Investigation Of Coaxial Permanent Magnet Halbach Circuit

High-power microwave (HPM) devices require the applied magnetic field to guide the propagation of intense electron beams, and indeed to inhibit the diffusion of the electron beams. Therefore, it is necessary to investigate the magnetic-field guide system. HPM technology development drives the need to investigate the periodic permanent-magnet (PPM) focusing system that is able to operate at a compact and practical size. The research work in this thesis mainly focus on the theoretical analysis and numerical simulation of a coaxial permanent magnet Halbach circuit. Meanwhile, investigate the propagation process of annular intense electron beam in this type of PPM focusing system, and free electron laser (FEL) amplifier with such kind of magnetic circuit as the wiggler with Particle-in-cell(PIC) simulation code.Firstly, the magnetic field of coaxial permanent magnet Halbach circuit is calculated using the finite element code (POISSON SUPERFISH). The approximate expressions for the components of the induction magnetic field are obtained. The problem of how to neglect the higher spatial harmonics of the field components is discussed and the initial phases of the field components are modified. Some properties of the magnet circuits are analyzed and summarized. The effects of structure parameters of the magnetic circuits, including the thickness of the magnetic ring, the gap between the coaxial circuits, and the period of the magnet, on the amplitude of the magnetic field are discussed.Secondly, the forces acting on intense annular electron beam in such kind of magnetic field are analyzed. The radial force equation with a modified Mathieu function form and the conditions for the electron beam stability are drawn. Then PIC simulation code is used to investigate the physical process of the beam propagation in such kind of magnetic field. The amplitude and period of the magnet field, the electron beam current, the thickness, the initial incident angle of electron beams and the initial emission position related to the stable propagation are discussed. The conclusion is made that several kilo-amperes intense annular electron beam could propagate stably, meanwhile the focusing form of such magnetic geometry provides action mechanism for the interaction between electron beams and microwave in the FEL.At last, this thesis studys FEL amplifier with coaxial permanent magnet Halbach circuit as the wiggler through particle simulation as the application of the circuit. The TE₀₁ mode is chosen as the operate mode of the FEL amplifier. The simulation results show the FEL amplifier, which is generated by a 525kV, 1kA electron beam with a 9.65 GHz, 12.4 kW input signal at TE₀₁ mode, operate at 9.65 GHz with averaged power of 100 MW. The efficiency is about 19% and the gain of the amplifier is 39 dB.