Mitigation of stochastic diffusion losses in optimized stellarators

The stochastic diffusion (SD) resulting from collisionless transformations between locally trapped and locally passing orbits may lead to a significant loss of energetic particles from stellarator plasmas (Beidler C D et al 2001 Phys. Plasmas 8 2731). In this work, a method to mitigate the consequences of SD is suggested. The method consists of changing the shapes of the separatrices between the locally trapped and locally passing states so that most separatrices do not cross the plasma boundary, which is achieved with a suitable modification of the magnetic configuration. It is demonstrated analytically that in this way the particle loss because of the SD may be reduced significantly, even though the SD still occurs. In particular, it is possible to protect fast particles inside a certain radius from SD loss. It is shown that a variation of the electric field can also affect the SD loss. The modifications of the magnetic configuration and the electric field required for the mitigation are quantified for a configuration of the stellarator Wendelstein 7-X.