Study On The Power Supplying System Of The 1MeV Intense Neutron Generator

The neutron generators which produce neutrons with energies of 14MeV or around 2.5MeV through T(d, n)⁴He or D(d, n)³He reactions by low-energy accelerators are widely used in the researches of neutron physics and its applications, furthermore, they have been being developed toward the "intense current" direction because of the needs of the anti-radiation researches on the materials of fusion reactors and strategic weapons, and cancer therapies by fast neutrons. Neutron generators whose neutron yields reach or exceed 10¹²n/s are generally called "intense neutron generators".The intense neutron generator being developed in Lanzhou University is designed to generate 8×10¹²14MeV neutrons per second. Analytical deuterium beam with higher energy of up to lMeV will be adopted in order to prolong the life of the T-Ti target and to get a 10 n/s yield of 2.5MeV neutrons produced by D(d, n) He reaction.In this paper, the physical and biological effects, the advantages and disadvantages, the developing histories and actualities of various kinds of cancer therapy means, are reviewed, including gamma and X rays, charged particle beams, intra-cavitary radiation by radioisotopes, neutron capture and fast neutrons. Combined with the technical needs of cancer therapies by fast neutrons, the main designing performances of the cancer therapy machine of Lanzhou University are given, and the bases and feasibilities of each index are analyzed qualitatively or quantitatively.The concrete demands for power supplies of each main components of the neutron generator are analyzed according to their working principles and influences to the capabilities of the neutron generator.This thesis has focused on the power supplying system of the intense neutron generator. The mixed deuterium beam extracted from the ECR ion source is first pre-accelerated to 120keV. The single-atomic ion D⁺ beam is analyzed from the mixed beam in the 90° magnetic analyzer and then accelerated to lMeV in the accelerating tube. The first difficulty of the power supplying system lies in the fact that the power supplies used in the ion source and pre-accelerating system have high powers and rigorous performance demands and, what ismore, they are located in the high potential terminal of near 1MV thus they may not be driven directly from ground. An intermediate frequency of 20kHz is adopted in the power supplying of the high voltage terminal in order to minish the dimension of the power supplying system, to assure the performances of the power supplies and to depress the noise. The powers generated by 5 20kHz power supplies are transported to 1MV high voltage terminal with 5 sets of isolation transformers, each of which have a insulation capability of 1MV. An intense-coupling technique we innovate is adopted in the isolation transformer so that the ratio of the voltage drop of the transformer between its non-load and full-load states to the output voltage can be reduced to less than 15%. This ratio can even be reduced to less than 5% by means of voltage sampling in the output terminal and modulation of the intermediate frequency output voltage through closed loop negative feed-back which is composed of electro-optical conversion, optical fiber transmission and photo-electric conversion. In some power supplies which require higher voltage or current stabilization, e.g., the pre-accelerating and the analytical magnet power supplies, dc voltage or current samplings and large closed loop negative feed-backs between the intermediate frequency transformers and the power supplies are introduced. Advanced photo-electrical coupling technique through optical fiber is adopted to transfer signals and carry out from ground the regulating and controlling of the power supplies standing on the high voltage potential. Some problems arisen from intermediate frequency supplying are simply discussed and the corresponding solving measures are put forward.Another difficulty of the power supplying system is the 1MV, 50mA dc high voltage power supply which requires a voltage stabilization of less than 0.1%. Techniques such as symmetrical voltage multiplier, RC filters, dual voltage stabilizing circuit, transformer compensation, fast voltage stabilization, and so on, as well as 20kHz intermediate frequency supplying, are adopted to guarantee the performances of this power supply.The neutron generator itself does not need a strict energy stabilization of the deuterium beam. The primary reason for the dc high voltage power supply to have a higher stabilization is that 4 experimental terminals are designed in this machine and deflection magnet is therefore needed. If the accelerating voltage is not stable, the excursion of the deuterium beam energy will result in an excursion of the beam trajectory. A method which can synchronize thevariations of the accelerating voltage and the exciting current of the deflection magnet is explored to assure the stability of the beam trajectory.The overvoltage and overcurrent protection is also of great importance. In this paper, the short circuit transient process of the Cockcroft-Walton circuit is studied thoroughly and systematically for the first time in the world. A simplified equivalent circuit based on some reasonable assumptions is given. Some formulae, which may enough be reasonably used toestimate the most possible maximum \i2(t)dt value of the rectifiers in the short circuitotransient process, and provide quantitative references for the selections of the parameters of the rectifiers and protective components in engineering design, are deducted. Conclusions that it is only necessary to connect protective resistors in series with the rectifiers situated in the locations of the highest and lowest potentials of the voltage multiplier and unnecessary to connect fast speed protective spark gap in parallel with the secondary winding of the step-up transformer, are drawn, which provide a theoretical basis for the simplification of the structure of the power supply.