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

Abstract

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 .

Electron characterization in weakly ionized collisional plasmas: from principles to techniques

“…A strong initial n e overshoot with a maximum of 7 × 10-13 cm −3 and a mean electron energy of 0.1-3.0 eV were observed in an argon plasma jet driven by a 3.5 kV positive dc pulse with a pulse width of 500 ns and a repetition frequency of 5 kHz [78]. Using a similar setup, Park et al [84] reported that the time-averaged n e was 1 × 10 9 cm −3 and T e was (1.0-2.5) eV via a comparison of the absolute intensity of continuum emission spectrum with the theoretically calculated neutral bremsstrahlung emissivity.…”

“…For further insights, the electrons in plasma jets has also been characterized using various electron diagnostic techniques, such as laser Thomson scattering [76][77][78], laser interferometry [79], Stark broadening of hydrogen atomic lines [80][81][82], and continuum radiation [84]. The line broadening method has been widely used for most electron studies in atmospheric-pressure plasmas, particularly in microwave-and radio-frequency-excited plasmas because of their high n e .…”

“…The 'white' intense appearance shown in the unfiltered image [ Figure 7(a)] seemingly implies the substantial continuum radiation in the visible wavelength range. The electron characteristics reported by earlier works [76][77][78]80,84,119,[153][154][155][156][157] in various atmospheric-pressure plasmas with the aforementioned diagnostic methods are summarized in Figure 8. For the sake of comparison, the electron data are grouped by dotted boxes with respect to the diagnostics presented in the figure, and valid ranges of each diagnostic technique for nonequilibrium plasmas at atmospheric pressure are depicted by dashed boxes.…”

“…Possible sources other than free electrons are listed in Table 3. Extra attention must be paid to continuum radiation from H 2 for Ar discharges and He 2 + for He discharges [84]. Continuum radiation from sources other than H 2 and He 2 + can also arise from the photorecombination of electrons with Ar þ 2 (Ar þ 2 þ e À !…”

“…In addition, the wide applicability of plasma technology to the food and agriculture industries has been confirmed by approaches that use plasma for the enhancement of seed germination rates, plant yields, and plant growth [19,20]; remediation Figure 1. Remarkable results of atmospheric-pressure plasma research, from plasma diagnostics to applications [17,23,24,66,84].…”