Electro-Acoustic Coupling in a Plasma Gas

Sutton, Yvonne (2012). Electro-Acoustic Coupling in a Plasma Gas. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f1d0


Sound emission using an ionised medium has been the subject of research since the beginning of the 20th century. The mechanism involves modulation at an audio frequency of an electrically sustained plasma discharge. In a similar effect to lightning, the charged particles in the plasma respond to the varying energy input. With this comes gas heating, molecular excitation, light emission from relaxation of excited molecular states and acoustic emission resulting from thermal expansion within, and external to, the discharge volume. Despite being the subject of research, there is scope for a deeper understanding of these kinetic processes within the plasma that transforms the input electrical energy into acoustic energy.

In this thesis, experimental measurements on an atmospheric pressure, radio frequency (RF) plasma are devised to explore the electro-acoustic mechanism that leads to sound generation. These measurement methods include: i) nitrogen spectroscopy for measurement of the species temperatures and emission intensities, ii) optical imaging of the visible emission for determining the discharge dimensions, iii) schlieren imaging for measuring the refractive index gradients within the discharge resulting from temperature and density variations and iv) electrical characterisation of the RF discharge. The work in this thesis builds on that reported previously by making direct measurements of the plasma itself to understand the dynamics of the plasma while under modulation. The work develops time-resolved measurement and non-invasive plasma diagnostic tehniques.

Under modulation the plasma behaves as a uniform sound emitter with the level of sound being dependent on the discharge size. A modulation of 15% on an rms conduction current produces a sound pressure level of approximately 67 dB at 3 kHz with around 4% distortion, based on an assessment of the harmonic content of the acoustic signal. In the steady-state, the RF plasma’s electrical and optical characteristics show a close correlation to several equivalent DC plasmas and to the results calculated from an adapted model of a DC glow discharge. For an rms conduction current range of 11-30 mA, the rotational temperature varies between 2800-3200 K; the vibrational temperature shows a change of 3500-4000 K with near equilibrium conditions to the rotational state occuring in the central region of the discharge. Spatial measurements identify the changes in the temperatures and dimensions along the vertical z-axis as well as the spatial dependence on the atomic and molecular species generated in the discharge. The widely acknowledged assumption of equilibrium between the rotational and translational states in nitrogen is challenged based on direct measurement of both parameters through spectrocopy and schlieren imaging. Similarly, the plasma dimensions provide contrasting behaviour depending On the spectral emission region used for measurement. Measurement of the discharge under modulation identifies the timescales for the various kinetic processes in the plasma; at 3 kHz the power leads the vibrational temperature by 34ps with a further delay of 70us between vibrational and rotatational temperatures and relate to the timescales for electron-vibration excitation and vibrational-translational relaxation in nitrogen. The source of the acoustic emission due to modulation is identified (and seen) within the central ionised channel.

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