Design Of Optic System For The Electron Cyclotron Emission Imaging Diagnostic On J-TEXT

There is a lot of instability mode affecting the confinement properties of plasma in tokamak. In order to understand various macroinstability and microinstability from experiment, it is required to get intuitionistic image of MHD evolution. It makes for realizing measuring fluctuation quantity, the fluctuation of the spatial coherence and the coherence between different fluctuation quantity at the same time. It needs a two-dimension plasma diagnostic tool with high space-time resolution, and electron cyclotron emission imaging system(ECEI) is one of the best two-dimension diagnostic tool. Over the past 20 years, a lot of nuclear fusion device has successively established ECEI system to measure a lot of MHD behavior like sawtooth magnetic reconnection,turbulent fluctuation structure, and so on.An electron cyclotron emission imaging(ECEI) system is being developed for J-TEXT. It is comprised of two antenna arrays that share the same toroidal window. Each antenna array are 16 channel in poloidal direction and 8 channel in radial direction. Two array can realize independent observation in different radial range at the same time. It is a set of 256 ECEI system in total. This article is focus on the design of optics system for ECEI.This article introduces the process of ECEI optical system design and optics system performance. ECEI optics system design is based on the Gaussian beam transmission theory. The front optics system is depend on modern three lenses design model, and the effective field curvature adjustment(FCA) lenses have been adopted for the ECEI imaging optic system. The front optics system is optimized for simplifying structure of overall lenses system. The lenses structure includes the zoom lenses group, the FCA lenses, high field side(HFS) focus lens, low field side(LFS) focus lens, H-Plane lenses.The front optics system has realized that the focus function, zoom function, field curvature adjustment function are combined. In poloidal high-resolution observation,spatial resolution at narrow zoom can achieve is 1.3 cm. In the poloidal zoom observation,zoom factor can changes from 1.16 to 1.99. For radial focusing function, radial focusing position cover from HFS edge to LFS edge in tokamak. The depth of field of focusfunction is from 38 mm to 100 mm. It is enough to guarantee radial focusing extension from HFS to LFS. More importantly, it is realized that adding FCA lens make optics system decouple field curvature from zoom function and radial focusing position. Field curvature at any radial position has been within 4.4 mm(less than 2.4% of the Rayleigh length). Therefore the ideal imaging surface are corrected to nearly flat plane. It is obviously improving the response of the antenna array to the plasma resonance layer.According to the pyramid horn detector size of F-band and W-band, the optical path design for F-band and W-band is within 1 meter and provides complete coverage upon the clear aperture antenna array for enough power.Material named hydrocarbon ceramics were selected for beamsplitter, and its refractive index is 2.480. As a result, the ratio of transmittance and reflectance is 45.4% to54.6%. It can be considered to 3 d B attenuation for power of the high side and low side signal.Finally, the front imaging optics performance are tested. It is tested that ECEI optics is suitable for different poloidal sampling volume. Scanning through the poloidal direction,the horn antenna detector could simulate the response of the antenna array to different poloidal sampling volume. Testing spacing and resolution in poloidal direction for imaging plane is close to design. It can be proven that laboratory test result is consistent with design analysis.In a conclusion, the optics design for ECEI on J-TEXT has been finished.