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High-resolution spectrometer for TOKAMAK plasma diagnostics

High - resolution spectrometer was designed to resolve the fine structure of discharge emitted radiation near hydrogenous line 4648.8 A. To achieve high resolution with relatively big aperture and only spherical optics the new method of the optical mounting calculation was applied. As it was mentioned1, some aberrations of the first part of the double monochromator can be compensated by its second part. For present device we chose the Rowland circle geometry for one wavelength. This geometry has no defocusing and meridional coma aberrations. The sagittal coma and the first order astigmatism were compensated using double monochromator mounting. To reduce aberrations for other wavelength of the spectral region the slight nonequidistancy of the grooves of two concave diffraction gratings was introduced. The device consists of two concave diffraction gratings with 2700 grooves per mm and the radius of curvature 500 mm. The theoretical limit of resolution for this case is 0.015 A. The aberration limit of resolution, calculated using the mathematical model of spectrometer is 0.006 - 0.03 A for the spectral region 4641.8 - 4661.6 A. Because of property of the double monochromator mounting to compensate the second order astigmatism aberration1, the entrance slit of spectrometer can be high, more than 10-20 mm. Then, it is possible to analyse a number of emitting points simultaneously.