Abstract

A 2-cm electron cyclotron resonance ion source has the advantages of long life and high specific impulse, which can meet the requirements for space gravitational waves detection. In the experiment on finding the lower limit of thrust, it is found that when the ion source operates under the extreme condition of 0.5-W microwave power and 0.1-sccm gas flow rate, increasing the voltages of grid system excessively may cause flameout. The plasma discharge level is controlled by the gas supply, microwave, and power supply system, and their small disturbances will make experimental results different, thus the flameout of the ion source appears randomly and transiently. Besides, it is difficult to observe the flameout phenomenon experimentally, because the probe diagnosis has big interference to low-density plasma, and the optical diagnosis is blocked by the grid system. Therefore, the integrative simulation with the full particle-in-cell method is used to simulate the operating process of the ion source, whose calculation range includes the discharge chamber, grid system, and plume. Through simulating the processes of plasma discharge and ion beam extraction continuously in space and time, the flameout phenomenon can be reproduced artificially after increasing the voltages of grid system. The simulation results show that the ambipolar diffusion between the antenna and discharge chamber is the fundamental reason for the flameout of the ion source. In the circuit, the antenna does not touch the discharge chamber but for bulk plasma, which makes its surface gradually accumulate charges until it reaches the floating potential. Because the increase of the voltage of antenna lags behind that of grid system, a strong electric field will appear between the antenna and chamber. Then, electrons and ions respectively move toward the chamber and antenna, the ambipolar diffusion helps the antenna reach the floating potential rapidly. When the plasma density inside the chamber is low, the ambipolar diffusion will cause flameout. In order to avoid the flameout of the ion source in such an extreme situation, an improvement measure that the voltage of antenna equals the voltage of chamber is proposed, which is verified by the integrative simulation. The study on the flameout phenomenon will provide a theoretical basis for the design and application of the ion source, which can help the ion source operate safely to meet the requirements for space gravitational wave detection.

Highlights

  • Charges accumulating on antenna. (a) Voltages of grid system increase continuously; (b) Voltages of grid system increase individually

  • 本文对 2 厘米 ECR 离子源进行了全粒子的一体化模拟,复现了 2 厘米离子源 在极限工况(0.1 sccm,0.5 W)下的猝灭现象。模拟结果表明,天线和放电室 之间的双极扩散是导致离子源猝灭的根本原因。单次大幅度增大栅极电压时,若 天线达到新的悬浮电压需要累积的电荷数大于初始电荷数,等离子体将在双极扩 散中消亡,离子源猝灭。多次小幅度增大栅极电压时,双极扩散引起的 Child-Langmuir 鞘层扩张降低了等离子体生成率,使放电室内等离子体密度降低, 直到离子源无法维持放电而猝灭。明晰猝灭机理后,本文提出了天线和屏栅等电 势的改进措施,并通过一体化模拟论证了可行性,为 2 厘米 ECR 离子源的设计和 应用提供理论依据。 参考文献 [1] Lemmer K 2017 Acta Astronaut

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Summary

Introduction

推力器[1,2]、微牛级霍尔推力器[3,4]、微牛级射频推力器[5,6]、离子液体推力器[7,8]。 有高比冲、长寿命的特点[9.10],且经实验初步探索[11],认为 ECR 离子推力器同样 具备执行引力波探测任务的可能性。 然而,当离子源处于极低功率(0.5 W)和极低流量(0.1 sccm)的极限工作状 态时,其内部气体放电及等离子体密度均处于低水平,要实现的宽范围的栅极电 压调节是对推力器性能的考验。因为在极限工作状态下,两种调节推力的方案都 可能引发离子源猝灭:一是多次小幅度改变栅极电压来缓慢调节推力;二是单次 大幅度改变栅极电压来快速调节推力[12]。需要特别说明的是,离子源在较高的等 离子体密度水平可以正常工作,不存在猝灭现象。由于猝灭现象具有随机性和瞬 时性,常常被归结为实验操作失误。采用实验手段对猝灭现象进行观测的难度很 大,因为一方面猝灭现象具有随机性和瞬时性;另一方面,介入式诊断[13,14]对弱 放电水平的等离子体干扰很大,而光学诊断又有栅极系统阻挡。鉴于引起猝灭的 具体原因尚未见报道,本文将采 PIC-MCC(particle-in-cell with Monte Carlo collision)方法[15,16,17]对离子源进行一体化模拟,探究猝灭机理。一体化模拟将放 电室、栅极系统和羽流整合到一个计算域里,可以时空连续的模拟等离子体放电 和离子加速引出的过程,非常适合观测改变栅极电压后,等离子体系统的演化过 程。通过模拟复现离子源在极限工况下的猝灭现象,分析猝灭机理,并提出改进 措施,可以为 2 厘米 ECR 离子源的设计和应用提供理论依据。 ECR离子源在不同工况下的放电图像如图2所示。可以看到,随着流量和微波 功率的降低,离子源内等离子体电离强度逐渐减弱。0.1 sccm和0.5 W是实验系 统所能给予的最低流量和微波功率。在这个工况下,离子源处于极限工作状态, 在调节栅极电压时可能出现猝灭现象。已知多次小幅度和单次大幅度地调节栅极 电压的方式都可能引起猝灭。后者相对于前者,离子源猝灭的栅极电压较低。需 要特别说明的是,猝灭仅出现在极限工作状态。

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