Progress on the ITER Toroidal Interferometer and Polarimeter (TIP)

The ITER Toroidal Interferometer and Polarimeter (TIP) is the primary diagnostic used for real-time feedback control of plasma density. In TIP, two-color vibration compensated interferometry is carried out at 10.59 μm and 4.6 μm using a CO2 and Quantum Cascade Laser respectively while a separate polarimetry measurement of the plasma-induced Faraday effect is made at 10.59 μm. Following tests of a full-scale TIP prototype on the DIII-D tokamak, which largely validated the measurement approach, work continues in advancing the engineering design and experimentally mitigating risks identified throughout the course of testing. Issues under investigation include atmospheric and window effects, beam refraction, and feedback alignment bandwidth. The TIP prototype has been upgraded with environmentally controlled "plasma" and "reference" leg chambers where humidity, pressure, temperature and species can be varied and the impact on measurements quantified. Numerical modeling suggests refraction is negligible for all but the highest density disruption mitigation scenarios; however, atmospheric and window effects, if not mitigated, will be issues and dominate TIP errors.