Engineering the Alcator C-Mod MSE diagnostic: Solutions to reactor-relevant diagnostic challenges
Deploying precise optical diagnostics in harsh reactor environments poses significant engineering challenges including limited access, large forces, time-varying systematic errors, reflections, low signal-to-background levels and a need for in-situ calibration. Fielding a polarization-based Motional Stark Effect (MSE) diagnostic on Alcator C-Mod has proven to be difficult due to similar challenges, which has led to the development of solutions that enable polarization angle measurements with an accuracy of 0.2 degrees.
Robotic calibration of the motional Stark effect diagnostic on Alcator C-Mod.
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.
Measurement of the plasma radial electric field by the motional Stark effect diagnostic on JET plasmas
The radial electric field gradient or the E×B flow shear has been pointed out as the underlying mechanism for turbulence suppression, responsible for an internal transport barrier formation in advanced tokamak scenarios. A comprehensive study on these subjects requires a direct measurement of the plasma radial electric field Er. The poloidal component of the magnetic field is assessed by the motional Stark effect (MSE) polarimeter, which is currently a standard diagnostic in fusion devices, allowing a local and nonperturbative measurement of the magnetic pitch angle. A precise measure to the state of polarization of the Stark components gives the information on the direction of the magnetic field. Due to the particular orientation of the Lorentz component, that is nearly perpendicular to Er, the MSE diagnostic is very sensitive to the plasma intrinsic radial electric field. This article describes a technique to measure Er involving the change of the polarization angle of the MSE emission, by using two bea...