A theoretic model of upstream dielectric multipactor in TEM microwaves, taking both the rf electric field and rf magnetic field into account, is proposed. In this theory, the growth rate of the window multipactor is predicted. The multipactor is approximately independent of the rf frequency, and much more serious than Kishek's model (1998 Phys. Rev. Lett. 80 193) in the regions of weak dc electric fields and strong rf electric fields. Then, an electromagnetic Particle-In-Cell method is carried out to simulate the multipactor in TEM microwaves. Simulation results show a good agreement with the theoretical results. Furthermore, the lower boundary of multipactor is derived analytically, while there is no upper boundary any more. Finally, a gradually transition between Kishek's model and our model is found by examining microwaves of TE modes. It is found that the upper boundary reappears when the rf frequency of TE modes trends to the cutoff frequency of waveguide.
The dynamic behaviors of electrons in a miniature Penning ion source model are analyzed by the particle in cell/Monte Carlo collision particle simulation method combined with the scalar magnetic potential finite-difference method. Mainly, the influence of magnetic shielding on the trajectory and spatial distribution of electrons in the discharge process is taken into consideration. The influence of magnetic shielding on the plasma impedance during the excitation discharge of the Penning source is further discussed. The calculated target current is in good agreement with the experimental value. The simulation results show that the addition of magnetic shielding can reduce the fluctuation of electron density, increase the uniformity of electron spatial distribution, and double the plasma impedance stabilization zone, which are beneficial to the stability of the discharge process in the source. However, the magnetic shielding can cause a decrease in the electron density, which limits the plasma ionization degree. The study on magnetic shielding helps to understand the electron dynamics characteristics of the Penning ion source and provides a reference for improving the source performance in the future.
In most of the simulations of the extraction region of negative hydrogen ion sources, the single-aperture simulation is often adopted by researchers to study the plasma phenomenon due to its small simulation domain and short calculation time. However, due to the complex three-dimensional magnetic field structure in the extraction region of the negative hydrogen ion source, the single aperture often does not meet the periodicity. In this paper, the complex three-dimensional magnetic field topology is established. The magnetic field includes the magnetic filter field and the magnetic deflection field. The influence of the plasma sheath is taken into account. The electron extraction process in the multi-aperture structure of the extraction region of a negative hydrogen ion source is numerically calculated using the PIC method. Besides, the magnetic field structure is optimized. Ultimately, the electron beam uniformity near the plasma grid is improved effectively, which has certain guiding significance for engineering application.
In most negative hydrogen ion sources, an external magnet is installed near the extraction region to reduce the electron temperature. In this paper, the self-developed CHIPIC code is used to simulate the mechanism of a magnetic filter system, in the expansion region of the negative hydrogen ion source, on “hot” electrons. The reflection and the filtering processes of “hot” electrons are analyzed in depth and the energy distribution of electrons on the extraction surface is calculated. Moreover, the effects of different collision types on the density distribution of “cold” electrons along the X-axis and the spatial distribution of “cold” electrons on the X−Z plane are discussed. The numerical results show that the electron reflection is caused by the magnetic mirror effect. The filtering of “hot” electrons is due to the fact that the magnetic field constrains most of the electrons from reaching the vicinity of the extraction surface, being that collisions cause a decay in electron energy. Excitation collision is the main decay mechanism for electron energy in the chamber. The numerical results help to explain the formation process of “cold” electrons at the extraction surface, thus providing a reference for reducing the loss probability of H−.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.