The framing camera is a two-dimensional (2D) ultrafast diagnostic device with high spatio-temporal resolution. In inertial confinement fusion (ICF) experiments, it can effectively acquire the duration and dynamic images at the stage of implosion compression. The pulse dilation framing camera is a new ultrafast diagnostic device with temporal resolution better than 10 ps. Firstly, this paper loads the dilating pulse between the photocathode (PC) and grid to achieve greater acceleration energy of the front edge of electron beams. Then, the temporal width of the electron beam is dilated through long-distance transmission. Finally, the dilated electron beam signal is measured by the microchannel plate (MCP) framing camera, and the temporal resolution is exponentially improved. However, in the pulse-dilation framing camera, with the transmission of dilating pulse along the PC, the voltage at different positions on the PC is changed, which results in temporal non-uniformity. This is one of the main factors restricting the development of framing cameras with large detection areas. To improve the restriction of temporal non-uniformity of the pulse-dilation framing camera, the gating pulse with picoseconds is designed, and the improvement of the temporal uniformity is studied by simultaneously loading the falling edge of the gating pulse and dilating pulse on the PC, which is based on the variational phenomenon of voltage during the transmission of dilating pulse along the PC.
This paper first deduces the computational equation of the dynamic dilating ratio of the electron beam along the PC by analyzing the causes of temporal non-uniformity and combines the principle of temporal dilation and the transmission characteristics of the electric pulse along the PC. Then, the high-voltage gating pulse with picoseconds is designed by the three-stage Marx pulse generator and the pulse shaping circuit. The Marx pulse generator is realized by the series and parallel of avalanche triodes, and the pulse shaping circuit is made up of the avalanche diode and the two-stage high pass filter with LC. In addition, based on the electric pulse superposition technology and the voltage variation characteristics of dilating pulse transmission along the PC, the improvement of temporal non-uniformity is studied with the optimization of the PC voltage distribution by simultaneously loading the falling edge of the gating pulse and the dilating pulse on the PC. Finally, when the falling edge of the gating pulse is loaded or not loaded on the PC, the temporal uniformity is numerically compared by calculating the dilating ratio of electron beams along the PC, and the temporal uniformity improvement is quantized via the difference percentage between the sampling points.
The picosecond pulse generator [Fig. 2(a)] is designed by the series and parallel of avalanche triodes, the series of avalanche diodes, and the two-stage high pass filter with LC. The gating pulse with an amplitude of -2.59 kV and full width at half maximum of 236 ps is obtained, and its rising and falling edges are 217 ps and 204 ps respectively [Fig. 2 (b)]. Temporal uniformity improvement is analyzed when the falling edge of the gating pulse is loaded or not loaded on the PC and then quantized by the difference percentage between the sampling points [Fig. 4 (d)]. While the falling edge of the gating pulse is not loaded on the PC, the dilating ratio of the electron beam along the PC increases from 11.74 to 39.04, and the difference percentage is 232.5%. When the falling edge is loaded, the dilating ratio rises from 11.28 to 14.23, and the difference percentage is descended to 40.21%. The temporal uniformity of the pulse-dilation framing camera is improved by loading the falling edge of the gating pulse.
The Marx pulse generator is designed by series and parallel of avalanche triodes. The high-voltage picosecond circuit is designed with avalanche diodes and high pass filters, and the high-voltage picosecond gating pulse is generated. Based on the pulse superposition technology, the temporal uniformity of the pulse dilation framing camera is improved by the falling edge of the gating pulse. The results show that when the PC voltage is -2 kV, the initial width of the electron beam is 5 ps, and the gradient of dilating pulse is 11.9 V/ps, with the transmission of the dilating pulse along the PC, the voltage is changed from -4.48 kV to -2.47 kV, and the dilating ratio of the electron beam grows from 11.74 to 39.04 with the difference percentage of 232.5%. When the falling edge of the gating pulse with the gradient of 12.7 V/ps is simultaneously loaded on the PC, the voltage along the PC is changed from -4.62 kV to -4.97 kV, and the dilating ratio of the electron beam rises from 11.28 to 14.23. The difference percentage is descended to 40.21%, and the temporal uniformity is improved. This study can provide an effective method for improving the temporal non-uniformity of pulse-dilation framing cameras, and a theoretical reference for the development of 10 ps framing cameras with large detection areas.
