Model Validation for Quantitative X-Ray Measurements

Soft–X-ray (SXR) brightness measurements contain information on a number of physics parameters in fusion plasmas; however, it is nearly impossible to extract the information without modeling. A validated forward model is therefore necessary for the accurate interpretation of SXR measurements and will be critical in the burning plasma era, where medium- and high-Z impurities are ever present. The Atomic Data and Analysis Structure (ADAS) database is a powerful interpretive tool that is extensively used to model and predict atomic spectra, level populations, and ionization balance for fusion plasmas. These predictions are in good agreement with experimental measurements. However, continuum radiation in the X-ray range, while also modeled in ADAS, has not been rigorously verified or tested against experimental data. We therefore performed a systematic comparison of ADAS to a simplified model called PFM. PFM only calculates continuum radiation but shows good agreement with experimental data when only continuum radiation is present. ADAS and the simplified model agree to within 1% to 2% indicating that ADAS is calculating continuum radiation correctly. We have also begun a validation of SXR brightness calculations from ADAS. The SXR brightness measurements modeled by ADAS agree well with experimental measurements from an extreme where the signal is dominated by line radiation continuously through another extreme where the signal is dominated by continuum emission. While this validation work is preliminary, it strongly suggests that ADAS accurately models the physics that lead to SXR radiation.