Multilayer mirror based line emission tomography for spherical Tokamaks

Due to their highly shaped plasma and possible poloidal asymmetry in impurity concentration, spherical Tokamaks will require tomographic reconstruction of local emissivities to assess impurity content and transport. To collect in an effective manner the data required for such reconstruction, we develop arrays of high throughput "mini-monochromators" using extreme ultraviolet multilayer mirrors as dispersive elements and filtered surface barrier diodes as detectors. We discuss monochromator optimization and show that by working at near normal incidence throughput and spectral resolution are simultaneously maximized. A system proposed for tomographic reconstruction of C V and C VI resonance emission at 33.7 and 40.5 Å respectively, achieves 0.9 Å spectral resolution, 2 cm spatial resolution, and 0.2 ms temporal resolution, together with good sensitivity and background rejection. Preliminary results obtained from CDX-U low aspect ratio tokamak are also presented.