The design and performance of a spectroscopic instrument able to simultaneo
usly measure spectral features, angular divergence, and conversion efficien
cy of high-order harmonics generated by an ultrashort pulsed laser focused
onto a gas jet are presented. It combines an advanced optical setup with a
EUV-sensitive bidimensional detector. The grazing-incidence spectrometer co
nsists of two optical elements: a toroidal mirror and a spherical variable-
line-spaced grating with flat-field properties. The mirror focuses the radi
ation tangentially on the grating entrance plane, in order to increase the
flux collected by the grating. The grating has a variable groove spacing wi
th parameters optimized to produce an almost flat focal surface in the 5-40
nm region. A 40 mm diameter microchannel plate intensifier is used to acqu
ire the spectrum. The mirror does not compensate for the grating astigmatis
m, so the length of the spectral lines on the spectrum is proportional to t
he sagittal angular divergence of the high-order harmonic emission. The abs
olute response of the system was measured in order to obtain the conversion
efficiency of the harmonic generation process. Some experimental examples
reported here show very high sensitivity, which makes the system able to wo
rk in the single-shot operation, thus allowing us to follow the dynamic evo
lution of the harmonic generation process. (C) 2001 American Institute of P
hysics.