Xenon charge exchange cross sections for electrostatic thruster models

Abstract

Charge exchange between xenon ions and xenon atoms is the source of a detrimental low energy plasma in the vicinity of electrostatic spacecraft thrusters. Proper modeling of charge-exchange induced spacecraft interactions requires knowledge of the respective charge-exchange cross sections. Guided-ion beam measurements and semiclassical calculations are presented for xenon atom charge-exchange collisions with Xe+ and Xe2+ at energies per ion charge ranging from 1 to 300 eV. The present measurements for the symmetric Xe++Xe exchange system are in good agreement with several earlier experimental studies and semiclassical calculations based on the most recently computed Xe2+ interaction potentials. The cross sections are ∼30% higher than predictions by the Rapp and Francis model [D. Rapp and W. E. Francis, J. Chem. Phys. 37, 2631 (1962)]. The present Xe2++Xe symmetric charge exchange measurements are the first to cover the ion energy range from 40 to 600 eV. The cross sections are in good agreement with low-energy drift tube measurements and are significantly lower than previous higher energy measurements. A simple model for symmetric two-electron transfer is proposed that is in good agreement with the present measurements. The onset for the asymmetric charge-exchange process, Xe2++Xe→2Xe+, is observed to be at 10 eV. For this process, a cross section of 2.8±0.9 Å2 is measured for a Xe2+ energy of 600 eV.

Cathode Effects in Cylindrical Hall Thrusters

“…Although negative potential wells may be destroyed by the buildup of slow charge exchange ions [31], in this case, the time for charge exchange ions [24] to accumulate in the negative potential well around the filament, (ν i cx ) −1 ∼ n 0 < σ i cx v i > 150µs, was substantially longer than the time for ions to be accelerated to the filament supports. This ion loss time was on the order of the filament length (∼ 10 cm) over the ion velocity gained from the potential gradient along the filament ( 10 V), τ i loss ∼ L/(2eV /m i ) 40µs.…”

Oxygen transport in the internal xenon plasma of a dispenser hollow cathode

“…O þ 2 þ Xe charge exchange is described using Xe þ þ Xe cross section data from Miller et al 41 Xenon 34)), and therefore, the charge exchange cross sections for O þ 2 þ Xe charge exchange should be higher than the reverse reaction. The Xe þ þ Xe charge exchange cross sections are approximately twice as large as the O þ 2 þ O 2 charge exchange cross sections.…”

Cathode effects in cylindrical Hall thrusters

“…Although negative potential wells may be destroyed by the buildup of slow charge exchange ions [31], in this case, the time for charge exchange ions [24] to accumulate in the negative potential well around the filament, (ν…”

“…The mean free path for ion charge exchange on background neutrals [24] was λ in cx = v i /(n a < σ in cx v i >) = 280 cm for 200 eV ions, characteristic of those produced in the thruster. A maximum background pressure of 20 µTorr was chosen so that the probability that a plume ion would undergo charge exchange before it reached the ion flux probe (1 − exp(−l/λ in cx ) ≈ 0.05) was nearly identical to previous experiments in the large thruster facility conducted at a higher flow rate (4 sccm through the anode and 2 sccm through the hollow cathode).…”