Emission-Excitation Cross Sections Relevant to Krypton-Propelled Electric Thrusters

摘要

Emission-excitation cross sections for collision processes relevant to krypton-propelled Hall effect and ion thrusters are measured using optical emission spectroscopy in the 350-1000 nm spectral range. Absolute spectral radiances were recorded for collisions between beams of krypton ions (either singly or doubly charged) and electrons with a Kr gas at charged particle energies relevant to electrostatic thruster plasmas. The most prominent emissions observed are in the near-infrared region from a low-lying excited electronic state of neutral Kr. In the case of Kr~+/Kr~(2+) + Kr systems, the emission-excitation cross section from this low-lying excited electronic state of neutral Kr has a threshold at approximately 12.5 eV in the center-of-mass frame, and it increases with collision energy until 200 eV, where it remains constant through 600 eV. The energy dependence of cross sections for emission from Kr Ⅱ varies with the colliding charged particle. The threshold for the radiative emission of Kr Ⅱ is estimated at, in the center-of-mass frame, between 40 and 50 eV for Kr~+ + Kr, ＜25 eV for Kr~(2+) + Kr and between electron energies of 29.9 and 34.4eV for e~- + Kr. The ion-impact rate coefficients indicate that near-infrared radiative emission resulting from ion-neutral collisions is important below an electron temperature of 10 eV. Potential diagnostic lines to be used for determination of electron temperatures and an initial two-line collisional-radiative model derived from these laboratory measurements are discussed.