Characteristics of Pulse Laser-Enhanced Guided Streamers Produced in a Helium Atmospheric Pressure Plasma Jet

Abstract

Atmospheric pressure plasma jet (APPJ) which is a plasma source has many advantages such as low electrical power supply, simple and small configuration. Because of the various applications of the APPJ, it takes a strong attention in research and industrials fields. The plasma generated by the APPJ are products of collisional processes and electrons play an important role in the plasma generation. Laser induced plasma (LIP) is also a plasma source in which plasmas with high electron temperature and density are produced by high power pulse laser. Recently, the LIP is applied to the APPJ to enhance the plasma radiation. Study on the interaction between the LIP and the APPJ is in its infancy and needs more study for understanding the mechanism of the plasma radiation enhancement. In this study, plasma parameters of the helium (He) APPJ were determined. Plasma characteristics due to the interaction between guided streamers produced in He APPJ and the ns pulse laser were investigated. An APPJ with two electrodes was configured for generating He atmospheric pressure plasma. An AC bias voltage with the frequency of 5 kHz and the amplitude of 10 kVp.p was applied to the electrodes, and a grounded target was placed outside of the plasma tube on the plasma flow axis. Absolute calibration for measuring the plasma emissivity was introduced. Helium atmospheric pressure plasma spectra were measured, and the electron temperature and the electron density of APPJ were diagnosed by the emissivity of the continuum radiation in the spectra. The electron temperatures and densities for helium atmospheric pressure plasmas with and without a target were compared each other. Spatial distributions of electron temperature and density were examined from the end of the quartz tube to the target to discuss the effects of target and target distance on the plasma parameters as well as the APPJ. In addition, OH radical was examined in the position outside the quartz tube of the APPJ. He APPJ with a single electrode was assembled to generate atmospheric pressure plasma, and various guided streamers in the APPJ were produced when the AC pulse was applied to the electrode. The LIP was produced in the APPJ by focusing a ns pulse laser on the plasma plume of the APPJ. The interaction of guided streamer with the pulsed laser led to the increase in the plasma radiation intensity measured by optical emission spectroscopy (OES). In addition to the OES results, fast ICCD imaging and Schlieren imaging were experimented to visually observe the plasma behavior resulting from the interaction between the induced streamer and the pulsed laser. The electron temperature and density were determined using collisional-radiative model. By the interaction regardless of the position of guided streamer, electron temperature and density increased. Despite an increase in electron temperature, the gas temperature determined from OH spectra remained constant. The measurement of the He metastable states population densities by the laser absorption spectroscopy (LAS) showed that the spectra enhancement was due to multi-step excitation by abundantly supplied electrons through the interaction with the pulsed laser. It was also found that helium metastable levels play a dominant role in the intensity increase of the plasma spectra.