# A Reaction Microscope for AMO Science at Shanghai Soft X-ray Free-Electron Laser Facility
## 2 Apparatus Setup
### Spectrometer  
Both the reaction between the supersonic gas jet and the laser and the collection of the reaction particles occur in the intersection chamber. The spectrometer, providing a uniform electric field to accelerate ions and electrons, is schematically shown in Figure 3. It consists of 23 pieces of 1-mm-thick stainless steel-ring electrodes of 120 mm inner diameter and 200 mm outer diameter. The rings are separated by an 11-mm distance between each other by insulating ceramics and electrically connected by a cascade of 30-k\(\Omega\) resistors. Thus, the voltages on each ring electrode are uniformly distributed simply by applying two voltages to the ends of the spectrometer. The length of ion-flight region is 100 mm, while that of electron is 184 mm. The shorter length of ion-acceleration region makes the ion detector more suitable for particles with higher energies, such as recoil ions from the Coulomb explosion of molecules. Two meshes are mounted at both ends of spectrometer to eliminate the influence of the high voltage from the MCPs and to ensure the uniformity of the electric field. The stainless steel grids (wire diameter: 25 \(\upmu\)m; mesh size: 292 \(\upmu\)m) provide a transmittance of 85%.  
For the same momentum, the electron and ion velocities differ by a large mass ratio. Thus, for increasing electron energies, the detection efficiency decreases. Increasing the electric field would reduce the electron-momentum resolution. In order to achieve a 4\(\pi\) solid-angle measurement for high-energetic electrons, a pair of Helmholtz coils is necessary to generate a uniform magnetic field along the spectrometer axis, constraining  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Chamber** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline before injection & \(1.5\times 10^{-9}\) & \(4.4\times 10^{-9}\) & \(1.3\times 10^{-9}\) & \(9.2\times 10^{-10}\) & \(1.2\times 10^{-10}\) & \(7.2\times 10^{-11}\) \\ after injection & \(2.7\times 10^{-3}\) & \(1.3\times 10^{-8}\) & \(6.2\times 10^{-9}\) & \(1.0\times 10^{-9}\) & \(1.2\times 10^{-10}\) & \(9.8\times 10^{-11}\) \\ \hline \end{tabular}
\end{table}
Table 1: Typical pressures of different chambers before and after gas injection. Numbers one through six represent the source, first differential, second differential, third differential, intersection and dumping chambers, respectively. The pressures are given in torr.  
Figure 2: Schematic diagram of supersonic gas-jet system structure. The green arrow represents the propagation direction of the gas jet passing through six chambers from the nozzle to the dumping chamber. After passing through two skimmers and two pairs of slits, the jet collides with the laser (pink) in the center zone of the interaction chamber (purple dot) and is collected in the dumping chamber.  
the trajectories of the electrons. In this way, electrons will fly in spiral tracks before reaching the detector. Helmholtz coils are a pair of parallel coils with the same radius, number of turns and resistance. The distance between the two coils and the radius of the coils are both 87.5 cm. According to the test, the coils can stably generate a maximum magnetic field of 17 Gs with no air cooling or water cooling.  
The effect of the electric and magnetic fields on particle trajectories can be seen in Figure 3. For electrons with an energy of 15 eV and oxygen ions with an energy of 0.2 eV, charged particles towards different directions are all collected by the detector with an electric field of 4 V/cm and a magnetic field of 6 Gs. Then, the three-dimensional momenta of ions and electrons can be reconstructed from the TOF, achieving the detection position of each event.