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

Abstract

Absorber foils have been installed in the TFTR X-Ray Imaging System to permit measurement of the electron temperature along 10-30 chords spaced at 5-12.5 cm with a time resolution of less than 100 Us. The technique uses the ratio t*f X-ray fluxes transmitted through two different foils. The ratio depends mainly on electron temperature. Simulations show that strong impurity line radiation can distort this ratio. To correct for these effects, special beryllium-scandium filters are employed to select the line-free region between 2 and 4.5 keV. Other filter pairs allow corrections for Fe L and Mi L line radiation as well as Ti K and Hi K erai ssion. Good accuracy is also obtained with simple beryllium filters, provided that impurity corrections are incorporated in the analysis, taking line intensities from thd X-ray pulseheight analysis diagnostic A description of modeling calculations and a comparison of temperature values from this diagnostic with data from the X-ray pulse height analysis, the electron cyclotron emission, and the Thomson scattering diagnostics are presented. Several applications of the absorber foil electron temperature 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 cause a redistribution of the plasma energy and may be partly responsible for deviations from adiabatic scaling.

Resistive wall mode identification by contrast enhancing technique of soft x-ray measurements on DIII-D

“…Although and can vary from 0.5 to 3 for TFTR case, the values = 1 and = 2 were used and we obtained the same results for the ratios of foil-detector signals as a function of electron temperature for a pure plasma. 21 Our calculation reconfirmed the conclusion that the ratio of the foil-detector system signals depends mainly on the peak electron temperature value along the chord and is relatively insensitive to the profile shape of temperature and density, and to the tangent radius.…”

“…First we tested our calculation with the reported results of modeling for electron temperature measurements by the x-ray absorber foil method on TFTR. 21 Electron density and temperature profiles were approximated by parabolas raised to the powers and , respectively, i.e., n e ͑r͒ = n e0 ͑1 − r 2 / a 2 ͒ and T e ͑r͒ = T e0 ͑1−r 2 / a 2 ͒ , where a is the plasma minor radius. Although and can vary from 0.5 to 3 for TFTR case, the values = 1 and = 2 were used and we obtained the same results for the ratios of foil-detector signals as a function of electron temperature for a pure plasma.…”

A ‘multi-colour’ SXR diagnostic for time and space-resolved measurements of electron temperature, MHD activity and particle transport in MCF plasmas

“…The appropriate selection of filters, as well as a low oxygen-to-carbon ratio of the order of a per cent or less, will enable the correct interpretation of the multi-colour ratios. In order to consistently check the very first assumption regarding the Maxwellain EEDF and thus the validity of the continuum radiation approximation, the amplitudes of the available signals can be routinely plotted against the foils cut-off energy as shown below [7][8][9]. The 'double foil' technique has been used successfully under a variety of plasma conditions and confinement schemes with a wide range of electron temperatures, from micro-pinches, macroscopic reversed field pinches (RFPs) and vacuum sparks, to modern tokamaks and stellarators.…”

Design and initial operation of a two-color soft x-ray camera system on the Compact Toroidal Hybrid experiment

“…If line radiation cannot be avoided, additional means of ascertaining the dominate impurity contribution to the SXR signal using x-ray pulse height analysis to provide impurity corrections to the twocolor measurements have been employed. 21 For the initial temperature estimates presented, the following effects on the calculated two-color temperature ratio are discussed: (i) a low energy transmission window that arises with single element thin filters, (ii) the effect of intrinsic impurity line radiation.…”