A cavity ring-down spectroscopy sensor for real-time Hall thruster erosion measurements

Area of Science:

• Plasma Physics
• Space Propulsion Engineering
• Spectroscopy

Background:

• Hall thrusters are crucial for spacecraft propulsion but suffer erosion of their insulating channels.
• Understanding the sputtering of channel material, like boron nitride, is key to predicting thruster lifetime.
• Real-time in-situ measurements of sputtered species are needed to validate erosion models.

Purpose of the Study:

• To develop and validate a continuous-wave cavity ring-down spectroscopy (CW-CRDS) sensor for measuring atomic boron density.
• To apply the sensor for real-time boron density measurements in the plumes of two different Hall thrusters.
• To compare sensor measurements with erosion models and experimental profilometry data.

Main Methods:

• Utilized a continuous-wave frequency-quadrupled diode laser at 250 nm for probing ground state atomic boron.
• Employed a high-finesse optical cavity for sensitive absorption measurements.
• Validated the sensor using a controlled ion beam setup and applied it to H6 and SPT-70 Hall thrusters.

Main Results:

• Achieved real-time measurements of peak boron densities up to 10 ± 2 × 10^14 m^-3 in the H6 thruster plume.
• Measured peak boron densities of 7.2 ± 1.1 × 10^14 m^-3 in the SPT-70 thruster plume.
• Estimated eroded channel volumes and erosion rates showed good agreement (within a factor of 2 and ~20%, respectively) with profilometry data.

Conclusions:

• The developed CW-CRDS sensor is effective for real-time monitoring of sputtered boron in Hall thruster plumes.
• The measurements provide valuable data for validating and improving Hall thruster erosion models.
• The study highlights the challenges and potential of using high-finesse cavities in energetic plasma environments.