Space Resolved Optical Emission Research Articles

Optical emission spectrum of filamentary nanosecond surface dielectric barrier discharge

Streamer-to-filament transition is a general feature of high pressure high voltage (HV) nanosecond surface dielectric barrier discharges. The transition was studied experimentally using time- and space-resolved optical emission in UV and visible parts of spectra. The discharge was initiated by HV pulses 20 ns in duration and 2 ns rise time, positive or negative polarity, 20–60 kV in amplitude on the HV electrode. The experiments were carried out in a single-shot regime at initial pressures P > 3 bar and ambient initial temperature in air, N2, H2:N2 and O2:Ar mixtures. It was shown that the transition to filamentary mode is accompanied by the appearance of intense continuous radiation and broad atomic lines. Electron density calculated from line broadening is characterized by high absolute values and long decay in the afterglow. The possible reasons for the continuous spectra were analyzed.

Plasma dynamics from laser ablated solid lithium

Emission plasma plume generated by pulsed laser ablation of a lithium solid target by a ruby laser (694 nm, 20 ns, 3 J) was subjected to optical emission spectroscopy: time and space resolved optical emission was characterised as a function of distance from the target surface. Propagation of the plume was studied through ambient background of argon gas. Spectroscopic observations can, in general, be used to analyse plume structure with respect to an appropriate theoretical plasma model. The plume expansion dynamics in this case could be explained through a shock wave propagation model wherein, the experimental observations made were seen to fit well with the theoretical predictions. Spectral information derived from measurement of peak intensity and line width determined the parameters, electron temperature (Te) and electron number density (ne), typically used to characterise laser produced plasma plume emission. These measurements were also used to validate the assumptions underlying the local thermodynamic equilibrium (LTE) model, invoked for the high density laser plasma under study. Some interesting results pertaining to the analysis of plume structure and spatio-temporal behaviour of Te and ne along the plume length will be presented and discussed.

Particle thermophoresis in low pressure glow discharges

Particles in glow discharges are charged negatively and are therefore often suspended electrostatically since the plasma potential is usually more positive than the potential of surfaces bounding the plasma. However, in addition to responding to electrostatic and gravitational forces, particles are sensitive to forces associated with gradients in neutral gas temperature. A series of experiments were performed varying the temperature of water circulated through planar, parallel electrodes. When particles are present and are suspended in the discharge, they move away from a heated electrode and towards a cooled electrode. In the absence of particles, the discharge optical emission spatial profile and electrical characteristics did not change significantly for any combination of electrode heating or cooling. Particles remaining in the interelectrode gap after the discharge is extinguished appear to be uncharged. With particles present, the discharge electrical properties and time- and space-resolved optical emission take on characteristics associated with discharges in electron attaching gases. We report on a simple particle-scavenging arrangement based on thermophoresis that can be used to remove particles from a discharge.