Date of Award

Fall 12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Program/Concentration

Physics

Committee Director

Charles Sukenik

Committee Member

Stephen Bueltmann

Committee Member

Alexander Godunov

Committee Member

Ted Rogers

Committee Member

Nikos Chrisochoides

Abstract

The overarching motivation of this work is the development of seedless, non-intrusive, laser-based diagnostics for supersonic airflow in wind tunnels. To advance this goal, this dissertation fo-cuses on three-photon excitation in a research-grade argon beam within a tabletop vacuum system, which offers a controlled environment for testing and refining the approach. The chosen excitation scheme drives argon atoms to the 3d[5/2]₃ state using a pulsed Ti:Sapphire laser system, enabling time-of-flight (ToF) measurements on atoms that subsequently undergo a multi-step decay to the metastable 4s[3/2]₂ state. Although full realization of this excitation scheme was hindered by technical challenges, including instability in the Ti:Sapphire oscillator and limitations of the ag-ing Nd:YAG pump laser, significant progress was made in beam characterization and diagnostic development. In particular, Doppler shift measurements were conducted using an 811.754 nm de-tection laser to extract the average velocity of an RF-induced metastable argon beam, yielding a value of approximately 440 m/s. Complementary ToF measurements were performed by pulsing a 801.699 nm quench laser with an acousto-optic modulator (AOM) to depopulate the metastable state; the temporal delay between the quench pulse and observed fluorescence dip provided a secondary estimate of the beam velocity yielding an average velocity of approximately 468 m/s. The number density of metastable argon in the atomic beam used for these measurements was com- pared to the number density of argon in a Mach 6 wind tunnel, indicating that if even a small

fraction of argon can be driven to the metastable state, detection in wind tunnels is viable. These measurements were instrumental in optimizing the detection geometry, estimating the characteristic velocities of the atomic beam, and establishing feasibility for implementation in wind tunnels. A Ti:Sapphire amplifier was also designed and constructed to overcome power limitations in the nonlinear excitation scheme. While a gain of 1.3 was achieved at low input energies, gain saturation at higher energies limited the amplifier’s utility for three-photon excitation. Nevertheless, the diagnostics and amplifier performance data developed in this work provide a strong foundation for future iterations of the experiment.

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DOI

10.25777/q3f2-8m34

ISBN

9798276042596

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