The mechanisms controlling the hydrodynamic effects induced by nanosecond pulsed discharges applied between two pin electrodes in air at atmospheric conditions are investigated experimentally and numerically. A cylindrical plasma kernel is formed between the electrodes during the discharge. After the discharge, schlieren images show that the cylindrical kernel evolves in different ways depending on the gap distance: (1) for short gap distances (), the cylindrical kernel collapses to form a torus-like structure. In this case, the flow field presents a significant source of vorticity; (2) for larger gaps (), the kernel retains its initial cylindrical shape and cools down primarily through heat diffusion. The objective of this work is to understand the mechanisms leading to the formation of these two hydrodynamic regimes. To this end, simulations were performed and compared with the experimental schlieren images. It is shown that the main source of vorticity is the baroclinic torque caused by the cylindrical blast wave that follows the fast energy addition during the discharge. Finally, a criterion is given to predict the occurrence of the two hydrodynamic regimes. This criterion is then validated against experimental results from the literature.

Presented herein is a feasibility study for absolute measurements of ground state atomic nitrogen inside of a nanosecond repetitive pulse (NRP) discharge using a femtosecond twophoton absorption laser induced fluorescence (TALIF) technique. A two-photon absorption scheme (λ = ×206.6 nm) is used to probe the ground state population of atomic nitrogen (2p 3 4 S) inside the discharge. The scheme involves the excitation of the 3p 4 S3/2 level with the subsequent fluorescence collection taking place from the radiative decay to the 3s 4 P triplet state around 745 nm. The main challenge is to demonstrate the ability to perform the measurements at above atmospheric conditions where the typical ns-TALIF diagnostics is unable to provide information due to the high rate of collisional quenching. As such, the current investigation focuses on high-pressure discharges in pure nitrogen. We demonstrate here the ability to measure fs-TALIF signals from atomic nitrogen in NRP discharges operated at pressures between 0.1-5 bar. Additionally, we perform fs-TALIF measurements in Krypton and we identify the main fluorescence channels using the λ = ×204.13 nm excitation scheme. We report that significant saturation effects was observed in both mixtures due to the presence of photoionization which resulted in a departure from the I 2 dependency of the TALIF intensity.

This work presents a femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) diagnostic for measuring ground-state atomic nitrogen in nanosecond repetitively pulsed (NRP) discharges. Absolute atom density is obtained from the TALIF signal via a novel calibration technique based on one-photon direct absorption measurements performed in a low-pressure DC discharge. The VUV measurements were done at the Soleil synchrotron facility using the high-resolution Fourier-transform spectrometer (minimum linewidth Δ̃ = 0.08 cm-1). The main goal of this work was to develop a quench-free diagnostic technique, which would allow measurements at elevated pressures with high spatial and temporal resolution. Here fs-TALIF measurements of N(4S) are demonstrated in the NRP post-discharge between 1-500 μs after the nanosecond high-voltage pulse. A maximum number density of N-atoms of × − was measured at 1 μs after the pulse when the discharge was operated at 1 bar in pure nitrogen. This corresponds to a dissociation fraction of ~ 0.1 %. The fs-TALIF technique at high laser intensity regime (> 1 TW cm-2) calibrated using VUV absorption was compared with the fs-TALIF at low laser intensity regime (< 100 MW cm-2) calibrated via the well-established non-saturated TALIF technique using krypton as an etalon gas. It was found that the two measurements of N(4S) in the NRP post-discharge agree within a factor of 3. Importantly, the limit of detection of the fs-TALIF at high laser intensity regime was determined to be ()~ e 1/. This is approximately one order of magnitude better than previously reported by ns-TALIF in low-pressure discharges.

Measurements of ground state atomic nitrogen inside of a nanosecond repetitively pulsed (NRP) discharge operating at pressures between 0.1-5 bar are performed using a femtosecond two-photon absorption laser induced fluorescence (fs-TALIF) technique. The main goal of this work is to develop a quench-free diagnostic technique which would allow measurements at elevated pressures with high spatial and temporal resolution. Quantitative information is extracted from the TALIF signal via a novel calibration technique based on direct absorption measurements performed in a low-pressure DC discharge. The VUV measurements were done at the Soleil synchrotron facility using their unique high-resolution Fourier-transform spectrometer (⁄ =). During this preliminary work, fs-TALIF measurements of N(4 S) are demonstrated in the post-discharge of the NRP between 1-500 µs after the nanosecond pulse. A maximum number density of N-atoms of × was measured at 1 µs after the pulse when the discharge was operated at 1 bar in pure nitrogen. Importantly, the limit of detection of the fs-TALIF technique was determined to be ()~. This is approximately two orders of magnitude lower than previously reported by ns-TALIF.

No abstract