Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496105
C. Limbach, C. Dumitrache, A. Yalin
This study describes the pressure dependence of laser plasma formation (breakdown) due to focused, ultraviolet (UV) nanosecond laser pulses, as manifested by gas heating (temperature increase) and the strength of resultant shock waves. These effects were explored over a range of pressure from 1 – 100 Torr in pure gases of nitrogen, oxygen, methane, carbon dioxide, argon and neon at 293 Kelvin. In all cases, experiments were conducted using 8 ns pulses at the 4thharmonic of Nd:YAG (266 nm) as a plasma source, with a constant pulse energy of 55 mJ. As a consequence of the laser wavelength and low pressure conditions, it is expected that plasma formation occurs predominantly through the multi-photon ionization process (rather than electron - impact / cascade ionization) and gas heating through subsequent electron thermalization (as opposed to inverse bremsstrahlung or electron-neutral collisions).
{"title":"Shock Wave Generation by Ultraviolet Nanosecond Laser Pulses at Reduced Pressure","authors":"C. Limbach, C. Dumitrache, A. Yalin","doi":"10.1109/PLASMA.2017.8496105","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496105","url":null,"abstract":"This study describes the pressure dependence of laser plasma formation (breakdown) due to focused, ultraviolet (UV) nanosecond laser pulses, as manifested by gas heating (temperature increase) and the strength of resultant shock waves. These effects were explored over a range of pressure from 1 – 100 Torr in pure gases of nitrogen, oxygen, methane, carbon dioxide, argon and neon at 293 Kelvin. In all cases, experiments were conducted using 8 ns pulses at the 4thharmonic of Nd:YAG (266 nm) as a plasma source, with a constant pulse energy of 55 mJ. As a consequence of the laser wavelength and low pressure conditions, it is expected that plasma formation occurs predominantly through the multi-photon ionization process (rather than electron - impact / cascade ionization) and gas heating through subsequent electron thermalization (as opposed to inverse bremsstrahlung or electron-neutral collisions).","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"39 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127094977","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496343
J. Kolb, Camelia Miron, R. Rataj, Jana Kredl, Tilo Schulz, P. Lukeš
Plasmas that are generated submerged in a liquid offer different possibilities for material processing, material synthesis and decontamination. Moreover fundamentals on generation and plasma properties are of great scientific interest for almost a century. In particular the mechanisms for the generation of discharges with pulsed high voltages is discussed controversial. The possibility for a breakdown development that is not mediated by an initial gaseous phase is disputed especially for the application of very short high voltage pulses of only a few nanoseconds or less. Associated with this specific excitation scheme are differences in plasma development, plasma parameters and reaction mechanisms. We have compared discharges in a point-to-plane geometry that were generated with 50-us, 100-ns, 10-ns or 1-ns high voltage pulses. Time-resolved shadowgraphy and spectroscopy were performed to evaluate discharge structures, plasma parameter and reactive species that were formed in water (in some cases ethanol). Different propagation modes, with velocities from 50 m/s to 6.7 km/s were observed. Optical emission spectroscopy has shown the formation of molecular bands of nitrogen, as well as strongly broadened atomic hydrogen and oxygen lines. Although for the very short pulses an initial bubble might not be observed, further studies are underway to verify this conclusion.
{"title":"Pulsed Discharges in Liquids: Generation and Applications*","authors":"J. Kolb, Camelia Miron, R. Rataj, Jana Kredl, Tilo Schulz, P. Lukeš","doi":"10.1109/PLASMA.2017.8496343","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496343","url":null,"abstract":"Plasmas that are generated submerged in a liquid offer different possibilities for material processing, material synthesis and decontamination. Moreover fundamentals on generation and plasma properties are of great scientific interest for almost a century. In particular the mechanisms for the generation of discharges with pulsed high voltages is discussed controversial. The possibility for a breakdown development that is not mediated by an initial gaseous phase is disputed especially for the application of very short high voltage pulses of only a few nanoseconds or less. Associated with this specific excitation scheme are differences in plasma development, plasma parameters and reaction mechanisms. We have compared discharges in a point-to-plane geometry that were generated with 50-us, 100-ns, 10-ns or 1-ns high voltage pulses. Time-resolved shadowgraphy and spectroscopy were performed to evaluate discharge structures, plasma parameter and reactive species that were formed in water (in some cases ethanol). Different propagation modes, with velocities from 50 m/s to 6.7 km/s were observed. Optical emission spectroscopy has shown the formation of molecular bands of nitrogen, as well as strongly broadened atomic hydrogen and oxygen lines. Although for the very short pulses an initial bubble might not be observed, further studies are underway to verify this conclusion.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"118 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127302253","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pulsed rf capactively-coupled discharge has attracted a lot of attention recently due to its widely applications in industrial film processes. In order to further optimize the plasma properties, a Particle-in-Cell simulation with Monte Carlo collisions was developed to explore the influence of pulsemodulated duty cycle on the sustaining of discharge and the fundamental plasma characteristics in the afterglow of pulse modulated RF capactively-coupled argon discharge at high pressure (0.3–1 Torr). It is found that, compared with the continuous wave rf plasma, the pulsed-modulated rf discharge can be sustained at a more broad driving frequency range, moreover, it can achieve higher electron density with lower electron energy for low driving frequency. At the pressure of 0.3 Torr, the electron density first decreases and then increases with the increasing duty ratio, with critical point around 2.5 MHz, while the electron temperature is inverse to that behavior. At the pressure of 1 Torr, the electron density shows the same pattern, with critical point around 6 MHz, while the electron temperature is always lower for the lower duty cycle.
{"title":"Influence Of Duty Cycle On Pulse Modulated Rf Capacitively-Coupled Argon Discharge","authors":"Lijie Chang, Xinpei Lu, Shali Yang, Xiang-mei Liu, Wei Jiang","doi":"10.1109/PLASMA.2017.8496297","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496297","url":null,"abstract":"Pulsed rf capactively-coupled discharge has attracted a lot of attention recently due to its widely applications in industrial film processes. In order to further optimize the plasma properties, a Particle-in-Cell simulation with Monte Carlo collisions was developed to explore the influence of pulsemodulated duty cycle on the sustaining of discharge and the fundamental plasma characteristics in the afterglow of pulse modulated RF capactively-coupled argon discharge at high pressure (0.3–1 Torr). It is found that, compared with the continuous wave rf plasma, the pulsed-modulated rf discharge can be sustained at a more broad driving frequency range, moreover, it can achieve higher electron density with lower electron energy for low driving frequency. At the pressure of 0.3 Torr, the electron density first decreases and then increases with the increasing duty ratio, with critical point around 2.5 MHz, while the electron temperature is inverse to that behavior. At the pressure of 1 Torr, the electron density shows the same pattern, with critical point around 6 MHz, while the electron temperature is always lower for the lower duty cycle.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"66 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129940878","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496188
C. Ciccarino, D. Savin
Space vehicles returning to Earth from beyond orbit enter the atmosphere at hypersonic velocities (greater than Mach 5). The resulting shock front generates a high temperature reactive plasma around the vehicle (with temperatures greater than 10,000 K). This intense heat is transferred to the capsule by radiative and convective processes. Designing vehicles to withstand these conditions requires an accurate understanding of the underlying non-equilibrium high temperature chemistry. Nitrogen chemistry is particularly important given the abundance of nitrogen in the atmosphere. Line emission by atomic nitrogen is a major source of radiative heating during reentry. Our ability to accurately calculate this heating is hindered by uncertainties in the electron-impact ionization (EII) rate coefficient for atomic nitrogen. 1
{"title":"New Data For Modeling Hypersonic Re-entry Into Earth’s Atmosphere: Electron-impact Ionization Of Atomic Nitrogen","authors":"C. Ciccarino, D. Savin","doi":"10.1109/PLASMA.2017.8496188","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496188","url":null,"abstract":"Space vehicles returning to Earth from beyond orbit enter the atmosphere at hypersonic velocities (greater than Mach 5). The resulting shock front generates a high temperature reactive plasma around the vehicle (with temperatures greater than 10,000 K). This intense heat is transferred to the capsule by radiative and convective processes. Designing vehicles to withstand these conditions requires an accurate understanding of the underlying non-equilibrium high temperature chemistry. Nitrogen chemistry is particularly important given the abundance of nitrogen in the atmosphere. Line emission by atomic nitrogen is a major source of radiative heating during reentry. Our ability to accurately calculate this heating is hindered by uncertainties in the electron-impact ionization (EII) rate coefficient for atomic nitrogen. 1","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"89 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129042844","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8495994
Cheng Liu, Guixin Zhang, Hong Xie, Lei Deng
Application of microwave plasma offers a potential method to produce faster combustion in internal combustion engine 1. In this paper, microwave multi-point ignition and spatial ignition had been confirmed via high-speed Schlieren imaging technique. The experiment was implemented with the microwave resonant ignition system and the Schlieren optical system. 2ms-3000W-2.45GHz microwave pulse was employed as the ignition energy source to produce initial flame kernel in the combustion chamber. The Schlieren imaging of reflected style was used to illustrate the flame development process with a high speed camera. A quartz glass coated with indium tin oxide (ITO), which ensured the sufficient microwave reflection characteristics and light transmission respectively 2, was used as the bottom of the microwave resonant chamber. Ignition experiments were conducted at high pressure of 2 bars of stoichiometric methane-air mixtures. It could be observed in Schlieren images that flame kernels were generated at more than one location simultaneously and flame propagated with different speeds in the combustion chamber. However, the number and the location of flame kernels seemed to be arbitrary.
{"title":"Microwave Plasma Multi-point Ignition Process in Methane-air Mixtures","authors":"Cheng Liu, Guixin Zhang, Hong Xie, Lei Deng","doi":"10.1109/PLASMA.2017.8495994","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8495994","url":null,"abstract":"Application of microwave plasma offers a potential method to produce faster combustion in internal combustion engine 1. In this paper, microwave multi-point ignition and spatial ignition had been confirmed via high-speed Schlieren imaging technique. The experiment was implemented with the microwave resonant ignition system and the Schlieren optical system. 2ms-3000W-2.45GHz microwave pulse was employed as the ignition energy source to produce initial flame kernel in the combustion chamber. The Schlieren imaging of reflected style was used to illustrate the flame development process with a high speed camera. A quartz glass coated with indium tin oxide (ITO), which ensured the sufficient microwave reflection characteristics and light transmission respectively 2, was used as the bottom of the microwave resonant chamber. Ignition experiments were conducted at high pressure of 2 bars of stoichiometric methane-air mixtures. It could be observed in Schlieren images that flame kernels were generated at more than one location simultaneously and flame propagated with different speeds in the combustion chamber. However, the number and the location of flame kernels seemed to be arbitrary.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126938576","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496068
F. Wessel, E. Ruskov, H. Rahman, P. Ney, J. Valenzuela, F. Conti, M. Ross, N. Aybar, J. Narkis, F. Beg, T. Darling
We report results from the latest staged Z-pinch experiments conducted on the 1MA Z-pinch Zebra facility at the University of Nevada, Reno. In these experiments a hollow shell of Krypton gas liner is injected through a supersonic nozzle (ID=2.0 cm), with a throat gap of 240 microns. The width of the anodecathode gap is 1 cm. The liner compresses a deuterium target plasma injected through a plasma gun with copper-tungsten walls. Axial magnetic field in the 1–2kG range, applied across the pinch region, stabilizes the Rayleigh-Taylor instabilities.
{"title":"New Staged Z-Pinch Experiments on the Mega-Ampere Current Driver Zebra*","authors":"F. Wessel, E. Ruskov, H. Rahman, P. Ney, J. Valenzuela, F. Conti, M. Ross, N. Aybar, J. Narkis, F. Beg, T. Darling","doi":"10.1109/PLASMA.2017.8496068","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496068","url":null,"abstract":"We report results from the latest staged Z-pinch experiments conducted on the 1MA Z-pinch Zebra facility at the University of Nevada, Reno. In these experiments a hollow shell of Krypton gas liner is injected through a supersonic nozzle (ID=2.0 cm), with a throat gap of 240 microns. The width of the anodecathode gap is 1 cm. The liner compresses a deuterium target plasma injected through a plasma gun with copper-tungsten walls. Axial magnetic field in the 1–2kG range, applied across the pinch region, stabilizes the Rayleigh-Taylor instabilities.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"189 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132495369","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496155
Lei Shi, T. Zhao, Yuantao Zhang, L. Zou, Li Zhang
Low-temperature atmospheric pressure plasmas are causing great attention for biomedical applications, the reactive species of which have been proven to be of great importance. However, there is very little basic research on the interaction mechanisms between the reactive species and biological components up to now. Simulating at the atomic level may be effective for acquiring a better insight in these processes. In this paper, reactive molecular dynamics simulations method is introduced to model the interaction of important reactive oxygen species, such as OH, O2 and O, with fungal chitin for a better understanding of plasma sterilization. It is found that among the reactive oxygen species mentioned above, OH radical and O atom can fracture important bonds of chitin (i.e., C-O, C-N, C-C), which subsequently results in the destruction of the fungal cell wall. All bond cleavages detected in the processes are initiated by a hydrogen-abstraction reaction from the chitin. Moreover, the OH radicals can react with each other and reduce the damage efficiency to the structure. It should also be mentioned that there is no bond cleavage events observed in the case of O2 molecules, which have only weak attractive non-bond interactions with the chitin. The simulation results are in good agreement with relevant experimental conclusions. This study can provide an important reference value for nonreversible destruction of the fungal chitin structure at the atomic level.
{"title":"Reactive Molecular Dynamics Simulation on Plasma-induced Destruction of Fungal Cell Wall Components","authors":"Lei Shi, T. Zhao, Yuantao Zhang, L. Zou, Li Zhang","doi":"10.1109/PLASMA.2017.8496155","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496155","url":null,"abstract":"Low-temperature atmospheric pressure plasmas are causing great attention for biomedical applications, the reactive species of which have been proven to be of great importance. However, there is very little basic research on the interaction mechanisms between the reactive species and biological components up to now. Simulating at the atomic level may be effective for acquiring a better insight in these processes. In this paper, reactive molecular dynamics simulations method is introduced to model the interaction of important reactive oxygen species, such as OH, O2 and O, with fungal chitin for a better understanding of plasma sterilization. It is found that among the reactive oxygen species mentioned above, OH radical and O atom can fracture important bonds of chitin (i.e., C-O, C-N, C-C), which subsequently results in the destruction of the fungal cell wall. All bond cleavages detected in the processes are initiated by a hydrogen-abstraction reaction from the chitin. Moreover, the OH radicals can react with each other and reduce the damage efficiency to the structure. It should also be mentioned that there is no bond cleavage events observed in the case of O2 molecules, which have only weak attractive non-bond interactions with the chitin. The simulation results are in good agreement with relevant experimental conclusions. This study can provide an important reference value for nonreversible destruction of the fungal chitin structure at the atomic level.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"56 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132517113","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496310
M. Talley, S. Shannon, Lee Chen, J. Verboncoeur
In industrial plasma processing for semiconductor fabrication, it is important to understand the characteristic properties of the plasma. The ion energy distribution function (IEDf) is one such property. The IEDf has a direct impact on process outcomes. Retarding field energy analyzers (RFEAs) have been used extensively to obtain IV curves for typical process conditions. These IV curves are often analyzed using an ideal RFEA model when calculating IEDfs. However, several factors can cause a measured IV curve to deviate from an ideal one, especially for higher grid voltages and plasma densities. Three factors to consider are voltage dip within the grid holes 1, electric field non-uniformity due to the probe geometry, and space charge build up between the grids 2. This last factor is a result of high ion flux or larger grid separation caused by high grid voltages. In this study, electrostatic simulations (EM Works) and particle-in-cell (PIC) simulations (XPDP1) were used to parametrize the impact of these factors on IV curves. Electrostatic simulation results led to a RFEA geometric design that minimized vertical electric field variations. The field uniformity was improved by 25x across the sensor area after optimization. In addition, the overestimation of the IEDf due to voltage dip within the grid holes was quantified. A shift of 2–2.5 eV was observed. Computed IEDfs were reconstructed from PIC generated IV curves using regularization methods. These simulations demonstrate how IV curves vary due to space charge build up. Space charge only affected lower energy ions. The specific energy is dependent on the grid separation distance. In this case, the IV curve begins to fall off at a lower voltage with a more gradual slope causing a larger low energy tail. This non-ideality in the curve can be corrected by limiting the flux of ions into the probe or through corrections during the regularization reconstruction. By taking these factors into account, it is possible to optimize a RFEA and modify measured IV curves to better represent an ideal curve.
{"title":"Retarding Field Energy Analyzer Optimization And Space Charge Effects","authors":"M. Talley, S. Shannon, Lee Chen, J. Verboncoeur","doi":"10.1109/PLASMA.2017.8496310","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496310","url":null,"abstract":"In industrial plasma processing for semiconductor fabrication, it is important to understand the characteristic properties of the plasma. The ion energy distribution function (IEDf) is one such property. The IEDf has a direct impact on process outcomes. Retarding field energy analyzers (RFEAs) have been used extensively to obtain IV curves for typical process conditions. These IV curves are often analyzed using an ideal RFEA model when calculating IEDfs. However, several factors can cause a measured IV curve to deviate from an ideal one, especially for higher grid voltages and plasma densities. Three factors to consider are voltage dip within the grid holes 1, electric field non-uniformity due to the probe geometry, and space charge build up between the grids 2. This last factor is a result of high ion flux or larger grid separation caused by high grid voltages. In this study, electrostatic simulations (EM Works) and particle-in-cell (PIC) simulations (XPDP1) were used to parametrize the impact of these factors on IV curves. Electrostatic simulation results led to a RFEA geometric design that minimized vertical electric field variations. The field uniformity was improved by 25x across the sensor area after optimization. In addition, the overestimation of the IEDf due to voltage dip within the grid holes was quantified. A shift of 2–2.5 eV was observed. Computed IEDfs were reconstructed from PIC generated IV curves using regularization methods. These simulations demonstrate how IV curves vary due to space charge build up. Space charge only affected lower energy ions. The specific energy is dependent on the grid separation distance. In this case, the IV curve begins to fall off at a lower voltage with a more gradual slope causing a larger low energy tail. This non-ideality in the curve can be corrected by limiting the flux of ions into the probe or through corrections during the regularization reconstruction. By taking these factors into account, it is possible to optimize a RFEA and modify measured IV curves to better represent an ideal curve.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131020457","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496075
M. Campanell, M. Umansky
In the recent literature, two distinct sheath solutions under strong electron emission have been reported in theoretical models, simulations, and in experiments [1], a “space-charge limited” (SCL) sheath and an “inverse” sheath. It is important to determine which sheath occurs under what conditions because they lead to sharply different particle and energy fluxes. Our current study [2] offers a unifying analysis of the strong emission problem, addressing both the presheath and sheath. We confirm from first principles that two equilibria, one with SCL sheaths and Bohm presheaths, and one with inverted sheaths/presheaths, are indeed possible whenever the emission coefficient exceeds unity, regardless of the plasma’s upstream properties (e.g., N and Te). However, we also show [3] that if cold ions are born in the potential dip of a SCL sheath, the accumulating ion space charge forces a transition to an inverse sheath. This explains why stable SCL sheaths were only observed in simulation studies without collisions in the plasma domain [4]. Assuming some ionization or CX collisions are always present in real sheaths, we predict only a monotonic inverse sheath should exist at any surface under strong emission conditions, whether a divertor plate, emissive probe, dust grain, Hall thruster channel wall, or sunlit object in space. Our new 1D simulations [2] illustrate that SCL and inverse equilibria have major differences of ion flow velocities and density gradients over presheath length scales. This will enable future experimental studies to identify the sheath state without having to probe inside the sheath itself.
{"title":"Is the Sheath Potential Positive or Negative at Strongly Emitting Surfaces?","authors":"M. Campanell, M. Umansky","doi":"10.1109/PLASMA.2017.8496075","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496075","url":null,"abstract":"In the recent literature, two distinct sheath solutions under strong electron emission have been reported in theoretical models, simulations, and in experiments [1], a “space-charge limited” (SCL) sheath and an “inverse” sheath. It is important to determine which sheath occurs under what conditions because they lead to sharply different particle and energy fluxes. Our current study [2] offers a unifying analysis of the strong emission problem, addressing both the presheath and sheath. We confirm from first principles that two equilibria, one with SCL sheaths and Bohm presheaths, and one with inverted sheaths/presheaths, are indeed possible whenever the emission coefficient exceeds unity, regardless of the plasma’s upstream properties (e.g., N and Te). However, we also show [3] that if cold ions are born in the potential dip of a SCL sheath, the accumulating ion space charge forces a transition to an inverse sheath. This explains why stable SCL sheaths were only observed in simulation studies without collisions in the plasma domain [4]. Assuming some ionization or CX collisions are always present in real sheaths, we predict only a monotonic inverse sheath should exist at any surface under strong emission conditions, whether a divertor plate, emissive probe, dust grain, Hall thruster channel wall, or sunlit object in space. Our new 1D simulations [2] illustrate that SCL and inverse equilibria have major differences of ion flow velocities and density gradients over presheath length scales. This will enable future experimental studies to identify the sheath state without having to probe inside the sheath itself.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"37 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128815696","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2017-05-01DOI: 10.1109/PLASMA.2017.8496025
Qing Xiong, S. Ji, Lingyu Zhu, Weifeng Lu, Shiying Chen
DC microdischarge sometimes occurs in aeronautic or space equipment, which put serious threat to the safety of the devices because it has no zero current point. Aeronautic and space equipment is subjected to low pressure or even vacuum. Therefore, it is of great significance to investigate the characteristics of DC microdischarge under low pressure. A test platform of DC microdischarge was built. The DC microdischarge generation setup was put in a vacuum chamber, and the pressure varied from 0.6 kPa to 96 kPa. The current and electromagnetic radiation signals when DC microdischarge occurred were measured by Hall current sensor and Hilbert curve fractal antenna, respectively. The influences of materials, shape and moving velocity of electrode, and pressure were investigated. FFT was applied to analyze the characteristic frequency of the electromagnetic radiation signals. And the characteristic parameters were extracted. When microdischarge generates, the current has high frequency pulses superimposed on the DC current. The fast change of current results in the electromagnetic radiation. The experimental results indicate that the amplitude of the electromagnetic radiation generated by microdischarge varies with the influential parameters. With the decrease of pressure, the amplitude of electromagnetic radiation pulse of DC microdischarge generated by brass, copper and aluminum shows a descending trend, while the change of stainless steel is not obvious. However, the electromagnetic radiation pulse of DC microdischarge has a characteristic frequency range (36–41 MHz), and is independent from pressure, electrode materials, shape and moving velocity.
{"title":"Characteristics of DC Microdischarge Under Low Pressure","authors":"Qing Xiong, S. Ji, Lingyu Zhu, Weifeng Lu, Shiying Chen","doi":"10.1109/PLASMA.2017.8496025","DOIUrl":"https://doi.org/10.1109/PLASMA.2017.8496025","url":null,"abstract":"DC microdischarge sometimes occurs in aeronautic or space equipment, which put serious threat to the safety of the devices because it has no zero current point. Aeronautic and space equipment is subjected to low pressure or even vacuum. Therefore, it is of great significance to investigate the characteristics of DC microdischarge under low pressure. A test platform of DC microdischarge was built. The DC microdischarge generation setup was put in a vacuum chamber, and the pressure varied from 0.6 kPa to 96 kPa. The current and electromagnetic radiation signals when DC microdischarge occurred were measured by Hall current sensor and Hilbert curve fractal antenna, respectively. The influences of materials, shape and moving velocity of electrode, and pressure were investigated. FFT was applied to analyze the characteristic frequency of the electromagnetic radiation signals. And the characteristic parameters were extracted. When microdischarge generates, the current has high frequency pulses superimposed on the DC current. The fast change of current results in the electromagnetic radiation. The experimental results indicate that the amplitude of the electromagnetic radiation generated by microdischarge varies with the influential parameters. With the decrease of pressure, the amplitude of electromagnetic radiation pulse of DC microdischarge generated by brass, copper and aluminum shows a descending trend, while the change of stainless steel is not obvious. However, the electromagnetic radiation pulse of DC microdischarge has a characteristic frequency range (36–41 MHz), and is independent from pressure, electrode materials, shape and moving velocity.","PeriodicalId":145705,"journal":{"name":"2017 IEEE International Conference on Plasma Science (ICOPS)","volume":"413 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2017-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123387814","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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