## 5 Conclusions  
We have presented the design and performance of a composite VMI spectrometer (CpVMI) optimized to measure both ions image and high-energy electrons image simultaneously at Shanghai soft X-ray free-electron laser. With the well-designed electric field, the spectrometer can detect the electrons with kinetic energies up to 1 keV and the ions with the kinetic energy of 20 eV simultaneously. With the experimental conditions investigated here, we reached a resolution of 1.5% at the electron kinetic energy of 500 eV. We also tested the momentum imaging capabilities of the ion arm at different voltage settings of the electron arm, reached a resolution of 4% at the ion kinetic energy of 5.6 eV when the electron arm is set for 100 eV. In a future upgrade, we plan to enable coincidence [20]  
Figure 5: The raw images of O\({}^{+}\) ions by dissociative ionization of O\({}_{2}\) with a 800 nm femtosecond laser with an intensity of \(1.5\times 10^{14}\) W/cm\({}^{2}\) at different sets of the electron arm: (**a**) 100 eV, (**b**) 300 eV, (**c**) 600 eV, respectively.  
Figure 6: (**a**) The slice of 2D momentum (at \(p_{z}=0\)) through 3D momentum distribution after a Abel inversion from the raw image of Figure 5a. (**b**) The kinetic energy spectrum of O\({}^{+}\) obtained by integration of the momentum distribution over the polar angle ranges between \(60^{\circ}\) and \(120^{\circ}\) with respect to the laser polarization.  
or covariance [17] detection of the high-energy electrons and the fragments ions using two novel timestamping fast optical cameras, Tpx3 Cam [31,32], which can simultaneously measure the position and time of all charged particles with excellent spatial resolution and temporal resolution.  
Conceptualization: X.L. ([PERSON]); Formal analysis: [PERSON], W.X., R.W., Y.F., L.T., Z.L. and X.L. ([PERSON]); Data curation: [PERSON], W.X., R.W., Y.F., L.T., X.L. ([PERSON]), [PERSON] and X.L. ([PERSON]); Investigation: [PERSON], W.X., R.W., Y.F., L.T. and X.L. ([PERSON]); Funding acquisition: [PERSON]. ([PERSON]); Project administration: [PERSON] and X.L. ([PERSON]); Supervision, [PERSON] and X.L. ([PERSON]); Writing, original draft: [PERSON], W.X. and X.L. ([PERSON]); Writing, review and editing: [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON], [PERSON] and [PERSON] ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Shanghai-XFEL beamline project (SBP), the National Natural Science Foundation of China (Grant Nos. 11574020, 12174259, 11604003), the Project of Thousand Youth Talents in China, and the starting grant of ShanghaiTech University.  
The experiment was performed at the BL02B beamline at SSRF, China. We are grateful to [PERSON], [PERSON], and [PERSON] for technical assistance and to the SSRF staff for stable operation of the equipment and of the storage ring during the experiments.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; nor in the decision to publish the results.