In order to investigate the process of optically triggered discharge formation,a model of ion space-charge formation based on classical plane electrodes and revised for a characteristic hollow-cathode discharge(HCD)configuration is proposed in this paper.The primary modified factor in our model is the penetrating electric-field parameter,which influences the ionization of trigger electrons and is calculated via particle simulation.Optical-trigger experiments are carried out using different voltages and under different seed-electron conditions,provided by two different photocathodes,Cu and Mg.The ion-accumulation rates calculated by our model are compared to the discharge-formation time,which is deduced from optical-trigger experiments.The results demonstrate that the process of positive space-charge formation is dominant in the HCD formation process or trigger delay,which is highly dependent on the seeding-electron density and applied voltage,and can therefore be quantitatively described by our model.Additionally,electron-beam generation is investigated by optically triggered HCD experiments on Mg-and Cu-photocathode-based devices.The results show that a more efficient trigger device is capable of generating an electron beam with higher amplitude and density.