A flat-field extreme ultraviolet(XUV) spectrograph, equipped with a varied line-spacing concave grating, a toroidal mirror, a 40-㎛ slit, and an XUV film on a spectral plane, was designed for a spectral range of 30-300 Å and constructed. The toroidal mirror is used to compensate for the astigmatism due to the grazing incidence of light at the grating. The components of the spectrograph were installed to correct the astigmatism at 100 Å, and their alignment method is suggested. The XUV emissions from a carbon plasma and an aluminum plasma, produced by an iodine laser with an energy of 0.5 J in 4 ns and a focusing intensity of $3×10^11 W/㎠$, were used as an XUV source for the spectrograph.
The principle on how the varied line-spacing concave grating produces a flat spectral plane is explained by investigating the grating equation at each position on the curved surface of the grating. The blaze function and the diffraction efficiency of the grating are calculated using the scalar diffraction theory and the reflectivity of gold. The efficiency has a maximum value of 15% near 100 Å in the first order of diffraction and varies significantly with wavelength and diffraction order. It is in good agreement with the experimental result in which the higher-order spectral lines at 30-40 Å showed similar intensities up to the third order, and the second-order line of 100 Å had the 70%-intensity of the first-order line.
The aberration of the spectrograph was analyzed by calculating of the wave front aberration. It confirmed the design of the grating which is optimized to obtain the best efficiency and spectral resolution near 100 Å. The spectral and spatial resolutions of the spectrograph were calculated. It showed that the spectrograph could produce a space-resolved spectral image with nearly constant spatial resolution throughout the entire wavelength range considered, and the spectral resolution did not significantly vary with the source height from the meridional plane. The measured spectral resolution(FWHM) of the spectrograph was 0.10 Å at 94.97 Å of Al VII with a 40-㎛ slit; thus, the spectrograph has a resolving power of about 1000 near 100 Å. As a result of the efficient correction of the astigmatism throughout the whole wavelength range, the spectrograph could record a spectrum on an XUV film from one shot of the iodine laser and could produce a space-resolved spectrum along the sagittal direction. From the comparison with other spectrograph systems using different gratings, it is found that the flat-field XUV spectrograph with the toroidal mirror is a system with a constant spatial resolution and a reasonably good spectral resolution in a wide wavelength range. The space-resolving spectrograph, having nearly the same spectral and spatial resolutions, will be very useful for the observation of the distributions of temperature, density, and ionization stage in plasmas.
The light scattering at $3ω_0/2$ was measured from X-ray laser plasmas produced by Gekko XII laser of Osaka University in order to diagnose the plasma. The spectral intensity profile near $3ω_0/2$ and the spatial intensity distribution of the $3ω_0/2$ emission from line-focused plasmas were obtained using a visible spectrometer coupled with a streak camera. In the case of a double-pulse pumping, the $3ω_0/2$ emission was observed with the second pumping pulse after a sufficient density scale length was developed, and the intensities of the X-ray laser and the $3ω_0/2$ emission increased as the energy of the first pumping pulse increased. The spatial imaging of the $3ω_0/2$ emission along the line-focused direction of the incident laser showed several localized emissions, which might come from a globally modulated two plasmon decay instability. The detailed analysis of the $3ω_0/2$ emission will help to resolve the difference still existing between the predicted gain, especially for the J=0-1 transition, and the observed value of the collisional-pumped X-ray laser.
