Feasibility study for a new high resolution Thomson scattering system for the ASDEX Upgrade pedestal

A new Thomson scattering diagnostic is proposed for the study of fast plasma dynamics in the pedestal of ASDEX Upgrade. The diagnostic will measure electron temperature and density profiles over a ∼ 3 cm wide area in the edge transport barrier region, with ∼ 1–2 mm spatial resolution and ∼ 10 kHz sampling rate. A challenging goal of the project is the study of the bootstrap current in the plasma pedestal by measuring the distortion and shift of the electron distribution along the toroidal direction. Expected spatial and time resolutions of the current density measurements are ∼ 3 mm and ∼ 1 ms correspondingly. The new diagnostic will be used to study the fast dynamic behaviour of the pedestal bootstrap current, where models indicate that it plays a key role in regulating edge stability, e.g. during ELMs. The diagnostic design is based on the intra-cavity multi-pass system currently in operation in TEXTOR, which uses a probing ruby laser, a grating spectrometer and two fast CMOS cameras for scattered light detection, and has achieved measuring accuracies of the order of ∼ 1% for ne and ∼ 2% for Te. Parts of that system will be reused in ASDEX Upgrade (some with significant modifications), but the laser multi-pass and light collection systems are entirely redesigned. Restrictions in space and line-of-sight availability have led to the adoption of a design which uses in-vessel multi-pass mirrors and light collection optics, requiring a number of innovative technical solutions to permit remote laser alignment and light collection. We give an overview of the project, discuss the underlying physics basis and present a number of technical solutions employed.