Optical characterisation of a low temperature plasma jet

  • Rachael Irwin

Student thesis: Doctoral ThesisDoctor of Philosophy


Low temperature plasma jets are of particular interest due to their unique way of producing reactive species even at room temperature and atmospheric pressure. They therefore have the potential to be used for various applications, in plasma medicine and surface modification in particular. The characterisation of these plasma jets is important in order to be able to control them for the desired application.

Various optical diagnostics have been implemented to establish the formation and propagation of the plasma, as well as to estimate the important parameters such as electron density, ne , and electron temperature, Te. Initially imaging on nanosecond timescale established that the plasma, although continuous to the naked eye, consisted of plasma bullets travelling at velocities of up to 105 m s−1. 2D quasi monchromatic images of the plasma, obtained by isolating the light produced by individual spectral transitions, allowed an estimation of the density of excited helium states within the plasma, in the region of 1 × 109 − 2 × 1010 cm−3.

Optical emission spectroscopy was used to identify the species present, to estimate the electron temperature of the plasma using helium line ratios, and to estimate the gas temperature using nitrogen emission. The electron temperature and gas temperature were found to be 0.3 eV and 303 K respectively, which confirmed that the plasma jet was operating in the low temperature regime. Emission from allowed and forbidden helium lines was also utilised, in this case to estimate the electric field present, which was calculated to be 32.1 kV cm−1.

Thomson scattering was also attempted in an effort to obtain electron density and temperature estimates. However, the electron density of the plasma in question was simply too low to detect using the experimental set-up at hand. However, using a model consisting of data from Raman scattering it was possible to put an upper limit on the electron density of the plasma of 9.95 × 1012 cm−3.
Date of AwardDec 2020
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsEngineering & Physical Sciences Research Council
SupervisorDavid Riley (Supervisor) & William Graham (Supervisor)

Cite this