## 9 The MID Instrument
### MID Instrumentation and Capabilities  
The MID instrument is mainly installed in two safety hutches, an optics hutch (OH) and an experiment hutch (EH), but several essential components are also placed inside the SASE2 photon beam transport tunnel (see Figure 8). The OH contains a Si(220) monochromator to reduce the bandwidth of the SASE radiation to \(\Delta\)E/E \(\sim\)6.1 \(\times\) 10\({}^{-5}\) if required. An additional Si(111) mono (\(\Delta\)E/E \(\sim\)1.4 \(\times\) 10\({}^{-4}\)) installed in the SASE2 tunnel can be used to pre-monochromatize or separately. Alternatively, the SASE beam can be applied directly (\(\Delta\)E/E \(\sim\)1 \(\times\) 10\({}^{-3}\)) or in self-seeded mode [104; 105] (\(\Delta\)E/E \(\sim\)1 \(\times\) 10\({}^{-5}\)) once self-seeding becomes available at SASE2. Together with undulator tapering this will allow achieving more than 10\({}^{12}\) ph/pulse and a record high spectral peak brightness of more than 10\({}^{14}\) ph/s/meV at 9 keV [106]. The OH contains beam attenuators and slits for further beam tailoring as well as an imager system to provide in-situ visualization of the beam size, shape, and intensity. A split-and-delay line (SDL) [107; 108] will also be installed in the OH and will give the possibility of modifying the time-structure of the beam. Normally, the European XFEL delivers ultrashort (\(\sim\)1-100 fs) pulses of photons every 220 ns (4.5 MHz), but with the SDL under construction for MID it is possible to reduce this spacing to any value from \(\sim\)10 fs to 800 ps [109]. This enables particular experiments requiring such an X-ray pulse pattern, e.g., speckle visibility techniques [110; 111] for ultrafast dynamics or X-ray pump X-ray probe, possibly in combination with an optical fs laser pump [112]. The latter gives the additional option of performing X-ray probe-Optical pump-X-ray probe measurements where the two pulses from the SDL are not only delayed in time but also are hitting the sample at different angles of incidence. This provides a unique possibility to distinguish the two X-ray diffraction patterns hitting the detector and a spatial encoding of ultrafast dynamics can hence be obtained to yield a time-resolution much better than the 4.5 MHz detector speed [112]. In the EH, the beam first passes through a double mirror system (if inserted) that allows reflecting the X-ray beam downwards for grazing incidence liquid surface scattering. Another mirror reflecting upwards provides the aforementioned option of different incidence angles for the two split beams from the SDL. Downstream of the mirror system the ultra-high vacuum (\(\sim\)10\({}^{-9}\) mbar) section of the instrument terminates and it is necessary to operate at a lower vacuum level or even at ambient conditions due to the presence of outgassing substances, sample environments, and electronics. This transition is ensured either by insertion of a beam transparent diamond window, or by use of the differential pumping section positioned immediately downstream of the mirror. A large multi-purpose sample chamber (MPC) hosts local optics for nano-focusing, a hexapod sample manipulation stage, as well as different sample environments, e.g., providing low-temperatures via He cryo-cooling, pulsed high magnetic fields, fast sample scanning, sample injection by liquid jets, aerosol injection, etc. To ensure a maximum of stability the stages carrying the nano-focusing setup and the sample hexapod are decoupled from the vacuum pipes and chamber walls and connected directly, via vacuum feedthroughs, to a several ton heavy granite block below the MPC. A focal spot down to 50 \(\times\) 50 nm\({}^{2}\) or smaller is enabled by the nano-focusing system [112]. Assuming full transmission of the FEL pulses this could enable peak intensities of beyond 10\({}^{21}\) W/cm\({}^{2}\)[39; 113; 114] allowing to explore non-linear X-ray interactions with matter, e.g., two- or multi-photon processes in scattering and absorption [115; 116; 117].