Tuesta Alfredo D, Fisher Brian T, Skiba Aaron W, Williams Logan T, Osborn Michael F
Appl Opt. 2021 Jan 20;60(3):773-784. doi: 10.1364/AO.412054.
Nonintrusive, quantitative measurements of thermodynamic properties of flows associated with propulsion systems are pivotal to advance their design and optimization. Laser-based diagnostics are ideal to provide quantitative results without influencing the flow; however, the environments in which such flows exist are often not conducive for such techniques. Namely, they often lack the optical accessibility required to facilitate the delivery of incident laser radiation and the subsequent collection of induced signals. A particularly challenging, yet crucial, task is to measure thermodynamic properties of plumes issuing from thrusters operating within a vacuum chamber. Large chambers used to simulate the vacuum of space generally lack optical ports that can facilitate complex laser-based measurements. Additionally, the near-vacuum environments within such chambers coupled with the ability of thrusters to efficiently expand the gas flowing through their nozzles lead to plumes with prohibitively low number densities (pressures below 1 Torr). Thus, there is a need to develop a diagnostic system that can offer high throughput without the use of free-space optical ports. Moreover, facilities where propulsion systems are tested typically lack vibrationally isolated space for diagnostic equipment and accurate climate control. As a result, such a high-throughput system must also be compact, versatile, and robust. To this end, the present work describes a fiber-coupled, multipass cell, spontaneous Raman scattering spectroscopy system. This system is intended to provide accurate temperature measurements within low-pressure environments via rotational Raman thermometry. Proof-of-principle measurements are successfully performed at pressures as low as 67 Pa (500 mTorr). Techniques to maintain the signal-to-noise ratio at lower pressures, and the trade-offs associated with them, are discussed and evaluated. Finally, the ability of this system to facilitate additional quantitative measurements is also discussed.
对与推进系统相关的流动进行非侵入式、定量的热力学性质测量对于推进其设计和优化至关重要。基于激光的诊断技术非常适合在不影响流动的情况下提供定量结果；然而，此类流动所处的环境通常不利于采用此类技术。也就是说，它们往往缺乏便于入射激光辐射的传输以及后续诱导信号收集所需的光学可达性。一项特别具有挑战性但又至关重要的任务是测量在真空室内运行的推进器喷出的羽流的热力学性质。用于模拟太空真空的大型腔室通常缺少能够便于进行复杂的基于激光测量的光学端口。此外，此类腔室内的近真空环境，再加上推进器能够有效地使流经其喷嘴的气体膨胀，导致羽流的数密度极低（压力低于1托）。因此，需要开发一种无需使用自由空间光学端口就能提供高通量的诊断系统。此外，测试推进系统的设施通常缺乏用于诊断设备的隔振空间和精确的气候控制。因此，这样的高通量系统还必须紧凑、通用且坚固。为此，本工作描述了一种光纤耦合的多程池自发拉曼散射光谱系统。该系统旨在通过旋转拉曼测温法在低压环境中提供精确的温度测量。在低至67帕（500毫托）的压力下成功进行了原理验证测量。讨论并评估了在更低压力下保持信噪比的技术以及与之相关的权衡。最后，还讨论了该系统促进其他定量测量的能力。
