  The microwave power absorption efficiency of the mu10 ECR ion thruster,
utilized in the Japanese asteroid explorers Hayabusa and Hayabusa2, is
investigated in order to allow performance measurement and provide information
for its improvement. A model detailing the local electron behavior in a real
ECR plasma discharge, based the magnetic field characteristics, is presented.
Three methods to evaluate the microwave power absorption efficiency are
proposed: an estimation based on the chamber geometry and magnetic field
characteristics, a measuremen based on performance parameters and a measurement
performed with Langmuir probes. The equations used for each method are
analytically derived. The local electron behavior model is confirmed with a
Langmuir probe experiment. Measurement of the microwave power absorption
efficiency is performed with the two independent methods proposed. Results from
the two experiments show good agreement with each other and with the theory.
Finally, a diffusion model explaining the different electron temperature
distributions observed in the chamber is proposed. The model and experiments
clarify the physics behind previously observed performance variations and give
valuable hints for future chamber improvement.
