The magnetic field structure of a snowflake divertor

The snowflake divertor exploits a tokamak geometry in which the poloidal magnetic field null approaches second order; the name stems from the characteristic hexagonal, snowflakelike shape of the separatrix for an exact second-order null. The proximity of the poloidal field structure to that of a second-order null substantially modifies edge magnetic properties compared to the standard X-point geometry (with a first-order null); this, in turn, affects the edge plasma behavior. Modifications include: (1) The flux expansion near the null-point becomes 2-3 times larger. (2) The connection length between the equatorial plane and divertor plate increases. (3) Magnetic shear just inside the separatrix becomes much larger. (4) In the open-field-line region, the squeezing of the flux-tubes near the null-point increases, thereby causing stronger decoupling of the plasma turbulence in the divertor legs and in the main scrape-off layer. These effects can be used to reduce the power load on the divertor plates and/or to suppress the "bursty" component of the heat flux. It is emphasized that the snowflake divertor can be created by a relatively simple set of poloidal field coils situated outside the toroidal field coils. Analysis of the robustness of the proposed divertor configuration with respect to changes of the plasma current distribution is presented and it is concluded that, even if the null is close to the second order, the configuration is robust. (c) 2008 American Institute of Physics.