Low Pressure Multiple-pass Raman Spectrometer
2020
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 H2
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.
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