Assessment of the two-filter soft x-ray method to infer electron temperature in the sub-100 eV regime

The two-filter soft x-ray (SXR) method has been widely used to infer the electron temperature (Te) in keV-scale fusion plasmas, but its application in the sub-100 eV regime has been relatively less explored. For application in this regime, several additional effects require consideration. This work assesses the two-filter SXR method to infer Te in the sub-100 eV regime using the Be/Al filter configuration of the versatile experiment spherical torus SXR arrays. In this process, the relevant effects are separated into two categories. Random uncertainty components, namely, the filter-thickness fabrication tolerance and detector noise, are propagated through the ratio-temperature response curve to quantify the uncertainty. Systematic effects, namely, native oxide layers on the filters and impurity line emission, modify the response and are incorporated into the response calculation. Overall, the results indicate that the various random and systematic effects must be properly accounted for to achieve two-filter Te measurements in the sub-100 eV regime.