Simple argon atmospheric pressure cold plasma jet has been designed and characterized. The spectral and electrical characteristics of the generated plasma jet were investigated. The effects of applied voltage and flow rate on plasma jet length were studied. A sinusoidal high voltage waveform was applied to ignite the plasma jet. Two current pulses were generated per each half cycle of the positive applied voltage while one current pulse was generated in the negative halves of the voltage waveforms. The time interval between the two positive current pulses decreased with increasing the applied voltage. The emission spectra of the plasma specified that the plasma included active species of O and OH. In addition, the estimated gas temperature of the generated plasma jet was in the range of 330 K, which nominated this jet to be used to treat heat sensitive materials. The obtained results showed that the length of the plasma jet increased by increasing any of the applied voltage or the flow rate at low values, whereas the length approached steady-state values at higher applied voltages and transient from laminar to turbulent flow mode at higher flow rates. This work is highly useful for applications of the required exposure to active species, charged ions, and UV photons at low operating temperatures and applied voltages.

Plasma is the fourth state of matter that represents about 99% of the universe and exhibits unique characteristics such as being a quasi-neutral gas with collective behavior.1 It includes simultaneously electrons, ions, neutral atoms/molecules and reactive species. Therefore, it contains a large density of electrically charged particles that can affect strongly the electrical properties of targets exposed to plasma. One of the main classes of plasma is non-thermal plasma, so-called cold plasma, in which the electron temperatures are much higher than the temperature of the remaining constituents, such as ions, atoms and molecules. The non-thermal plasma can be generated at low temperatures close to room temperature utilizing gas breakdown in noble gases. Therefore, it would be a powerful medium that has a wide variety of applications in different technologies in our daily life such as biological, medical, and textile.2–4 Atmospheric-pressure cold plasma has been gained a lot of interest due to its wide range of applications such as sterilization, cancer treatment, wound healing, skin treatments, seeds germination and surface modifications.5 Also, the parameters that affect the generation of atmospheric cold plasma such as the applied voltage and the used gas can be easily controlled in addition to the low cost and flexibility to be used in different applications.6 

Plasma jet is one of the most important and utilized kind of atmospheric pressure plasma where the plasma is shaped as a jet out of a nozzle. This kind of plasma jet can be generated by MW, RF, AC, or pulsed power supplies. Currently, the researchers are attracted to plasma jet due to its unique properties.7 Firstly, in the accessibility of atmospheric pressure plasma jet, there is no need to any vacuum equipment or any complex materials to generate the plasma jet. Secondly, in the flexibility of the generated jet, it is like a pen connected to the power supply. This property gives users the possibility to bring the jet system into the potential target by simple means. Thirdly, low gas temperature opens the potential use of these jets to treat the heat-sensitive materials and biological samples.8 Fourthly, in low fabrication cost for plasma jet system, there is no vacuum equipment required to operate the atmospheric plasma jet.9,10 Finally, the controllability of operating parameters such as, applied voltage and frequency as well as different types of gases and power sources can be used which gives a wide range of applications.11 

Cold atmospheric pressure plasma jet (CAPPJ) can be operated using different types of gases such as; the noble gas plasma jet, N2 plasma jet and air. The noble gases have the lowest breakdown voltage less than the air and N2, however, they are costly and not as reactive as air plasma jets. Thus, many researches mixed different gases for various applications.12 

In the present work, a detailed investigation on atmospheric-cold- argon-plasma jet is presented, which can generate two current pulses per each half a cycle of positive applied voltage. The electrical and spectroscopical properties of the generated plasma jet have been measured to investigate the plasma properties and deduce plasma generated species and their gas temperature. The effect of the applied voltage and flow rate on plasma jet length were studied. This study can be nominated for the generated plasma jet to be used for treating heat sensitive materials.

A schematic diagram of the atmospheric pressure plasma jet experimental setup system is depicted in Fig. 1. The atmospheric pressure argon plasma jet system consists of a quartz tube of 2 mm and 1 mm outer and inner diameters, respectively. A hollow needle, of about 1 mm and 0.5 mm outer and inner diameters respectively, was inserted in the quartz tube and connected to a high voltage power supply. The needle tip was placed at 20 mm from the tube nozzle.

FIG. 1.

Experimental set up of the generation and characterizing atmospheric-pressure- argon -plasma jet.

FIG. 1.

Experimental set up of the generation and characterizing atmospheric-pressure- argon -plasma jet.

Close modal

A grounded copper stripe with 8 mm width was wrapped around the tube such that its lower edge is above the tube nozzle by 7 mm and its upper edge is below the needle tip by 5 mm. An argon gas of 99.999% purity is blown through needle hole (gas flow rates are ranged from 0.1 to 5.4 L/Min, slpm are available). An AC high voltages were applied between the two electrodes (the needle and the copper stripe) at a frequency of 27 kHz. The applied voltage in this set up was varied from 0 to 30 kV, whereas the frequency was changed from 20 to 30 kHz.

A CCD camera (AVT Manta G-504) with video lens (VZM 450I) system was used to detect the plasma images to study the plasma formation. The argon flow rate was measured and controlled using a mass flow meter (AlicatScientfic MC-5 slpm-D and MC-100 slpm-D). A high voltage AC power supply (Plasma Driver PVM500 Information Unlimited Co) was used to generate the plasma jets which was characterized electrically and spectroscopically. A non-inductive 33.33 kΩ resistor (Shenzhen Zenith sun Electronics Tech. CO., LTD.) was used to limit the current. The voltage and current waveforms were recorded using 1 GHz, 5 GS/s digital phosphor oscilloscope (Tektronix DPO 4104B). The applied voltage was probed at the HV electrode (the needle) using a Tektronix P6015A high voltage probe. The current waveforms were measured, using a Pearson current probes (model 6585), through two paths. First, the total current; It, that was measured through the high voltage electrode. Second, the current passed through the grounded electrode, Ig. Then, the plasma jet current, Ij, was calculated as the difference between the total current (It) and the ground currents (Ig) using the relation Ij = It – Ig.

The key for many technological applications of atmospheric pressure plasma jets is the gas temperature. Therefore, the gas temperature determination has attracted the attention of many researchers.13 The rotational temperature of molecules in atmospheric pressure plasma is considered to be the gas temperature due to fast energy transfer between rotational and translational energies. Therefore, a hydroxyl radical; OH (A2Σ+→X2π), in the range of 305 nm 315 nm were used to determine the rotational temperature through the comparison between its experimental and simulated spectra. The measured experimental spectra of the hydroxyl radical, were fitted and matched with the simulated spectra to determine the more closely matched temperature.

Typical current-voltage waveforms of the atmospheric-pressure-argon- plasma jet at argon flow rate of 1.67 splm and at applied frequency of 27.2 kHz are shown in Fig. 2. A strong single current pulse was formed, (at low voltage of 3.5 kV, Fig. 2a) per each half a cycle of applied voltage, which indicated the formation of a homogenous discharge in the tube between its electrodes. These strong current pulses were due to the quick increase in gas conductivity when the plasma jet ignited, which was followed by the accumulation charges on the insulating surface to extinguish the discharge and reduce the gas voltage.14 The voltage waveforms given in Fig. 2(b) indicates that the discharge occurred at low applied voltages other than its peak value. The space charge accumulated on the dielectric barrier surface was in the opposite direction with the applied voltage during the first half cycle of the applied voltage. However, in the next half cycle both fields were in the same direction as the applied voltage reversed its direction and the resultant electric field was strong enough to produce discharge at low external applied field.

FIG. 2.

Current-voltage waveforms of the atmospheric-pressure-argon- plasma jet after ignition at different peak to peak applied voltages of 3.5, 5.0 and 8.5 kV.

FIG. 2.

Current-voltage waveforms of the atmospheric-pressure-argon- plasma jet after ignition at different peak to peak applied voltages of 3.5, 5.0 and 8.5 kV.

Close modal

A two current pulses, per each positive half cycle of applied voltage, were generated as the applied voltage was increased as presented in Fig. 2(b) and (c). The time duration of the first pulse is about 0.3 μs, whereas the duration of the second pulse was about 1 μs. Fig. 3 shows the variation in the time interval between the two pulses of Ig at different applied voltages. At low voltage value up to 4 kV a single pulse per each half a cycle of applied voltage was formed.

FIG. 3.

The applied voltage effect on the time interval between the two current pulses.

FIG. 3.

The applied voltage effect on the time interval between the two current pulses.

Close modal

However, for higher applied voltage two current pulses were formed per each positive half a cycle of applied voltage while one current pulse was formed per each negative half a cycle. Once the applied voltage reached a higher value of 8.4 kV the second pulse started to form in the negative half of the current waveform. The streamer breakdown was the dominant mechanisms for atmospheric pressure plasma.15 Therefore, the difference between positive and negative streamer formation and propagation was the main reason for the formation of the second current pulse through positive half of the current waveform but not in the negative one at lower voltage values. The decrease in the time intervals between the two current pulses as a function of increasing the voltage was due to the increase in the residual charge carries after the first pulse and the increase in the induced charges on the dielectric surface.

The effects of the argon gas flow rate and the applied voltage variations on the generated plasma jet formation were investigated by studying plasma jet images as shown in Figs. 4 and 5, respectively. As shown in Fig. 6, the length of the plasma jet was plotted against the flow rate and the applied voltage.

FIG. 4.

Photographs of plasma jet at different flow rates at 3.5 kV and frequency 27.2 kHz.

FIG. 4.

Photographs of plasma jet at different flow rates at 3.5 kV and frequency 27.2 kHz.

Close modal
FIG. 5.

Photographs of plasma jet at flow rate 1.67 slpm and frequency 27.2 kHz at different applied voltages.

FIG. 5.

Photographs of plasma jet at flow rate 1.67 slpm and frequency 27.2 kHz at different applied voltages.

Close modal
FIG. 6.

The plasma length versus (a) argon gas flow rate and (b) applied voltage.

FIG. 6.

The plasma length versus (a) argon gas flow rate and (b) applied voltage.

Close modal

The length of the jet increased with the flow rate to reach a maximum value at flow rate of 1.67 slpm and then decreased with the increase of the flow rate. The change of the plasma jet length from the increase at low flow rate to the decrease at high rate above 1.67 slpm is due to the transition from laminar to turbulent flow mode. The plasma jet images in Fig. 4 shows this transformation at the end of plasma jet edge. Fig. 7 confirms the existence of two modes laminar and turbulent modes when comparing the scanned intensity of the jet at a certain distance for flow rates 1.67 and 5.4 slpm.16,17

FIG. 7.

Scan of the plasma jet images along the white light shown in Fig. 3(c) at flow rate of 1.67 slpm and Fig. 3(j) at flow rate of 5.4 slpm.

FIG. 7.

Scan of the plasma jet images along the white light shown in Fig. 3(c) at flow rate of 1.67 slpm and Fig. 3(j) at flow rate of 5.4 slpm.

Close modal

The jet is wider at higher flow rate. These two modes were generated as filaments inside diffuse plasmas. The laminar mode was dominant at low flow rates, whereas the turbulent mode was dominant at high argon flow rates. Fig. 6 shows that the discharge at large flow rate looked much more diffuse at the end of the plume. This may not actually be a diffuse plasma, but instead due to an average of filaments at different positions randomly that can took place in turbulent atmospheric pressure argon jets as had been observed using planar laser-induced fluorescence (PLIF).18 It was shown that the streamer path was correlated to the turbulent flow pattern.

The increase of the length of the plasma jet shown in Fig. 5 with the increase of applied voltage was due to more energy in discharge delivered to charges, which allowed the jet to elongate in surrounding air.19,20 However, the rate by which the jet length increased had lower value at high applied voltages, which might be due to the increase of the plasma jet gas temperature because of higher consumed power.

The emission spectrum of the generated plasma jet in the range (250-880) using a half-meter spectrograph with 1800 g/mm grating is shown in Fig. 8. The spectra show the presence of N2 second positive system (C3Πu→B3Πg), hydroxyl (OH) band at 308.24 nm (A2Σ →X2 Π) and oxygen ions at 777.28 nm radicles with Ar lines, which have the maximum intensities. The use of these radical species are valuable with biological treatments and its effective importance in the sterilization and disinfection processing for different surfaces. The spectra are rich of different strong argon lines. In addition, spectral lines for OH and O indicated the existence of these radicals due to impurities in the gas or ambient air that could enter the discharge zone.

FIG. 8.

The emission spectrum of the plasma jet in the spectral range 200-900 nm.

FIG. 8.

The emission spectrum of the plasma jet in the spectral range 200-900 nm.

Close modal

The emission spectra are not only important to specify different ions and radicals that exist in the plasma jet, but also to determine the gas temperature. The latter can be carried out by comparing the experimentally measured emission spectra with the simulated rotational emission spectra for the second positive system of nitrogen molecules or OH. The energy transfer from rotational to translational energy occurred in a very short time in the range of microseconds at atmospheric pressure due high collision rate between plasma species.20 Therefore, the rotational temperature can be considered as the translational temperature. The OH originated from entrance of ambient air to the discharge zone, which was used to estimate the gas temperature.21Fig. 9 shows an example of this comparison to show that the estimated gas temperature is 330 K. The temperature was measured at 2 mm below the jet nozzle and flow rate of 1.67 slpm at 3.5 kV and frequency 27.2 kHz.

FIG. 9.

Estimation of gas temperature for emission spectra taken at frequency of 27.2 kHz, applied voltage at 3.5 kV and 1.67 slpm flow rate.

FIG. 9.

Estimation of gas temperature for emission spectra taken at frequency of 27.2 kHz, applied voltage at 3.5 kV and 1.67 slpm flow rate.

Close modal

The generated atmospheric-pressure argon plasma jet at different operating conditions was investigated. The current-voltage waveforms at different argon flow rates and applied voltages were measured. A current waveform of two positive pulses was detected, which increased the productivity of plasma species per each positive half a cycle of applied voltage. Also, the length of the plasma jet was measured at different flow rates and applied voltages and the maximum length was obtained at flow rate of 1.67 slpm at an applied voltage of 3.5 kV and frequency 27.2 kHz. The photos of the plasma jet confirmed the existence of the two modes laminar and turbulent filamentary. The measured spectra of the generated plasma ensured that the plasma temperature was at room temperature that made it very useful in medical and biological applications. Moreover, the spectra indicated the generated plasma OH, O, and many Ar spectral lines. The generated cold-atmospheric plasma jet is promising technology for the applications that require exposure to reactive species, charged ions, and UV photons at low operating temperatures and applied voltages.

The financial support from the Deanship of Scientific Research at Taibah University is greatly acknowledged.

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