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Coherent Terahertz Acoustic Phonons: A Novel Diagnostic for Erosion in Plasma Thruster
Discharge Chamber Walls

Thomas Wilson (Marshall University, Dept. of Physics), Iain D. Boyd (University of Michigan, Department of Aerospace Engineering)

The study is based on the success in obtaining the first experimental evidence for the direct excitation of coherent nanosecond-pulsed high-frequency acoustic phonons in semiconducting doping superstructures by electromagnetic fields of the same frequency. Acoustic phonons are detected by a superconducting bolometer, with nanosecond resolution, at the appropriate time-of-flight across a (100) silicon substrate for ballistic longitudinal phonons when a silicon delta-doped doping superlattice is illuminated with grating-coupled nanosecond-pulsed 246-GHz laser radiation with an approximate power density of 1 kW/mm². The absorbed phonon power density in the bolometer detector is estimated to be 10 μW/mm², in agreement with theory. The absence of any detected transverse acoustic phonon signal by the superconducting bolometer is particularly striking – implying that coherent THz longitudinal acoustic phonons can be generated in silicon doping superlattices with negligible associated heat pulse generation. As a first application of this novel phonon source but at higher frequency (due to enhanced scattering), team propose to undertake a study of 1 THz acoustic phonon transmission through thin layers of plasma thruster wall materials, in an attempt to quantify the level of defect generation resulting from plasma exposure. A key life-limiting mechanism for most plasma thrusters is their internal erosion through nonequilibrium plasma-material interactions mainly in the form of energetic ion impact. It is well known that lattice defects can strongly scatter acoustic phonons.