Fast Ion Loss Diagnostic for the Wendelstein 7-X Stellarator

Abstract

An important issue of future reactor-like fusion devices is the confinement of fusion born alpha particles required for a burning plasma. A fast ion loss diagnostic, like proposed here, could be an adequate diagnostic tool to study the fast ion confinement properties of W7-X and, thus, could confirm at least one of the 7 optimization criteria given in Ref. [1]. The objectives of such measurements can be classified into 3 categories: (i) The confinement of fusion alphas directly depends on the magnetic field structure. Generally, the loss cone of the configuration should be as narrow as possible to avoid first orbit losses, otherwise alpha particle and auxiliary heating efficiency may be deteriorated. Although alpha particle confinement is expected to be sufficient in a HELIAS reactor (R = 20 m, a = 2 m), alpha particle confinement studies should be performed in W7-X (R = 5 m, a = 0.5 m) with 60 keV protons as a substitute. First orbit losses of ions injected by neutral beam heating (NBI) are caused in cases where the particle sources reside close to the lost/confined boundary in phase space or even are located inside the loss cone. Ion cyclotron heating (ICH) drives mainly perpendicular momentum of the ions, thus a transport into the loss cone is likely. (ii) Even in magnetic fields with perfect ion confinement losses may occur when ions are affected by fluctuating electric and magnetic fields. Shear Alfven modes as an example are accompanied by poloidal and radial electric field oscillations that change the ion orbit trajectory, provided that particle and wave propagation are roughly in resonance. In this case, a subensemble of fast ions suffer from radial diffusion. Such modes can be excited by inverse Landau-damping with either fast ions of auxiliary heating or fusion alphas. In contrast to this, fusion alphas may be expelled from the plasma by fast ion driven modes, which could endanger reactor operation. However, if specific modes are excited in a controlled way, ion-wave interaction may be beneficial for helium ash removal. In this case, fusion alphas have to pass a resonance layer at a certain velocity during their slow-down at which enhanced radial transport is induced. (iii) Knowledge about fast ion losses is also essential to improve plasma performance. Strong fluxes of suprathermal ions are an additional drive mechanism for radial electric fields. Consequently, the lost/confined boundary may changes in a positive way and plasma performance can be improved, at least for thermal ions. The fast ion loss diagnostic provides experimental evidence whether such a drive is caused by fast or thermal ions. Another important issue is the influence of impurities. Enhanced impurity concentration increases pitch angle scattering of fast ions. This leads to an enhanced diffusion in phase space, and subsequently, to increased ion loss rates. Strong ion outward fluxes are not desirable, because they cause strong local heat loads on the first wall, which are not acceptable in a steady state plasma operation. Additionally, new impurity sources are generated by sputtering or by ion impact induced desorption. Thereby, the impurity concentration is increased and pitch angle scattering gets further enhanced leading to a non-stationary behavior in all plasma parameter.