Ionization waves in microgravity dusty plasma experiments. — The PK-4 system is a micro-gravity dusty plasma experiment, utilizing a long DC discharge in neon or argon gases, currently in operation on the board of the International Space Station. The local plasma parameters that serve as input data for dust dynamics models were calculated by our recently developed 2D particle-in-cell with Monte Carlo collisions discharge. The simulations further show that on the microsecond time scales the positive column is highly inhomogeneous: ionization waves with phase velocities in the range between 500 m/s and 1200 m/s dominate the structure, where the electric field and charged particle densities can reach amplitudes up to 10 times of their average value. In our ground-based PK-4 replica system the direction of the DC current can be alternated, which favours the dust particle chain formation.
Soliton propagation in a strongly coupled dusty plasma layer. — We have investigated by Molecular Dynamics simulations the propagation of solitons in a two-dimensional many-body system with Yukawa potential. The solitons were created in an equilibrated system by short electric field pulses. These pulses were found to generate pairs of solitons, with a positive and a negative density peak, respectively, which propagated into opposite directions. At small perturbations, these features were confirmed to propagate with the longitudinal sound speed, from which an increasing deviation was found at higher amplitude perturbations. The application of an external magnetic field was found to block the propagation of the solitons, which were, however, found to get released upon the termination of the magnetic field and could propagate further into directions that depend on the time of trapping and the magnetic field strength.
Control of heavy particle’s energy and flux in low-pressure capacitively coupled radio frequency (rf) discharges. — Particle-in-Cell/Monte Carlo Collisions simulations of capacitively coupled Ar discharges driven by multi-frequency tailored voltage waveforms were performed, in order to clarify the effects of surface processes on the discharge characteristics. The simulations were based on a discharge model in which realistic approaches were implemented for the description of the secondary electron emission induced by electrons and heavy-particles at the electrodes, as well as for the sputtering of the electrodes. The simulations showed that the mean energy of Ar+ ions and fast Ar atoms, as well as the flux of sputtered atoms can be controlled at both electrodes by changing the phases of the even harmonics. The domain over which the sputtered atom flux can be varied was found to be enlarged by adding more harmonics to the driving voltage waveform.
Electron power absorption in low-pressure capacitively coupled rf argon discharges — We conducted a spatio-temporally resolved analysis of the electron power absorption in capacitively coupled argon plasmas (CCPs) at low pressures (1–10 Pa), based on the 1D momentum balance equation embedded into 1d3v particle-in-cell/Monte Carlo collisions simulations. In contrast to the predictions of theoretical models, the 'Ohmic heating' is found to be the dominant electron power absorption mechanism on time average at the lowest pressures, and not the 'stochastic' or 'Pressure heating', as one would expect. This is attributed to the attenuation of the electron power absorption due to the electron acceleration by the 'ambipolar' electric field on time average at low pressure, which is a consequence of the collisionless transit of energetic beam electrons generated during sheath expansion at one electrode to the opposite electrode. These energetic electrons arrive during the local sheath collapse and thus can be lost on the surface. As a consequence, the plasma density decreases and the electron temperature becomes temporally more symmetric within the RF period compared to that in discharges operated at higher pressures, which causes a reduction of 'Pressure heating' on time average.
Electron power absorption mode transitions in neon capacitively coupled rf discharges — The spatio-temporal ionization and excitation dynamics in low-pressure radiofrequency (RF) discharges operated in neon were studied by phase resolved optical emission spectroscopy (PROES) and by particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations. At a fixed frequency and peak-to-peak voltage, the spatio-temporal distribution of the ionization rate obtained from PIC/MCC showed a transition of the discharge operation mode from the α-mode to the γ-mode with pressure. In the spatio-temporal distribution of the excitation rate obtained from the PROES and PIC/MCC the α-peak (the intensity maximum at the bulk side of the expanding sheath edge) was dominant, while a γ-peak (a maximum near the edge of the fully expanded sheath) became visible only at higher values of the pressure (500 Pa) or at the lowest frequency of 3.39 MHz. On the other hand, a γ-peak was visible in the ionization rate for all operation conditions, and it dominated the ionization in the vast majority of the cases investigated.
Figure 1. Spatio-temporal plots of the electron-impact excitation rate measured by PROES [a.u.] (left) and obtained from PIC/MCC [1019 m−3s−1] (center), and the ionization rate obtained from PIC/MCC [1020 m−3s−1] (right). The sheath edges are shown as white lines. The powered electrode is located at x/L = 0, while the grounded electrode is at x/L = 1. Discharge conditions: f = 6.78 MHz, L = 2.5 cm, Vpp = 330 V, p = 500 Pa. TRF = 1/f.
Results in 2019
Gas discharge physics. — Particle-in-Cell/Monte Carlo Collision (PIC/MCC) simulations have been performed to investigate the characteristics of geometrically symmetric capacitively coupled plasma sources in various gases and excited by different (single- and multi-harmonic) waveforms. In particular, we have investigated the effects of heavy-particle induced secondary electrons (SEs) on the ionization dynamics and on the control of ion properties (flux and energy distribution) at the electrodes , the dependence of the computed discharge characteristics on the assumptions on secondary electron emission coefficients of the electrodes and on the electrode distance , as well as the electron kinetics and electron power absorption as a function of the mixing ratio of an electropositive (Ar) and an electronegative (CF4) gas [4].
In our studies of the effect of heavy-particle induced secondary electrons we have used radiofrequency excitation waveforms composed of up to four harmonics of the fundamental frequency of 13.56 MHz and “tailored” the driving voltage waveform by adjusting the identical phase angles of the even harmonics, θ. The simulations were carried out at neutral gas pressures of 3 Pa (nearly collisionless low-pressure regime) and 100 Pa (collisional high-pressure regime). Different approaches were used in the simulations to describe the secondary electron emission (SEE) at the electrodes: we adopted (i) constant ion-induced secondary electron emission coefficients (SEECs), γ, and (ii) realistic, energy-dependent SE yields for ions and fast neutrals. The mean ion energy at the electrodes was found to be controlled by θ at both pressures, for both approaches adopted to describe the SEE in the simulations. At a low pressure of 3 Pa, we obtained largely different dependencies of the ion flux at the electrodes on θ, depending on the value of the γ-coefficient. We have found that (i) at both pressures the surface conditions affect the plasma parameters and the quality of the separate control of ion properties at the electrodes and that (ii) adopting realistic, energy-dependent SE yields for heavy particles in the simulations can lead to significantly different results compared to those obtained by assuming constant SEECs. In , more details about the effect of γ on the electron power absorption dynamics, the plasma parameters and the quality of the separate control of ion flux and mean ion energy at the electrodes have been addressed. To demonstrate the effect of the choice of γ on modelling results, we carried out Particle-in-Cell/Monte Carlo Collision simulations of 13.56 MHz, single-frequency argon and helium capacitive discharges. The experimental investigations of the electrode distance on the plasma characteristics were carried out in oxygen gas, at a fixed pressure of 2.66 Pa and a driving frequency of 13.56 MHz. We have found an increase of the central electron density with an increased electrode gap, while the time averaged optical emission of atomic oxygen lines decreased. These results were reproduced and understood by the PIC/MCC simulations performed under identical conditions. The simulations showed that the electron density increases due to a mode transition from the drift-ambipolar-mode (where the limited conductivity results in power absorption in the bulk part of the plasma) to the alpha-mode (where power absorption occurs near the edge of the expanding plasma sheath) induced by increasing the electrode gap. This mode transition was attributed to a drastic change of the electronegativity and the mean electron energy, which lead to the observed reduction of the emission intensity of an atomic oxygen line. More details about the electron power absorption mode transitions (the basic mechanism of energy coupling from an external power supply into the plasma) were uncovered using CF4+Ar gas mixture plasmas excited by tailored voltage waveforms in experimental investigations by phase-resolved optical emission spectroscopy, in conjunction with kinetic simulations and an analytical model. Single- and triple-frequency ‘peaks’- and ‘valleys’-type excitation waveforms (generated as a superposition of multiple consecutive harmonics of 13.56 MHz) were used at pressures of 20 and 60 Pa with 25 mm electrode gap and 150 V total driving voltage amplitude to determine the effects of the tailored driving voltage waveform in different gas mixtures. As the argon content in the buffer gas was increased the discharge switched from the drift-ambipolar power absorption mode to the alpha-mode (see Fig. 1). This transition was proven to occur due to the disappearance of the bulk and ambipolar electric fields as the electronegativity of the plasma decreased with increasing argon content.
Figure 1. Spatio-temporal distribution of the electron impact excitation rate for the 703.7 nm fluorine line (excitation threshold energy: 14.56 eV) obtained from PIC/MCC simulations for the ‘peaks’-waveform (3 harmonics, 150 V total voltage amplitude) at 20 Pa, in pure CF4 gas (a) and in 50% CF4 – 50% Ar mixture (b). The sheath edges are marked by the white lines, the powered electrode is located at x=0 mm, while the grounded electrode is at x=25 mm. The panels in the second row illustrate the driving voltage waveform. Note the high excitation rate over the whole inner part of the plasma in (a) (characteristic for the drift-ambipolar mode) and the sharp maxima upon sheath expansion in (b) (characteristic for the alpha-mode) .
Complex plasmas. — The self-diffusion phenomenon in a two-dimensional dusty plasma at extremely strong (effective) magnetic fields was studied experimentally and by means of molecular dynamics simulations. In the experiment the high magnetic field was introduced by rotating the particle cloud and observing the particle trajectories in a corotating frame, which allows reaching effective magnetic fields up to 3000 T. The experimental results confirm the predictions of the simulations: (i) super-diffusive behavior is found at intermediate timescales and (ii) the dependence of the self-diffusion coefficient on the magnetic field is well reproduced .
In the many-body system of charged particles interacting via a pairwise Yukawa potential, the so-called Yukawa one-component plasma (YOCP), in the long-wavelength limit, the linear term in the dispersion of collective excitations defines the sound speed. The evolution of this from the weak-, through the strong-coupling regimes was investigated by analyzing the dynamic structure function S(k, ω) in the low-frequency domain. Depending on the values of Coulomb coupling and Yukawa screening five domains were identified in which the physical behavior of the YOCP exhibits different features. The competing physical processes are the collective Coulomb like versus binary-collision-dominated behavior and the individual particle motion versus quasilocalization. The theoretical results based on various models were compared in order to see which one provides the most cogent physical description and the best agreement with simulation data in the different domains .
A web-based platform was developed that allows users to perform molecular dynamics simulations, visualize the system for selected system parameters, and obtain results for the pair correlation function and the dispersion relation of waves in the system .
Reactive plasmas. – In the last decade plasma-activated water (PAW), or more generally plasma-activated liquid (PAL) has received a lot of attention from the plasma medicine and plasma agriculture community due to its potential to induce oxidative stress to cells. By PAL it is meant the liquid which contains reactive species, mostly reactive oxygen and nitrogen species (RONS), generated by the interaction of active or afterglow plasma with the liquid. PALs have been found to have antimicrobial and antibacterial effect, to have potentials for cancer therapy, and to improve the seeds germination and plant growth. The main long-lived RONS produced in PAL have been identified to be the H2O2, NO2− and NO3−. In order to be able to identify the role of different species and to clarify the synergy effects in the interaction of PAL with biological systems, PAL with different compositions would be welcomed, in what concerns the density ratios of different RONS. We have shown that a surface-wave microwave discharge is suitable to tune the ratio of active species concentrations over three orders of magnitude in deionized water (DIW), which can be preserved during months of storage at room temperature . In order to control the ageing of PAWs, we have suggested the control of the H2O2 concentration by calling for a Fenton type of reaction with using copper surfaces.
Results in 2018
Gas discharge physics. — We developed a particle-based simulation code for the description of short-pulsed (~ns) discharges at atmospheric-pressure helium gas with an admixture of molecular nitrogen (at concentrations ≤1%). In this code, we have also included the photon treatment of the VUV resonance radiation of helium. We have explored the spatiotemporal evolution of charged species densities, reaction rates, and the fluxes of ”active” species at the surfaces. We have investigated, as well, the behaviour of the electron velocity distribution function and the electron energy probability function, and concluded that these deviate significantly from the Maxwell-Boltzmann distribution, especially in the cathode region of the discharge. These observations demonstrated the usefulness (and uniqueness) of particle simulations of similar physical systems. We have found that the VUV resonance radiation of He-atoms is heavily trapped within the high-pressure gaseous environment and photons are absorbed/re-emitted typically several hundred times before leaving the plasma. Nonetheless, the escaping photons were found to contribute significantly to the electron emission process at the electrodes. For most conditions studied, an increase of around a factor of two of the current pulse peak was observed when VUV photons were included in the simulations, in comparison to those cases when their effect was neglected. Fig. 1 shows results for a plane-parallel electrode configuration with a gap of 1 mm, to which a high voltage pulse with an amplitude of 1000 V is applied with a trapezoidal shape. Current pulses in the order of 10 A are generated over the electrode surface of 1 cm2, when VUV photons are not considered in the simulation (at an initial charged particle density of 1.5×1011 cm−3). The peak current grows to ~20 A, when VUV photons are included. The figure also shows the time integrated wavelength-resolved radiation from the plasma, in the vicinity of the theoretical wavelength of the 21P → 11S resonant transition.
We have performed systematic investigation of the influence of various surface processes - such as the secondary electron (SE) emission induced by ions and electrons, and electron reflection at the electrodes - on the discharge characteristics in low-pressure, capacitively coupled radiofrequency discharges. By using a realistic (energy-dependent) model for the description of the electron-surface interaction in our Particle-in-Cell/Monte-Carlo Collisions (PIC/MCC) simulation code, we have studied the effect of the electron-induced SEs on the discharge characteristics in the 0.5 Pa–3 Pa pressure range, for voltage amplitudes between 100 V–1500 V, assuming different SE yields for ions (γ–coefficient). Such discharge conditions are typical in industrial applications, such as plasma etching, sputtering and plasma-immersion ion implantation. We demonstrated that the realistic description of the electron-surface interaction significantly alters the computed plasma parameters, compared to results obtained based on a simple model (which completely neglects the emission of SEs due to electron impact) for the description of the electron-surface interaction, widely used in PIC/MCC simulations of low-pressure capacitively coupled plasmas.
Figure 1. Physical setting, simulated species, parameter values, and results of the numerical studies of nanosecond discharges (applied voltage and computed discharge current waveforms, as well as the lineshape of the He resonance radiation).
We have found that both the gas pressure and the value of the γ-coefficient affect the role of electron induced SEs (δ-electrons) in shaping the discharge characteristics at different voltage amplitudes. Their effect on the ionization dynamics has been found to be most striking at low pressures, high voltage amplitudes and high values of the γ-coefficient (see Fig. 2). At low pressures, the energetic ion-induced SEs (γ -electrons) generated at one electrode and accelerated towards the bulk by the local sheath electric field, can cross the bulk without collisions and generate a high number of electron-induced SEs (δ-electrons) upon their impact at the opposite electrode. Depending on the instantaneous local sheath voltage, these δ-electrons are accelerated into the plasma bulk, where they generate significant ionization and enhancement of the plasma density. On the other hand, at high pressures due to the more frequent collisions, the electrons reach the electrodes at lower energies, which leads to a decrease of the number of SEs emitted by electron impact at the electrodes, therefore, to a decrease of the contribution of δ-electrons to the ionization.
Figure 2. Contribution of electron-induced secondary electrons (δ-electrons) to the total ionization at different pressures and voltage amplitudes for different values of the ion-induced secondary electron emission coefficent, γ.
Dusty plasmas. — In the field of dusty plasma physics, we have realized a rotating electrode experiment to measure the diffusion of highly charged dust particles in large external quasi-magnetic field. We have shown that with increasing magnetization, the transport becomes hindered and the non-Maxwellian nature becomes more prominent as strong super-diffusion is observed. Our experimental results are supported by molecular dynamics simulations of magnetized Yukawa systems. In relation to the PK-4 dusty plasma experiment onboard the International Space Station (ISS), we have implemented a cylindrical 2D particle-in-cell with Monte Carlo collisions (PIC/MCC) numerical simulation for direct-current neon discharges. Our results indicate that the apparently quiet positive column as observed in the experiment with long exposure time imaging is in fact made of quickly moving ionization waves, as shown in Fig. 3. Our numerical prediction has been confirmed by fast camera imaging experiments. The presence of ionization waves explains the unexpected observation of dominant dust particle chain formation in the micro-gravity setup.
Figure 3. Electron impact excitation source from the PK-4 neon discharge simulation at long and short exposure times.
Reactive plasmas. — In the last decade, plasma-activated water (PAW), or, more generally, plasma-activated liquid (PAL) has received a lot of attention from the plasma medicine and plasma agriculture community due to its potential to induce oxidative stress to cells. By PAL it is meant the liquid which contains reactive species, mostly reactive oxygen and nitrogen species (RONS) generated by the interaction of active or afterglow plasma with the liquid. PAW has been found to have antimicrobial and antibacterial effect, which is thought to occur due to the synergetic effect between the RONS and/or pH of the solution. Plasma activated buffered solution and cell culture media have also been studied for therapeutical aims, and its potentials for cancer therapy have also been shown. In the field of agriculture with PAW, the improvement of seeds germination and plant growth have been targeted. The main long-lived RONS produced in PAL have been identified to be H2O2, NO2− and NO3−. The usual studies report typically one or two PAL conditions, giving no further suggestions for PAL composition tuning. In order to be able to identify the role of different species and to clarify the synergy effects in the interaction of PAL with biological systems, PAL with different compositions would be welcomed, in what concerns the density ratios of different RONS.
With our study we have shown the possibility of tuning the ratio of active species concentrations over three orders of magnitude in deionized water (DIW) activated with a surface-wave microwave discharge (see Fig. 4) by varying the gas flow rate, initial gas mixture composition and treatment distance. The surface-wave microwave discharge is generated with the help of a surfatron launcher in a quartz tube of outer diameter 6 mm and inner diameter 4 mm using as a main gas Ar at gas flow rates of 1500 and 2000 sccm (Ar_x conditions in Figure 4). Ar-N2/O2 binary (ArN2_x and ArO2_x conditions) and ternary (ArN2O2_x conditions) mixtures are also used with the O2 and N2 gas flow rates ranging between 10-100 sccm. The input power is varied between 25 and 30 W, and the DIW is positioned below the plasma plume with the water surface being at d = 5.5 - 10.5 mm distance from the edge of the quartz tube, see Fig. 5. By changing the mixture composition and the treatment distance, at the plume - water surface interaction point the electron density is changed, which determines the concentration of H2O2 created in the liquid phase.
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Figure 4. The ageing of the nitrate to hydrogen peroxide concentration ratio in the case of PAWs produced by Ar/N2/O2 surface-wave microwave discharges at different discharge and treatment conditions.
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Figure 5. A surface-wave microwave discharge in interaction with water surface.
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Results in 2017
Gas discharge physics. – We have addressed several aspects of charged particle kinetics and transport in low-temperature plasmas. Particular attention has been devoted to the non-linear effects, like the spontaneous pattern formation in electronegative discharge plasmas driven by radiofrequency (RF) voltage sources. In strongly electronegative gases, like carbon-tetraflouride, a positive ion – negative ion plasma is formed with a strongly depleted electron density, due to the (dissociative) electron attachment to molecules, which this way form negative ions. We have shown that a resonance between the excitation (radio) frequency and the eigenfrequency of the ion-ion plasma leads to an instability, which, in turn, creates prominent non-linear structures. The dependence of this pattern formation mechanism on the discharge conditions, such as the driving voltage amplitude and frequency, as well as the electrode separation and gas pressure, has been studied in detail.
By using our Particle-in-Cell/Monte-Carlo Collisions (PIC/MCC) simulation code, we have investigated the electron heating and ionization dynamics in capacitively coupled oxygen discharges driven by tailored voltage waveforms at different fundamental frequencies and at different pressures. We have found transitions of the discharge electron heating mode from the drift-ambipolar mode to the α-mode, induced by changing the number of consecutive harmonics included in the driving voltage waveform or by changing the gas pressure. We have found that changing the number of harmonics in the waveform has a strong effect on the electronegativity of the discharge, on the generation of the DC self-bias and on the control of ion properties at the electrodes. Furthermore, we have investigated the effect of the surface quenching rate of O2(a) metastable molecules on the spatio-temporal excitation patterns. We have obtained good agreement between the spatio-temporal distributions of the excitation rates obtained from the simulations and those derived from phase-resolved optical emission spectroscopy measurements. This benchmarking study was complemented with a sensitivity analysis of the results on the rates of selected plasma-chemical reaction processes.
We have developed a realistic model for the description of the electron-surface interaction in capacitively coupled plasmas and incorporated this model into our PIC/MCC simulation code. This realistic model considers the elastic reflection and the inelastic backscattering of electrons, as well as the emission of electron-induced secondary electrons taking into account the properties of the surface. By using this model, we have studied the influence of the electron-induced secondary electrons on the plasma parameters in argon gas at low pressures, for SiO2 electrodes. Compared to the results obtained by using a simplified model for the electron-surface interaction, we have found that the electron-surface interactions strongly influence the electron power absorption and ionization dynamics (see Fig. 1).
Figure 1. PIC/MCC simulation results on the spatio-temporal distributions of the ionization rate [1021m-3s-1], based on a simplified model (left plot) and a realistic model (right plot) for the electron-surface interaction. Discharge conditions: argon, SiO2 electrodes, 6.7 cm electrode gap, 0.5 Pa, 13.56 MHz, 1000 V. The horizontal axis corresponds to two RF periods. The vertical axis shows the normalized distance from the powered to the grounded electrode.
Strongly coupled plasmas. – In the field of strongly coupled plasmas (SCP), we have contributed by molecular dynamics simulations to the validation of the theoretical “method of moments” approach, which allows the determination of the characteristics of the collective modes (including their damping), based solely on static characteristics (i.e., the static structure factor, or the pair correlation function) of the plasma. We presented the first experimental measurement of the 3-point static structure factor, S(3)(k1,k2,k0), of a 2-dimensional dusty plasma liquid. The higher-order structure factor was as well computed from molecular dynamics simulations and very good agreement was obtained between the two sets of data (see Fig. 2). Both the measurements and the simulations confirmed the existence of negative values of S(3)(k1,k2,k0); this indicates the breakdown of the convolution approximation that gives S(3)(k1,k2,k0) in a factorized form of S(2)(2-point) functions. According to the quadratic fluctuation-dissipation theorem, a changing sign of S(3)(k1,k2,k0) implies a sign change of the quadratic part of the density response function of the system and an intriguing vanishing quadratic response at a certain wavenumber.
Dusty plasmas. – In the field of dusty plasma physics, we have developed a new, very simple and sensitive method to measure the sputtering rate of solid materials in stationary low-pressure gas discharges. The method is based on the balance of the centrifugal force and the confinement electric force acting on a single electrically charged dust particle in a rotating environment. We have demonstrated the use and sensitivity of this method in a capacitively coupled radio frequency argon discharge. We were able to detect a reduction of 10 nm in the diameter of a single dust particle and have measured the reduction rate of 6 nm/min of the particle radius.
A magnetic field was recently shown to enhance the field-parallel heat conduction in a strongly correlated plasma whereas cross-field conduction is reduced. With three-dimensional molecular dynamics simulations relevant to dusty plasmas, we have shown that in such plasmas, the magnetic field has the additional effect of inhibiting the isotropization process between field-parallel and cross-field temperature components, thus leading to the emergence of strong and long-lived temperature anisotropies when the plasma is locally perturbed. We have presented an extended heat equation, which is able to describe this process accurately.
Figure 2. Maps of the full S(3)(k1,k2,k0) 3-point static structure factor of strongly coupled Yukawa-liquids at a wave vector k1a = (1.85,0). Left: experimental data obtained on a 2-dimensional dusty plasma, right: results of molecular dynamics simulations at the same plasma parameters (a coupling coefficient of 95 and a screening coefficient of 0.7).
Technological application of high-frequency discharge systems. – Based on our experience gained during the biological decontamination studies on afterglow plasmas, we have joined another fast-developing field, namely plasma agriculture, which is aimed to develop new technology for agriculture. We used the afterglow of a surface-wave microwave discharge to investigate the effect of different afterglow plasmas on cereal crops. In our study, we treated non-infected and infected cereal crops, respectively, in the afterglow of Ar/N2-O2 surface-wave microwave discharges at 2-8 mbar pressure, using the following initial gas mixtures: (i) N2-20%O2, (ii) N2-10%O2, (iii) N2-2%O2, (iv) Ar-20%O2, (v) Ar-40%O2 and (vi) Ar-20%O2 + N2-2%O2, which made possible to isolate different species and identify their role in the process. We have shown that the germination and vigour of non-infected seeds are not significantly effected when barley is treated max 120 s at 2 mbar and maize 240 s at 4 mbar. On the other hand, seeds can be disinfected from the germination inhibitors F. graminearum and F. verticillioides. The most efficient treatment, which also increases the germination of infected seeds above 80%, is the 3 min Ar-20%O2 afterglow at 4 mbar for barley, while for maize the 4 min Ar-20%O2+2 min N2-2%O2 afterglow at 8 mbar. The high NO-content mixtures and the heating of seed surface by the recombination of O and N-atoms inhibit barley germination.
Figure 3. The post-discharge system with the surface-wave microwave discharge operating in N2-20%O2 mixture during seed treatments.
We have studied the formation of oxide structures on copper plates in the discharge sheath and in the afterglow region of an inductively coupled rf discharge at different gas mixtures, input power and treatment time, as well as in the afterglow of a surface-wave microwave (mw) discharge, and compared the two systems. In the sheath of the rf discharge, regular shapes have been formed with incipient growth of nanowires as shown in Fig. 4 (a). Higher power, which results in higher temperature, contributed to thicker layer formation, while lower powers to the structuring of the oxide layer. The oxidation in the afterglow was found to be much faster, in few minutes a thick layer was formed which detached after a threshold thickness. Depending on the oxygen content and gas temperature, different structures could be created. At lower O2 content mixture (50 sccmAr-10 sccm O2), larger individual structures have been formed, with the attempt of wires to grow on them. At the same low flow rate, with further decrease of the input power, wall structures were found, and, similarly, also in the afterglow of the mw discharge at 500 sccm N2 – 120 sccm O2. Fig. 4 (b)-(c) show the restructuring of the copper-oxide layer created in RF afterglow with the N2-O2 mw afterglow, showing the wall shape structuring of the initial structures. In case of Ar-O2 mw discharge, the oxidation rate is very low due to the lower temperature compared to the N2-O2. We have found that the wall structure, which is the basic element of the structures, can be created at lower oxidation rate, which is related to lower temperature and lower O-atom density. In case of a surface-wave microwave discharge system, this can be easily tuned with the gas flow rate and the position of the wave launcher along the discharge tube.
Figure 4. (a) Copper-oxide surfaces created in the discharge region of the 50 sccm O2, 50 W rf discharge. (b) The copper-oxide surface created in the afterglow of the 10 sccm O2, 20 W rf discharge. (c) The (b) surface restructured in the N2-O2 mw afterglow.