Electric field and higher harmonics of RF plasma slit jet measured by antennas and VI probes

Plasma Sources Science and Technology Plasma Sources Sci. Technol. 33 (2024) 055017 (16pp) https://doi.org/10.1088/1361-6595/ad48b5 Electric field and higher harmonics of RF plasma slit jet measured by antennas and VI probes K Polᡠsková1,2, P Drexler3, M Klíma3, J Machácˇ4, D Necˇas1, M ˇ Svanda4 and L Zajícˇková1,2,∗ 1Plasma Technologies, Central European Institute of Technology—CEITEC, Brno University of Technology, Purkyˇ nova 123, CZ-61200 Brno, Czech Republic 2Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotlᡠrská 2, CZ-61137 Brno, Czech Republic 3Department of Theoretical and Experimental Electrical Engineering, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 12, CZ-61600 Brno, Czech Republic 4Department of Electromagnetic Field, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, CZ-16000 Prague 6, Czech Republic E-mail: [email protected] Received 14 November 2023, revised 4 April 2024 Accepted for publication 8 May 2024 Published 29 May 2024 Abstract The cold atmospheric plasma jets change their character when interacting with the different surfaces. Since such interaction is the primary area of plasma jet applications, it is essential to monitor the process. The non-linearity of the RF plasma slit jet (PSJ) was analyzed using the VI probes and a novel method, the non-intrusive antenna measurements. Regardless of the experimental setup and gas mixture (Ar, Ar/O2, Ar/N2), the PSJ frequency spectrum consisted of the following main features: dominant fundamental frequency peak, relatively strong odd harmonics, and significantly weaker even harmonics. The lowest degree of non-linearity was recorded for the Ar PSJ ignited against a grounded target. Admixing a molecular gas increased the discharge non-linearity. It was attributed to the enhancement of secondary electron emission from the dielectric surfaces. In addition to the non-linearity analysis, the antenna spectra were for the first time used to determine the semi-quantitative values of the PSJ-radiated electric field. The electric fields decreased by a factor of 2 after the admixing of nitrogen and oxygen molecular gases. Out of the studied targets, the highest electric fields were observed when plasma impinged on the grounded targets, followed by the floating target (2×lower) and the PSJ ignited in the open space configuration (4×lower than in the grounded target configuration). Supplementary material for this article is available online Keywords: radio frequency plasma jet, atmospheric pressure, electrical parameters, electromagnetic field, higher harmonics ∗Author to whom any correspondence should be addressed. Original Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1 © 2024 The Author(s). Published by IOP Publishing Ltd Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al 1. Introduction The atmospheric pressure plasma discharges have been successfully utilized in many different applications, such as the treatment of polymer surfaces for improved adhesion [1–5], cleaning of metal surfaces [6], plasma activation of water [7, 8], plasma gas conversion [9,10] or wound healing [11–13]. This incomprehensive list demonstrates the uses of plasma in varying configurations—open space and ignited against dielectric, metal, or liquid targets. Naturally, the question of if and how the choice of the target affects the fundamental plasma properties (electric field, charge density) has arisen. In recent years, several groups have studied the interaction of cold atmospheric pressure plasma jets (APPJs) with different targets, either through modeling [14–16] or experimentally, by monitoring the flow patterns [17,18] and measuring plasma properties such as electric field [19–21] and densities of electrons and different plasma-generated species (He∗, OH, O, ...) [22–24]. However, these studies were usually limited to single-filament He APPJs driven by a kHz power supply, with the information on the Ar or the RF-driven discharges being almost non-existent. The characteristics of the He APPJ can significantly differ with the electrical properties of the target (conductivity, dielectric permittivity) or with the proximity of the electrical ground. Of the two options, the choice of the target material is the more impactful. Upon contact with a low-permittivity (ϵr< 10) dielectric, the initial bulk ionization wave (IW) propagating from the powered electrode towards the target transitions into an outwards spreading surface IW (SIW). The speed and area of the SIW spreading depend on the dielectric constant, with the higher speeds and larger areas observed at the targets with the lower permittivity [14,15,25]. On the contrary, the SIW does not develop when plasma interacts with a metallic target. Instead, a return stroke (reflected IW) caused by the impedance mismatch between the electrical system with the HV electrode at its end, the conductive channel, and the target is induced [14,21,23,26]. The third possible target material choice, a high-permittivity (ϵr>10) dielectric, results in the hybrid behavior as both the SIW and the return stroke occur [14,23]. The choice of the target material also affects the plasma properties. The speed of the IW propagation, the magnitude of charge density, and the electric field all increased with the increasing relative permittivity of the target [14,16]. The proximity of the electrical ground, i. e. the grounding of targets, does not significantly influence the development of the SIW or the return strokes. However, it affects the plasma parameters with the higher velocity and the intensity of IW corresponding to the grounded target. Overall, the influence of the ground on the plasma parameter is stronger than the influence of the target material, as the IW has higher velocity and intensity when the APPJ impinges on the grounded low-permittivity dielectric target than on the floating metallic target [25]. The electric field is one of the most crucial parameters of plasma discharges because its magnitude determines the production and energy of the charged particles, hence the behavior and gas chemistry of the discharge [27]. Despite the importance, measurements of the electric field of the APPJs are infrequent due to the experimental difficulties. The electric field induced second harmonic generation [28,29] and coherent anti-Stokes Raman scattering (CARS) measurements [30,31] both require a ps laser. Furthermore, the CARS method is limited to the molecular species (mainly N2). The methods based on optical emission spectroscopy, although less equipment demanding, are restricted to the bright part of the discharge and a specific plasma gas chemistry. The Stark polarization spectroscopy method is limited to the He-containing discharges [27,32], whereas the determination of electric field from the intensity ratio of N2(C–B) and N+ 2(B–X) emissions requires a valid collision-radiative model in addition to the excitation of both molecular bands [33–35]. The measurement by electrooptic sensors based on the Pockels effect is not species-specific or overly experimentally demanding [20,36,37]. Although nonintrusive and nonperturbative measurements are possible [38,39], electro-optic sensors are standardly used in contact with the plasma, which can, as discussed above, affect the plasma parameters. In this work, we investigated the electromagnetic characteristics of the atmospheric pressure RF plasma slit jet (PSJ) ignited in argon flow to which oxygen and nitrogen gas or water aerosol were admixed. Compared to the other APPJs, the RF PSJ has a unique power coupling principle, a combination of capacitive and inductive coupling. Thus, we first used the current and voltage probe measurements to improve our understanding of the PSJ electrical character and how it is affected by the gas feed composition. The second part of the paper is dedicated to the novel plasma diagnostics method, measurement of the radiated electromagnetic field by the antennas, which was used to study the interaction of RF PSJ with different targets. The analysis of obtained frequency spectra showed that the effect of a target on the PSJ non-linearity differs with the used gas feed (Ar, Ar/O2, and Ar/N2). The electric antenna measurements can also be used to determine the semi-quantitative values of macroscopic PSJ electric field. We demonstrate the feasibility of using the antenna measurements as a possible alternative to other experimentally more challenging methods of electric field determination. 2. Experimental details 2.1. RF PSJ and choice of substrates The RF PSJ construction (figure 1(a)) is unique in comparison with most of the cold APPJs [40]. In the PSJ, the plasma is ignited inside a mica composite slit (151 ×3 mm) placed in a specially designed coil serving as the periodic deceleration structure. This co-joined component is situated inside the tunable cavity composed of the fixed metal cover and movable conductive plates used as the resonance matching circuit. Therefore, adjusting the matching plate positions changed the discharge impedance, allowing us to match it to the RF generator 50 Ωoutput impedance without requiring a separate 2 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 1. (a) Schematic drawing of the RF plasma slit jet (side view) and (b) of the experimental setups used for the electromagnetic characterisation of the discharge: 1 – combined RF voltage/current sensor or separate voltage and current probes , 2 – coaxial cable, 3 – RF plasma slit jet, 4 – current probe, 5 – dipole antenna positioned above a substrate, 6 – substrate, 7 – biconical antenna, and 8 – dipole or magnetic antennas positioned under a substrate. matching unit. The PSJ is designed to work at the frequency of 13.56 MHz in the width of 150 mm. The applied power of 500 or 600 W was delivered by the CESAR 136 generator (Advanced Energy). High voltage, required to sustain the discharge, is induced on the last three turns of the coil serving as a system of high-voltage (HV) high-frequency electrodes. The bottom part of the PSJ metal cover/shielding (see figure 1(a)) serves as the grounded electrodes. The gases (and water aerosol) are injected into the PSJ through a tube with evenly distributed holes facing the topmost metal cover. This configuration results in the formation of random turbulences that ensure the intermixing of discrete flows leaving the outlets. As the gas flows downwards through the slit, it homogenizes, leading to the uniform laminar flow at the slit exit. Four working gas feeds were studied: Ar, Ar/O2, Ar/N2, and Ar/H2O aerosol. The Ar flow rate was set to either 67 or 100 slm, the O2flow rate was kept constant at 1 slm, and N2flow rate was varied between 0.5 and 3.5 slm. Water aerosol was produced from demineralized water (50 ml) by an ultrasonic nebulizer working at the 1.7 MHz frequency. It was gathered by a separate Ar flow (flow rates 1–3 slm) and intermixed with the main Ar gas stream before entering the PSJ body. The measurements were carried out with the jet freely expanding to open air or impinging on five different targets placed 10 mm away from the slit exit. Two targets, the tables (0.22 m in height) with differently designed tops, were on a floating potential: •The target denoted as the dielectric table had the top made of a 6 mm thick mica composite plate (ϵr=6). •In the table with glass window, plasma impinged on a 1 mm thick sheet of quartz glass (ϵr=3.8) covering a slightly larger 3 mm thick mica composite plate with a rectangular hole (95 ×35 mm) cut in its center. The PSJ was aligned with the longer principal axis of the rectangular cut-out. Three remaining targets consisted of a mica composite plate in contact with plasma filaments and a ground beneath it: •In the table with a mesh, a grounded metallic mesh was held in between two 3 mm thick mica composite plates. •In the conveyor belt, the plasma impinged on a 3 mm thick mica composite plate positioned above a 2 mm thick aluminum plate inserted into the grounded conveyor belt cover. •The cryo-table was composed of a 4 mm thick mica composite plate and a grounded aluminum table placed inside two nested vessels made of expanded polypropylene (PP). The top of the metallic table was made of a perforated Al sheet. The smaller PP vessel could be filled with liquid nitrogen, allowing us to monitor the effect of low temperature. 2.2. Electrical characterization of PSJ The VI characteristics of the PSJ were measured by two sets of probes—a combined RF voltage/current sensor Octiv VI Probe (Impedans Ltd) and separate voltage and current probes. The Octiv VI Probe was placed directly after the RF generator (figure 1(b)). The probe recorded root mean square (RMS) values of current and voltage, their phase 3 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 2. The determination the power dissipated in the Ar PSJ discharge as the difference of power delivered with and without plasma at the same RMS current. shift, and the delivered power. The impedance was also monitored, though, it was ∼50Ωin all the measurements with the ignited plasma as the discharge was matched to this value. Due to the insufficient time resolution (0.1 s), the Octiv VI sensor could not follow the VI waveforms (the wave period of 13.56 MHz signal is 73.7 ns). Therefore, we measured the voltage and current waveforms with high-voltage (Cal Test CT4028) and current (Pearson 6585) probes connected to the oscilloscope (Agilent DSO9054A). The current was recorded in two parts of the circuit—directly after the RF generator and at the grounded electrode, where the measured signal corresponded to the discharge current. The voltage was measured only at the RF generator output because the highvoltage probe was not connectible to the high-voltage RF electrodes inside the PSJ body where the discharge voltage is induced. The power dissipated directly in the plasma is standardly determined from the VI characteristics measured after the matching box. In our experimental setup, the matching unit is not a separate box but an integral part of the PSJ body (figure 1). Therefore, we would need to connect the highvoltage probe to the high-voltage RF electrodes inside the PSJ body, which, as explained above, was not viable. Instead, we used an indirect power measurement method proposed by Hofmann et al [41] using the VI data measured directly after the generator before the matching. Presuming the power dissipated in the matching unit with and without the discharge was the same for the same RMS current, the power dissipated in the plasma is equal to the difference of power delivered by the RF generator with and without the ignited plasma (figure 2). 2.3. Antenna measurements The electromagnetic field generated by the PSJ was measured using electric and magnetic antennas placed in several different positions (figure 1(b)) relative to the PSJ. The electric field was assessed by two differently sized dipole antennas. The smaller custom-made dipole antenna (further in the text denoted simply as the dipole antenna) had a diameter of 10 mm and was used in two different positions, below and above the target. In the below-target position, the dipole antenna was situated directly beneath the plasma at a 1 mm distance between the topmost antenna part and the bottom target surface. In the above-target position, the dipole antenna was placed on the longer PSJ side, most often 20 mm away from the PSJ and 10 mm above the target (i. e. the center of the antenna was at the level of the PSJ slit outlet). The exception was the setup with the cryo-table, where the dipole antenna was positioned 45 mm above the substrate and 100 mm away from the slit exit to prevent its malfunction. The larger dipole antenna was the commercial biconical antenna BicoLOG 20300 (biconical antenna), 350 mm in a diameter. Thanks to its much larger diameter, it could be placed further away from the PSJ without losing the sensitivity even to weak overtone signals. In the utilized experimental configuration, the biconical antenna was placed 460 mm away from the shorter side of PSJ with the antenna center (maximum gain) leveled with the slit exit. The magnetic field (results can be found in S2 section of supplementary information) was probed by Rigol antennas NFP-3-P1 (further referred to as the ring magnetic antenna) and NFP-3-P4 (small magnetic antenna). Both antennas were utilized in the same below-target configuration as the dipole antenna. The ring magnetic antenna was used oriented perpendicular to the plasma. The PSJ electromagnetic radiation (detected by the antennas) was recorded as power frequency spectra using the frequency analyzer ZVL-6 (Rohde & Schwarz) set to the maximum peak regime. The presented data were measured over frequency range 10–700 MHz using 50 accumulations (figure 3). The total acquisition time was 35.5 s. The y-axis (intensity) of recorded power frequency spectra was calibrated using frequency-dependent antenna factors. The commercial antennas (biconical, ring magnetic, small magnetic) antenna factors were provided by the manufacturers. The antenna factors of the custom-built dipole antenna were measured in the transversal electromagnetic mode cell using the standard field method without a reference electric field probe. The dipole was placed between the plates of the parallel plate waveguide with a defined impedance. The antenna factor of the dipole antenna was obtained as the ratio of the theoretically calculated electric field induced between the waveguide plates (for the known applied power) and the effective voltage measured at the dipole antenna connector. The biconical antenna measurements were calibrated using two different antenna factors depending on the presence of ground beneath the target. The original manufacturer-supplied 4 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 3. A segment of uncalibrated frequency spectrum (full range 10–700 MHz) measured by the biconical antenna for the Ar/N2PSJ (67 slm Ar/1.5 slm N2, 500 W) ignited against the conveyor belt covered with mica substrate. The distance between the slit outlet and the surface was 10 mm. antenna factors were used to calibrate the data obtained with the PSJ in open space configuration and ignited against the floating targets. In the remaining cases, the presence of the ground beneath a dielectric target distorted the relative intensity of the first and second harmonics compared to the current probe measurement. Since the biconical antenna is further away from the PSJ, the distortion is presumably not the result of a capacitive coupling between the discharge and the antenna. Therefore, the antenna factors were corrected by 15 dB for the first and second harmonics. The influence of antenna directivity was neglected, as all the dipole antennas had relatively high radiation pattern widths. Besides, the measurements were carried out in a room with metal walls where the real antenna directivity is likely considerably different from the theoretical values, and its determination would require complex simulations far out of this study’s scope. The laboratory room also acted as a resonance cavity. However, there was no overlap between the frequencies of higher harmonics generated by the discharge and the room resonance frequencies, and no harmonics data needed to be discarded. The intensity of each higher harmonic was determined from the calibrated frequency spectrum by integrating its peak area. The average standard deviation of antenna measurements was 3.5 dB, with the maximum errors reaching up to 10 dB. The error bars are not shown in any graphs to maintain clarity and ease of orientation. 2.4. Determination of electric field intensity The antenna factor AF used to calibrate the measured antenna signal is defined as the ratio of electric field intensity amplitude Eto the effective voltage induced at the antenna connector U by the said electric field AF [m−1]=E[Vm−1] U[V].(1) The influence of antenna directivity is again neglected. As a logarithm, the equation (1) takes the form of AF [dBm1]=E[dBVm−1]+U[dBV].(2) Thus, the calibrated antenna signal is directly proportional to the strength of the measured electric field, i. e. to the radiated electric field of the PSJ system (a mix of applied, plasma and surface-charge-induced electric fields). The spectrum analyzer recorded (and displayed) the signal induced by the electric field in the form of power level P[dBm] expressed in decibels with reference to one milliwatt. The effective voltage Ufound in the equations (1) and (2) was derived from the power level using a basic electrical power equation P[mW] = U2 RMS [V2] Z0[Ω] 1000 (3) where Z0=50Ωis the spectrum analyzer input connector impedance. Substituting the Z0value into the formula and using a logarithm, the power level expression becomes P[dBm] = U[dBV] + 13dB,(4) and the formula used to calculate the amplitude of electric field intensity measured by the antenna at its position can be rewritten using only the known quantities E[Vm−1]=10(AF [dBm−1]+P[dBm]−13dB)/20 .(5) All the antenna measurements were carried out in the reactive near-field region (outer boundary of the region at the 13.56 MHz is ∼3.5 m) where the phase shift is frequencydependent. Therefore, the values of the electric field derived from the harmonics (subharmonics and ultraharmonics) peak could not be summed up, and the total amplitude of radiated electric field intensity was approximated by a value derived from the most intense contributor, the fundamental frequency peak. An antenna measures the amplitude of electric field intensity Eain its position. In the reactive near field region, the decrease of electric field intensity with the distance from the source can be approximated by the inverse-cube law [42], the same as for the dipole field. Therefore, the antenna position (i. e. the distance between the antenna and the plasma) has to be factored in the calculation. The electric field recorded by 5 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al the antenna consists of three contributions: the applied electric field, the electric field induced by the space charges (plasma), and the electric field induced by the surface charges deposited on the dielectric target surface by the discharge. Of the three contributions, the applied electric field is decidedly the weakest (the antenna signal measured when the RF generator delivers the power, but the discharge is not yet switched on is one to two orders of magnitude lower than with the ignited discharge) and thus was neglected. The electric field induced by the surface charges is also excluded from the calculation because it differs based on the target setup (dielectric type, ground proximity), and its estimation would require advanced numerical simulations. Thus, only the plasma and plasma sheath contributions are considered further on. Since plasma is conductive and the potential gradient is concentrated in the thin plasma sheath, for an approximate estimate, plasma can also be disregarded, considering only the sheath of a constant height. Such approximation is possible because the electric field was measured after the discharge had stabilized (at least five minutes passed between the PSJ ignition and the spectrum recording), and the measurement duration (38 s) was much longer than the RF cycle (74 ns). The field within the sheath was modeled as a spatially extended dipole (similar to the field inside a capacitor), and so was the field outside. The recalculation ratio from electric field Eaat the antenna position to Epinside the sheath was then obtained as the ratio of electric fields inside the extended dipole and at the antenna position. The effective geometry of the dipole was obtained using previous results of filament fast camera video analysis [43]. Because of rapid horizontal filament movement and many repetitions in the antenna measurements, a simple rectangular cuboid geometry was used to represent the mean of all possible filament states. The cuboid length was the slit length (150 mm), its height was the sheath thickness (0.3 mm [44]), and its width was computed to match the estimated average area of the plasma sheath (1.2 mm for Ar). The electric field calculation was based on the biconical antenna data. The remaining valid setup, the dipole antenna in the below-target position, was too complex for the approximation as it does not account for the dampening of the electromagnetic radiation by the target. 3. Results 3.1. VI waveforms and power dissipated in PSJ The RF PSJ is a filamentary discharge (figure 4). The appearance, number, and movement of PSJ filaments differ depending on the working conditions, especially on the gas feed. The constricted filaments of the Ar PSJ were the most numerous and shuffled along the length of the slit (except for stationary filaments situated at both ends of the slit). Admixing of oxygen (Ar/O2PSJ) did not affect the constricted nature of the filaments, although their number decreased, and they became almost stationary. In the Ar/N2gas mixture, the filaments were composed of short constricted centers surrounded by longer diffuse plasma double plumes. Their number and the degree of movement were between the Ar and Ar/O2 PSJ. In addition to the motion, filament formation and annihilation events happened. Most crucially, the PSJ filaments formed self-organized patterns characterized by inter-filament distance. The behavior resembles the self-organizing patterns of a quasi-1D dielectric barrier discharge (DBD) [43,45]. Electrical characteristics provide valuable information about the discharge conditions. For example, the VI waveforms are often used to distinguish between the filamentary and homogeneous modes in DBDs, as the filamentary mode is easily recognizable by sharp current peaks, each corresponding to a newly ignited filament superimposed on the discharge current waveform [46,47]. Despite the similarities between the PSJ and a DBD, this was not the case for the PSJ, whose discharge current waveforms were smooth with no sharp peaks (figure 5), although the plasma was decidedly filamentary. The RF PSJ operated at three orders of magnitude higher frequency than a typical DBD. Therefore, it cannot be understood as a series of consecutive discharges with reversed electrode polarity. At the RF frequency, the breakdown occurred only once. The filaments were not extinguished during the individual half cycles because the fast polarity switching prevented the charge build-ups on dielectrics. Hence, the current waveforms do not have sharp peaks in the RF PSJ or the standard RF-driven filamentary DBDs [48–50]. Admixing oxygen or nitrogen gas into the Ar PSJ led to a decrease in discharge current amplitude and a higher distortion of the applied sinusoidal waveform (figure 5). In other words, the PSJ load behaved more non-linearly, and a higher number of more intense harmonics of the fundamental frequency 13.56 MHz was generated. The phase shift was probed on the coaxial cable after the RF generator before the PSJ. Prior to the breakdown, current lagged the voltage by circa −83◦regardless of the matching conductive plates position (figure 1), meaning the load of PSJ body (and the coaxial cable) was inductive. With the plasma on, the load of the whole system, i. e. plasma, PSJ body, and coaxial cable, became mostly resistive, with phase shifts in the range of −22◦to 18◦(table 1). Approaching the zero phase shift for the switched-on plasma was expected because the voltage and current waveforms should be in phase in the matched circuit. Increasing the applied power and admixing oxygen and nitrogen gases had the opposite effect on the phase shift. The phase shift had a more positive value (table 1) when the applied power was changed from 500 to 600 W. On the other hand, admixing of molecular gas into the Ar working gas flow moved the phase shift to more negative values, with nitrogen admixing having a more pronounced influence. Changes in plasma conductivity can explain the influence of both the applied power and gas feed. Increasing the applied power from 500 to 600 W increased the power dissipated in the plasma, resulting in a higher ionization rate and, hence, higher electron density, directly proportional to plasma conductivity. Molecular gas admixing had the opposite effect due to a loss of energy in dissociation and excitation of numerous rotational and vibrational levels and a loss of electrons due to oxygen electronegativity. 6 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 4. Front (left) and side (right) views of the RF plasma slit jet discharge ignited in three different gas mixtures at 500 W: (a) 67 slm Ar, (b) 67 slm Ar/1 slm O2, and (c) 67 slm Ar/1.5 slm N2. The distance between the slit outlet and the dielectric surface (mica composite, thickness of 6 mm) was kept at 10 mm. The exposure time of the images was 25 µs, corresponding to approximately 340 RF cycles. Reproduced from [43]. © IOP Publishing Ltd. All rights reserved. Figure 5. Applied voltage and grounded electrode current waveforms taken at the fixed applied power 500 W and flow rates 67 slm Ar, 67 slm Ar/1.5 slm N2and 67 slm Ar/1 slm O2. Table 1. Electrical parameters of the PSJ operated in Ar (67 slm Ar), Ar/O2(67 slm Ar/1 slm O2, and Ar/N2(67 slm Ar/1.5 slm N2) gas feeds at two different applied powers (500 and 600 W). The RMS current IRMS, voltage URMS, and their phase shift were measured on the coaxial cable after the RF generator before the PSJ (and the matching). Gas P(appl.) P(fwd.) IRMS URMS Phase Pdiss Power transf. feed (W) (W) (A) (V) shift (◦) (W) efficiency Ar 500 491 2.81 176 12.2 459 0.95 600 570 3.18 184 17.9 529 0.95 Ar/O2500 492 2.79 178 −9.0 463 0.94 600 598 3.13 189 5.1 553 0.94 Ar/N2500 480 3.18 164 −21.8 448 0.92 600 600 3.33 179 −1.9 554 0.93 The power dissipated in the PSJ discharge Pdiss was more or less the same for all the studied gas feeds, 450–460 W at 500 W, and 530–550 W at 600 W applied power (table 1). The corresponding power transfer efficiencies, calculated as the ratio of power dissipated in plasma and the applied power measured by the VI probe at the RF generator output, were 0.92–0.95, with the Ar PSJ having the highest efficiency and the Ar/N2PSJ the lowest. The measured Pdiss and thus the power transfer efficiency values are likely somewhat overestimated compared to reality because the used indirect measurement method expects the same heat dissipation (i. e. the same power losses induced by Joule heating) with and without plasma. This assumption was not fulfilled in the PSJ, as the PSJ body (mainly the coil) was heated up more when the discharge was on. Furthermore, the temperature of the PSJ body with the ignited plasma depended on the position of the aluminum plates used for the impedance matching. The heating was notably stronger when the upper part of the coil was situated between the matching plates (the pure Ar discharge). The influence of plasma gas chemistry (e. g. vibrational excitation transfer in the Ar/N2PSJ) on the PSJ body heating can be disregarded as the rotational temperature used to approximate the plasma gas temperature was the same in all the gas feeds (Trot ≈750 K). 7 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 6. Relative intensities of higher harmonics present in the antenna spectra taken for PSJ ignited against the conveyor belt using different gas feeds, flow rates, and power. 3.2. Antenna measurement of radiated electric field The electric antenna measurements provided information about the distortion of applied sinusoidal waveforms by the plasma and the amplitude of the PSJ system’s radiated electric field. Focusing on the analysis of the PSJ system non-linearity, the advantage of the antenna measurement over the standard VI probes lies in the non-intrusivity of the former method. An antenna monitors the distortion without being connected to the system, whereas VI probes must be included in the electrical circuit, which is not always possible. Additionally, the antennas usually have higher efficiency at the higher frequencies (starting at ∼100–150 MHz), enabling a precise intensity measurement of very weak higher harmonics (seventh and higher in our setup). At the fundamental frequency 13.56 MHz (wavelength λ=22.12 m), all the antenna measurements were carried out in the reactive near field region (outer boundary ∼3.5 m). In this region, adjacent conductive objects can absorb and re-emit the radiation, distorting the initial electromagnetic wave radiated by the PSJ. More crucially to our measurements, placing antennas too close to the plasma may affect its parameters, thus the emitted spectrum. Therefore, a procedure based on comparing the relative intensities of frequency spectra obtained by different antennas and from the discharge current recorded by the current probe was used to validate the measurements, see the S1 section of supplementary information. It uncovered capacitive coupling between the PSJ plasma and the dipole antenna situated above a target, whose data were therefore discarded. As for the validity of the obtained electric field values, the electric antennas measured the radiated electric field of the PSJ system, which consisted of applied, plasma, and surface-charge-induced contributions. Due to the limitation of the experimental setup and several approximations used in the calculation (section 2.4), the obtained values are only semiquantitative. However, despite the uncertainty in absolute values, the obtained trends are reliable as they are reproducible by all the antennas. 3.2.1. Generation of higher harmonics, subharmonics, and ultraharmonics. The gas feed induced non-linearity was studied in the configuration with the PSJ ignited against a 3 mm thick mica composite plate placed on the grounded conveyor belt, i. e. in the standard setup for plasma treatment experiments. In all the tested working conditions, the recorded frequency spectra had the same main features (figure 6) – an intense fundamental frequency and significantly weaker higher harmonics. Moreover, the odd harmonics were markedly stronger than the even ones. In the Ar/N2PSJ, this trend persisted over the whole frequency range, while in the Ar and Ar/O2PSJ, their intensities became comparable starting from the sixth harmonic. 8 Plasma Sources Sci. Technol. 33 (2024) 055017 K Polᡠsková et al Figure 7. The influence of N2flow rate on the relative intensities of higher harmonics present in the frequency spectrum of Ar/N2PSJ (67 slm Ar/0.5–3 slm N2, 500 W) impinging on different targets (table with mesh, dielectric table, and table with glass window). The ratio of spectral weights of higher harmonics and the fundamental frequency 13.56 MHz was used to describe a distortion of generator-supplied sinusoidal signal by the plasma. The results obtained by the antennas correspond well with the discharge current measurements, with the lowest ratio observed in Ar (0.005–0.03), followed by Ar/O2(0.05–0.10), and the Ar/N2(0.08–0.16) gas mixtures. In the Ar discharge, changing the power and Ar flow rate did not affect the relative intensities of harmonics. Similar to the Ar PSJ, the frequency spectra of Ar/O2PSJ were within the experimental errors the same for all the tested combinations of O2:Ar flow rate ratio and applied power. The non-linearity of the PSJ load induced by nitrogen admixing changed with the N2:Ar flow rate ratio. Overall, the N2flow rate did not affect the relative intensities of odd harmonics within the experimental errors (figure 7). The same does not apply to the even harmonics whose relative intensities increased with the higher N2flow rate. In addition to higher harmonics, subharmonics (partial multiples of the fundamental frequency) and ultraharmonics (integer multiples of subharmonics) were generated by the PSJ ignited in nonstandard working conditions, i. e. against an atypically rough mica composite target with and without admixing of water aerosol into Ar gas flow. The increase in target roughness led to the excitation of weak half-harmonic and its ultraharmonics. Contrary to the higher harmonics, changing the working parameters (gas feed, flow rates, power) did not affect the subharmonic and ultraharmonics relative intensities, except for the water aerosol admixing that increased their strength by 10–20 dB probably due to the droplets deposited on the surface increasing its roughness. The addition of the water aerosol also generated the fifth subharmonic and its multiples (1/5, 2/5, 3/5, 4/5, 6/5, ...) once a threshold concentration of aerosol in the plasma was crossed (2 slm of Ar carrier gas in our PSJ setup). We hypothesize that the fifth suband ultraharmonics originated from the oscillation asymmetric sheaths formed around the water droplets [51]. The hypothesis is based on the analogy with a generation of sub and ultraharmonics by the insonated microbubbles in the liquid ultrasound contrast agents that have been attributed to the oscillation of microbubbles diameters [52,53]. As for the effect of roughness, two possibilities exist. The generation of the half subharmonic and its multiples can be caused by the inhomogeneity of plasma density (plasma density is higher at the apex of roughness modulations). 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