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In optimized stellarators of the Wendelstein line prompt losses of fast ions in superbanana orbits are reduced through a modification of the magnetic configuration by high β (ratio of the plasma pressure to the magnetic field pressure), so that most contours of the longitudinal adiabatic invariant of locally trapped particles are closed inside the plasma volume. However, magnetic drift leads to the transformation of some of these particles into the locally passing ones and vice versa. Orbit transformation upon crossing the separatrix between locally trapped and locally passing states is accompanied by jumps in that lead to collisionless stochastic diffusion (SD), which was shown to be an important mechanism of the loss of energetic ions in the optimized Wendelstein-line stellarators (C D Beidler et al 2001 Phys. Plasmas 8 2731). The theory of SD that took into account the phase space asymmetry between locally passing particles with opposite signs of was developed in (A V Tykhyy 2018 Ukr. J. Phys. 63 (6) 495). This work extends that theory to consider the effect of radial electric field Er on SD. It is found that negative Er of such magnitudes as those observed in W7-X and predicted in Helias reactor can mitigate SD losses from the plasma core of NBI ions in W7-X and α-particles in Helias. This loss mitigation occurs because added drift increases the range of pitch angles for which separatrices are closed inside the plasma volume, even while it increases the SD coefficient. This means that Er can be used in addition to magnetic configuration changes for both loss mitigation and energy deposition profile optimization. [ABSTRACT FROM AUTHOR]

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 al2001 Phys. Plasmas82731). 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. [ABSTRACT FROM AUTHOR]

It is found that the presence of the so-called non-axisymmetric resonances of wave–particle interaction in stellarators (which are associated with the lack of axial symmetry of the magnetic configuration, Kolesnichenko et al 2002 Phys. Plasmas9 517) may have a strong stabilizing influence through Landau mechanism on the toroidicity-induced Alfvén Eigenmodes (TAE) and isomon modes (Alfvénic modes with equal poloidal and toroidal mode numbers and frequencies in the continuum region) destabilized by the energetic ions. These resonances involve largest harmonics of the equilibrium magnetic field of stellarators and lead to absorption of the mode energy by thermal ions in medium pressure plasma, in which case the effect is large. On the other hand, at the high pressure attributed to, e.g., a Helias reactor, thermal ions can interact also with high frequency Alfvén gap modes (helicity-induced Alfvén Eigenmodes and mirror-induced Alfvén Eigenmodes), leading to a considerable damping of these modes. Only resonances with passing particles are considered. The developed theory is applied to various modes in the Wendelstein 7-X stellarator and a Helias reactor, and to two TAE modes in the LHD helical device. [ABSTRACT FROM AUTHOR]