Application of phase-modulated dispersion interferometry to electron-density diagnostics of high-pressure plasma

Phase-modulated dispersion interferometry (PMDI) is a technique for measuring the electron density in plasmas that was first developed for large fusion reactors. In this paper, we demonstrate the potential of PMDI for the diagnostics of microplasma generated at high pressures. PMDI can eliminate the effect of nondispersive components in the refractive-index variation on the measurement; therefore, most of the variation of the refractive index induced by the variation of gas density is eliminated by signal processing, contributing to accurate electron-density determination in microplasmas. The measurement results for a pulsed-dc microplasma in an atmospheric-pressure helium gas flow revealed that the electron density of the microplasma was in the range between 4 × 10 13 and 1.4 × 10 14 cm −3 , and our PMDI system had a temporal resolution of 110 µ s and a sensitivity of the line-integrated electron density of 7 × 10 11 cm −2 .