Preliminary Research Of Neutron Flux Detecting In ITER

International Thermonuclear Experimental Reactor(ITER)is the major international cooperation project which carries out thermonuclear fusion reaction by means of magnetic constraint and is jointly participated by China,the European Union,the United States,Russia,Japan,Korea and India.As an important research topic of plasma diagnosis in ITER project,the neutron flux detection/monitor(NFM)may reflect a series of important parameters in ITER fusion process in real time,such as the neutron yield,the fusion energy,and the energy density by detecting the neutron flux.During the process of thermonuclear fusion reaction in ITER,massive neutron,Gamma particle,Alpha particle and so on will be generated together with the release of a large number of neutrons.When the neutron signal is required,the effect of the other non-neutron signal must be eliminated.The research of neutron flux detection has initially been divided into the following three stages:(1)the research of the key problems in the neutron flux detection,including the preliminary study,selection and research of detector,the design of preamplifier,the feasibility study of the pulse shape discrimination and the verification of Campbell integral principle.(2)The development of the neutron flux detection prototype system.The so-called "prototype" is the principle prototype.The main objective of this stage is to complete the development of the prototype system,to carry out the joint debugging of the prototype system and experimentalizing in neutron radiation environmental,and to identify and summarize the problems in the development process,so as to provide valuable technical accumulation for ultimately completing the procurement package project of neutron flux detection system in ITER.(3)The development of the ultimate procurement package product of neutron flux detection system.This thesis mainly introduces the research of the first two stages.The neutron flux detection system mainly consists of the fission ionization chamber detector,the preamplifier and the main electronics system of the neutron flux detection.The main electronics system of the neutron flux detection consists of hardware and software systems.The hardware system is composed by the n-y pulse discriminator,Campbell integrator,baseline restorer,time division data acquisition,the supporting cable and the host computer and software system runs on the host computer.In this thesis,the main work and innovation are as follows:(1)The high-speed and high-gain preamplifier has been developed for fission ionization chamber detector by exploiting the multi-stage cascade of the high-speed current feedback integrated op-amp.The voltage gain of the preamplifier can be 60dB,and the bandwidth is better than 300MHz,which solves the key technical problem of easy self-oscillation in the high-speed and high-gain current-feedback amplifier.(2)High-speed baseline restorer has been developed to solve the problem of the baseline offset of the preamplifier output signal in the neutron flux detection system,in which the output signal baseline offset is maintained at a zero level,bandwidth of which is more than 200MHz,and the effective baseline recovery range is±3V when keeping the high-speed one-way nuclear pulse signal waveform basically unchanged.(3)The double constant fraction discriminator(CFD)measurement pulse rise time method is put forward to realize the neutron signal discrimination,supplemented by the pulse amplitude discriminator,based on which the n-γ pulse discriminator is designed to detect the neutron signal in the range of 0～106cps.The amplitude discrimination accuracy of n-γ pulse discriminator can be up to less than 10mV,and the screening accuracy of rise time ±1ns.(4)Automatic gain adjustment Campbell integrator has been developed to solve the problem of integrator overflow in large dynamic range and to realize Campbell integral linearity in the neutron count rate detection range of 104～108cps.Campbell integrator,together with the n-y pulse discriminator,can make the neutron count rate detection range of neutron flux detection system reach up to 0～108cps.(5)A new multi-channel time-scaler and waveform recorder have been developed,and the shortest time of time-sharing is up to 1ms.The linear correlation coefficient of time-scaler and waveform recorder both are R≈1.The neutron flux detection prototype system has carried on the union testing in the steady neutron radiation field of CNNC(Beijing)nuclear instrument factory and in the neutron sources and proton accelerator environment of China Institute of Atomic Energy.Experimental result is in accordance with the anticipated desired design goal,and the experimental data,which passed the unified data processing by the national radioactive measurement station,has been recognized by the standardized analysis research group.