| Around the enhancement and suppression of multipacting in pulsed power technology,some exploratory problems are theoretically and numerically studied in this dissertation.Suppression problem is opposite to enhancement probelm,but the two problems are all relevant to multipacting.Thereby,there is a similarity betweent two problems.For the multipacting enhancement,by using one-sided multipacting on dielectric and investigating its effective enhancement ways,a novel multipacting cathode for high-current diode is designed and studied in this dissertation.For the multipacting suppression,the effective suppression ways of two-sided multipacting in accelerator rf cavity during the field-buildup are studied in this dissertation.The main contents of this dissertation are as follows:For the electron-beam production with high emission current density,explosive emission could support the ultra-high current of electron emission,but the plasma diffusion and surface erosion will shorten cathode working life and degrade cathode performance.Field induced emission could also support the ultra-high current of electron emission,but the total output current is not strong enough due to the small effective area of emitter.Secondary emission has many potentials in development and application,but the micro-pulsed electron gun does not adapt for high-power microwave sources duo to complicated structure and controlling.How to efficiently improve the emission current density and prolong the working life of cathode is the core-problems in the industry of high-power microwave.In this dissertation,a novel multipacting cathode producing a ring-like electron beam for high-current diode is presented,by using the triple-junction,combining the field-induced emission and multipacting and effectively improve multipacting to achieve the high emission current density.The prototype and configuration design are detailed introduced(including pulsed power input structure,cathode socket,anode barrel,multipacting dielectric cathode and field-induced array).The mechanism of this novel multipacting cathode is also analyzed(using the effects of emission by radial electric-field,acceleration by axial electric-field and deflection by axial magnetic-field).The results of electron trajectory,transit time,impact energy and working range are put forward by using dynamic theory.The theoretical results are verificated and validated by Monte Carlo simulation,the theoretical results agree with the Monte Carlo simulated results well.The feasibility of multipacting forming,keeping and controlling on the cathode surface is proved.The emission current density is roughly estimated to the level of kA/cm~2,and the design procedure of this multipacting cathode is introduced and discussed in detail.In this dissertation,by using the particle-in-cell method,a 2D3V PIC code is self-programmed to simulate the multipacting forming,developing and saturation processes of this novel multipacting cathode.The simulated results of electron trajectory,transit time,impact energy agree with the theoretical and Monte Carlo results well.In addition,the multipacting saturation mechanism is analyzed due to charge-deposition.The simulated result of emission current density agrees with the rough estimation in theory.Some effective ways of multipacting enhancement for improving the performance of this novel multipacting cathode are investigated by using the self-programmed 2D3V PIC code.The simulation demonstrates that the most effective way of improving the performance of this novel multipacting cathode is synchronously improving the axial and radial electric-field and the axial magnetic-field.Finally,by using the particle-in-cell and Monte Carlo collision methods,a 2D3V PIC-MCC code is self-programmed to estimate the performance variability and deterioration caused by flashover.For the dynamic field-buildup of accelerator rf cavity,the voltage in rf cavity is a successively increasing process till saturation.The field-buildup time is very long especially in superconductivity cavity.If there is a serious multipacting phenomenon in the process of field-buildup,the cavity energy-storage will be decreased and the resonant-frequency will be influenced,sometimes the field-buildup process will fail.Nowadays,most studies are still in experiments.The failure phenomenon of field-buildup in high-Q cavity is generally found in many experiments,but the mechanism is not deep understood.Meanwhile,there is lack of effective ways of multipacting suppression.In this dissertation,by using the particle-in-cell and Monte-Carlo sampling methods,a 1D3V PIC code is self-programmed to investigate the temporal characteristics of high order two-sided multipacting in cavity and the influence of material secondary electron yield(SEY)on two-sided multipacting in cavity.The simulated results illustrate that the value of discharge power in the high order(3rd order)mode is about 1%of that in the 1st order mode.Therefore,the high order mode is not significant in accelerator field-building process compared with the 1st order mode.Under the circumstances of lower value of SEY,the saturation characteristics is determined by both“debunching”and“reverse field”of space charge effects.But under the circumstances of higher value of SEY,the multipacting mechanism tends to be one-sided mode in the steady stage which can be obviously determined by“reverse-field”of space charge effect.In this dissertation,the coupled physical model is established based on the equivalent circuit for describing dynamic field buildup in accelerator rf cavity and particle-in-cell method for describing two-sided multipacting in rf cavity.By using the self-programmed 1D3V-PIC code for multipacting and fully equivalent circuit code for rf field buildup,the influence of multipacting on the dynamic process of field buildup in rf cavity and effective multipacting suppressing ways are numerically investigated.The simulated results illustrate that the higher Q-value of cavity or bigger area of secondary electron emission,the lower probability of rf field buildup success is.Decreasing the initial emission current-density of background seed electron will help to reduce the total electron number which triggers the multipacting.The probability of rf field buildup success could be efficiently improved in this way.The multipacting interaction interval could be efficiently shortened by increasing the loading gap-voltage,decreasing the rising-time of driven source and slightly mismatching the eigen-frequency of rf cavity.The shorter multipacting interaction interval,the higher probability of rf field buildup success is. |
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