| dc.description.abstract | Charged particle beams have intensive applications on science, medicine, industry and military. Hence, much more information and understandings from charged particle beams, much more advantages on the practical designs and explanations of phenomena. Space charge effect limits the total number of charges and affects the beam quality of the charged particle beam directly, so that the study of this research topic becomes an important research issue in vacuum electronics. Basically, the injection, acceleration and the formation of a charged
particle beams are operated in accelerating regions or drift regions. The charge particle sources are usually generated from cathode and accelerated up to the designed energy in the accelerating region, and then injected into the drift region for the practical use. However, Space Charge Limited (SCL) Current would limit the maximum current across the accelerating
region or drift region. The SCL current results from the limit due to the space charge effect of the charge particle beam in the system. Thanks to the improvements of science and technology, the charge particle beams have much higher energy and thinner beam width. The traditional formula to estimate the SCL current is no longer available, and thus needs to be modi ed. In the past few decades, the theory of the SCL current has been revised extensively to consider various effects such as nite emission area, short pulse length, relativistic effects, and etc. However, there are still many issues need to be discussed. First of all, the current theories only consider
the electrostatic (ES) model and ignore the electromagnetic (EM) effects. Thus we rst used the two-dimensional (2D) EM particle-in-cell (PIC) simulation to study the EM effects on the SCL current in an accelerating region. We found that EM effects cannot be ignored when a short-pulse and nite-width charge particle beam is considered. Our research results
also indicated that ES model is only available for long-pulse cases or short-pulse beams but with larger width.
The most of the research regarding the SCL current focus on the accelerating region, but the drift region draws less attentions. For this reason, we developed a 2D ES relativistic SCL current theory for drift region, and also studied the dynamical behavior when the injected current exceeds the
threshold. It is veri ed by 2D ES PIC simulation, and the determination
of SCL current in the simulation was also discussed.
We further developed a 2D long-pulse uni ed theory to combine the accelerating region and drift region. The SCL current and the dynamical behavior when the injected current exceeds the threshold were derived, and the transition from the accelerating region to the drift region also studied. The theoretical analysis shows that the 2D SCL current is mainly determined by the geometrical effects, but the dynamical behaviors of the current flow are mainly determined by the initial velocity at higher current density exceeding the SCL current density.
Besides the long pulse charge particle beam, a charge sheet model was also proposed for the study of the maximum charge density of an electron pulse train injected to in an accelerating region or a drift region with uniform temporal pulse separation. An electron pulse train can be applied to the time resolved electron microscopy, Smith-Purcell radiation and free electron lasers, thus the space charge effect of an electron pulse train plays an important role for the applications. We developed a efficient way to determine the maximum charge density without the temporal distortion when the pulse train arriving at the anode. The method can also be applied in relativistic regime and has wide range applications.
This dissertation deepens and widens the understanding of SCL current. Moreover, these theoretical models and simulation methods are useful for the researches and developments of the future corresponding applications. | en_US |