Two-dimensional electron density imaging of nanosecond-pulsed arc plasma in supercritical CO2

Abstract Until now, the plasma parameters of supercritical (SC) fluids have remained largely unknown. In this study, two-dimensional electron density distributions were observed using Shack-Hartmann sensors for nanosecond-pulsed arc plasma in SC-CO2 (7.38 MPa) and high-pressure CO2 (1.59 MPa) generated in a 1 mm needle-to-plane electrode. The maximum electron density in SC-CO2 reached 1026 m-3, which was several times higher than that under the high-pressure CO2 (1.59 MPa), and was several-order higher than that of a typical arc discharge under vacuum (1019-22 m-3) and the atmospheric-pressure gas phase (1022-24 m-3). We investigated difference of the electron density decay characteritics between the SC and high-pressure CO2. The electron density for the SC CO2 has a longer decayig time constant than the high-pressure CO2, because the arc discharge under the SC CO2 maintains the arc temperature due to radiative re-absorption and one-order-smaller diffusion coefficient of the SC medium in contact with the plasma than that of the high-pressure phase. The results provide a database for numerical simulations of the pulsed-arc discharge in supercritical CO2 in the future.