given for simple magnetic geometries The aim of this paper is to demonstrate that the SF concept will also work for a strongly shaped plasma. The other set of issues considered in the present paper relates to the possible presence of the toroidal current near the null-point. To find a set of divertor coils for a system with a strongly shaped plasma, we use the following strategy. We start from a configuration with the flux surfaces similar to the desired ones in the plasma core. Then we identify the point where the poloidal field null is desired and introduce divertor coils generating this null, be it first or second order. This, of course, somewhat changes the shape of the flux surfaces in the plasma core but, if the divertor is compact, the change is modest (except for the surfaces close to the separatrix). We will illustrate this procedure in the limit of a low-aspect-ratio tokamak, replacing it by a 'rectified' torus. In this 'rectified' geometry, the direction of the plasma current and the current in poloidal field coils is z, with the axes (x, y, z) forming the right-hand triplet. The generalization to the toroidal geometry is straightforward but leads to lengthy equations. General properties of a toroidal field have been discussed in a review paper [5].

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6 p. (0.5 MB)

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PDF-file: 6 pages; size: 0.5 Mbytes

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  • Presented at: 34th Plasma Physics Conference of the European Physical Society, Warsaw, Poland, Jul 02 - Jul 06, 2007

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  • Report No.: UCRL-CONF-232094
  • Grant Number: W-7405-ENG-48
  • Office of Scientific & Technical Information Report Number: 920490
  • Archival Resource Key: ark:/67531/metadc893680

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  • June 21, 2007

Added to The UNT Digital Library

  • Sept. 27, 2016, 1:39 a.m.

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  • April 17, 2017, 12:50 p.m.

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Ryutov, D. A "Snowflake" Divertor and its Properties, article, June 21, 2007; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc893680/: accessed November 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.