Abstract
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.
Laser ion acceleration in the high laser energy and high laser intensity regimes
