# A Reaction Microscope for AMO Science at Shanghai Soft X-ray Free-Electron Laser Facility
## 2 Apparatus Setup
### Supersonic Gas Jet  
At room temperature, the thermal motion of atoms and molecules in a gas target would bring a great error to the measurement of momentum. Therefore, the formation of a well-localized and internally cold gas target is one of the key factors to ensure high-resolution measurement. In addition, it is desirable to ionize less than one molecule per laser pulse in experiments, in order to make accurate ion-electron coincidence measurements, so, a thin target is required, considering the high power of SXFEL.  
The composition of the supersonic gas jet system is depicted in Figure 2. A 50 \(\upmu\)m nozzle is installed on a three-dimensional adjustable manipulator. The high-pressure (1-5 bar) gas froma conventional gas cylinder passes into the low-pressure vacuum chamber through the nozzle. The gas in the low-pressure region expands quasistatically and adiabatically so that the enthalpy of the gas is converted into directional kinetic energy. A conical skimmer of 0.2 mm inner diameter is placed at 15-mm downstream of the nozzle to extract the centerline gas beam from the so-called zone of silence. The length of zone of silence during the experiments is estimated to be 30 mm by equation [43]  
\[L=\frac{2}{3}\sqrt{\frac{p_{0}}{p_{b}}}d_{nozzle}, \tag{1}\]  
where \(p_{0}\) is the initial gas pressure and \(p_{b}\) is the background pressure in the first stage. Subsequently, a skimmer of 0.4 mm in diameter and mounted parallel to the first skimmer selects particles with a small momentum spread in the x and z planes. After that, two pairs of adjustable slits are arranged to tune the size of the gas beam in the direction perpendicular to the jet's propagation direction. An internally cold supersonic gas target is then obtained and the remaining gas is collected in the dumping chamber, which is an independent residual gas extraction system (not shown here). The distance between the nozzle and reaction center is about 1045 mm.  
As is shown in Table 1, the whole supersonic gas jet system has six vacuum stages. The low vacuum pressurein the source chamber is gradually transitioned to ultra-high vacuum conditions when reaching the intersection chamber via several differential pumping stages. At the intersection region, the maximum diameter of the gas jet(uncollimated) in the x and z directions is around 4.0 mm, and the size adjustment accuracy is 0.02 mm,  
Figure 1: Overview of the REMI at SXFEL. A mutually perpendicular gas jet line (green arrow) and FEL line (purple arrow) meet in the intersection chamber, which contains the spectrometer and detectors. A pair of coils (orange) is used to constrain the electron trajectory.  
which is determined by the adjustment accuracy of the slits. The density of gas target is estimated to be about \(10^{8}/\)cm\({}^{3}\). From the speed ratio, S, of 38.7 in the experiment of the single ionization of He, which is the mean jet velocity divided by the thermal spread in velocities, it is estimated that the gas jet is eventually cooled to 0.5 K [44]. Ultra-high vacuum conditions and a thin gas-jet target make the coincidence measurement of ions and electrons in the strong field of free electron laser feasible.