Laser Induced Fluorescence to measure flow velocity of Argon neutrals in a plasma environment

Laser Induced Fluorescence (LIF) is being implemented on the Caltech Water-Ice Dusty Plasma Experiment to measure the neutral particle velocity. Argon neutrals are excited by a chopped 696 nm tunable diode laser having \textless 1 MHz line width. An unchopped sample of the laser beam passes through the plasma to a photodiode to provide a signal for a PID controller that stabilizes the laser frequency.~The 772 nm fluorescence emitted by the excited argon neutrals is detected by a photomultiplier connected to a lock-in amplifier synchronized to the chopper. The system now resolves~1 m/s but the flow velocity appears to be much slower as the measurements are irreproducible other than showing that the velocity is less than 1 m/s. This irreproducibility (large error bar)~results from a small drift of the lock-in signal. A search for the cause of this drift is underway by looking for correlations of drifts of various parameters with the lock-in drift. At the time of writing a prime suspect is the slightly changing laser wavelength that might be caused by plasma variations as it is used to lock the wavelength to a defined absorption level. Finding the cause of the lock-in drift and then eliminating it should enable resolving much slower velocities.