Simulation Analysis And Study Of Comprehensive Photoemission Characterristics Of CsI Photocathode

X-ray streak cameras have been widely used to diagnose important quantities of laser-produced plasmas and high-energy-density physics in inertial confinement fusion(ICF) which requires high spatially and temporally resolved measurements. At present, CsI is the most commonly used material in x-ray streak cameras as the photocathode, so the characteristics of high energy x-ray induced secondary electron(SE) emission from a CsI photocathode is an essential factor that affects the spatial and temporal resolution and the measurement accuracy of the x-ray streak camera.The energy spectrum of x-rays that generated in the ICF gradually moves to the high energy region with the ICF experiments progress. The peak position of the x-ray energy spectrum has achieved 10 ke V for a long time in National Ignition Facility(NIF). But there is no existing theoretical or experimental evidence to prove whether the CsI photocathode is able to meet the requirements for high spatially and temporally resolved measurements in the energy range that higher than 10 ke V. Thus it is necessary to make a study of the characteristics of high energy x-ray induced secondary electron emission from CsI photocathodes.In addition, CsI is known to be very hygroscopic. The quantum yield of a CsI photocathode decreases in a nonlinear way even after a short exposure to air. Unfortunately, a CsI photocathode will be unavoidably exposed to the air for some time during the transportation and installation process in the ICF experiment, leading to an inaccurate imaging taken by the x-ray streak camera.In order to solve the problems mentioned above, the main work and main innovations are as follows:1. After studying quantities of high energy particle collision theories, we draw valuable contents from those theories to conduct numerical calculation and develop a Monte Carlo program to calculate the characteristics of high energy x-ray induced secondary electron emission from a CsI photocathode used in an x-ray streak camera. According to the energy range of the x-rays generated in ICF, the simulation energy in the Monte Carlo program is set in the range of 1- 30 ke V.The simulation results agree well with existing experimental data.2. Time distributions of emitted SEs have been investigated with an incident x-ray energy range from 1 to 30 ke V and a CsI thickness range from 100 to 1000 nm. Simulation results indicate that SE time distribution curves have no significant dependence on the incident x-ray energy and CsI thickness. The calculated time dispersion within CsI photocathode is around 70 fs, which should be the temporal resolution limit of x-ray streak cameras that use CsI as photocathode material.3. Energy distributions of emitted SEs have been investigated with an incident x-ray energy range from 1 to 30 ke V and a CsI thickness range from 100 to 1000 nm. Simulation results indicate that the ratio of fast electrons(>50 e V) increases with the energy of the incident x-ray and the thickness of the CsI photocathode. When the energy of the incident x-ray is 30 ke V and the thickness of the CsI photocathode is 1000 nm, the ratio of the fast electrons achieves 10.8%. However, when the thickness of the CsI photocathode is 100 nm and the energy of the incident x-ray is higher than 15 ke V, the ratio of the fast electrons is about 3.4% and does not increase with the energy of the incident x-ray.This conclusion indicates that it is a good choice to maintain the CsI photocathode thickness which is now 100 nm.4. The quantum yield of the CsI photocathode has been investigated. Simulation results indicate that the quantum yield of a CsI photocathode decreses rapidly to below 0.1 after the x-ray energy higher than 10 ke V.The reason of this rapid decrease is that the x-ray absorption length increase with x-ray energy in an exponent form, which lead to less photo absorption.Increasing the CsI photocathode thickness can help increase only a little quantum yield, so it is doubtful that whether CsI photocathode can meet the sensitivity requirement of x-ray streak cameras in the high energy region.5. The surface topographies of a CsI photocahode have been observed before and after air exposure via Scanning Probe Microscope(SPM). We assume that the decrease of quantum yield is caused by the crystal size expansion brought by CsI photocathode's exposure to the air. With this assumption, the relationship between the quantum yield and the crystal size is acquired through the Monte Carlo program. According to this relationship, we propose a method to solve the quantitative measurement problem of x-ray streak camera.