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

Abstract

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 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 , for and between electron energies of 29.9 and 34.4 eV for . 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.