In recent years, inertial confinement fusion (ICF) technology has developed rapidly and exhibited its great potential for applications. In ICF experiments, the pellet will radiate a large amount of X-rays, and the nuclear fusion process can be analyzed by studying the spatio-temporal properties of X-rays. However, the nuclear fusion duration is short (nanosecond-picosecond order), with requirements of high spatial resolution and large dynamic range. However, the commonly applied ultra-rapid diagnostic instruments are more or less defective, among which the optomechanical high-speed camera cannot monitor ultrafast phenomena below the nanosecond order, with sufficient temporal resolution. Electro-optical or magneto-optical shutter high-speed camera makes it difficult to monitor weak signals due to a shutter resulting in incident light loss. Therefore, the study of streak cameras (streak tubes) with ultra-high spatio-temporal and light intensity resolution capabilities is of significance for detecting X-rays in ICF experiments. The anisotropic focusing streak tubes can achieve anisotropic focusing of electron beams by making the temporal-directed focusing system and the spatial-directed focusing system independent of each other. This tube type can not only improve the spatial resolution by increasing the magnification of the streak tube, but also suppress the space charge effect, reduce the aberration in the spatial direction, and improve the dynamic range and temporal resolution of the streak tubes.

At present, it is difficult for the existing simulation software to completely simulate the whole physical process of streak tubes. Although CST (CST Studio Suite) and other electromagnetic simulation software can suitably reflect the electron transport and the interaction between electrons and electromagnetic fields, less credible results are given for the photoelectron generation process. Therefore, when designing a streak tube, researchers generally need to specially program to calculate the photoelectron distribution of the photocathode based on the Monte Carlo method. However, generally, a programme can only be adopted for one or a few cases, which makes it less portable. Additionally, it is more difficult to verify the results of purely theoretical calculations. Therefore, we employ a high-energy particle simulation software Geant4 (GEometry ANd Tracking) developed by the European Organization for Nuclear Research based on the Monte Carlo method to simulate the photoelectron generation process. Then, based on the simulation results of Geant4, we leverage CST to simulate the subsequent electro-optical system. Finally, the design of an anisotropic focusing streak tube is realized by the software co-simulation.

The high-energy particle simulation software Geant4 is introduced to ultrafast diagnostics, the co-simulation from Geant4 to CST is realized, and an anisotropic focusing streak tube design that encompasses the entire process of photoelectron generation, transmission, focusing, imaging, and interaction between electrons and electromagnetic fields is yielded. Compared with the traditional simulation method, the photoelectron generation process is visualized in this scheme, and the photoelectron distribution is more consistent with the actual experimental situation. Meanwhile, since Geant4 can provide models for the electromagnetic, strong, and weak interaction between substances and particles of different energy to simulate the complete physical process, this scheme can be adapted to a wider range of photoelectron generation situations and is highly portable.

By adopting the co-simulation from Geant4 to CST, an anisotropic focusing streak tube with a CsI photocathode is designed, and the magnification ratio is 2 in both the sagittal and meridional directions, which can meet the practical engineering needs. The secondary electron emission from CsI photocathodes with a thickness of 50-300 nm irradiated by X-rays in the energy range of 1-10 keV is investigated by simulation in Geant4. In this process, the peak of secondary electron energy is around 1 eV, the proportion of secondary electrons is around 85%, and the half-height width of the secondary electron emission time is about 3.0 fs, with the angle of the secondary electron emission sinusoidally distributed from 0° to 90°, and the outline of emission electron being nearly a circular diffuse spot. The Geant4 results are subsequently imported into CST to explore the imaging of anisotropic focusing streak tubes in this case. Additionally, the effects of the temporal focusing system and spatial focusing system on the imaging results of the streak tube are obtained. By optimizing each structure parameter in the electro-optical system, the imaging aberration is wiped out to realize a uniform image-surface electron distribution. The electro-optical system with electron distribution generated by the CST self-generator and the Geant4 is simulated respectively, and the distributions of the electron beams on the object-surface and image-surface obtained for each of the two cases are analyzed. Finally, we find that the imaging results obtained by Geant4 are more uniform and reasonable, and this simulation scheme is more consistent with the actual situation.