Local, High Resolution, Electron Density and Magnetic Field Measurements via Doppler-free Saturation Spectroscopy

The extreme environment encountered in fusion relevant devices provide a challenging obstacle for most diagnostics. Traditional approaches are either (1) unable to endure the harsh conditions, (2) are constrained in areas they can access, or (3) suffer from poor measurement resolution. At Oak Ridge National Laboratory, we have developed a diagnostic method that allows us to overcome these limitations. The diagnostic is based on measuring the spectral line profile using Doppler-free saturation spectroscopy (DFSS). DFSS is a laser-based absorption technique that greatly reduces Doppler broadening and essentially eliminates instrument broadening by crossing two counter-propagating beams. This results in high resolution spectroscopic measurements that can be localized along the line of sight. Plasma parameters are then extracted by fitting the measured spectrum to a quantum mechanical model using the Explicit Zeeman Stark Spectral Simulator (EZSSS) code. The effects of Zeeman-splitting and Stark broadening on the spectrum allow for diagnosing the magnetic field and electron density, respectively. DFSS has been successfully employed to diagnose a magnetized (500-800 G), low temperature (~5 eV), low density (10¹⁶-10¹⁸ m^-3), helium plasma. The results and accuracy are presented here.