Abstract:

This thesis demonstrates a novel diagnostic technique: coherent microwave Rayleigh scattering from a small plasma region generated by resonance enhanced multiphoton ionization (REMPI), Radar REMPI. The technique combines the high sensitivity of microwave detection and the extreme selectivity of nonlinear optical processes. Time accurate, high precision and non-intrusive spectroscopy is achieved.

Properties of microwave scattering from the small laser plasma are first explored to verify its coherent nature. Experimental results show that microwave scattering is in the Rayleigh regime when both the microwave wavelength and skin layer thickness are greater than the size of the plasma. A general solution of Mie scattering is needed when either assumption is not satisfied.

Then the concept of Radar REMPI is demonstrated both experimentally and theoretically in inert gases. Theoretical plasma dynamic and gas dynamic models are developed. The computational results match the experimental data satisfactorily. REMPI spectra obtained by microwave scattering are verified by comparison to the classical electron collection method. The highly time accurate, in-situ and non-intrusive nature of Radar REMPI is expected to attract numerous applications in fluid dynamics, combustion, chemistry and semiconductor physics.

The applications of Radar REMPI are illustrated here by trace species detection and weakly ionized plasma amplification by REMPI enhanced avalanche ionization. Trace species detection is shown by detecting below 1 ppm (parts per million) nitric oxide in nitrogen. The nanosecond response of the excitation and detection makes Radar REMPI robust to quenching and useful for the reacting environment. The time accuracy and quick response of Radar REMPI is finally exploited in the plasma amplification. Sequential ionization by REMPI of argon and avalanche ionization of xenon in the argon and xenon mixture is observed experimentally and verified theoretically.