Infrared laser polarimetry for electron density measurement in tokamak plasmas

The infrared CO2 laser polarimetry for electron density measurement based on the tangential Faraday rotation has been progressed in JT-60U tokamak. The reliability of polarimetry in a pellet-injected plasma was confirmed. The Faraday rotation was measured with good angle resolution of ∼0.01° under the sufficiently fast temporal resolution of 4 ms. Electron density evaluated from the tangential Faraday rotation measured by polarimetry shows good agreement with that measured by interferometry for an identical probing laser beam. The long-time continuous measurement was successful with good stability and accuracy. The basic feasibility of the infrared CO2 laser polarimetry for electron density measurement in long-time and steady-state operation in large fusion devices was demonstrated.