Considerations from the Viewpoint of Neoclassical Transport Towards Higher Ion Temperature Heliotron Plasmas

Abstract

The neoclassical (NC) transport analyses have been performed to elucidate the plausible approaches towards higher ion-temperature heliotron plasmas. Avoidance of the ripple transport is the key issue, for which the neoclassical ambipolar radial electric field (Er) can be utilized. The ion-root scenario and the electron-root scenario are expected to be effective according to the experimental situation (especially, the temperature ratio between ions and electrons). The impact of the ion mass on the neoclassical ambipolar Er is also investigated to reveal the easier realization of electron-root Er in heavier ion plasmas.

Parameter Regime of Ion Internal Transport Barrier Formation in the Large Helical Device

“…The formation of an ion ITB is observed in the low-collisional regime of 1/ν, where helical ripple transport dominates the neoclassical transport. In this lowcollisional regime, a significant improvement in neoclassical transport with a positive radial electric field (neoclassical electron root) is predicted at higher temperatures [10]. From these results, we conclude that extension of the ion ITB regime may be possible, if the ion heating power is increased, which will be experimentally confirmed by the new installation of perpendicular NBI (BL5) in the near future.…”

Effects of equilibrium-scale radial electric fields on zonal flows and turbulence in helical configurations

“…In this study, effects of E r0 reducing the radial drift motions of helical-ripple-trapped particles are introduced in the gyrokinetic equation through M p that is larger for heavier ion mass in the case with the same E r0 and T i . The magnitude of E r0 caused by the neoclassical transport in the deuterium discharge in LHD is almost identical to that in the hydrogen plasma [23]. In contrast to axisymmetric configurations, accordingly, the uniform and constant E r0 can lead to the favourable isotope effect on the ZF response enhancement in non-axisymmetric systems.…”

“…If the ion mass is changed with the ion temperature and the radial electric field fixed, the E r0 dependence of the ZF response is translated into an isotope effect through M p . Indeed, a neoclassical transport analysis for the LHD confirms that the magnitude of the equilibrium radial electric field estimated for the deuterium discharge is almost identical to that in the hydrogen plasma, if other parameters are the same [23]. The isotope effect on ZF response is, thus, expected to appear through E r0 generated by the neoclassical transport, and would play a favourable role in reducing the turbulent transport.…”

Moderation of neoclassical impurity accumulation in high temperature plasmas of helical devices

“…Of course, temperatures even higher will cause a positive radial electric field [3] and an outward impurity flux. Plasmas of small and negative E r have been briefly discussed in predictive studies of bulk particle transport in Wendelstein 7-X (W7-X) [21] and of energy transport in heliotron plasmas of high temperature [3,22]; in this work we focus in its relevance for impurity transport.…”