
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39300147
70966
10.1038/s41598-024-70966-7
Article
The ring-shaped spatial distribution of the argon excimer, Ar2∗, in the effluent of the kINPen-sci
Nave Andy S. C. 12
Mattern Philipp 1
van Helden Jean-Pierre H. jean-pierre.vanhelden@inp-greifswald.de

1
1 https://ror.org/004hd5y14 grid.461720.6 0000 0000 9263 3446 Leibniz Institute for Plasma Science and Technology (INP), Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany
2 grid.510739.9 0000 0004 7707 1130 Silicon Austria Labs GmbH, Europastraße 12, 9524 Villach, Austria
19 9 2024
19 9 2024
2024
14 2185915 3 2024
22 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Spatially resolved measurements of the argon excimer, Ar2∗, were performed in the effluent of a cold atmospheric plasma jet (CAPJ), the so called kINPen-sci, using cavity ringdown spectroscopy. The spatial distribution of the density of Ar2∗ could be distinguished into two distinct populations: a Gaussian and a toroidally shaped distribution. The production mechanisms of these populations seem to differ. On the one hand, a strong correlation was found between the Gaussian Ar2∗ population and the spatial distribution of the filaments produced in the effluent of the kINPen-sci. On the other hand, measurements performed while varying the experimental conditions under which the kINPen-sci was operated indicate that the gas flow velocity must play a major role in the formation of the toroidal Ar2∗ population. However, the mechanism of formation of the toroid Ar2∗ population remains unclear.

Subject terms

Plasma physics
Optical spectroscopy
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

In recent years, many cold atmospheric plasma jets (CAPJs) have been developed to treat surfaces for biomedical and industrial applications, e.g. to decontaminate, activate and/or coat a given substrate1–3. Such a CAPJ, the kINPen, was first developed for the treatment of sensitive biological surfaces4,5, and one of its version the kINPen MED was the first CAPJ to obtain certification as a medical device6. One of the main fields of research and development of CAPJs is to correlate a certain biological response to the quantities of plasma generated species, notably the reactive oxygen and nitrogen species (RONS)7. To this aim, shielding gas devices have been developed to envelop the effluent of the kINPen with a known O2/N2 gas mixture to control the RONS produced8,9. Hence, quantitative measurements of the RONS and the energetic argon species generated by the kINPen are paramount to assess the dose delivered to a given sample, but also to investigate the physicochemical reaction pathways, whose understanding would enable the tailoring of the chemical composition.

In fact, many investigations addressing the quantification of the plasma generated species in the kINPen have been performed10–20. In particular, absorption spectroscopy techniques have been prominently employed as they permit the determination of the absolute densities of the generated species without calibration21. Furthermore, the higher sensitivity of cavity ring-down spectroscopy (CRDS) techniques has been increasingly exploited to measure small densities of the generated species in-situ, i.e. across the small distance of the plasma jet’s effluent14–16,19,20. Although, absorption spectroscopy techniques are inherently yielding line-of-sight integrated measurements, numerical techniques such as Abel inversion can be employed to retrieve the spatially resolved densities16,19.

Recently, we demonstrated the possibility of measuring the argon excimer (Ar2∗) in the effluent of an atmospheric argon plasma source, the kINPen-Sci, using CRDS20. The measurement of Ar2∗ is crucial to characterize atmospheric argon plasma sources as it is predicted, due to efficient three-body association collisions, to be the most abundant energetic argon species12,22. Moreover, the Ar2∗ a3Σu+ state has an energy level of 10.6 eV and an effective lifetime of 2.9 μs in pure argon at 1 bar23, therefore it is particularly efficient at producing RONS through quenching and penning ionization of O2 and N2 molecules. Furthermore, Ar2∗ are a source of ionizing radiation, thus the determination of its density allows to assess the amount of harmful radiation delivered to a patient.

In the current work, the spatial distribution of Ar2∗ in the effluent of the kINPen-sci was investigated. We found that a model assuming two distinct Ar2∗ populations, a Gaussian and a toroidal distribution, was best fitting the spatial profile measured. To verify this model and further investigate these phenomena, Ar2∗ spatial profiles were measured at various distances from the nozzle while operating the kINPen-sci under various experimental conditions. Hence, we establish a strong correlation between the Gaussian Ar2∗ population and the spatial distributions of the filaments produced by the kINPen-sci24,25. Moreover, we found that gas flow velocity must play an important role in the formation of the toroidal Ar2∗ population.

Results

The spatial distribution of Ar2∗ in the effluent of the kINPen-Sci

An absorption spectrum of Ar2∗ can be found in our previous work20. In the current work, to expedite the measurements of the line-of-sight integrated density, 200 ringdown times, τ, were recorded for a reduced sets of wavelengths: from 511 to 511.9 nm (step: 0.1 nm) and from 516.9 to 517.8 nm (step: 0.1 nm) for determination of the baseline, τ0,λ=512, and between 512.03 and 512.08 nm (step: 0.005 nm) to measure the peak absorption of the 7pσ3Σg+ ← a3Σu+ (ν′ - ν′′) systems of Ar2∗. Subsequently, the line-of-sight integrated absorbance, Aint,λ=512, can be calculated using:1 Aint,λ=512=Lc×1τλ=512-1τ0,λ=512

where L = 58 cm is the cavity length, c the speed of light in vacuum, τλ=512 and τ0,λ=512 the ring-down time with and without Ar2∗ absorption, respectively. Hence, measuring the peak integrated absorbance at 512 nm, Aint,λ=512, the integrated Ar2∗ densities along the y-axis, Nint, can be determined using:2 Nint=Aint,λ=512σλ=512=∫[Ar2∗](y)dy

where σλ=512=3.3±0.8.10-16 cm2 molecule-1 is the cross-section for the predominant feature at 512 nm of the 7pσ3Σg+ ← a3Σu+ (0–0) system determined by Kunz et al.26 at atmospheric pressure in a pure argon environment and [Ar2∗](y) is the spatially resolved density of Ar2∗ along the y-axis. Although, peak cross-section values are dependent on the gas mixture and the kINPen-Sci is operated in ambient air, we shown in our previous work20 that the measured absorption are unchanged with or without an argon environment surrounding the plasma jet. Hence, we assume that the peak cross-section value given by Kunz et al.26 is unchanged under our experimental conditions.

In the current work, the kINPen-sci is mounted vertically in the middle of the cavity where the y-axis is parallel to the cavity axis, the x-axis corresponds to the horizontal axis, moving the kINPen-sci perpendicularly to its gas flow and the z-axis corresponds to the vertical axis, moving the kINPen-Sci parallelly to its gas flow. In Fig. 1a, the average values and standard deviations of Nint determined, operating the kINPen-sci with 3 slm Ar, using three distinct measurements are shown as a function of the position along the x-axis.Fig. 1 (a) The Ar2∗ integrated densities, Nint, measured approximately 1.2 mm below the nozzle tip of the kINPen-Sci operated with a flow of 3 slm Ar is shown as the function of the position along the x-axis. (b) Sketch of the toroidal + Gaussian model proposed to fit the Ar2∗ spatial distribution measurement.

Compared to our previous work20, more data points along the x-axis were recorded allowing to clearly identify more complex spatial features than previously reported. The profile is roughly symmetric with slow increases from -1.6 to -0.8 mm and from + 0.8 to + 1.6 mm, sharp increases from -0.8 to -0.7 mm and from + 0.7 to + 0.8 mm, small peaks between -0.7 and -0.4 mm and between + 0.4 and + 0.7 mm and a flat top between -0.4 and +0.4 mm. This profile has complex features and it is not straightforward to determine the underlying Ar2∗ spatial distribution. However, assuming circular symmetry of the Ar2∗ density around the z-axis, together with the observation of the two maxima at -0.7 and + 0.7 mm, indicate the presence of a toroidal component.

Hence, using the simplest possible toroid profile, a homogeneous Ar2∗ toroidal distribution, a toroid + Gaussian model is proposed to fit the measured Ar2∗ spatial profile. A sketch of this model is illustrated in Fig. 1b, where nt(x,y) and ng(x,y) are, for a given z position, the spatially resolved Ar2∗ density functions for a toroid and a Gaussian, respectively. Hence, the sum of the integrated Ar2∗ density profiles, nint(x), can be written as:3 nint(x)=ng,int(x)+nt,int(x)

where ng,int(x) and nt,int(x), expressed in molecule cm-2, are the integrated density profiles along the y-axis of the Gaussian and of the toroid, respectively. Equation (3) is further developed as:4 nint(x)=Ng·e-2(x-xg)2wg2+Nt·dt(x,rt,tt,xt)

where Ng, xg and wg are the Gaussian peak density, offset and width, respectively. Furthermore, the toroid density, Nt, at a given z position is assumed homogeneous and the distance across the toroid, dt, is expressed for -rt+tt<x<rt-tt as:5 dt(x)=2rt2-(x-xt)2-2(rt-tt)2-(x-xt)2

for -rt<x≤-rt-tt and rt-tt≥x>-rt as:6 dt(x)=2rt2-(x-xt)2

and for x≤-rt and x≥rt as:7 dt(x)=0

where rt, tt and xt are the toroid radius, thickness and offset, respectively. We should note that all parameters described above have a dependency on the axial position, z-axis. Moreover, whereas the toroid density is considered homogeneous and, Nt is expressed in molecule cm-3, the Gaussian density is intrinsically inhomogeneous and Ng is expressed as an integrated density in molecule cm-2.Fig. 2 The Ar2∗ integrated density profiles are shown at the distances, z, from 0.4 to 4.6 mm below the nozzle tip of the kINPen-sci operated with a flow of 3 slm Ar. The toroid + Gaussian model fit are represented by the red curves. The green and blue curves correspond to the Gaussian and toroidal distributions, respectively.

In Fig. 2, the Ar2∗ spatial distributions measured at different axial positions are shown to be fitted remarkably well by this toroid + Gaussian model. The red curve corresponds to the toroid + Gaussian model fit and the green and blue curves to their Gaussian and toroidal components, respectively. Notably, this model can also fit the slight asymmetry observed, with higher density at -0.6 than at +0.6 mm, by adjusting the offset values for the two distributions. This asymmetry arises from the technical difficulty to place the central electrode exactly in the center of the dielectric. Indeed, the central electrode is never placed perfectly in the center of the dielectric and, therefore, the produced filaments have a preferred path, causing the observed unsymmetrical spatial profile of Ar2∗.

Remarkably, at every measured axial position we observed the same sharp increases from -0.8 and -0.7 mm and from +0.7 to +0.8 mm and small peaks between -0.7 and -0.4 mm and between +0.4 and +0.7 mm. These features are the clear signature of a toroid integrated along an axis perpendicular to its rotation axis, see the schematic in Fig. 1b. We should note here, that a competing model consisting of a revolving Gaussian distribution around the axial axis was also investigated. However, fitting of this model to the measured Ar2∗ density profiles lead to significantly worst chi-squared values, notably, it was incapable to properly fit the sharp increase observed from -0.8 to -0.7 mm and from +0.7 to +0.8 mm. Finally, at z = 0.4 near the center of the effluent, the determined standard deviations are significantly larger than at other positions, we correlate this to a higher instability of the cavity mode at this position, the possible sources of which will be discussed below.Fig. 3 Toroid and Gaussian parameters shown as a function of the axial position, z, retrieved from the fit of the Ar2∗ spatial distribution shown in Fig. 2. The total number of molecules of the Gaussian and toroid are shown in (a) and the thickness, radius and widths in (b).

In Fig. 3, the parameters retrieved from the Gaussian + toroid model fit are shown as a function of the axial position, z. Figure 3a, allows to compare the densities and Fig. 3b the widths, radius and thickness of the Gaussian and toroidal distributions. However, the parameters Ng and Nt cannot be straightforwardly compared as they are defined as a peak density expressed in molecule cm-2 and a homogeneous density expressed in molecule cm-3, respectively. Therefore, the total number of molecules for the Gaussian and toroidal parts were determined for axial slices of size Δz=0.27mm, corresponding to the width of the lowest order cavity mode, TEM00. The total number of molecules of the toroid slice, Nt,tot is given by:8 Nt,tot=Nt×(πrt2-(π(rt-tt)2))×Δz

On the other hand, to determine the total number of molecules of the Gaussian slice, Ng,tot, the 2D-Gaussian density distribution expressed in polar coordinates, ng(r), must be first retrieved from its projection on the x-axis, namely the measured integrated ng,int(x) densities which can be expressed according to the Abel transform as:9 ng,int(x)=2∫|x|+∞ng(r)rr2-x2dr

For a Gaussian function, the Abel inversion has an analytic solution and we can express ng(r) as a function of the parameters of ng,int(x) as follow:10 ng(r)=Ngwgπ2×exp-2r2wg2

Hence, given the volume of a 2D-Gaussian we can express Ng,tot as:11 Ng,tot=Ngwgπ2×2π×wg2×Δz

Finally, in Fig. 3a, the Nt,tot values were multiplied by a factor 10, allowing to compare their trends.

Hence, we can clearly observe the distinct behaviors of the Gaussian and toroid populations with Ng,tot decreasing linearly and Nt,tot decreasing with a second order polynomial. Furthermore, in Fig. 3b, we observe that the thickness and radius of the toroid stay constant in contrast to the width of the Gaussian, which clearly increase down to 2 mm and then starts to decrease. These differences in trends between the Gaussian and toroid distributions suggest that these distributions correspond to two distinct Ar2∗ populations whose production mechanisms differ.

On the formation of the Ar2∗ Gaussian population

Fig. 4 Side picture of the kINPen-sci operated with a flow of 3 slm Ar. The cyan curves correspond to the sum of the light intensity integrated along the z-axis, with an integrating length Δz=0.6mm for each curve.

In the kINPen-sci, a mechanism which can be responsible for the formation of Ar2∗ in the argon effluent is the stochastic propagation of filaments as shown by Iseni et al.24,25 Indeed, argon excimer can be produced in the wake of these filaments where the electron energy is sufficiently high (> 11 eV) to generate the excited atomic Ar(4S) state and, in turn, produce Ar2∗ through three-body association collisions:12 Ar(4S)+Ar+M→Ar2∗+M

In Fig. 4, a side picture of the kINPen-sci operated with a flow of 3 slm Ar shows the average of the light emitted over a period of 18 ms. The cyan curves correspond to the sum of the light intensity integrated along the z-axis, with an integrating length Δz=0.6mm for each curve. These curves can be very well fitted by Gaussian distributions. Remarkably, the retrieved widths follow a similar trend as for the Ar2∗ Gaussian population with a maximum width at z ≈ 2 mm. This indicates a strong correlation between the filaments distribution and the Ar2∗ Gaussian population.

On the formation of the Ar2∗ toroid population

The Ar2∗ toroid population does not seem to be produced by the filaments as no ring shape is observed outside of the kINPen-sci nozzle. Moreover, if a filamentary process would be responsible for the formation of the toroidal population we would expect the thickness and/or the radius, see Fig. 3b, to vary with the distance from the nozzle as the filament propagation has been shown to have a stochastic nature24,25. On the other hand, the rather slow decay of the Ar2∗ toroidal population shown in Fig. 3a indicates that the measured Ar2∗ cannot be produced in the kINPen-sci nozzle and must be produced directly in the effluent. Indeed, halving of Ar2∗ density measured at 0.4 mm occurs approximately at 1.9 mm, therefore, with a lifetime of 2.9 μs, Ar2∗ would require an inconceivable speed of 500 m/s to explain the measured decay. Moreover, the species responsible for the production of Ar2∗, the Ar(4S) states, have significantly shorter lifetimes. Indeed, it has been shown by Klose et al.18 that the longest lived Ar(4S) state, the metastable Ar (3P2) state, has a lifetime below 200 ns in the effluent of the kINPen-sci. Hence, the production mechanism of the population occupying the toroid remains unclear.

To further investigate this phenomenon a set of spatial distribution measurements, at the same axial positions as in Fig. 2, were performed varying the experimental conditions under which the kINPen-sci was operated, see Fig. 5. In Fig. 5a, the kINPen-sci was operated using a gas curtain device allowing to envelop the 3 slm Ar central gas flow with a 5 slm gas mixture of 20% O2 and 80% N2. In Fig. 5b, the kINPen-sci was operated with an admixture of 50 ppm water to the 3 slm Ar central gas flow. Finally, in Fig. 5c, the kINPen-sci was operated with a 5 slm Ar central flow, only. First of all, we remark that the proposed Gaussian + toroid model is particularly robust as it is fitting properly all the measured Ar2∗ spatial distributions regardless of the experimental conditions under which the kINPen-sci was operated.

In Fig. 5a, the gas curtain device extended further down, cutting the cavity mode at z = 0 mm between x=-0.5 and +0.5 mm. As a result the model could not be fitted at this axial position. Nonetheless, comparing with the measurement shown in Fig. 2 we observed no significant difference with or without the curtain device down to 1.6 mm. This indicates no interaction occurs between the gas curtain and the central effluent for the first 1.6 mm. In contrast, from 2.6 mm we observe progressively lower Ar2∗ densities with the gas curtain than without. In fact, at 2.6 mm the densities are reduced by only 20% while at 4.6 mm the densities are reduced by around 70% with only slight hints of the toroid remaining. This could be explained by an increased transport of the gas curtain O2 and N2 molecules into the central gas effluent with larger distances from the nozzle, as Ar2∗ can be very efficiently quenched by N2 or O2, via for example:13 Ar2∗+O2→2Ar+2O

14 Ar2∗+N2→2Ar+N2(A)

Moreover, we should note here that further experiments were performed to investigate the effect of the mixture of the gas curtain on the Ar2∗ densities. Indeed, at 3.6 mm below the nozzle tip, where the Ar2∗ densities are approximately reduced by 50%, the densities were measured at x = -0.5 and +0.5 mm as a function of the O2 to N2 ratio of the gas curtain. We observed a reduction of around 75% of the Ar2∗ densities when a pure O2 curtain was employed, indicating better Ar2∗ quenching with O2 than with N2.Fig. 5 The Ar2∗ integrated profiles are shown at the distances, z, from 0.4 to 4.6 mm below the nozzle tip of the kINPen-sci. The kINPen-sci was operated under various experimental conditions: (a) using the gas curtain device operated using a gas flow of 5 slm and with a mixture of 20% O2 and 80% N2 surrounding the 3 slm Ar central gas flow, (b) with 50 ppm water admixed to the 3 slm Ar central gas flow and (c) with 5 slm Ar central gas flow. The toroid + Gaussian model fits are represented by the red curves. The green and blue curves correspond to the Gaussian and toroid distributions, respectively.

In Fig. 5b, a roughly 50% reduction of the Ar2∗ densities is observed at all axial positions compared to when the kINPen-sci was operated with a dry effluent. This strong impact on Ar2∗ densities with relatively small water quantities could be explained by electron and excited Argon species consumption and/or direct quenching of Ar2∗ by water as follow:15 H2O+e-→H2O∗+e-

16 H2O+Ar(4S)→H2O∗+Ar

17 H2O+Ar2∗→H2O∗+2Ar

In Fig. 5c, the Ar2∗ densities are roughly 30% higher between 0.4 and 2.6 mm, 40% higher at 3.6 mm and 60% higher at 4.6 mm compared to when the kINPen-sci was operated with 3 slm Ar. Moreover, for measurements at z = 0.4 mm between -0.5 and +0.5 the standard deviations on the Ar2∗ densities are substantially smaller for 5 slm, Fig. 5c, than for 3 slm Ar, Fig. 2. The only difference being the gas flow rate of the feeding gas, this suggests that despite the higher gas flow rate typically increasing turbulence, the flow and/or discharge is more stable at 5 slm. Indeed, the stability of the cavity mode is directly correlated to the index of refraction being more stable over time. Finally, we note that the toroid signature is clearly more visible with higher gas flow.Fig. 6 Parameters of the Gaussian and toroid retrieved from the fit of the Ar2∗ spatial distribution shown in Figs. 2 and 5. The parameters of the Gaussian distribution are on the left, with the total number of molecules in (a) and width in (c) and the parameters of the toroid distribution on the right, with the total number of molecules in (b) and thickness (empty symbol) and radius (full symbol) in (d).

In Fig. 6, the parameters retrieved from the fit of the Gaussian + toroid model to the the Ar2∗ spatial distributions measured with 3 slm Ar, 3 slm Ar + 50 ppm H2O, 3 slm Ar + 5 slm curtain 20% O2 + 80% N2 and with 5 slm Ar are shown as a function of the axial position, z. The Gaussian total number of molecules, Ng,tot, is shown in Fig. 6a, the toroid total number of molecules, Nt,tot, in Fig. 6b, the Gaussian width in Fig. 6c and the thickness and radius of the toroid in Fig. 6d. Equations (8) to (11) shown above described how the total number of molecules for the Gaussian and toroid were retrieved.

The total number of molecules of the Gaussian, and the thickness and radius of the toroid are fitted with straight lines, while the total number of molecules of the toroid and the Gaussian width are fitted with second order polynomials. Albeit, the parameters retrieved from the gas curtain measurement are not as well fitted by these functions than for the parameters retrieved from the other experimental conditions. The extra flow introduced by the gas curtain can cause more instability to the cavity mode explaining the observed larger uncertainty. Moreover, we can clearly observe that the gas curtain influence on the Ar2∗ profiles is not constant with the axial position. Indeed, as previously observed in Fig. 5a, the parameters are mostly unaffected by the curtain down to z=1.6 mm and are significantly lower thereafter.

It is interesting to note that the Gaussian total number of molecules with 3 and 5 slm Ar decrease with similar slope, − 8.2 × 107 and − 7.7 × 107 molecule mm-1 respectively, however with H2O admixture the slope is significantly smaller, − 4.5 × 107 molecule mm-1. In contrast, the second order polynomials fitting the toroid total number of molecules with and without water have similar coefficients, within 20%, however with 5 slm Ar the coefficients are a factor of 3 higher or lower. Hence, with water admixture the Gaussian Ar2∗ population trend with z can be altered without affecting the toroid behavior. Inversely, with a change in gas flow the trend of the toroid Ar2∗ population with z can be altered without affecting the Gaussian behavior. This clearly demonstrates that the Gaussian and toroid Ar2∗ populations are manifestation of distinct phenomena which can be influenced independently.

Although, the uncertainty on the Gaussian width hinders a clear statement, it seems that the widths at 5 slm are slightly larger than under the other experimental conditions. As for the radii of the toroids, they are constant across all conditions and with the nozzle distance. Moreover, their average value is 0.8 mm, the same as the inner radius of the kINPen-sci capillary. Furthermore, the thickness of the toroid is also staying mostly stable with the distance from the nozzle. Notably, the maximal slope retrieved was from the water admixture condition where the slope is 10% of the average value which is on par with the uncertainty of the values. Hence, determining averages and standard deviations of the thicknesses is considered reasonable. The average values obtained are 0.22 ± 0.03 mm for 3 slm Ar, 0.24 ± 0.08 mm with the gas curtain, 0.27 ± 0.06 mm with water admixture and 0.35 ± 0.05 mm for 5 slm Ar. Remarkably, this shows again that a change in gas flow has a clear impact on the toroid distribution. However, it remains unclear under which process the toroid Ar2∗ population is produced.

Discussion

Although, a density spatial distribution occupying a toroid can seem improbable, such a ring feature was already reported in numerous studies, mostly for CAPJs operated with helium27–31 but also with argon32. Subsequently, many experimental and theoretical studies were performed with the sole purpose to attempt to explain this phenomenon in helium33–40. We should note that in our case, there are significant differences in the plasma source geometry, the gas employed and the ring shape was not observed in the light emitted by the plasma source outside the nozzle. However, we can still assess whether the proposed interpretation could explain the ring shape in the case of the kINPen-sci.

On this matter, Naidis34,39 proposed radial non-uniformity of the gas mixture and gas densities which in turn cause variation in the radial direction of the rate of ionization of molecules by electron impact. In our work, it is clear that gas impurities on the edge of the argon gas flow are not the cause of the formation of the toroid, as we measure Ar2∗ and this species is very effectively quenched by any impurities. This is shown in Figs. 5b and 6, where an admixture of only 50 ppm of water caused a reduction of the densities by a factor 2. Moreover, as discussed earlier about the gas curtain measurement below 2.6 mm where the N2/O2 molecules seem to be able to transport toward the central gas flow, the retrieved toroid thickness, Fig. 6d, are within our error bar not significantly different from the measurements without curtain.

On the other hand, it very interesting to note that Chang et al.40, similarly to our finding, reported that the gas flow played a major role in the formation of the ring shape. However, the mechanism they propose to explain the toroid is probably not the same in our case. Their study focused on laminar flow regimes (Re = 164) and a “micro-hybrid interface zone” between the carrier gas channel and ambient air. They attribute this hollow structure formation primarily to guided laminar gas flow, operating their jets within a defined laminar regime. In contrast, our study involves a different flow situation. At 3 slm, Schmidt-Bleker et al.41 documented a turbulent flow regime in the kINPen-sci with a Reynolds number of 2980, which is even higher at 5 slm. Our setup includes an inner electrode that disrupts the flow upstream, creating additional instabilities. Therefore, the laminar-based ring-shaping mechanism by Chang et al.40 likely does not apply to our turbulent conditions.

Post-nozzle, the flow behaves as a jet, but standard jet theory is not fully applicable due to differences in fluid and flow profiles. However, within the first few millimetres after the nozzle, the lack of flow-induced mixing within the initial measurement regime is still relevant. Flow forces play a significant role in shaping the plasma ring, as show comparing the 3 slm and 5 slm fitted parameters in Fig. 6. This reveals variations in parameters that extend beyond what would be expected from the linear scaling of fully developed turbulent flow profiles. While the Gaussian profile in Fig. 6a might initially suggest such scaling solely attributed to flow dynamics, the remaining plots show more complex tendencies.

This deviation from expected flow behavior stems from the internal geometry of the kINPen, particularly the electrode’s influence on the flow. Unlike carefully designed nozzles in fluid dynamics studies, the kINPen’s electrode disrupts the flow, preventing the formation of a fully developed profile at the nozzle exit. This aligns with observations by Zhang et al.42, which emphasizes the potential for electrodes to alter initial flow conditions and trigger additional disturbances in plasma jets.Fig. 7 Preliminary CFD simulation of the flow within a geometry resembling the kINPen. (a) The contour plot illustrates the axial velocity magnitude (Uax), with black lines indicating particle pathlines to highlight secondary flow effects. (b) Radial profiles of Uax are extracted at z = 0 (electrode tip), 1D, and 2.8D downstream, showing the development of an annular (“M-shaped”) velocity profile after the electrode at a flow rate of 3 slm. Here, D = 1.6 mm is the characteristic length scale of the kINPen. This profile slowly decays but does not fully transition into a fully developed flow profile within the simulated domain.

In Fig. 7, preliminary results revealing annular (“M-shaped”) velocity profiles at the nozzle exit, a consequence of the kINPen’s internal geometry, particularly the electrode. This leads to complex flow regimes within the capillary, specifically influencing gradients and shear forces internally, which later in turn affect how the plasma mixes with ambient air post-nozzle. Understanding these complex profiles is crucial for optimizing plasma jet performance and tailoring its properties for various applications. Finally, as the current study was only performed at 3 and 5 slm, we cannot generalise the observed Ar2∗ population to other flow conditions and further research is needed to fully elucidate the impact of these internal flow conditions on the external characteristics of the effluent of the kINPen.

Methods

A schematic and a detailed description of the CRDS experimental set-up can be found in our previous work20. Briefly, a Nd:Yag pumped dye laser (Sirah Lasertechnik, Cobra Stretch) is coupled into an optical cavity consisting of two high reflective mirrors, with R > 99.99 % (Layetertec, 139207). A photomultiplier (Hammamatsu, H10721-110) is used to collect the light leaking out of the cavity, the signal is digitized by an oscilloscope (Teledyne Lecroy, HD06104) and analysed on a computer to retrieve a ring-down time, τ, for each laser pulse.

In this work the kINPen version employed, the kINPen-sci was specifically developed to gain better access and control over its physical parameters43. Notably, a high-voltage probe (Tektronix, P6015A) was used to continuously monitor the applied voltage and controlling that all Ar2∗ measurements were performed at around 2.5 kVpp. The kINPen-sci is mounted vertically in the middle of the cavity (y-direction) and positioned relatively to the cavity mode with xz-translation stages (Thorlabs, NRT100/M). The kINPen-sci consists of a stainless-steel needle electrode centered in a quartz dielectric capillary with an inner diameter of 1.6 mm, itself ringed by a grounded electrode. The central electrode is sharpened to a tip with a maximum diameter of 1 mm and is powered with around 1 W at a frequency of 860 kHz. The kINPen-sci exhibits a plasma plume of approximately 1 cm in length. Detailed information about the kINPen-sci can be found in Reuter et. al.6.

The kINPen-sci was operated under various operating conditions: varying the Ar gas flow, adding water to the gas flow and ensheathing the plasma jet with a O2/N2 gas curtain. The water content was regulated by passing part of the Ar flow through a bubbler containing distilled water and measured with a hygrometer(Edgetech, DewMaster). The leftover humidity in the dry argon gas line was measured to be approximately 20 ppm. To shield the central gas flow, a device, first introduced by Reuter et al.8,9, made of polypropylene is mounted on the kINPen-sci. The shielding gas is supplied from the top of the gas curtain device and operated with a total flow rate of 5 slm. The central gas flow and gas curtain mixture were regulated by a set of mass flow controllers (MKS, GE50A013104SBV020, 10000 sccm and 1179BX21CM1BK, 20 sccm) which were operated using a 946 Vacuum system controller (MKS).

Finally, pictures of the kINPen-sci were taken with a camera (IDS, U3-3080CP) and are the average of 600 raw images each taken with an exposure time of approximately 30 μs.

Acknowledgements

The authors would like to thank M. Stankov, R. Brandenburg and T. Gerling for fruitful discussion and F. Weichbrodt for its technical expertise.

Author contributions

A.N.: conceived and conducted the CRDS measurements, performed the formal data analyze and wrote and edited the manuscript, P.M. produced the pictures of the kINPen-sci and the preliminary CFD flow simulation, conducted the flow dynamic discussion and reviewed the manuscript and J.H. conceptualized the experiment, secured the funding and reviewed the manuscript.

Data availability

The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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