Heavy ion beam probing—diagnostics to study potential and turbulence in toroidal plasmas

Abstract

Heavy ion beam probing (HIBP) is a unique diagnostics to study the core plasma potential and turbulence. Advanced HIBPs operate in the T-10 tokamak and TJ-II flexible heliac with fine focused (<1 cm) and intense (100 µA) beams. They provide measurements in the wide density interval = (0.3–5) × 1019 m−3, in a wide range of Ohmic and electron cyclotron resonance heated (ECRH) discharges with various currents at T-10, and in the wide range of magnetic configurations with ECR and neutral beam injection (NBI) heating at TJ-II. Time evolution of the radial profiles and/or local values of plasma parameters from high field side (HFS) to low field side (LFS), −1 < ρ < 1, is observed in TJ-II by 125 keV Cs+ ions in a single shot, while LFS (+0.2 < ρ < 1) is observed in T-10 by 300 keV Tl+ ions. Multi-slit energy analyzers provide simultaneously data on the plasma potential φ (by the beam extra energy), plasma density ne (by the beam current), poloidal magnetic field Bpol (by the beam toroidal shift), poloidal electric filed Epol that allows one to derive the electrostatic turbulent particle flux ΓE×B. The cross-phase of density oscillations produces the phase velocity of their poloidal propagation or rotation; also it gives the poloidal mode number. Dual HIBP, consisting of two identical HIBPs located ¼ torus apart provide the long-range correlations of core plasma parameters. Low-noise high-gain electronics allows us to study broadband turbulence and quasi-coherent modes like geodesic acoustic modes and Alfvén eigenmodes.