Power dissipation, gas temperatures and electron densities of cold atmospheric pressure helium and argon RF plasma jets

A set of diagnostic methods to obtain the plasma parameters including power dissipation, gas temperature and electron density is evaluated for an atmospheric pressure helium or argon radio frequency (RF) plasma needle for biomedical applications operated in open air. The power density of the plasma is more or less constant and equal to 1.3 × 10⁹ W m^-3. Different methods are investigated and evaluated to obtain the gas temperature. In this paper the gas temperatures obtained by rotational spectra of OH(A-X) and (B-X) are compared with Rayleigh scattering measurements and measurements of the line broadening of hydrogen and helium emission lines. The obtained gas temperature ranges from 300 to 650 K, depending on the gas. The electron densities are estimated from the Stark broadening of the hydrogen α and β lines which yield values between 10¹⁹ and 10²⁰ m^-3. In the case of helium, this is an overestimate as is shown by a power balance from the measured power density in the plasma jet. The obtained plasma parameters enable us to explain the radial contraction of the argon plasma compared with the more diffuse helium plasma. The accuracy of all considered diagnostics is discussed in detail.