Scenarios with predominant heating of electrons and ions in Wendelstein 7-X

Using a 0D paradigm for energy balance equations, it is shown that in plasmas with predominant electron heating, in particular, during electron cyclotron resonance heating, the ion temperature is always capped (restricted) by a temperature below the electron temperature.When ion thermal conductivity is anomalous (e.g., due to turbulence caused by the ion temperature gradient (ITG) instability) the capping can have features of the ion temperature clamping observed in stellarators and tokamaks, in particular, in the Wendelstein 7-X stellarator, which is the focus of this paper.Therefore, the used paradigm can serve as a tool for the description of the clamping.Relations are obtained, which can be used for diagnostics of the plasma energy confinement time during the clamping.Employing the same paradigm, plasmas heated by neutral beam injection (NBI) are considered, with the aim of investigating the possibility of achieving a steady state ion temperature exceeding the electron one (T i > T e ) in W7-X.It is revealed that in the steady state, the fraction of NBI power received by the bulk plasma ions has a maximum at a certain electron temperature, 3 4 keV, when protons with the maximum energy 55 keV are injected into a hydrogen plasma.This tends to break the necessary condition for T i > T e obtained in this work, i.e., the heating rate of the electrons should be less than their energy loss rate.Nevertheless, steady state scenarios with T i > T e are possible at least in plasmas with relatively low density and sufficiently high energy confinement time of the ions (when ITG turbulence is absent or mitigated).