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

近年来，包括申请者在内的世界范围内的一些小组已经证明了太赫兹时域光谱分析在测量低温非平衡等离子体中等离子体密度的一般可行性。与经典的微波干涉测量法相比，这种新的诊断方法具有一定的优势。特别是，并行探测了非常宽的光谱范围，包括太赫兹范围内的更高频率，并且有可能在ps范围内实现超高的时间分辨率。最后但并非最不重要的一点是，该技术对振动有效地不敏感。然而，到目前为止，这种方法的灵敏度很低，而且在所有情况下，等离子体都是专门定制的，以便达到不寻常的高密度。应用于现场应用和研究的标准放电需要将灵敏度提高几个数量级。此外，到目前为止，时间分辨测量从未利用过潜在的ps分辨率，而是更多地在100 ns尺度上进行。最后，所有的研究都是在低压下进行的，对于诊断来说，电子中性碰撞频率可以忽略不计。超快锁定放大器数据采集方面的新发展现在为实际实现所需的增强提供了基础。因此，该方案的目标是：首先，将灵敏度提高至少两个数量级，达到10^11 cm^-2(线积分值)或更好。这一点将在低压放电中实现和展示。其次，首次应用这一新技术测量了大气压射频微等离子体射流中的电子密度。第三，在高密度自脉冲模式下对压缩射流的点火、自脉冲和传播进行时间分辨测量。这里，特别利用了ps脉冲持续时间和脉冲的10 ns间隔重复。因此，在三个时间尺度上进行动力学解析：对于点火过程，ps和ns并行；通过在传播过程中的不同位置测量，得到ms时间尺度。最终的愿景是一种基于飞秒光纤激光器的紧凑型设备，可以像朗缪尔探测器一样轻松地应用。然而，这里使用的激光器是标准的钛：锑激光器，建议的重点是诊断技术的发展，而不是技术的发展。这可能会在未来的第二步实现。

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.