Helium line ratio spectroscopy for high spatiotemporal resolution plasma edge profile measurements at ASDEX Upgrade (invited)

TLDR

Line‑ratio spectroscopy of neutral helium provides simultaneous electron temperature and density measurements in plasmas. The upgraded diagnostic uses an optical head with a novel fiber‑clamping system to distribute 32 lines of sight across an 8.5 cm radial span, and a polychromator that simultaneously records the 587 nm, 667 nm, 706 nm, and 728 nm helium lines with filter‑photomultiplier arrays. With a 900 kHz temporal resolution and 3 mm spatial resolution, the system delivers fast, high‑resolution electron temperature and density profiles of the edge and scrape‑off layer, resolving plasma fluctuations, ELMs, filaments, and turbulence for frequency, localization, and propagation analysis.

Abstract

The thermal helium beam edge diagnostic has recently been upgraded at the ASDEX Upgrade (AUG) tokamak experiment. Line ratio spectroscopy on neutral helium is a valuable tool for simultaneous determination of the electron temperature and density of plasmas. The diagnostic now offers a temporal resolution of 900 kHz with a spatial resolution of up to 3 mm at 32 lines of sight (LOS) simultaneously. The LOS covers a radial region of 8.5 cm, starting at the limiter radius and reaching into the confined region beyond the separatrix. Two components are of particular importance for the aforementioned hardware improvements. The first is the optical head, which collects the light from the experiment. Equipped with an innovative clamping system for optical fiber ends, an arbitrary distribution pattern of LOS can be achieved to gain radial and poloidal profiles. The second major development is a new polychromator system that measures the intensity of the 587 nm, 667 nm, 706 nm, and 728 nm helium lines simultaneously for 32 channels with filter-photomultiplier tube arrays. Thus, the thermal helium beam diagnostic supplements the AUG edge diagnostics, offering fast and spatially highly resolved electron temperature and density profile measurements that cover the plasma edge and scrape-off layer region. Plasma fluctuations, edge localized modes, filaments, and other turbulent structures are resolved, allowing analysis of their frequency and localization or their propagation velocity.