Grazing-incidence flat-field spectrometer for high-order harmonic diagnostics

The design and performances of an EUV spectrometer for diagnostics of high-order harmonics generated by an ultrashort (<30 fs) pulsed laser focused onto a gas jet are presented. The system consists of a toroidal mirror, an entrance slit, a spherical variable-line-spaced grating and a bidimensional detector. The mirror focuses astigmatically the radiation from the gas jet, with the tangential focus on the entrance slit. The grating diffracts the radiation coming out from the slit, spectrally focusing the 5 to 40 nm wavelength range in an almost straight focal curve, where the detector is located. The latter is a 40 mm diameter microchannel-plate intensifier. The spatial focal point of the toroidal mirror lies before the spectral focal plane of the spectrometer, so measurement of the height of the spectral lines gives the beam divergence. The spectral resolution of the spectrometer was measured using the narrow line spectra obtained by a microfocus soft x-ray source. Also the absolute response of the system, that is grating efficiency plus detector efficiency, was measured.