Charge exchange ion number density distribution in Hall thruster plume Shigeru Yokota a, * , Daichi Sakoh a , Makoto Matsui a , Kimiya Komurasaki b , Yoshihiro Arakawa a a Department of Aeronautics and Astronautics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan b Department of Advanced Energy, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan Keywords: Electric propulsion Hall thruster Plume shield Charge exchange PIC–DSMC method Laser absorption spectroscopy abstract The charge exchange (CEX) ions by the collisions between energetic ions and slow neutral particles in a Hall thruster plume are thought a source of the ions coming back to the satellite. Therefore, mechanical plume shielding has been proposed to prevent the CEX ions from spreading out from near exit region. In order to optimize the shield length, CEX reactions in the plume were evaluated. Number density distributions of neutral particles were measured by the laser absorption spectroscopy (LAS) targeting the absorption line of XeI 823.16 nm (6s[3/2] 2 0 / 6p[3/2] 2 ). Ambient neutral gas effect in a test chamber was cancelled out using the measurements at two different back pressures. Measured distribution was compared with the Particle-in-Cell (PIC) and Direct Simulation Monte Carlo (DSMC) analyses. As a result, normalized mean outflow velocity of neutral particles v z =v th was identified at 0.6, and the computed CEX ion density was found to be decreased from the channel exit by 50% at 80 mm distance. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction A Hall thruster is a promising thruster in the electric propulsion systems for near earth applications because its thrust efficiency is higher than those of other thrusters at the specific impulse in the range of 1000–3000 s [1–3]. In the practical use on a spacecraft, the interactions between the plasma exhausted from the propulsion and the host spacecraft cause serious problems: abnormal arcing on solar panels, erosion, sputtering, degradation, and increment of temperature, etc. [4–6]. High-energy main beam ions generated and accelerated in the acceleration channel collide with unionized propellant neutral particles in the plume, resulting in the production of low-energy ions and high-energy neutral particles by charge exchange (CEX) reactions. Because CEX ions have low-energy, these ions spread in the radial and upstream directions due to the radial electric fields near the spacecraft which is charged negatively. Therefore, mechanical plume shielding has been proposed to protect spacecraft surfaces from the CEX ions spreading out from the plume [7]. In order to optimize the shield length, it is important to obtain the neutral density distribution for the estimation of CEX ion distribution. In this study, a fully kinetic 2D3V Particle-in-Cell (PIC) and Direct Simulation Monte Carlo (DSMC) analyses were conducted. In order to characterize the neutral particle flows exhausted from the channel, its density distribution was measured by the laser absorption spectroscopy (LAS) targeting the absorption line of XeI 823.16 nm (6s[3/2] 2 0 / 6p[3/2] 2 ). Using this result, CEX ion distri- bution was also computed. 2. Neutral density measurement 2.1. Measurement theory The relationship between probe laser intensity I and absorption coefficient k(x) is expressed by the Beer–Lambert law as dI dx ¼kx ðÞI (1) Here, x is the coordinate in the laser pass direction. Because distributions of absorption properties in plumes would be axisymmetric, local absorption coefficient k(r , n) is obtained by the Abel inversion [8], where r and n are the radial coordinate and the frequency, respectively. Assuming partial local thermal equilibrium in the plume, integrated absorption coefficient K(r) is expressed as a function of the number density at the absorbing state n i (r) as Kr ðÞ¼ Z N N kr; n ð Þ dn ¼ c 2 8pn 2 0 g j g i A ji n i r ðÞ 1  exp  DE ij k B T e     (2) Here, subscripts i and j denote the absorbing and excited states, respectively. n 0 , c, g, A, E, k B and T e represent the absorption frequency, velocity of light, statistical weight, Einstein coefficient, energy gap between the states, the Boltzmann constant and elec- tronic excitation temperature, respectively. On the ground test in a vacuum chamber, an effect of back- ground particles attributed to ambient pressure is not negligible. In * Corresponding author. Fax: þ81 03 5841 6559. E-mail address: yokota@al.t.u-tokyo.ac.jp (S. Yokota). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum Vacuum 83 (2009) 57–60 Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.024