## 3 Double-Stage Monochromator for Ultrashort Pulses  
The two geometries discussed above can be joined in an innovative design of a monochromator for ultrashort XUV pulses. The instrument has two different and interchangeable diffracting stages both used at grazing incidence, one with the gratings in the CDM and the other in the OPM. The use of two stages gives great flexibility: the OPM stage is used for sub-50 fs time response and low spectral resolution (\(\lambda/\Delta\lambda\leq 100\)) and the CDM stage is used for 100-300 fs time response and medium spectral resolution (\(\lambda/\Delta\lambda\geq 500\)). The design overcomes the limits of the two configurations and presents the advantages of both of them, giving on the same instrument either ultrafast time response with low spectral resolution or longer time response with medium resolution. Furthermore, it has broad tunability and high efficiency. In particular, a monochromator is described here that is being realized for the spectral selection of HHs in the 12-120 nm (100-10 eV) interval. It has several gratings optimized in different intervals within the region of operation and adopts the new proposed design to support both ultrashort temporal response with low resolution and longer response with high resolution. A block diagram is shown in Figure 6. The first block is the fixed toroidal mirror to collimate the light coming from the HH source. The second section consists of three gratings operated in the OPM mounted on a motorized stage to select one of them. The gratings of the second section can be removed from the optical path to use the gratings of the third section. The latter has three gratings operated in the CDM at a constant deviation angle. The forth section has two different toroidal mirrors that can be inserted in the optical path to focus the light on the exit slit: the first one is mounted to intercept the light diffracted by the grating in the OPM, the other is used with the CDM. Finally, the exit slit block has two different variable-width slits: one is mounted vertically for the CDM, the other horizontally for the OPM. The executive drawings to realize the instrument are shown in Figure 7. The parameters of the monochromator are resumed in Table 2. The expected performances are shown in Figure 8.  
Figure 6: Block diagram of the double-stage monochromator: (**a**) OPM configuration; (**b**) CDM configuration. This design allows us to use both the grating geometries with their peculiar characteristics. The OPM stage gives fast temporal response, that is, a pulse-front tilt in the 10-50 fs range, and low resolution, that is, \begin{table}
\begin{tabular}{l l l l} \hline _MIRRORS_ & \multicolumn{3}{c}{_GRATINGS_} \\ \hline
**BLOCK 1** & Collimating mirror & **BLOCK 2** & Off-plane gratings \\ \hline Source-to-mirror & 400 mm & Altitude & 5\({}^{\circ}\) \\ \hline Incident angle & 87\({}^{\circ}\) & _Grating 1_ & 30–120 nm 40–10 eV \\ \hline  & & Groove density & 200 gr/mm \\ \hline
**BLOCK 4** & Focusing mirrors & _Grating 2_ & 20–60 nm 60–20 eV \\ \hline Mirror-to-slit distance & 400 mm & Groove density & 400 gr/mm \\ \hline _Mirror 1_ & Off-plane geometry & _Grating 3_ & 12–30 nm 100–40 eV \\ \hline Incident angle & 87\({}^{\circ}\) & Groove density & 800 gr/mm \\ \hline _Mirror 2_ & Classical geometry & & \\ \hline Incident angle & 80\({}^{\circ}\) & **BLOCK 3** & Classical gratings \\ \hline  & & Deviation angle & 154\({}^{\circ}\) \\ \hline _SLITS_ & & _Grating 1_ & 30–120 nm 40–10 eV \\ \hline
**BLOCK 5** & Horizontal/vertical & Groove density & 300 gr/mm \\ \hline Slit aperture & 20 μm–2 mm & _Grating 2_ & 20–60 nm 60–20 eV \\ \hline  & & Groove density & 600 gr/mm \\ \hline  & & _Grating 3_ & 12–30 nm 100–40 eV \\ \hline  & & Groove density & 1200 gr/mm \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of the monochromator. Figure 6: _Cont._  
Figure 7: Executive drawings of the monochromator main chamber; (**a**) OPM; (**b**) CDM. \(\lambda/\Delta\lambda=\mathrm{E}/\Delta\mathrm{E}\approx 30\)-100.