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

近年来，包括申请人在内的世界范围内的少数团体已经证明了太赫兹时域光谱法用于测量低温非平衡等离子体中的等离子体密度的一般可行性。这种新的诊断相比，经典的微波干涉测量具有一定的优势。特别地，并行探测非常宽的光谱范围，包括THz范围中的较高频率，并且ps范围中的超高时间分辨率是可能的。最后但并非最不重要的是，该技术对振动有效地不敏感。然而，到目前为止，该方法的灵敏度较低，并且在所有情况下，等离子体都是专门定制的，以达到不寻常的高密度。应用于标准放电应用和调查领域需要提高几个数量级的灵敏度。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。最后，所有的调查都是在低压力下，电子中性碰撞频率是可以忽略不计的低diagnostics.Novel的发展，在数据采集超快速锁定放大器提供了基础，现在实际上实现所需的增强。因此，该提案的目标是：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）或更高。这将在低压排放中实现和证明。第二，应用该新技术首次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、自脉冲模式下对收缩射流的点火、自脉冲和传播进行时间分辨测量。在此，特别利用ps脉冲持续时间和脉冲的10 ns间隔重复。因此，动力学是解决三个时间尺度：ps和ns并行的点火过程和ms的时间尺度，通过测量在传播过程中的不同位置。最终的愿景是一个紧凑的设备的基础上fs光纤激光器，可以应用类似的朗缪尔探针。然而，这里使用的激光器是标准的Ti：Sa激光器，并且该建议专注于诊断而不是技术的开发。这可能在未来的第二个步骤中实现。

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.