Refractive and reflective collection optic designs for the JT60-SA high-field-side Thomson scattering system

JT60SA will be equipped with three collection optics for Thomson scattering measurements on the same laser beam tangentially crossing the plasma: one for the edge plasma region, one for the plasma core, and one, still in the preliminary design phase, for the high field side (HFS TS). The baseline design of HFS TS employed a refractive collection optic system with various lens materials including fluorites. The glass material cannot be used for large size lens (>200mm in diameter). To overcome this issue, two alternative designs are presented here. The first is fully based on reflective elements. Its design is reminiscent of a standard Maksutov system, although the primary and secondary mirrors in this case are manufactured as aspherical surfaces, in order to retain sufficient spatial resolution all over the field. The use of reflective optics results in a system which is more compact and features a higher radiation resistance and higher transmission, at a price of lower imaging performance. A 20% reduction in the collection solid angle is expected due to secondary mirror obstruction. The second is a simplified refractive system with a reduced number of lenses: at least 4 elements are required. It has been optimized either for a standard straight or a periscope-folded layout. Optical analysis for both refractive and reflective systems includes spots, vignetting and imaging characterization, as well as a detailed comparison of the expected spectral transmission. Spurious light collection, fiber coupling and scattering volume/laser beam radius optimization are also discussed. A possible integration in JT60SA port plug is finally discussed.