Electron extraction mechanisms of a micro-ECR neutralizer

Abstract

13Three-dimensional particle simulations have been conducted to analyze the mechanisms of 14 electron extraction through the orifices of a 4.2 GHz microwave discharge microneutralizer, 15 using a xenon electron cyclotron resonance plasma. The dimensions of the neutralizer are 16 20×20×4 mm 3 , and a ring-shaped microwave antenna and permanent magnets are employed for 17 its discharges. The numerical model is composed of a particle-in-cell simulation with a Monte 18 Carlo collision algorithm for charged particle motions, a finite-difference time-domain method 19 for microwaves, and a finite element analysis for magnetostatic fields. The simulation results 20 microspacecraft, HODOYOSHI-4. 8) The microspacecraft was launched on June 19, 2014 and 36 the MIPS was operated successfully in space on October 28, 2014 for the first time in the world. 37The neutralizer of the MIPS is based on a low-power microwave neutralizer for the 150-mA-38 class ion beam exhausted from an ion thruster, [9][10][11] and employs the same frequency of 4.2 GHz 39 and ring-shaped permanent magnets, but a different type of microwave antenna is used because 40 of the small size of the MIPS neutralizer. 12) 41Although the MIPS has already been operated in space, the mechanism of electron 42 extraction from its neutralizer is still unclear and needs to be elucidated for a better performance. 43Owing to its small size, the neutralizer operates using an identical discharge chamber, the same 44 4 microwave power, and a half gas flow rate compared with the ion source, which indicates that 45 the MIPS neutralizer consumes resources (space, power, and propellant) more significantly than 46 conventional ion propulsion systems. One of the reasons for its poor performance is considered 47 24) and elastic scattering and charge exchange for ions, 25) where the null-collision method is 79 employed to reduce the calculation time. 26) The motion of excited-state atoms and Coulomb 80 collisions are not taken into account. Neutral particles are assumed to be spatially uniform 81 throughout the simulation and have a Maxwellian velocity distribution at a gas temperature of 82 300 K. (ii) The magnetic fields of microwaves are neglected compared with the magnetostatic 83 fields of the permanent magnets. (iii) The effect of plasma current is not taken into account 84 127The macroscopic parameters, such as electron density and electron temperature, were 128 determined by averaging over 50000 microwave cycles (11.9 µs) after the steady state was 129 reached. The peak plasma density is located in the ECR layer on the right side of the antenna, 130where the maximum electron density is 1.6×10 17 m −3 , and their distributions spread along the 131 magnetic field lines, producing the ring-shaped profile of plasma density. Such a distribution 132 was also confirmed in the experiment. This result indicates that the plasma is well confined 133 because of the mirror magnetic fields. The distributions of electron temperature and plasma 134 potential are almost the same as the d...

Electron extraction enhancement via the magnetic field in a miniature microwave discharge neutralizer

“…To improve the electron extraction efficiency, we first focused on the orifice shapes. In previous studies, 17,18 the E × B drift induced not only electron extraction on the clockwise-side of each orifice but also electron backflow on the counterclockwise side and some electrons were also extracted along the magnetic field lines through the orifice, which primarily occurred in the region approximately |r| ≥ 5 mm inside the orifices. Figure 3 shows schematics of the conventional and two redesigned orifice plates, which are referred to as the cases with four circular orifices, four arc orifices, and two arc orifices, respectively.…”

“…The red line in Fig. 10(a) shows an electron in MF-1 that moved along the magnetic field line, experienced cross-B field transport due to the E × B drift inside the orifice, 17 and was then extracted through the orifice into the vacuum. As shown by the blue line in Fig.…”

“…10(a), another electron was reflected by the sheath potential at the outside wall of the discharge chamber and flowed back inside the discharge chamber owing to motion along the magnetic field lines or to E × B drift. 17 When an electron had sufficiently high energy to overcome the sheath barrier at the outside wall, it was lost to the outside wall (green line). The red line in Fig.…”

“…8 This type of result cannot be obtained using two-dimensional models, which have often been employed to model other E × B devices, such as Hall thrusters 9-14 and magnetron discharges. 15,16 In addition, we numerically investigated the electron extraction mechanisms through the orifices of the neutralizers, which is also difficult to measure experimentally, and found that the E × B drift caused electron extraction mainly on one side of each orifice 17 and that electrons were also extracted along magnetic field lines. 18 Therefore, it was implied that the electron extraction efficiency depends on the orifice shapes and the magnetic fields.…”

Effect of Orifice Diameter on the Performance of a Miniature Microwave Neutralizer

“…2,[4][5][6] The operation of such propulsion systems require the use of an electron source, referred to as the "neutralizer." [7][8][9][10][11][12] The various limitations imposed upon the geometry, power, and material requirements of the neutralizers for space propulsion make their development challenging.…”