Development of a Ross Filter-Based Aluminum Line Radiation Detector (NICKAL2) for the MST Toroidal Plasma Experiment

Modeling and data from the new NICKAL2 Ross filter-based x-ray diagnostic, installed on the MST, are presented. NICKAL2 has three filtered detectors sensitive to soft x-ray (SXR) radiation. Because MST has an Al plasma-facing surface, Al line radiation is present in the SXR spectrum, complicating the spectrum. The filters are designed to isolate Al line radiation and quantify their absolute intensities. Simulations show that the NICKAL2 passbands are dominanted by this Al line radiation. This results in a strong constraint on Al density at electron temperatures (T_e) as low as 250 eV. NICKAL2 will aid the two-color SXR tomography (SXT) diagnostic, which can not distinguish between continuum and line radiation. Line radiation is typically filtered out of the SXT signal using thick Be filters to ensure only continuum radiation contributes, thus restricting the SXT measurement range to T_e ≥ 1 keV. Using NICKAL2 to constrain Al line radiation independently permits the use of thinner filters on SXT diagnostic, enabling better inference of T_e and extending measurement cababilities to lower T_e. NICKAL2 data also provide important information to infer Z_eff profiles, particularly when combined with charge exchange recombination spectroscopy data.