Continuum emission-based electron diagnostics for atmospheric pressure plasmas and characteristics of nanosecond-pulsed argon plasma jets

Electron diagnostics based on electron–neutral atom (e–a) bremsstrahlung in the UV and visible range emitted from atmospheric pressure plasmas is presented. Since the spectral emissivity of the e–a bremsstrahlung is determined by electron density (n_e) and mean electron temperature (T_e) representing the Maxwellian electron energy distribution, their diagnostics is possible. As an example, emission spectra measured from capacitive discharges are presented, which show good agreement with the theoretically calculated emissivity of the e–a bremsstrahlung. For a single pin electrode nanosecond-pulsed plasma jet (n-PPJ) in argon, we investigate the electron properties and the temporal behavior of the positive streamers. Streamers with many branches are clearly observed inside the dielectric tube, while a few main streamers propagate outside the tube along the jet axis. A two-dimensional (2D) measurement of the time-averaged T_e distribution was developed using a commercial digital camera and optical band pass filters based on the emissivity ratio of two wavelengths of the e–a bremsstrahlung. The viable measurement range of T_e is 0.5–7 eV for the choice of two wavelengths of 300s and 900s nm and 0.5–4 eV for two wavelengths of 400s and 900s nm, which are uncontaminated by the atomic and/or molecular spectra. The 2D T_e distribution obtained using 514.5 and 632.8 nm emissions helps to reveal the role of electrons in streamer characteristics in the argon n-PPJ. Time-averaged T_e of 2.0 eV and 1.0 eV inside and outside the tube, respectively, were measured. The streamer dynamics of the n-PPJ is shown to be dependent on T_e. (paper)