Unmonitored changes in the transmission of viewing windows owing to deposited films can produce errors in Thomson scattering temperature measurements. This effect is illustrated by a recent run on the Tokamak Fusion Test Reactor (TFTR) where apparent errors of over 25% in peak temperatures of 9 keV owing to carbon films were noted. Since coatings can also be removed by hydrogen discharges, the transmission of a window may change with time, resulting in variable errors in the temperature. It is proposed that these changes be monitored by calibration in situ with the aid of a low-pressure hydrogen glow discharge.
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D. H. Mcneill (1990) studied this question.
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