Development of a wide bandwidth heterodyne dispersion interferometer for electron density measurement of atmospheric pressure plasmas*

 

A CO2 laser Heterodyne DI (HDI) is being developed to measure the dynamics of the electron density in atmospheric pressure plasmas (APPs) with a high temporal resolution. Laser based interferometry is a common tool for measurement of electron density in plasmas; however, for application to APPs, the phase shift caused by the plasma can often be masked by the large phase shift associated with changes in neutral gas density. A Dispersion Interferometer (DI), whose probe beam is a mixture of the fundamental and the second harmonic components of laser light, potentially overcomes this neutral gas effect. DIs inherently cancel a significant portion of the large phase shifts caused by neutral gas density as well as by mechanical vibration. As a result, DIs can measure the relatively small electron density induced phase shifts even in the presence of vibration and atmospheric conditions. To achieve the necessary bandwidth, a heterodyne approach, as first demonstrated on the DIII-D tokamak fusion device, has been implemented in a CO2 laser heterodyne dispersion interferometer. The HDI is targeted to measure APPs in an electron density range of 10²⁰ to 10²⁴ m^-3 with a time resolution of 1 microsecond. The optical system is currently being optimized for APP measurements and the achievable density resolution is being characterized.