Study On The Key Technology Of Diagnosing The Surface Density Of Explosive Plasma Fuel By Scattering Method

Fuel areal density (pR) is an important diagnosis parameter in the experimental research of inertial confinement fusion (ICF), which can be utilized for characterizing the ignition condition. The compression symmetry of fuel capsule can be understood through areal density measurements in different directions.The key techniques of the diagnosis of fuel areal density via down-scattered neutron measurements are discussed in detail. The value of fuel areal density is proposed to be derived from the linear relationship between pR and the yield ratio of down-scattered neutrons to primary neutrons providing it is practically possible to measure down-scattered neutrons accurately. Firstly, the impact factors which may have an influence on the down-scattered neutron spectrum are discussed. Secondly, the collimator of the detection system is designed according to experimental environment of Shenguang-Ⅲ (SG-Ⅲ) target bay for the purpose of reducing the influence of neutron backgrounds on the measurement of down-scattered neutrons. Lastly, two types of detection system are designed.The impact factors which may have an influence on the down-scattered neutron spectrum, for example, thermal motion of fuel nucleus, hot-spot sizes, hot-spot distributions and the pusher of fuel capsule, are discussed. Through mathematical derivation and numerical simulation, the effect of thermal motion of fuel nucleus on the down-scattered neutron spectrum is discussed and proved to be negligible when the hot-spot model is under consideration. With this approximation, the down-scattered neutron spectra for different areal densities and different hot-spot sizes are simulated by using Monte Carlo method. The simulation results indicate that there is a nearly linear relationship between fuel areal density and the yield ratio of down-scattered neutrons to primary neutrons. Further, the coefficient of the linear relationship depends both on the selection of energy range of down-scattered neutron measurement and on the hot-spot size. The effects of hot-spot distribution on the down-scattered neutron measurements in different directions are analysed. A brief guidance on how to choose energy range of down-scattered neutron measurement and primary neutron measurement is also presented. For typical indirect-drive target at SG-Ⅲ prototype, direct-drive targe at OMEGA and exploding-pusher target at SG-Ⅲ, the effect of the pusher on the down-scattered neutron spectrum is discussed.The collimator of detection system is designed according to experimental environment of SG-III facility. The radiation background at SG-Ⅲ target bay is analyzed in detail for D2 fuel target via Monte Carlo simulations. The simulation results indicate that the backgrounds are mostly from scattering of primary neutrons with vacuum chamber, surrounded diagnostic platforms, ceiling and floor. According to the analysis, appropriate materials and shielding solution are optimized. And Monte Carlo calculations indicate that the optimized structure of collimator meets the experimental requirements.Some key parameters of plastic scintillator and MCP-PMT, which are needed in the design of detector system, are measured. Fast and slow components of plastic scintillator fluorescence are measured via the time correlated single photon counting technique (TCSPC). And MCP-PMT’s linear output for intensive short pulse light is measured via waveform comparison method. The device based on TCSPC has a dynamic range of over 104 and an intrinsic time resolution of less than 0.8 ns. The decay time of fast and slow components are 2.9 ns and 50 ns for ST401,1.6 ns and 30 ns for EJ232 according to double exponential fitting. A new parameter called linear output charge which doesn’t depend on the input pulse width is demonstrated when MCP-PMT’s linear output is measured. Two types of laser pulses with FWHM of 20 ns and 10 ns, respectively, are used in the experiment. The linear output charges of MCP-PMT for the two pulses are obtained when the gain of MCP-PMT decreases by 10%. Both results are consistent.Current-mode detection system and imaging detection system are designed according to the radiation level at 5.5 m from the target with shielding collimator and experimental parameters of plastic scintillator and MCNP-PMT. The effects of slow components, areal densities and detection location on the signal to noise ratio (SNR) of the two systems are discussed via Geant4 simulation. If the plastic scintillator contains only the fast component and the fastest slow component or otherwise its slow components are all weak enough, current-mode detection system can be used for fuel areal density diagnosis with a SNR of 40:1 at 5.5 m from the target when areal density is 10 mg/cm2 and primary neutron yield is 1012. If the plastic scintillator contains a fast component and two significant slow components, current-mode detection system can not be used for fuel areal density diagnosis at 5.5 m from the target. However, because the SNR increases as detector-target distance increases, current-mode detection system can be used for fuel areal density diagnosis with a SNR of 20:1 if detector location is at 20 m from the target and when areal density is 100 mg/cm2 and primary neutron yield is 1012. For imaging system consisting of scintillator with all its slow components weak enough, the SNR turns out to be 10:1 at 5.5 m from target when areal density is 10 mg/cm2 and primary neutron yield is 1011. If areal density increases, the SNR is better. However, when areal density is 100 mg/cm2 and primary neutron yield is 1012, the statistical counts of down-scattered neutrons decreases since the overlapping probability of multiple neutron scattering events occuring in a single scintillator pixel (called overlapping probability) increases. The overlapping probability can be reduced by shortening the exposure time or decreasing the thickness of plastic scintillator. Therefore, the fluorescence of slow components which are excited by primary neutrons and scattered neutrons with high energy has a signifcant influence on the measurement of down-scattered neutrons.In conclusion, the key techniques of the diagnosis of fuel areal density via down-scattered neutron measurement are studied. The feasibility of the diagnosis of fuel areal density via down-scattered neutrons measurement at SG-Ⅲ facility is numerically proved. The preliminary design of diagnostic system is completed and its characteristics are evaluated based on numerical simulation.