The Technical Challenges of using Cavity Ring - Down Spectroscopy to Study Hall

We present the initial results from a cavity ring-down sensor for measuring the density of sputtered boron atoms originating from the discharge channel of a 6-kW Hall thruster. The sensor traps 250 nm light in a high-finesse cavity to greatly increase its sensitivity to boron atoms. Measurements were obtained with the thruster operating at seven conditions spanning discharge voltages from 150 to 600 V, and anode mass flow rates from 10 to 30 mg/s. Power level at these operating conditions ranged from 1.5 to 10 kW. Boron density was found to vary from <1×10 to ~1×10 m with the higher density measurements being found near the two channel walls. We apply a simple two-dimensional velocity model to obtain boron flux. The boron flux is then correlated to the boron nitride sputter rate, which is found to vary from 4×10 to 6×10 mm/s. This paper then goes on to describe a number of technical challenges encountered in the course of developing the sensor. These challenges include cavity alignment maintenance in the presence of mechanical vibration and mirror degradation. The paper presents the solutions used during this experiment to address the said challenges as well as suggests more permanent solutions for future research. The experiment presented in this paper clearly demonstrates the potential for the use of cavity ring-down spectroscopy to study the problem of Hall thruster channel wall erosion, as well as provide a roadmap for future research in this area.