Geometrical properties of a “snowflake” divertor

Abstract

Using a simple set of poloidal field coils, one can reach the situation in which the null of the poloidal magnetic field in the divertor region is of second order, not of 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 look like a snowflake (hence the name). This arrangement allows one to spread the heat load over a much broader area than in the case of a standard divertor. A disadvantage of this configuration is 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 the mode with two X-points close to each other. To avoid this problem, it is suggested to have a current in the divertor coils that is roughly 5% higher than in an “optimum” regime (the one in which a snowflake 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; on the other hand, a strong flaring of the scrape-off layer still remains in force. Geometrical properties of this configuration are analyzed. Potential advantages and disadvantages of this scheme are discussed.

On description of magnetic stochasticity in poloidal divertor tokamaks

“…This kind of divertor tokamaks with the second order of null of the poloidal magnetic field have been recently proposed in Ref. 46 in order to expand the flux of heat load over a much wider area than in the case of the standard divertor discussed above. The expansion of the Hamiltonian near the points ͑z = z s , p z = p s ͒ then contain the third order terms, and can be presented as…”

Plasma turbulence, suprathermal ion dynamics and code validation on the basic plasma physics device TORPEX

“…Several new magnetic field configurations are possible, Figs 2(c) and (d) and Fig. 8(a), from simple plasmas limited on the low-field side (LFS) by the vessel, to more complex magnetic geometries, such as single or double-null X-points, as well as advanced divertor concepts like snowflakes (Ryutov 2007;Piras et al 2009). …”

Advanced divertor configurations with large flux expansion

“…Recently, new divertor magnetic geometry concepts have emerged: the Super-X (SX) divertor [11] and the snowflake (SF) divertor [12] configurations. Both concepts enable the divertor plasma-wetted area, effective connection length and divertor volumetric power loss to increase beyond those in the standard divertor, potentially reducing heat flux and plasma temperature at the target.…”

“…The SF magnetic configuration uses a second-order null-point created by bringing two first-order null-points of the standard divertor together [12,15,16]. Poloidal magnetic flux surfaces in the vicinity of the second-order null point have hexagonal separatrix branches with an appearance of a snowflake.…”