Investigation On The Discharge Characteristics Of A Rod-to-Ring Plasma Jet Excited By Different Types Of Voltage Waveforms

Atmospheric pressure plasma jet can generate a plasma plume with a variety of reactive species in open space,which results in no limit of the work piece by the gas gap.Therefore,plasma jet has extensively application potentials for biomedicine,industry and so on.In this paper,using a rod-to-ring plasma jet with argon used as working gas,discharge characteristics are investigated for the plasma plumes excited by different excitations.For direct-current excitation,it is found for the first time that the plume operates in a single-pulsed mode,which is different from the self-pulsed mode reported previously;For alternating-current excitation,the interactions between upstream plume and downstream one are investigated in detail,which is qualitatively explained based on a streamer mechanism influenced by residual charges;At last,discharge characteristics of the plasma plumes excited by sinusoidal,triangular and square waveforms are comparatively investigated.The main contents are as follows:Under the circumstances that the rod is placed inside the gas tube,the plume can only be generated in the downstream region of the argon.Investigation indicates that the plume operates in a single-pulsed mode or a continuous one depending on the value of the applied direct-current voltage.For the singe-pulsed mode,discharge frequency increases with increasing the applied voltage or the air concentration,while it keeps almost constant with increasing the argon flow rate.Through temporally and spatially resolved measurements,it is found that the discharge process includes a breakdown stage followed by an afterglow stage.The discharge propagates along the gas flow direction at the breakdown stage.Moreover,the afterglow manifests a stratification into dark and bright luminous regions;For the continuous mode,the emission intensity gradually decreases along the gas flow.It is found that the continuous discharge is in a Townsend discharge regime judged from both the small current density on the cathode surface and the positive slop of the voltage-current curve.By optical emission spectroscopy,it is found that with increasing the applied voltage,excited electron temperature decreases for the single-pulsed mode until it reaches almost constant for the continuous mode.Moreover,gas temperature is slightly higher for the single-pulsed mode than that for the continuous mode.Under the circumstances that the rod is placed inside the gas tube,discharge characteristics are compared for the plasma plumes excited by sinusoidal,triangular and square waveforms.Results indicate that the discharge pluses excited by the three waveforms only appear in the positive half cycle of the applied voltage,however,there is not any discharge pulse in the negative half cycle.Moreover,the pulse number per half voltage cycle increases with increasing the peak voltage.For the multi-pulsed discharge,the pulse intensity of the discharge increases in the rising edge of the applied voltage for the sinusoidal and triangular waveforms,while it keeps constant in the positive half cycle of the square voltage.Furthermore,the time interval of adjacent pulses is larger for the square voltage than those for the sinusoidal or triangular voltages.By high-speed photography,it is found that the plasma plumes excited by the sinusoidal and triangular voltages are composed of a breakdown stage and an afterglow stage.Under the circumstances that the rod is placed outside gas tube,the rod-to-ring jet excited by a sinusoidal voltage can generate plasma plumes in both the upstream region and the downstream one of the argon flow.Moreover,the jet is neither similar to a linear-field jet,nor a cross-field jet,whose angle between the electric filed and the gas flow can be changed easily.Results indicate that the upstream plume length increases and the downstream plume length decreases with increasing the applied voltage or the angle.Moreover,with increasing the peak voltage,the pulse number tends to increase for every voltage cycle,while the inception voltage decreases during this process.Dynamics of the plasma plumes are also investigated in the downstream and upstream regions by high-speed photography,which is qualitatively explained by a streamer mechanism influenced by residual charges.By optical emission spectroscopy,the intensity ratio of two spectral lines is investigated to obtain the electron density and the excited electron temperature for the upstream and downstream plumes.As the upstream plume has certain effect on the propagation of the downstream plume,experimental parameters are selected to avoid generation of the upstream plume.Therefore,only the influence of the angle is investigated on the discharge behavior of the downstream plume.Results indicate that the downstream plume length decreases with increasing the angle.For different angles,the positive discharge or the negative one per half voltage cycle corresponds to a discharge pulse.Moreover,the pulse intensity decreases with increasing the angle.By fast photography,it is found that every discharge pulse corresponds to a propagating process of one plasma bullet.The bullet velocity increases firstly,then decreases during its propagating process.Moreover,its maximal velocity decreases with increasing the angle.