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Laser Based Sensor for Hall Thruster Erosion Measurement

        Hall thrusters are becoming more and more extensively used in space propulsion applications due to their high specific impulse, high thrust efficiency, and high thrust density. Hall thrusters experience sputter erosion in the insulator channel, which is the primary factor that limits the lifetime of the thruster. In order to avoid the unacceptably risky alternative of doing no life testing, it is crucial to develop accelerated testing capabilities allowing rapid measurements of low sputter erosion rates. Continuous wave cavity ring-down spectroscopy (CW-CRDS) is an ultra-sensitive laser absorption technique which is well suited to make measurements of low density sputtered boron nitride, the material of interest for Hall thrusters. This laser based sensor can provide an in situ diagnostic suitable to study thruster erosion and lifetime and aid in thruster design in future. This research is primarily supported by the Michigan/Air Force Center for Excellence in Electric Propulsion (MACEEP).  
Hall thruster in operation
        Here at Colorado State University, we develop the basic knowledge needed to perform sensitive absorption measurements of ground-state atomic boron (the sputtered material) at 250 nm. A frequency quadrupled laser, partially shown below, is needed to generate the light resonant with boron. This light is then used to excite a high-finesse optical cavity which is formed around the exhaust plume of the Hall thruster. By scanning on and off of resonance with boron, the path-integrated number density of sputtered boron can be found. Using this technique, we have demonstrated absorption sensitivities of 5.1 x 10-9 cm-1 Hz-1/2, which is orders of magnitude below convention absorption spectroscopy.  
Frequency quadrupled laser
        After testing at CSU, the sensor design is transferred to our collaborators at the University of Michigan who have performed the first sputtered boron measurements from a Hall thruster using CRDS. Their experimental setups are shown below.  
        It is our vision that, with a bit more time to mature, this CRDS technique will provide rapid, in-situ diagnostics for Hall thruster lifetime assessments, thereby accelerating the development of this revolutionary technology.  
 

Links related to Hall thrusters:

http://pepl.engin.umich.edu/ 		 
       

     References:

 
  • L. Tao, B. Lee, N. Yamamoto, A. Gallimore, A. Yalin, "Development of a Cavity Ring-Down Spectroscopy Sensor for Boron Nitride Erosion in Hall Thrusters," 31st International Electric Propulsion Conference, IEPC-2009-146, Ann Arbor, MI, (2009)
     
  • W. Huang, A. Gallimore, and T. Smith, "The Technical Challenges of using Cavity Ring-Down Spectroscopy to Study Hall Thruster Channel Erosion," 32nd International Electric Propulsion Conference, IEPC-2011-030, Weisbaden, Germany (2011)
     
  • N. Yamamoto, L. Tao, B. Rubin, J. D. Williams, A. P. Yalin, "Sputter Erosion Sensor for Anode Layer Type Hall Thrusters Using Cavity Ring-Down Spectroscopy," Journal of Propulsion and Power 26, 142-148 (2010)
     
 
       

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