Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ionic compound wikipedia , lookup
Membrane potential wikipedia , lookup
State of matter wikipedia , lookup
Strangeness production wikipedia , lookup
Two-dimensional nuclear magnetic resonance spectroscopy wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Plasma (physics) wikipedia , lookup
22nd International Symposium on Plasma Chemistry July 5-10, 2015; Antwerp, Belgium Mass spectrometry and iCCD analysis of coupled and uncoupled mode in a Gatling-gun like plasma source V. Colombo1, M. Gherardi1, Z.Lj. Petrović2, N. Puač2, N. Selaković2 and A. Stancampiano1 1 2 Alma Mater Studiorum – Università di Bologna, viale Risorgimento 2, IT-40136, Bologna, Italy Institute of Physics Belgrade, University of Belgrade, Pegrevica 118, RS-11080 Belgrade, Serbia Abstract: In the present study, the jet-to-jet coupling phenomenon occurring in a novel type of plasma source, composed of an array of seven plasma jets arranged adjacent to one another (Gatling-gun like) was investigated by using mass spectrometry, iCCD imaging and electric measurements. Results shows that jet-to-jet coupling induces emission intensities and ion concentrations at least one order of magnitude higher than uncoupled jets for similar operating conditions. Keywords: jet-to-jet coupling, mass spectrometry, multijet, ions spectra, iCCD 1. Introduction A novel type of plasma source, composed of an array of seven plasma jets arranged adjacent to one another, similar in shape to a Gatling machine gun, was recently developed to take advantage of the jet-to-jet coupling phenomenon and to generate atmospheric pressure cold plasmas with higher intensity and energy with respect to singular plasma jets [1]. This source can be operated either in “uncoupled” mode, where seven plasma jets are independently produced, or in “coupled” mode, where plasma jets merge in a single combined very intense jet. Recent studies reported on the use of the coupled mode of operation to overcome some of the limitations of atmospheric cold plasmas as the ability to achieve etching [2]. Previous experiments by some of the authors also demonstrated a higher antibacterial potential and surface activation efficacy of this source when operated in the coupled mode than in uncoupled one [3]. In the present study, the coupling phenomenon occurring in a Gatling plasma source was investigated by using mass spectrometry, iCCD imaging and electric measurements. Results show that in the coupled mode ion concentrations are at least one order of magnitude higher as compared to the uncoupled mode for similar operating conditions. Consistently, ICCD acquisitions shows higher emission intensity in coupled than uncoupled mode and differences in plasma front propagation. 2. Gatling plasma source The Gatling source adopted in this paper is an array of seven PTFE capillaries (Ø ext 1.6 mm, Ø int 1 mm) arranged adjacent to one another in an axisymmetric structure with one in the centre and six surrounding it. A schematic of the source is reported in Fig.1. The Gatling source was driven by high voltage sinusoidal waveforms (80 KHz, up to 6.3 kVpeak-peak). Helium was used as working gas and different modes of operation were P-I-2-65 achieved by varying the gas flow between 2 slm (coupled mode) and 4.5 slm (uncoupled mode). To isolate the effect of the presence of the surrounding jets on the ions composition of the central jet discharge in coupled mode, a comparison with only one jet with the same gas mass flow and excitation was performed. These tests were achieved using a modified plasma source with only one central jet ignited while the other jets had no electrodes and were directed away from the substrate. Fig.1 Schematic views of the plasma source and spectrometer set up 3. Mass spectrometry setup A molecular beam mass spectrometer (HIDEN HPR60) was used to detect mass spectra of plasmas. The secondary ion mass spectrometry mode (SIMS+/-) was used to investigate the mass spectra of positive and negative ions respectively. In all measurements the Gatling gun was placed in front of the HPR60 5 mm and 9 mm away from the detection orifice (Ø 0.3 mm). The pressures in all three stages (P 1 = 6.5x100 Torr; P 2 = 2.4x10-5 Torr; P 3 = 4.1x10-7 Torr;) were kept constant 1 during the measurements. A photo of the Gatling plasma source during spectrometry analysis is shown in Fig. 2. As the mass spectrometer detection orifice has a diameter of only 0.3 mm and is therefore smaller than the area affected by the plasma generated by the source, the detection of only one jet at a time in uncoupled mode is possible. The coupled mode presenting only one jet may be more easily detected by the mass spectrometer. Nevertheless the uncoupled mode ions signal even multiplied by a factor of 7 (to take into account the undetected jets) results lower than the coupled mode signal. 107 Fig.2 Photo of the Gatling plasma source in operation during mass spectrometry analysis Coupled 2 slm He Uncoupled 4.5 slm He Positive ions 6.08 kV, 4.1 Vpp, 9 mm 106 5 Counts [c/s] 4. iCCD and electrical measurements setup The plasma structure in the two modes of operations was investigated by means of an ICCD camera (Andor iStar DH734I) with 25 ns exposure time synchronized with the sinusoidal excitation waveform. The temporal evolution of the discharge was scanned for an entire period (12.5 µs) with a time step of 0.5 µs. A wide range of operating conditions that granted coupled mode of operation was also investigated by means of iCCD acquisitions of 12.5 µs (one period) exposure time. 10 N+ O+ + H NO+ + O2 + OH H2O+ N2+ H3O+ 104 103 102 0 5 10 15 20 25 30 35 40 45 50 Mass [amu] For this analysis the Gatling plasma source was positioned in front of a copper plate grounded through a 100 kΩ resistance. The recorded voltage waveform was achieved by means of a high voltage probe (Agilent N2771B) in contact with the high voltage electrode while the current measurement were obtained measuring the voltage drop at the 100 kΩ resistance with a differential voltage probe (Agilent 10076A). Fig.3 Positive ions spectra for Gatling plasma source in coupled and uncoupled mode of operation 107 Coupled 2slm He Uncoupled 4.5 slm He Negative ions F6.08 kV, 4.1 Vpp, OH9 mm - 106 O - H From negative ions spectra it can be noticed that the most abundant species are F-, OH-, O-, for both operating modes. The presence of F- shows significant material release from the PTFE capillaries wall. As for positive ions spectra, the composition of negative ions does not change with the change of the Gatling plasma source operation mode. For negative ions the amount of detected species is in some cases several orders of magnitude higher in coupled mode than for uncoupled one. A significant presence of heavy molecules (<35 amu) was also detected in negative ion spectra for coupled mode. 2 Counts [c/s] 5. Mass spectrometry results A selection of obtained positive and negative ions mass spectra for the Gatling source during coupled and uncoupled mode of operation is shown in Fig. 3 and 4. For positive ions the most abundant species are N and O ions and relative compounds. In the two modes of operation the composition of positive ions is similar, nevertheless the amount of positive ions detected is nearly one order of magnitude higher for coupled mode than for uncoupled one. NO2- O2- H3O 5 10 NO4 10 103 102 0 5 10 15 20 25 30 35 40 45 50 Mass [amu] Fig.4 Negative ions spectra for Gatling plasma source in coupled and uncoupled mode of operation In Fig. 5 and 6 are reported the positive and negative ions spectra for case where the Gatling source in coupled mode was compared with the modified plasma source presenting only the central jet ignited. During these test the distance between the plasma source and the detection orifice was reduced to 5 mm. Similarly to what was previously discussed also in this comparison the coupled mode presents higher positive ions concentrations than the single jet source even if multiplied by a factor of 7 to take into account the P-I-2-65 missing surrounding jets. For negative ions this trend is even more accentuated as the difference in the recorded signal is of several orders of magnitude. The presence of heavy molecules in particular is observed only for the coupled case. As expected both for negative and positive ions the coupled mode signal results higher than that previously shown as the distance from the orifice was reduced from 9 mm to 5 mm. 107 Counts [c/s] Coupled Single Jet Positive ions 5.04 kV, 4.14 Vpp 5 mm, 4.5 slm He 106 uncoupled mode, even if not all the jets are detected at the same time, while only one front is observed for couple mode. 105 104 103 102 0 5 10 15 20 25 30 35 40 45 50 Mass [amu] Fig.5 Positive ions spectra for Gatling source in coupled mode compared with the modified single jet source 107 106 Counts [c/s] Coupled Single jet Negative ions 5.04 kV, 4.14 Vpp 5 mm, 4.5 slm He 105 104 103 102 0 5 10 15 20 25 30 35 40 45 50 Mass [amu] Fig.6 Positive ions spectra for Gatling source in coupled mode compared with the modified single jet source 6. iCCD and electrical measurements results The temporal evolution of the plasma discharge generated by the Gatling plasma source was investigated by means of an iCCD camera. In Fig. 7 and 8 time resolved images for the coupled and uncoupled mode of operation are shown. In both picture are shown six acquisition related to different instants of the voltage pulse (t= 0 µs at the start of the positive half-period of the voltage pulse). As it can be observed in the pictures during the positive half period the plasma propagates from the source outlet toward the grounded target. Light emission in coupled mode starts later than in uncoupled one. As expected multiple ionizing fronts are visible in P-I-2-65 Fig.7 Recorded voltage and current waveforms (top) with iCCD acquisitions (25 ns gate) of the Gatling plasma source (blue square on the right) in coupled mode (He 2 slm, 9mm, 6.08 kVpp) at different instant of the high voltage pulse waveform. 3 analysis the emission intensity of the coupled mode appears to be always higher than the uncoupled one (different colour scale in the figures). Deeper investigations showed that for the coupled mode of operation there is not always a direct correlation between the voltage imposed by the pulse generator and the voltage recorded at the high voltage electrode. In fact, after a certain value the recorded voltage does not increase with the imposed voltage but reaches a plateau, while the current constantly increases. iCCD acquisitions showed plasma discharges of increasing intensity and cross section as the imposed voltage was increased and a significant change in the plasma discharge structure as the recorded voltage plateau was reached. Fig.9 Recorded voltage and current values for different pulse generator imposed voltage during operation in coupled mode (He 2 slm, 9 mm gap) 7. Conclusion Results show that in the coupled mode ion concentrations are higher (at least one order of magnitude) than in the uncoupled mode for similar operating conditions. This is more pronounced for negative ions and may suggest a key role of these species in the coupling phenomenon. ICCD acquisitions also show higher emission intensity in coupled than uncoupled mode and differences in plasma front propagation. This work presents new insights on the Gatling-like plasma sources and more in general on the jet-to-jet coupling phenomenon suggesting that the jet produced by coupling phenomenon results to be different from simply being the sum of the single jets. Fig.8 Recorded voltage and current waveforms (top) with iCCD acquisitions (25 ns gate) of the Gatling plasma source (blue square on the right) in uncoupled mode (He 4.5 slm, 9mm, 6.08 kVpp) at different instant of the high voltage pulse waveform. In the negative half period, for both cases and with similar timing, is possible to observe a counter propagating front (repopulation of the PAPS tail). Consistently with the mass spectrometry ions mass 4 8. Acknowledgments Work partially supported by COST Action MP1101 “Biomedical Applications of Atmospheric Pressure Plasma Technology” 9. References [1] J. Kim et al, Plasma Process Polymers, 9, (2012) [2] J. Kim et al, IEEE Transactions on Plasma Science, 39, 2338-2339 (2011) [3] V. Colombo et al, IEEE Transactions on Plasma Science, 42, 10 (2014) P-I-2-65