| The Large-Area-Bremsstrahlung(LAB)diode driven by the pulsed power machine is usually employed to generate large-area and high-dose uniform X-ray field.It is widely used in X-ray field simulation,anti-radiation reinforcement research and has significant application prospects in the national defense and space science.The low-impedance LAB diode driven by a single module usually has a large radius electrode and a small A-K gap.However,additional outer control device must be employed in this type of diode to regulate the transmission process of the electrons and make the electrons inject to the anode surface of diode as uniform as possible which is crucial to obtaining the large-area uniform X-ray field.The disadvantages of this diode are perhaps slightly complex structures and unstable performance.A low-impedance LAB diode with tapered magnetically insulated transmission line and flat cathode with hole in the center area was proposed in this dissertation.The large-area uniform X-ray field could be achieved with this diode without aid of additional outer control device.The main work and achievements of this dissertation are as follows:1)The transmission process of photons and electrons in the target was studied by theoretical analysis and Monte Carlo simulation.The relationship between the X-ray dose and the thickness of convert target(tantalum)was studied under different electron energy and the target thickness influence on uniformity was also studied.The electrons and photons absorption in the graphite target,which is used to absorb escaped electrons from tantalum target,was studied.The results showed that the optimum thickness of tantalum target and the energy of the electron were almost linear in a certain range.For 2-MeV electrons,the optimum thickness of tantalum target was about 300 ?m and electrons could rarely escape from a 5-6 mm graphite target while photons will be slightly decayed.2)Many diode structures from plane diode to the novel diode proposed in this dissertation were studied by 2-D Particle-In-Cell(PIC)and Monte Carlo simulation.The electrons distribution and output X-ray performance under corresponding structures were obtained.The results showed that the new proposed diode exhibited a better X-ray performance.For the 2 MV/75 ns pulsed voltage,the characteristic impedance of the diode is about 3.5 ?.A uniform area with peak dose of 6457 R and radius of 22.2 cm and a volume with peak dose of 5652 R and ?24 cmx 12 cm can be found with uniformity 2:1.The optimized composite target is 250 ?m tantalum+5 mm graphite.3)Based on the novel diode structure,the influence of A-K gap,radius of cathode hole,slope of tapered transmission line and space ion current on the diode performance were studied by 2-D PIC simulation.Optimized results of each parameter were found respectively.The study of space ion current based on Monte Carlo simulation showed that with the increasing of ion current the pinch of electrons will become stronger and X-ray uniformity along the axial direction will slightly get worse,but have little influence on the uniform plane.So,in order to get good X-ray performance,the production of ion current should be suppressed in the experiment. |
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