Lia Strong Numerical Simulation Of Beam Focusing And Lens Design

The solenoid lens is used in Induction Linear Accelerators to focus the intense relativistic electron beam. In this article, a non-laminar and non-collision numerical simulation model, which is based on a third order trajectory equation is presented. The trajectory equation is derived with space charge added in the form of discrete annular current elements in axisymmetric system.In this model, the injection angles of beamlets are considered as a uniformity random distribution with which the emittance of beam is well treated. The electron beam is divided into several pieces along z-axis. Every piece is arranged with energy sweep and Corkscrew motion data according to experiment results. Simulation result shows that X-ray spot size is almost certainly convergent when the number of beamlets is increasing gradually. If the maximum magnetic field of lens is 9500Gs and the FWHM width of field is about 11.6cm, the X-ray spot size will finally be 1.03mm approximately.X-ray spot size obtained from the simulation model is consistent with the experimental results. The uncertainty of calculation is in range of ± 0.05mm. The maximum absolute error of the X-ray spot size is -0.4mm and that of focusing length is +3.5cm. The difference of X-ray spot size affected by the parameters of the electron beam is discussed in detail.The magnetic field of solenoid lens can be calculated by ANSYS program and the result is nearly consistent with the measurement results. Through calculation, the dimension of pole shoe of the lens is given determinately. Calculation results indicate that the FWHM width of the magnetic field is independent of magnetic conductivity of pole shoe and the magnetizing current of solenoid. On the contrary, the maximum magnetic field lies on both of the two factors.The configure dimension of solenoid lens is optimized on two aspects. One is to increase the maximum magnetic field of lens; the other is to extent the FWHM width of the magnetic field. Numerical simulation results prove that the later can obtain smaller X-ray spot size. If the same X-ray spot size is demanded the later can reduce the requirement of the parameters of the electron beam. Simultaneously, it also obtains a higher maximum magnetic field when the magnetizing current of solenoid is enhanced appreciably.