Impurity Control in Helical Reactor

Yu. Igitkhanov^a, A. Sagara^b, S. Murakami^c , S. Sudo^b and O. Motojima^b

^aForschungszentrum Karlsruhe, IHM, Box 3640, 76021 Karlsruhe, Germany

^bNational Institute for Fusion Science, Toki 509-5292, Japan

^cKyoto University, Japan

e-mail of main author: yli@ipp.mpg.de or Juri.Igitkhanov@ihm.fzk.de

Impurity accumulation is the crucial issue for Helical reactor, FFHR, because it can limit the reactor operation time. The impurity accumulation can occur due to the lack of a temperature screening effect in helical devices, the large negative electric field and the large surface area due to typically large aspect ratios of the Helical reactor. However, the impurity accumulation can be mitigated due to stochastic screening layer at the plasma edge and the existence of islands. Since all this issues are rather uncertain, the reliability of rector can be insured only by developing the active control of impurity.

A new technique of bulk plasma screening from the penetration of intrinsic impurity ions, originating at the first wall of the FFHR, has been suggested. By launching repetitively small pellets at the plasma edge the perturbation of density and temperature can capture the impurities and push them back into the scrape-off layer. The resulting screening of the bulk plasma from heavy impurities partly is similar to that seen in ELMing H-mode tokamak plasmas. Calculations show that periodic plasma and energy outflow to the SOL changes the impurity behavior along the open magnetic field lines, where the conduction and convection alternate to enhance the efficiency of the impurity screening in the SOL. For the LHD case calculation gives the required pellet frequency and size to be about 10Hz, and 10²¹/s, respectively. This amount of pellets, which should be injected continuously, are supplemental to the larger pellets needed to initiate the super dense core regime in a helical reactor and are not detrimental to the achievement of the internal diffusion barrier and furthermore do not deteriorate the confinement time.

For the helical reactor it could be shown, that these small pellets pose no threat to ignition. In a tokamak pace making ELMs are used to reduce the power load to the divertor. In contrast we propose these artificially induced ELMs as a means of impurity control without a threatening increase of the power loading of the divertor in a helical reactor.