Correcting Thomson Scattering Electron Density Measurements Using Alignment Monitors

Electron density measurements via Thomson scattering rely on an absolute calibration, relating the number of scattered photons to signal levels in spectral detectors. Misalignment between the calibrated positional relationship of the scattering volume in the plasma and the collecting optics may lead to fewer photons collected and thus an increased error in the density measurement. Scattered light collected with a dedicated and specialized optical fiber bundle that spans the volume — monitoring the accuracy of the alignment — is used to estimate the reduction of collected photons in the main collection fiber bundles. The validity and results of correcting the electron density measurements via these alignment monitor data are investigated.