Optical Diagnostics of a Vacuum Arc Thruster Generated Plasma Plume: Development of an indirect thrust measurement method based on optical imaging and spectrometry

The Vacuum Arc Thruster (VAT) is a compact, low-cost electric propulsion system that generates micro-Newton-level thrust by accelerating a plasma plume generated from the thrusters cathode material. Accurate characterization of this plume is crucial for optimization of the thruster performance and predicting in-orbit behavior, particularly for this type of low-thrust systems where direct thrust measurements are challenging. This thesis addresses the challenge of characterizing the VAT generated plasma plume and estimating its thrust vector and magnitude through non-invasive, optical-based methods. Existing techniques are incompatible with small-scale and pulsating thrusters like the VAT, making the development of alternative thrust measurement methods crucial for precise characterization of these low-thrust systems. To overcome these limitations, a novel optical diagnostic method was developed using high-speed imaging and spectrometry for indirect thrust measurement, along with 3D tomographic reconstruction via the Simultaneous Algebraic Reconstruction Technique (SART). Six simultaneous optical projections were used to reconstruct the plasma plume in three dimensions, and estimate its thrust vector orientation and the plasma bulk velocity. Additional spectrometry measurements confirmed that the plume consists predominantly of titanium ions emitted from the cathode. The results show that the thrust vector fluctuates between 0◦ and 30◦ from the optimal direction, depending on operating conditions, and a plasma bulk velocity approximately 320 m/s directly after thruster ignition. High temporal resolution imaging was also achieved, though with reduced spatial resolution due to camera limitations. The thrust magnitude was was calculated with a new proposed indirect thrust estimation method which resulted in measured thrust levels of 7.028 ± 5.706 μN. The indirect thrust estimation method was validated against direct thrust measurements using a micro-Newton torsional balance.