Speaker
Mr
Abdullah Zafar
(North Carolina State University)
Description
Passive spectroscopic measurements of Stark broadening have been reliably used to determine electron density for decades. However, a low-density limit ($\sim 10^{13}$ cm$^{-3}$) exists due to Doppler and instrument broadening of the spectral line profile. A synthetic diagnostic for measuring electron density capable of high temporal (ms) and spatial (mm) resolution is currently under development at Oak Ridge National Laboratory. The diagnostic is based on applying Doppler-free saturation spectroscopy (DFSS) to measure the Stark broadened, Doppler-free, spectral line profile of a Balmer series transition. The experimental data is fit to a quantum mechanical model to extract relevant parameters. The quasi-static approximation with the Holtsmark distribution is used to model the Stark broadening.
DFSS uses two counter-propagating laser beams to excite atoms with a velocity vector perpendicular to the beams, resulting in a Doppler-free measurement. The extreme reduction in line broadening allows access to the Stark broadening regimes not previously available through passive spectroscopy. This technique has been successfully employed to measure spectral data in an electron cyclotron resonance (ECR) source for an electron density range of 10$^{11}$ - 10$^{12}$ cm$^{-3}$. Theoretical modeling continues to improve as crossover peaks, an artifact of the diagnostic, are better understood and captured in the simulations. Details of diagnostic implementation and agreement between experimental data and theoretical results is discussed.
Primary author
Mr
Abdullah Zafar
(North Carolina State University)
Co-authors
Dr
Elijah Martin
(Oak Ridge National Lab)
Dr
Steve Shannon
(North Carolina State University)
