Investigation on the Characteristics of Argon Diffuse Plasma Plume by Photoelectric Methods
Plasma plume produced by an atmospheric pressure plasma jet(APPJ)can be either diffuse or filamentary.Filament discharge is violent,and a large amount of joule heat generated in the filaments will damage fragile samples,so diffuse discharge can better meet application needs than filament discharge.Plasma parameters,i.e.is the representation of plasma plume state,determine whether APPJ is suitable for specific applications and its application efficiency.It is of great practical significance to study plasma parameters,including:electron temperature(is approximately equal to electron excitation temperature(Texc)),electron density,molecular vibration temperature and molecular rotation temperature(Trot)(is approximately about equal to gas temperature(Tg)),etc.In this work,an argon diffuse plasma plume is generated by a plasma jet excited by a direct current power with a needle-ring struc-ture supply at atmospheric pressure.Optical imaging shows that the plasma plume is diffuse.Photoelectric signals indicate:depend-ing on the voltage between the needle and the ring electrodes(Vp),the discharge may be pulsed mode or continuous mode.In the pulsed mode,Vp remains almost unchanged,while the discharge current and the total optical signal oscillate periodically.Obviously,these pulses are Trichel pulses.The difference is that,all the waveforms including Vp,discharge current and total optical signal remain almost unchanged with time in the continuous mode.Trends of the plasma plume length in two modes are different:the plasma plume length increases with the absolute value of Vp(abs(Vp))in the pulsed mode,but remain almost constant in the whole continuous mode.The discharge frequency of the plasma plume increases as abs(Vp)increases in the pulsed mode.The plasma plume length is almost not affected by airflow,and the discharge frequency is also unaffected by airflow in the pulsed mode.Two identical photomultiplier tubes are used to collect optical signals at different locations of the plasma plume in the pulsed mode.It can be seen from the optical signal waveforms that luminescence of the discharge propagates in the form of plasma bul-let,and its propagation speed is about 2.8×103 m/s.Therefore,the discharge in the pulsed mode operates in a negative streamer mechanism.Due to asymmetric of electrode structure,electric field reaches a maximum at the tip of the needle cathode,and decreases along argon stream,so discharge occurs first near the tip of the needle.Electrons in primary avalanche leave the tip of the needle and travel along the argon stream.On the other hand,the voltage-current characteristic curve has a positive slope,and the current density on the needle cathode surface is too small in the continuous mode.Therefore,the discharge in the continuous mode is probably in a Townsend discharge mechanism.Optical emission spectrum of the total discharge in both modes has been collected.Spectral lines,which including argon atom(Ar Ⅰ)and argon ion(Ar Ⅱ)appear in both discharge modes.Besides,the following spectral lines:N2,N+2,and OH Π(A2Σ+→X2Π)can also be recognized,whose intensities are much lower than that of Ar Ⅰ.Based on optical emission spectra of two modes,electron exci-tation temperature(ten spectral lines containing Ar Ⅰ and Ar Ⅱ are selected to estimate Texc)and gas temperature are estimated taking advantage of Boltzmann fitting and OH(A2Σ+→X2Π)free radical spectral lines fitting,respectively.Results manifest that Texc decreases as abs(Vp)increases in the pulsed mode,but remains almost unchanged with the increase of abs(Vp)in the continuous mode.Both the gas temperatures in the pulsed mode(310±20)K and the continuous mode(290±20)K are close to room temperature,in which Tg in the pulsed mode is slightly higher.A thermometer is also used to measure Tg of the plasma plume in two modes,and Tg obtained by the two methods is compared.All of the above results have been qualitatively explained.The lower gas temperature indi-cates that the diffuse plasma plume produced in this work is suitable for sterilization and surface treatments of biopolymers.
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