Detection of electron temperature anisotropy by an x-ray crystal spectrometer in the Large Helical device

Abstract We examine Ar XVI spectra measured by an x-ray crystal spectrometer to estimate electron temperature anisotropy in high-temperature electron cyclotron heated plasmas in the Large Helical Device (LHD). We calculate the atomic structure and electron impact excitation cross section between magnetic sublevels of Ar XVI. By assuming a bi-Maxwellian electron distribution function and considering the mount parameters of the x-ray spectrometer in LHD, we model the expected intensity ratio q/r as a function of T e ⊥ B and T e ∣∣ B , where ⊥ B and ∣∣ B denote perpendicular and parallel components to the magnetic axis of toroidal plasma, respectively. The calculation results show that the intensity ratio of q and r, which are formed by inner-shell excitation from the ground state of Ar 15+ , is sensitive to electron temperature anisotropy. We apply the calculation results to the LHD experiments. The ratio of q/r changes with variations in electron density and collision frequency. In the core and low ν e region, T e ⊥ B is predominant, and the electron temperature becomes isotropic above ν e > 10 4 Hz. By combining electron temperature measurements from Thomson scattering and radial profile of Ar 15+ ions estimated using the extreme ultraviolet spectrometer, local values of T e ⊥ B / T e ∣∣ B are quantitatively estimated using the q/r ratio. The derived T e ⊥ B / T e ∣∣ B is compared with collision frequency, radial electric field, and effective helical ripple, and the experimental results are explained qualitatively.