In situ millimeter wave spectroscopy of microplasma within a photonic crystal
Abstract
Microplasma generated within a millimeter wave (MMW) photonic crystal (PhC) is analyzed by direct measurement of the electron density and the wave transmission spectra (S21) of the crystal. A continuous wave (cw) drive frequency of 43.66 GHz maintains plasma, while a low power frequency sweep (43.5–44.1 GHz) simultaneously probes the wave transmission in the presence of microplasma. Rotational gas temperature and electron density are measured from the CH emission spectrum and the Stark broadening of the Hβ atomic transition. The permittivity of the plasma depends on the electron density. As higher cw power and argon gas pressure increase the electron density in the PhC vacancy, the resonant frequency of the PhC shifts upward in accordance with the measured electron density and plasma permittivity. As the PhC obscures the central core of the microplasma, we present a simple diffusion model that approximates the electron density distribution within the microplasma. The diffuse electron density is then used in a numerical model for S21(ω). The measured and modeled transmission spectra can only be reconciled using the diffusion density gradient of the microplasma, thus resolving previous discrepancies observed when using simplified, abrupt-boundary plasma models.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Jan 15, 2021
- Source ID
- 10.1063/5.0032522
Entities
People
- Hyunjun Kim
- Jeffrey A. Hopwood
Organizations
- Air Force Office of Scientific Research
- Tufts University