Structural characteristics of the upstream sheath of the ion optics and its application in evaluating the beam performance of an ion thruster

Abstract

To evaluate the extraction capabilities of ion optics and promote the generation of highly collimated ion beams for propulsion, the properties of the upstream sheath of the ion optics and how those properties relate to the beam divergence are investigated numerically and theoretically. The characteristics of the beam divergence at different grid parameters are studied from the behaviors of the impingement current and divergence angle obtained by simulations. Additionally, the simulations indicate the existence of an optimal structure for the upstream sheath of the ion optics, one that corresponds to a moderate focusing effect and a relatively small divergence angle. The plasma densities at the dividing points of different sheath structures are then derived with the matching model of the ion optics and the Child–Langmuir law, coupled with semi-empirical approaches based on the simulation results. According to the theoretical analyses, the range of existence of the most-desirable sheath structure depends on the strength of the penetration of the extraction field, the voltage between the grid apertures, and the distance between the upstream surfaces of the grids. Also, sensitivity analyses are performed with the numerical partial derivatives of the models to investigate how the grid parameters affect the sheath structures. The plasma densities at the dividing points generally vary synchronously with the changes of grid parameters, but the ranges of variations are different. Consequently, the desirable sheath structure and operating conditions of the ion optics can be achieved by correctly adjusting the grid parameters.

On the space-charge effects in the beam extraction process of ion thrusters: the roles of compensating electrons and changing beam radius

“…Then the corresponding beam current I b,pl = J CL,pl π r 2 s . The main factors affecting the perveance limit of an ion optics can be found from equations ( 11)-( 13): I b,pl can be improved by increasing φ sg − φ ag and r a or decreasing d s + d sa and r s , which is consistent with the previous studies [54]. The applicability of the modified CL law is verified by the comparison with the simulation results at different grid parameters.…”

“…We refer to Li and Sun [28] for more details on the simulation method. Besides, the codes, which have been validated and benchmarked [51][52][53][54], are written in Fortran90 and parallelized by using the MPI interface and domain decomposition technique [55].…”

“…Accordingly, the perveance limit where the ion optics turns to the over-perveance state can be regarded as the upper limit of the normal operating range of ion optics [18]. Since the plasma densities corresponding to the planar and convex meniscuses shape are higher than that of the perveance limit [54], the meniscus will not enter the aperture of SG when the ion optics works at a normal operating point. Then it can be known from section 3.1 that the electrons in the bulk plasma generally do not enter the aperture of SG and the region beyond.…”

“…V CL should equal to Vsa which is the average voltage between the surfaces z = 0 and z = d s + d sa and can be calculated by Vsa = ´rs 0 φ sg,a (r) dr r s − ´ra 0 φ ag,a (r) dr r a , (10) where φ sg,a (r) and φ ag,a (r) are the potential at the surfaces z = 0 and z = d s + d sa , respectively. A semi-empirical model was proposed and verified to estimate φ ag,a (r) [54] φ ag,a (r…”

A minimized electron cyclotron resonance ion thruster for China’s space-borne gravitational wave detection missions

“…The detail of the specific impulse and its influence will be discussed in section 4. In addition, the curvature of the sheath, concave or convex, determines the wear of the acceleration grid, as well as the lifetime of the thruster [54][55][56]. Therefore, the thruster can only work in the concave case to ensure a long lifetime.…”