Absolute electron density measurements in a dielectric barrier discharge using an optical cavity

We have developed an optical phase measurement platform for directly measuring electron densities in dielectric barrier discharge (DBD) plasmas. We use this to map the electron density for a DBD in He (3.3 kV, 1 kHz, 1.5 sccm). The optical phase measurement is sensitive not only to index of refraction changes from free electrons in the plasma, but also to the shock wave created by the plasma itself. For our plasma conditions, we measure a peak electron density of 10^15/cm^-3. The peak pressure in the shock wave is 250 Pa. The Mach number of the shock is 4. We show the time-evolution of the electron density and shock wave.