Ultra fast and sensitive electron density measurement by THz time domain spectroscopy

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In recent years a small number of groups worldwide, including the applicant, has demonstrated the general feasibility of Terahertz time domain spectroscopy for measuring plasma densities in low-temperature non-equilibrium plasmas. This novel diagnostics has certain advantages compared to classical microwave interferometry. In particular, a very wide spectral range is probed in parallel, including higher frequencies in the THz range, and ultra high temporal resolution in the ps range is possible. Last but not least, the technique is effectively insensitive to vibrations. However, so far the sensitivity of the method is low and in all cases the plasma was specially tailored in order to reach unusual high densities. Application to standard discharges applied and investigated in the field requires an enhancement of the sensitivity by several orders of magnitude. Further, so far time resolved measurements have never taken advantage of the potential ps resolution but have been more on the 100 ns scale. Finally, all investigations have been at low pressures where the electron-neutral collision frequency is negligibly low for the diagnostics.Novel developments in data acquisition by ultra-fast lock-in amplifiers provide now the basis to actually realize the required enhancement. Therefore, objectives of the proposal are: Firstly, enhancing the sensitivity by at least two orders of magnitude to 10^11 cm^-2 (line integrated value) or better. This will be realized and demonstrated in low-pressure discharges. Secondly, application of the novel technique for first-time determination of the electron density in an atmospheric pressure RF micro-plasma jet. Thirdly, performing temporally resolved measurements of the ignition, self-pulsing, and propagation of the constricted jet in the high-density, self-pulsing mode. Here, particular advantage is taken of both, the ps pulse duration and the 10 ns interval repetition of the pulses. Therefore, the dynamics is resolved on three time scales: ps and ns in parallel for the ignition process and ms time scale by measuring at different positions during propagation. The ultimate vision is a compact device on the basis of fs-fibre lasers that could be applied with similar ease as a Langmuir probe. However, the laser used here is a standard Ti:Sa laser and the proposal is focused on the development of the diagnostics and not of the technology. This might be realized in a second step in the future.