Robotic calibration of the motional Stark effect diagnostic on Alcator C-Mod

Abstract

The capability to calibrate diagnostics, such as the Motional Stark Effect (MSE) diagnostic, without using plasma or beam-into-gas discharges will become increasingly important on next step fusion facilities due to machine availability and operational constraints. A robotic calibration system consisting of a motorized three-axis positioning system and a polarization light source capable of generating arbitrary polarization states with a linear polarization angle accuracy of <0.05° has been constructed and has been used to calibrate the MSE diagnostic deployed on Alcator C-Mod. The polarization response of the complex diagnostic is shown to be fully captured using a Fourier expansion of the detector signals in terms of even harmonics of the input polarization angle. The system's high precision robotic control of position and orientation allow it to be used also to calibrate the geometry of the instrument's view. Combined with careful measurements of the narrow bandpass spectral filters, this system fully calibrates the diagnostic without any plasma discharges. The system's high repeatability, flexibility, and speed has been exploited to quantify several systematics in the MSE diagnostic response, providing a more complete understanding of the diagnostic performance.

Development of an 11-channel multi wavelength imaging diagnostic for divertor plasmas in MAST Upgrade

“…spherical aberration) in the system are accumulating with channel number leading to the deterioration of image quality. The polychromator has the capability to self-cancel certain off-axis aberrations like coma which depend on the angle of field of view due to the flip of the image by the concave mirror on consecutive channels, as described in detail in reference 18 . However, the spherical aberration, which dominates other aberrations due to the wide field of view of the system, is accumulated and the best way to minimize such aberrations is to reduce the numerical aperture by stopping down the camera lens.…”

“…This intermediate image can be used for in-situ focus adjustment and image quality assessment for all the cameras. The polychromator, which has the same configuration as MANTIS 14,15 , is in turn based on the original polychromator design for the Motional-Stark Effect diagnostic on Alcator C-MOD 18 . Each polychromator in the MWI is composed of a field lens with focal length of 750mm (achromatic doublet lens), a bandpass interference filter, a dielectric coating concave mirror with a curvature radius of 768mm, camera lens and a CMOS camera.…”

MANTIS: A real-time quantitative multispectral imaging system for fusion plasmas

“…Previous efforts of the detachment control efforts employed sensors based on rt-thomson 10 and rt-bolomtery. 11 The system's optical design 12 is based on a 4-channel Multispectral Imaging System (MSI) 9 previously installed on TCV. The design was extended to 10 channels with a completely new mechanical design, relay optic, and cameras.…”

Motional Stark Effect Measurements Of The Local Magnetic Field In High Temperature Fusion Plasmas

“…Crucial for any MSE polarimeter is the absolute accuracy of the polarization angle measurement and the calibration to achieve this 43 . Issues involved with the calibration of the polarimeter setups are the Faraday rotation induced by the magnetic field inside the observation optics, reflections from (non-ideal) mirrors, any type of spurious birefringence in the observation optics, or coatings of first mirrors or windows from plasma vessel conditioning or from plasma operation.…”