Multichord time resolved electron temperature measurements by the X-ray absorber foil method on TFTR

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Abstract
Absorber foils have been installed in the TFTR X-Ray Imaging System to permit measurement of the electron temperature (Te) along 10–30 chords, spaced at 5–12.5 cm, with a time resolution of less than 100 μs. The technique uses the ratio of X-ray fluxes transmitted through two different foils, which depends mainly on Te. Simulations show that strong impurity line radiation can distort this ratio. Good accuracy is obtained with simple beryllium filters, provided that impurity corrections are incorporated in the analysis, taking line intensities from the X-ray pulse height analysis diagnostic. The paper presents a description of modelling calculations and a comparison of Te values from this diagnostic with data from X-ray pulse height analysis, electron cyclotron emission and Thomson scattering diagnostics. Several applications of the absorber foil Te diagnostic on TFTR are discussed. In particular, it was found in compression discharges that unusually large internal sawtooth disruptions occurred during or shortly after compression. The large sawteeth caused a redistribution of the plasma energy and may be partly responsible for deviations from adiabatic scaling.