Core radial electric field and transport in Wendelstein 7-X plasmas

Abstract

Results from the investigation of neoclassical core transport and the role of the radial electric field profile (E r) in the first operational phase of the Wendelstein 7-X (W7-X) stellarator are presented. In stellarator plasmas, the details of the E r profile are expected to have a strong effect on both the particle and heat fluxes. Investigation of the radial electric field is important in understanding neoclassical transport and in validation of neoclassical calculations. The radial electric field is closely related to the perpendicular plasma flow (u ⊥) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity, which can be measured using the x-ray imaging crystal spectrometer (XICS) and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of ∆u ⊥ ∼ 5 km/s (∆E r ∼ 12 kV /m), have been observed within a set of experiments where the heating power was stepped down from 2 M W to 0.6 M W. These experiments are examined in detail to explore the relationship between heating power, temperature and density profiles and the radial electric field. Finally the inferred E r profiles are compared to initial neoclassical calculations using measured plasma profiles. The results from several neoclassical codes, sfincs, fortec-3d and dkes, are compared both with each other and the measurements. These comparisons show good agreement, giving confidence in the applicability of the neoclassical calculations to the W7-X configuration.

Charge exchange recombination spectroscopy at Wendelstein 7-X

“…For the detailed studies of the particle and energy transport required to assess the optimisation, accurate diagnosis of key plasma properties are necessary, particularly the electron and main ion temperature and density profiles T e , n e , T i , n i . A further critical ingredient is the radial electric field (E r ) that develops to equalise the electron and ion particle fluxes, which plays a central role in stellarator neoclassical particle transport 4 .…”

“…Lineaveraged ion temperatures and radial electric field information was provided by an X-Ray Imaging Crystal Spectrometer (XICS) 4,8,9 based on low levels of argon injection. Ion density information was available only in the form of some preliminary Bremsstrahlung based effective charge Z eff measurements 10 .…”

Stellarators Resist Turbulent Transport on the Electron Larmor Scale

“…1 is consistent with the destabilization of the ETG mode with increasing values of the temperature gradient and temperature ratio (T i =T e ), while the subsequent reduction of χ gk e in the outer region can be attributed to mode stabilization by the density gradient. It should be noted that the experimental diffusivities contain significant contribution from neoclassical fluxes [20] (also shown in the top middle panel), but the subtraction of this contribution only amplifies the observed trend [21].…”

“…Electron heat diffusivities (middle and right columns, top) from power balance analysis (χ eff e ) and gyrokinetic simulations (χ gk e ) are presented for comparison. The electron density, electron temperature, and ion temperature were measured by Thomsen scattering, ECE radiometry, and an XICS diagnostic, respectively, assuming equal argon and hydrogen ion temperatures [20].…”

In situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X

“…The ion temperature is found from the Doppler broadening of spectral lines; the electron temperature from the ratio of different lines; the flow velocity from the Doppler shift of the lines; and the impurity ion density from the line intensities. In a stellarator, the radial electric field, E r , can be inferred through measurements of the perpendicular plasma flow, u ⊥ 6 . Measured E r profiles are important for many aspects of stellarator physics including the study of neoclassical optimization in W7-X.…”