Beam emission polarimetry

The beam emission polarimetry diagnostic measures the Stark split emission from the Balmer line Dα (n = 3 → 2 transition, λ = 656.1 nm) of deuterium atoms from beam source #6 of neutral beam injection (NBI) box 2 at AUG. The Stark effect provokes a wavelength splitting and a polarization of the emitted radiation. The Dα emission line splits into 15 lines, 9 strong and 6 very weak lines. The Stark lines are polarized perpendicular to the electric field (three σ lines for Δm = ±1, with m the magnetic quantum number) and polarized parallel to the electric field (six π lines for Δm = 0). Simultaneous spectral and polarization measurements of the direction of the linearly polarized π and σ component emission allows for a direct measurement of the magnetic field line angle γ = arctan (Bθ/Bφ), where Bθ and Bφ denote the poloidal and toroidal magnetic field. The edge current density is derived from γ through Bθ using Ampère’s law.

The diagnostic consists three optical heads mounted at sector 6 inside the AUG vessel. Each optical head is equipped with five optical fibers, a lens and a polarizer. We use three independent observations of the same five radial positions with polarizer angles α ≈ 0°, 45°, and 90°, where 0° serves as a reference for the π lines, 90° for the σ lines, and 45° measures a combination of both σ and π lines.

[1] R. Dux et al., A forward model for beam emission spectroscopy at ASDEX Upgrade, 42nd EPS Conference, P1.121, Lisbon (2015).
[2] E. Viezzer et al.,
A new beam emission polarimetry diagnostic for measuring the magnetic field line angle at the plasma edge of ASDEX Upgrade, RSI 87, 11E528 (2016).