Line ratio spectroscopy on thermal helium at ASDEX Upgrade

The new thermal helium diagnostic provides highly resolved electron temperature and density profiles in the plasma edge region of ASDEX Upgrade.

CAD representation of the in-vessel components for line ratio spectroscopy on thermal helium.

The plasma edge region of magnetically confined fusion plasmas is of highest importance for the overall plasma performance. To investigate the transport processes in this region, a thermal helium beam diagnostic has been implemented at ASDEX Upgrade.

For this diagnostic, helium is locally injected into the plasma by a piezoelectric valve. Due to collisions with plasma particles, neutral helium gets excited and specific line radiation is emitted. This process is described by collisional radiative models which enable the reconstruction of the underlying density and temperature values from the measurement of line intensity ratios, performed with a newly developed four-color 32-channel polychromator system.

Thus the diagnostic provides simultaneous measurements of the electron temperature and density with high spatial (3 mm) and temporal (900 kHz) resolution. This allowed for the first time the determination of plasma parameters within single turbulent structures.

For this work Michael Griener received his doctorate from TUM on the 19th of July 2018.          

Details of the diagnostic can be found in:
PhD Thesis Griener,
Review of Scientific Instruments 2017,
Plasma Physics and Controlled Fusion 2018,
Review of Scientific Instruments 2018

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