18th, Laser Aided Plasma Diagnostics

Design Advances of the Core Plasma Thomson Scattering Diagnostic for ITER

Sep 25, 2017, 11:50 AM

40m

INVITED TALK High-temperature plasmas General

Speaker

Dr Rory Scannell (UKAEA / CCFE)

Description

The Core Plasma Thomson Scattering (CPTS) diagnostic on ITER performs measurements of the electron temperature and density profiles which are critical to the understanding of the ITER plasma. The diagnostic must satisfy the ITER project requirements, which translate to requirements on performance as well as reliability, safety and engineering requirements. The implications of these requirements are particularly challenging for beam dump lifetime, the need for continuous active alignment of the diagnostic during operation, the requirement for low neutron flux through the diagnostic drawer and the protection of the first mirror from plasma deposition. The CPTS design has been evolving over a number of years. One recent improvement is that the collection optics have been modified to include new freeform surfaces. These freeform surfaces introduce extra complexity to the manufacturing of the optics but provide greater flexibility in the design. The greater flexibility introduced allows for example to lower neutron throughput or use fewer surfaces while conserving optical performance. Compared with existing TS systems, the CPTS diagnostic has a relatively small collection solid angle, long scattering lengths, high laser energy with low integration time. Performance assessment have shown that scattering from a 1064 nm laser will be sufficient to meet the measurement requirements, at least for the system at the start of operations. The optical transmission in the lower wavelength region, below 600 nm, is expected to degrade over the ITER lifetime due to neutrons and deposition on the first mirror. For this reason, it is proposed that the diagnostic shall additionally include measurements of TS ‘depolarised light’ and a 1318 nm laser system. These additional techniques have different spectral and polarisation dependencies compared to scattering from a 1064 nm laser and hence these additional measurements introduce greater robustness into the inferred measurements of electron temperature and density.