Application of polarization interferometers for Thomson scattering

Wide field-of-view, high transparency birefringent filters (essentially fixed delay interferometers) are proposed for incoherent Thomson scattering measurements of the temperature and density of plasma free electrons. For thermal electrons, the optical coherence of the Thomson scattered light at an appropriately chosen optical path delay, is a unique function of the electron temperature and density. The detection system utilizes a single filter combined with imaging optics and dual detector arrays to simultaneously observe both dark and bright scattered light interference fringes. The system delivers two signals that allow the recovery of the two unknowns—temperature and density. It is shown that the normalized intensity difference between the bright and dark interference fringes gives a direct measure of the electron temperature, even for strongly blue shifted high temperature spectra. As usual, the total scattered light flux (the sum of bright and dark signals) is proportional to the number of illuminated electrons. For multi-pulse systems, an electronically switchable ferroelectric liquid crystal delay plate synchronized with the laser repetition rate can allow density and temperature to be obtained using a single detector array. The use of a time-multiplex approach both simplifies relative channel calibration issues and opens the possibility for 2D temperature imaging. This paper describes the measurement principle and presents the results of numerical simulations for both low and high temperature scenarios.