Time-resolved electron density and electron temperature measurements in nanosecond pulse discharges in helium

Abstract

Thomson scattering is used to study temporal evolution of electron density and electron temperature in nanosecond pulse discharges in helium sustained in two different configurations, (i) diffuse filament discharge between two spherical electrodes, and (ii) surface discharge over plane quartz surface. In the diffuse filament discharge, the experimental results are compared with the predictions of a 2D plasma fluid model. Electron densities are put on an absolute scale using pure rotational Raman spectra in nitrogen, taken without the plasma, for calibration. In the diffuse filament discharge, electron density and electron temperature increase rapidly after breakdown, peaking at ne ≈ 3.5 · 1015 cm−3 and Te ≈ 4.0 eV. After the primary discharge pulse, both electron density and electron temperature decrease (to ne ~ 1014 cm−3 over ~1 µs and to Te ~ 0.5 eV over ~200 ns), with a brief transient rise produced by the secondary discharge pulse. At the present conditions, the dominant recombination mechanism is dissociative recombination of electrons with molecular ions, . In the afterglow, the electron temperature does not relax to gas temperature, due to superelastic collisions. Electron energy distribution functions (EEDFs) inferred from the Thomson scattering spectra are nearly Maxwellian, which is expected at high ionization fractions, when the shape of EEDF is controlled primarily by electron–electron collisions. The kinetic model predictions agree well with the temporal trends detected in the experiment, although peak electron temperature and electron density are overpredicted. Heavy species temperature predicted during the discharge and the early afterglow remains low and does not exceed T = 400 K, due to relatively slow quenching of metastable He* atoms in two-body and three-body processes. In the surface discharge, peak electron density and electron temperature are ne ≈ 3 · 1014 cm3 and Te ≈ 4.25 eV, attained after the surface ionization wave reaches the grounded electrode. The sensitivity of the present diagnostics is too low to measure electron density in the plasma during surface ionization wave propagation (estimated to be below ne ≈ 1013 cm−3). After peaking during the primary current pulse, the electron density decays due to dissociative recombination. Electron temperature decreases rapidly over ~150 ns after the primary current pulse rise, to Te ≈ 0.5 eV, followed by a much more gradual electron cooling between the primary and the secondary discharge pulses, due to superelastic collisions providing moderate electron heating in the afterglow.