title: A cavity ring-down spectroscopy sensor for real-time Hall thruster erosion measurements.
abstract: A continuous-wave cavity ring-down spectroscopy sensor for real-time measurements of sputtered boron from Hall thrusters has been developed. The sensor uses a continuous-wave frequency-quadrupled diode laser at 250 nm to probe ground state atomic boron sputtered from the boron nitride insulating channel. Validation results from a controlled setup using an ion beam and target showed good agreement with a simple finite-element model. Application of the sensor for measurements of two Hall thrusters, the H6 and SPT-70, is described. The H6 was tested at power levels ranging from 1.5 to 10 kW. Peak boron densities of 10 ± 2 × 10(14) m(-3) were measured in the thruster plume, and the estimated eroded channel volume agreed within a factor of 2 of profilometry. The SPT-70 was tested at 600 and 660 W, yielding peak boron densities of 7.2 ± 1.1 × 10(14) m(-3), and the estimated erosion rate agreed within ~20% of profilometry. Technical challenges associated with operating a high-finesse cavity in the presence of energetic plasma are also discussed.
authors.affiliation: Physics Department, Colorado State University, Fort Collins, Colorado 80521, USA.
authors.forename: B C
authors.initials: BC
authors.lastname: Lee
authors.affiliation: NASA Glenn Research Center, 2100 Brookpark Rd., Cleveland, Ohio 44135, USA.
authors.forename: W
authors.initials: W
authors.lastname: Huang
authors.affiliation: Mechanical Engineering Department, Colorado State University, Fort Collins, Colorado 80521, USA.
authors.forename: L
authors.initials: L
authors.lastname: Tao
authors.affiliation: Mechanical Engineering Department, Colorado State University, Fort Collins, Colorado 80521, USA.
authors.forename: N
authors.initials: N
authors.lastname: Yamamoto
authors.affiliation: Aerospace Engineering Department, University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: A D
authors.initials: AD
authors.lastname: Gallimore
authors.affiliation: Mechanical Engineering Department, Colorado State University, Fort Collins, Colorado 80521, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Yalin