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.
First mirror contamination studies for polarimetry motional Stark effect measurements for ITER
The motional Stark effect (MSE) diagnostic on the International Thermonuclear Experimental Reactor will need to guide the light through a labyrinth of mirrors to provide neutron shielding. Knowledge of how the mirrors change the polarization is essential for accurate determination of the q profile. The optical properties of the plasma facing mirror are also expected to change with time due to deposition/erosion. For the purpose of examining this experimentally a detector system, identical to the JET MSE system, using twin photoelastic modulators was constructed. Measurements have been performed on freshly prepared mirrors, on mirrors after exposure to plasmas in Tore Supra, and labyrinth designs. The result shows a significant effect on the optical properties and demonstrate the need for in situ monitoring. The measured properties of the labyrinth closely follow the Mueller matrix formalism. With a correct choice of material the angle change introduced by the four mirrors furthest away from the plasma wil...
Motional Stark effect upgrades on DIII-D
The measurement and control of the plasma current density profile (or q profile) is critical to the advanced tokamak program on DIII‐D. A complete understanding of the stability and transport properties of advanced operating regimes requires detail poloidal field measurements over the entire plasma radius from the core to the edge. In support of this effort, we have recently completed an upgrade of the existing motional Stark effect (MSE) diagnostic, increasing the number of channels from 8 to 16. A new viewing geometry has been added to the outer edge of the plasma which improves the radial resolution in this region from 13 to ∼2 cm. This view requires the use of a reflector that has been designed to minimize polarization effects. Vacuum‐compatible polarizers have also been added to the instrument for in situ calibration. Future use of the MSE diagnostic for feedback control of the q profile will also be discussed.