Engineering upgrades to the Motional Stark Effect diagnostic on Alcator C-Mod

[摘要] The Motional Stark Effect (mse) diagnostic deployed on the alcator c-mod tokamak previously experienced unacceptable calibration drift and sensitivity to partially-polarized background light that limited its ability to measure magnetic field pitch-angles. The need to identify the cause of the polarization drift and to characterize sources of error motivated the development of a robotic calibration system consisting of a motorized three axis positioning system and a light source capable of generating arbitrary polarization states. The system produces linear polarization angles with accuracy of < 0.05°, operates during maintenance periods, has a Lambertian and laser illumination source, and captures the complex diagnostic polarization response using a Fourier expansion of the detector signals in terms of even harmonics of the input polarization angle. This study guided the fielding of a first-of-a-kind inter-shot calibration (isc) system for the mse diagnostic -- a long sought capability. The isc inputs high quality polarized light at four carefully chosen polarization angles into the diagnostic objective lens within ten seconds of the plasma discharge using a single high-precision moving part internal to the vacuum vessel. Fiber optics are used to sequentially illuminate the calibration polarizers via backlight scattering similar to an LCD screen, fully filling the view of each of the ten mse sightlines with polarization angles repeatable to better than 0.05°. The system has operated inside the vacuum for over 8500 cycles and is used to reconstruct the diagnostic calibration on a variety of timescales. This system was used to identify thermal-stress induced birefringence in several of the diagnostic;;s lenses as the cause of polarization calibration drift. Gold-plated radiative heat-shields and low-conduction lens and periscope mounts thermally isolate the in-vacuum periscope, and a forced-flow active thermal control system regulates the ex-vessel periscope and vacuum window. This successfully controls the optic stress in thermal environment ranging ±150°C. A high-throughput narrow-bandpass filter-based polychromator was developed as a new configuration for mse detectors. The system allows high-spectral selectivity at an étendue of > 15mm2 sr for four simultaneous polarization measurements on the same viewing sightline. It sequentially passes light among four different < 0.9nm FWHM temperature-tuned filters to custom avalanche photodiode detectors operating at NA = 0.6. The polychromator is imaging, non-vignetting, high transmission, utilizes mostly off-the shelf optics, is easily aligned, remote-controlled and can be replicated at relatively low cost. This enables wavelength-interpolation of the mse background light and simultaneous detection of different parts of the Stark manifold.

[发布机构] Massachusetts Institute of Technology