To reduce the threshold and increase the conversion efficiency,the multi-pass configuration was applied. present the curves of the S1 photon conversion efficiency in relation to the
pump energies at different pressures (pass-number fixed at 3) and different pass numbers (D2 pressure fixed at 42 bar), respectively.It can be seen that the S1 thresholds were significantly decreased by increasing the pass number. From the 3-pass to 9-pass configuration, the S1 threshold was decreased from 14.9 mJ to 5.0 mJ (corresponding to the peak power from 3 MW to 1 MW). The S1
thresholds and the maximum efficiency of all the curves in Figs. 2 and 3 are listed in Tables 1 and 2, respectively. When the photon conversion efficiency reached 1%, the corresponding pump energy is defined as threshold.