潘宁离子源放电过程仿真分析

Simulation Analysis of the Discharge Process in a Penning Ion Source

摘要: 为了进一步研究低气压条件下潘宁离子源内部等离子体的形成过程及空间分布特征，本文建立了简化的氢潘宁离子源三维物理模型，并采用粒子模拟与蒙特卡罗碰撞相结合的PIC/MCC方法，对离子源内部带电粒子的输运、碰撞和电离过程进行了数值模拟。模型中考虑了电子与氢气分子之间的弹性碰撞、激发和电离过程，以及相关离子和中性粒子的输运行为。在阳极筒施加约2 kV电压、轴向磁场强度为0.2 T的条件下，实现了离子源内部的自持放电，并分析了电势、磁场以及$H_2^{+}$ 离子密度的空间分布特征。模拟结果表明，电场和轴向磁场共同决定了电子在离子源内部的运动轨迹，其中磁场能够显著延长电子在放电区域内的有效运动路径，提高电子与中性氢气发生碰撞和电离的概率。随着放电过程逐渐建立，离子源中心区域形成较高的等离子体密度，$H_2^{+}$ 离子主要集中于轴向中心区域，并在径向上表现出较明显的中心峰值分布。该分布特征有利于后续采用轴向引出方式获得离子束。研究结果可为进一步理解潘宁放电机理、优化离子源结构以及设计离子引出系统提供一定的理论依据。

Abstract: To further investigate the formation process and spatial distribution characteristics of plasma inside a Penning ion source under low-pressure conditions, a simplified three-dimensional physical model of a hydrogen Penning ion source was established. Numerical simulations of charged-particle transport, collisions, and ionization processes within the ion source were performed using the particle-in-cell/Monte Carlo collision (PIC/MCC) method. The model accounts for elastic collisions between electrons and hydrogen molecules, and excitation and ionization of hydrogen molecules by electrons, as well as the transport behavior of the associated ions and neutral particles. Under an applied anode-cylinder voltage of approximately 2 kV and an axial magnetic field strength of 0.2 T, self-sustained discharge was achieved within the ion source, and the spatial distributions of the electric potential, magnetic field, and $H_2^{+}$ ion density were analyzed. The simulation results show that the electron trajectories inside the ion source are jointly governed by the electric field and the axial magnetic field. In particular, the magnetic field significantly extends the effective electron path length within the discharge region, thereby increasing the probability of electron collisions with and ionization of neutral hydrogen molecules. As the discharge gradually develops, a relatively high plasma density is established in the central region of the ion source. The $H_2^{+}$ ions are mainly concentrated around the axial center and exhibit a pronounced centrally peaked distribution in the radial direction. Such a distribution is favorable for subsequent ion-beam extraction along the axial direction. The results provide a theoretical basis for further understanding the Penning discharge mechanism, optimizing the ion-source structure, and designing the ion-extraction system.