Abstract
As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL). The stable output of S-band MPG has been achieved in many labs. To establish reliable foundation for the future application of it, the propagation of picosecond electron bunch produced by MPG should be studied in detail. In this article, the MPG which was working on the rising stage of total effective Secondary Electron Yield (SEY) curve was introduced. The self-bunching mechanism was discussed in depth both in the multipacting amplifying state and the steady working state. The bunch length broadening induced by the longitudinal space-charge (SC) effects was investigated by different theoretical models in different regions. The 2D PIC codes MAGIC and beam dynamic codes TraceWin simulations were also performed in the propagation. The result shows an excellent agreement between the simulation and the theoretical analysis for bunch length evolution.
Highlights
Since the multipacting effect was firstly discovered by Farnsworth in 1934,1 it has been deeply investigated in many areas, such as RF structure related accelerators,[2,3,4,5] high power microwave generators,[6,7] etc
As an important electron source, Micro-Pulse electron Gun (MPG) which is qualified for producing high average current, short pulse, low emittance electron bunches steadily holds promise to use as an electron source of Coherent Smith-Purcell Radiation (CSPR), Free Electron Laser (FEL)
Considering that the average beam current produced by MPG is not very high in our simulation and the magnetic force generated by the beam will restrict the beam size, we ignore the variety of the transverse beam size and focus on the bunch length broadening
Summary
Since the multipacting effect was firstly discovered by Farnsworth in 1934,1 it has been deeply investigated in many areas, such as RF structure related accelerators,[2,3,4,5] high power microwave generators,[6,7] etc. CSPR was discovered in 1990s as well.[21] Compared with incoherent Smith-Purcell radiation, it can generate a more powerful and coherent emission with locked frequency due to displaying all these three enhancements, Ng (the number of grating period), Ne (the number of electron in the bunch), Nb (the number of electron bunch).[22] Obviously, MPG is one of ideal electron sources of CSPR for that S-band electron source can increase energy density at these frequencies by increasing Nb with more electron bunches passing the granting in a given distance. High frequency and short bunch length which are the most two important features of MPG make it a useful electron gun in CSPR and many other fields. As the frequency is locked, exploring the evolution of the electron bunch length becomes the primary task in transmission designing
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