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J. R. Myra  One of the best experts on this subject based on the ideXlab platform.

a Sheath boundary condition for fast wave propagation near conducting surfaces
Physics of Plasmas, 2012CoAuthors: D A Dippolito, J. R. MyraAbstract:Radiofrequency (rf) waves can accelerate electrons into material boundaries, so that electron confinement requires rf Sheath potentials that are significantly larger than the thermal Bohm Sheath. The condition for rf Sheaths to occur is that the rf electric field has a component E parallel to the equilibrium magnetic field. Thus, a proper treatment of rf wave propagation requires an accurate description of the geometry of the magnetic field and of the bounding surfaces, and a boundary condition (BC) that includes the effect on the waves of the electronpoor Sheath. When the static magnetic field has a component at an angle to the Sheath, the propagating fast wave (with E=0) is coupled to a slow wave (with E≠0) in order to satisfy the boundary condition at the metal wall, and the timeaveraged Sheath potential has a strong component from rectification of the rf Sheath. In this brief communication, a previously derived Sheath BC is reformulated to treat the coupling of the fast wave to the slow wave a...

numerical analysis of radio frequency Sheath plasma interactions in the ion cyclotron range of frequencies
Physics of Plasmas, 2012CoAuthors: Haruhiko Kohno, J. R. Myra, D A DippolitoAbstract:A new finite element numerical scheme for analyzing selfconsistent radiofrequency (RF) Sheathplasma interaction problems in the ion cyclotron range of frequencies is applied to various problems represented by simplified models for the tokamak scrapeoff layer. The present code incorporates a modified boundary condition, which is called a Sheath boundary condition, that couples the radiofrequency waves and Sheaths at the material boundaries by treating the Sheath as a thin vacuum layer. A series of numerical analyses in one and twodimensional domains show several important physical properties, such as the existence of multiple roots, hysteresis effects, presence and characteristics of the Sheathplasma waves, and the phase shift of a reflected slow wave, some of which are newly identified by introducing a spatially varying plasma density and background magnetic field.

Analytic model of nearfield radiofrequency Sheaths. I. Tenuous plasma limit
Physics of Plasmas, 2009CoAuthors: D. A. D’ippolito, J. R. MyraAbstract:An analytic model is derived for electromagnetic radiofrequency (rf) wave propagation in a waveguide filled by a tenuous plasma with a slightly tilted equilibrium magnetic field B, i.e., by=By/B⪡1. The calculation includes the selfconsistent coupling between the rf fields and the Sheaths at the Sheathplasma interface and can be used to describe antenna Sheath formation in the ion cyclotron range of frequencies. The Sheaths are treated as thin vacuum regions separating the plasma and metal wall. It is shown that (i) the launched fast wave is coupled parasitically to the slow wave by the magnetic field structure when by≠0 and by the Sheath boundary condition, (ii) the Sheath voltage Vsh is dependent on the wave parity (the “antenna phasing”), and (iii) integrating the vacuum rf fields, Vvac=−∫dzE∥(vac), gives an overestimate of the Sheath voltage. An expression for the selfconsistent Vsh including plasma effects and satisfying the Child–Langmuir law is obtained.

far field Sheaths due to fast waves incident on material boundaries
Physics of Plasmas, 2008CoAuthors: D A Dippolito, J. R. Myra, E F Jaeger, L A BerryAbstract:The problem of “farfield” Sheath formation is studied with a new quantitative onedimensional model. These radiofrequency (rf) Sheaths occur when unabsorbed fast waves in the ion cyclotron range of frequencies are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear Sheath boundary condition gives selfconsistent solutions for the wave fields and Sheath characteristics, and it introduces a Sheathplasmawave resonance which can enhance the Sheath potential. The model is used to compute the parametric dependence of the farfield Sheath potential. Its application to postprocess the rf fields computed by a fullwave code for a typical D(H) minority heating scenario is also discussed. This work shows that twodimensional effects (included heuristically) are essential in determining whether farfield Sheath potentials are strong enough to cause significant edge interactions, such as impurity generation and reduced heating efficiency.

far field Sheaths due to fast waves incident on material boundaries
RADIO FREQUENCY POWER IN PLASMAS: 17th Topical Conference on Radio Frequency Power in Plasmas, 2007CoAuthors: D A Dippolito, J. R. Myra, E F Jaeger, L A BerryAbstract:The problem of far‐field Sheath formation is studied with a new quantitative 1D model. These Sheaths occur when unabsorbed fast waves (FW) are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear Sheath BC gives self‐consistent solutions for the wave fields and Sheath, and incorporates a Sheath plasma wave (SPW) resonance which enhances the Sheath potential. The model is applied to edge fields computed by the AORSA‐1D full‐wave code for a typical D(H) minority heating scenario. This work indicates the conditions under which far‐field Sheaths can explain some of the “missing power” (low heating efficiency) and rf‐specific impurity generation in ICRF experiments.
Mark J Kushner  One of the best experts on this subject based on the ideXlab platform.

a semianalytic radio frequency Sheath model integrated into a two dimensional hybrid model for plasma processing reactors
Journal of Applied Physics, 1997CoAuthors: Michael J Grapperhaus, Mark J KushnerAbstract:In high plasma density ([e]>1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often <100 s μm while the reactor dimensions are 10 s cm. Resolving the Sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the Sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency (rf) biased Sheaths which has been integrated into a twodimensional model for plasma etching reactors. The basis of the Sheath model is to track the charging and discharging of the Sheath in time, and use a onedimensional analytical model to obtain the instantaneous Sheath voltage drop based on the Sheath charge and the plasma conditions at the Sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200–800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl2 will ...

a semianalytic radio frequency Sheath model integrated into a two dimensional hybrid model for plasma processing reactors
Journal of Applied Physics, 1997CoAuthors: Michael J Grapperhaus, Mark J KushnerAbstract:In high plasma density ([e]>1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often 1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often <100 s μm while the reactor dimensions are 10 s cm. Resolving the Sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the Sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency (rf) biased Sheaths which has been integrated into a twodimensional model for plasma etching reactors. The basis of the Sheath model is to track the charging and discharging of the Sheath in time, and use a onedimensional analytical model to obtain the instantaneous Sheath voltage drop based on the Sheath charge and the plasma conditions at the Sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200–800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl2 will ...
Alalahti Matti  One of the best experts on this subject based on the ideXlab platform.

Magnetic field fluctuation properties of coronal mass ejectiondriven Sheath regions in the nearEarth solar wind
'Copernicus GmbH', 2020CoAuthors: Kilpua Emilia, Fontaine Dominique, Alalahti Matti, Osmane Adnane, Palmerio Erika, Yordanova Emiliya, Moissard Clement, Good Simon, Hadid Lina, Janvier MihoAbstract:International audienceIn this work, we investigate magnetic field fluctuations in three coronal mass ejection (CME)driven Sheath regions at 1 AU, with their speeds ranging from slow to fast. The data set we use consists primarily of highresolution (0.092 s) magnetic field measurements from the Wind spacecraft. We analyse magnetic field fluctuation amplitudes, compressibility, and spectral properties of fluctuations. We also analyse intermittency using various approaches; we apply the partial variance of increments (PVIs) method, investigate probability distribution functions of fluctuations, including their skewness and kurtosis, and perform a structure function analysis. Our analysis is conducted separately for three different subregions within the Sheath and one in the solar wind ahead of it, each 1 h in duration. We find that, for all cases, the transition from the solar wind ahead to the Sheath generates new fluctuations, and the intermittency and compressibility increase, while the region closest to the ejecta leading edge resembled the solar wind ahead. The spectral indices exhibit large variability in different parts of the Sheath but are typically steeper than Kolmogorov's in the inertial range. The structure function analysis produced generally the best fit with the extended p model, suggesting that turbulence is not fully developed in CME Sheaths near Earth's orbit. Both KraichnanIroshinikov and Kolmogorov's forms yielded high intermittency but different spectral slopes, thus questioning how well these models can describe turbulence in Sheaths. At the smallest timescales investigated, the spectral indices indicate shallower than expected slopes in the dissipation range (between −2 and −2.5), suggesting that, in CMEdriven Sheaths at 1 AU, the energy cascade from larger to smaller scales could still be ongoing through the ion scale. Many turbulent properties of Sheaths (e.g. spectral indices and compressibility) resemble those of the slow wind rather than the fast. They are also partly similar to properties reported in the terrestrial magnetoSheath, in particular regarding their intermittency, compressibility, and absence of Kolmogorov's type turbulence. Our study also reveals that turbulent properties can vary considerably within the Sheath. This was particularly the case for the fast Sheath behind the strong and quasiparallel shock, including a small, coherent structure embedded close to its midpoint. Our results support the view of the complex formation of the Sheath and different physical mechanisms playing a role in generating fluctuations in them

Magnetic field fluctuation properties of coronal mass ejectiondriven Sheath regions in the nearEarth solar wind
'Copernicus GmbH', 2020CoAuthors: Kilpua, Emilia K. J., Fontaine Dominique, Alalahti Matti, Good, Simon W., Osmane Adnane, Palmerio Erika, Yordanova Emiliya, Moissard Clement, Hadid, Lina Z., Janvier MihoAbstract:In this work, we investigate magnetic field fluctuations in three coronal mass ejection (CME)driven Sheath regions at 1 AU, with their speeds ranging from slow to fast. The data set we use consists primarily of highresolution (0.092 s) magnetic field measurements from the Wind spacecraft. We analyse magnetic field fluctuation amplitudes, compressibility, and spectral properties of fluctuations. We also analyse intermittency using various approaches; we apply the partial variance of increments (PVIs) method, investigate probability distribution functions of fluctuations, including their skewness and kurtosis, and perform a structure function analysis. Our analysis is conducted separately for three different subregions within the Sheath and one in the solar wind ahead of it, each 1 h in duration. We find that, for all cases, the transition from the solar wind ahead to the Sheath generates new fluctuations, and the intermittency and compressibility increase, while the region closest to the ejecta leading edge resembled the solar wind ahead. The spectral indices exhibit large variability in different parts of the Sheath but are typically steeper than Kolmogorov's in the inertial range. The structure function analysis produced generally the best fit with the extended p model, suggesting that turbulence is not fully developed in CME Sheaths near Earth's orbit. Both KraichnanIroshinikov and Kolmogorov's forms yielded high intermittency but different spectral slopes, thus questioning how well these models can describe turbulence in Sheaths. At the smallest timescales investigated, the spectral indices indicate shallower than expected slopes in the dissipation range (between 2 and 2 :5), suggesting that, in CMEdriven Sheaths at 1 AU, the energy cascade from larger to smaller scales could still be ongoing through the ion scale. Many turbulent properties of Sheaths (e.g. spectral indices and compressibility) resemble those of the slow wind rather than the fast. They are also partly similar to properties reported in the terrestrial magnetoSheath, in particular regarding their intermittency, compressibility, and absence of Kolmogorov's type turbulence. Our study also reveals that turbulent properties can vary considerably within the Sheath. This was particularly the case for the fast Sheath behind the strong and quasiparallel shock, including a small, coherent structure embedded close to its midpoint. Our results support the view of the complex formation of the Sheath and different physical mechanisms playing a role in generating fluctuations in them.Peer reviewe

Alfvén Ion Cyclotron Waves in Sheath Regions Driven by Interplanetary Coronal Mass Ejections
'American Geophysical Union (AGU)', 2019CoAuthors: Alalahti Matti, Souček Jan, Pulkkinen Tuija, Kilpua, Emilia K.j., Dimmock, Andrew P.Abstract:We report on a statistical analysis of the occurrence and properties of Alfvén ion cyclotron (AIC) waves in Sheath regions driven by interplanetary coronal mass ejections (ICMEs). We have developed an automated algorithm to identify AIC wave events from magnetic field data and apply it to investigate 91 ICME Sheath regions recorded by the Wind spacecraft. Our analysis focuses on waves generated by the ion cyclotron instability. AIC waves are observed to be frequent structures in ICMEdriven Sheaths, and their occurrence is the highest in the vicinity of the shock. Together with previous studies, our results imply that the shock compression has a crucial role in generating wave activity in ICME Sheaths. AIC waves tend to have their frequency below the ion cyclotron frequency, and, in general, occur in plasma that is stable with respect to the ion cyclotron instability and has lower ion β‖ than mirror modes. The results suggest that the ion beta anisotropy β⊥/β‖>1 appearing in ICME Sheaths is regulated by both ion cyclotron and mirror instabilities.Peer reviewe

Alfvén Ion Cyclotron Waves in Sheath Regions Driven by Interplanetary Coronal Mass Ejections
'American Geophysical Union (AGU)', 2019CoAuthors: Alalahti Matti, Kilpua, Emilia K. J., Souček Jan, Pulkkinen Tuija, Dimmock, Andrew P.Abstract:We report on a statistical analysis of the occurrence and properties of Alfven ion cyclotron (AIC) waves in Sheath regions driven by interplanetary coronal mass ejections (ICMEs). We have developed an automated algorithm to identify AIC wave events from magnetic field data and apply it to investigate 91 ICME Sheath regions recorded by the Wind spacecraft. Our analysis focuses on waves generated by the ion cyclotron instability. AIC waves are observed to be frequent structures in ICMEdriven Sheaths, and their occurrence is the highest in the vicinity of the shock. Together with previous studies, our results imply that the shock compression has a crucial role in generating wave activity in ICME Sheaths. AIC waves tend to have their frequency below the ion cyclotron frequency, and, in general, occur in plasma that is stable with respect to the ion cyclotron instability and has lower ion beta(parallel to) than mirror modes. The results suggest that the ion beta anisotropy beta(perpendicular to)/beta(parallel to) > 1 appearing in ICME Sheaths is regulated by both ion cyclotron and mirror instabilities.Peer reviewe

Mirror mode waves in coronal mass ejectiondriven Sheath regions
Helsingin yliopisto, 2017CoAuthors: Alalahti MattiAbstract:Mirror mode waves arise from the antiphase, low frequency fluctuations of the magnetic field and plasma density when the energy is conserved and a sufficient temperature anisotropy is present in the plasma. These waves are linearly polarized and they are frequently observed in heliospheric plasma, in particular in different Sheath structures. They are the most widely studied in the planetary magnetoSheaths, but also found in cometoSheaths and in the helioSheath. In addition, mirror mode waves are reported to also occur in CMEdriven Sheaths. The knowledge of mirror modes in CMEdriven Sheaths is, however, very limited despite of the fact that they might contribute to regulating the CME Sheath plasma on a global scale, and also a ect the geoeffectivity of CMEdriven Sheaths as well as the modulation and acceleration of energetic particles. As of yet, no statistical studies of mirror modes in CMEdriven Sheaths exist. In this thesis, a background to the basic physical plasma phenomena and structures in the heliosphere is given by briegly discussing the solar wind, interplanetary shocks and Sheath regions. The central focus of this thesis is however on CMEdriven Sheath regions and the mirror mode wave occurrence in them. CMEdriven Sheaths are turbulent plasma regions between the CME ejecta and its preceding interplanetary shock. This thesis discusses the differences between CMEdriven Sheaths and other heliospheric Sheaths. In addition, mirror modes are considered in detail by presenting the theory of mirror instability in both fluid and kinetic descriptions and by discussing the fundamental features of mirror modes in other heliospheric Sheaths regions. The previous studies of mirror modes and the methods applied in them are also widely presented. A program that identifies mirror mode structures from the magnetic field data of a spacecraft is constructed for this thesis. In the identification process, the program applies the linear polarization of mirror modes and the knowledge of the angular change of the magnetic field direction across a mirror mode structure. This new, almost fully automatic program combines previous mirror mode identification methods in a novel way, thus creating a new method for detecting and studying mirror modes in CMEdriven Sheath regions, as well as in other Sheath regions. In this thesis, the constructed program is applied to perform a statistical study of mirror mode waves in CMEdriven Sheaths. Mirror modes were discovered to be common structures, but similar to the planetary magnetoSheaths. They occupy only a relatively small part of the CME Sheath. The results show that in CMEdriven Sheaths mirror modes are generally low amplitude structures that typically occur as trains of two or three mirror mode waves. In addition, the Sheath plasma was noted to have notable temperature anisotropies, being generally mirror unstable when mirror modes were detected. The properties of the preceding shock of a CMEdriven Sheath were deduced to a ect mirror mode occurrence and the shock compression was concluded to provide a source of free energy
D A Dippolito  One of the best experts on this subject based on the ideXlab platform.

a Sheath boundary condition for fast wave propagation near conducting surfaces
Physics of Plasmas, 2012CoAuthors: D A Dippolito, J. R. MyraAbstract:Radiofrequency (rf) waves can accelerate electrons into material boundaries, so that electron confinement requires rf Sheath potentials that are significantly larger than the thermal Bohm Sheath. The condition for rf Sheaths to occur is that the rf electric field has a component E parallel to the equilibrium magnetic field. Thus, a proper treatment of rf wave propagation requires an accurate description of the geometry of the magnetic field and of the bounding surfaces, and a boundary condition (BC) that includes the effect on the waves of the electronpoor Sheath. When the static magnetic field has a component at an angle to the Sheath, the propagating fast wave (with E=0) is coupled to a slow wave (with E≠0) in order to satisfy the boundary condition at the metal wall, and the timeaveraged Sheath potential has a strong component from rectification of the rf Sheath. In this brief communication, a previously derived Sheath BC is reformulated to treat the coupling of the fast wave to the slow wave a...

numerical analysis of radio frequency Sheath plasma interactions in the ion cyclotron range of frequencies
Physics of Plasmas, 2012CoAuthors: Haruhiko Kohno, J. R. Myra, D A DippolitoAbstract:A new finite element numerical scheme for analyzing selfconsistent radiofrequency (RF) Sheathplasma interaction problems in the ion cyclotron range of frequencies is applied to various problems represented by simplified models for the tokamak scrapeoff layer. The present code incorporates a modified boundary condition, which is called a Sheath boundary condition, that couples the radiofrequency waves and Sheaths at the material boundaries by treating the Sheath as a thin vacuum layer. A series of numerical analyses in one and twodimensional domains show several important physical properties, such as the existence of multiple roots, hysteresis effects, presence and characteristics of the Sheathplasma waves, and the phase shift of a reflected slow wave, some of which are newly identified by introducing a spatially varying plasma density and background magnetic field.

far field Sheaths due to fast waves incident on material boundaries
Physics of Plasmas, 2008CoAuthors: D A Dippolito, J. R. Myra, E F Jaeger, L A BerryAbstract:The problem of “farfield” Sheath formation is studied with a new quantitative onedimensional model. These radiofrequency (rf) Sheaths occur when unabsorbed fast waves in the ion cyclotron range of frequencies are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear Sheath boundary condition gives selfconsistent solutions for the wave fields and Sheath characteristics, and it introduces a Sheathplasmawave resonance which can enhance the Sheath potential. The model is used to compute the parametric dependence of the farfield Sheath potential. Its application to postprocess the rf fields computed by a fullwave code for a typical D(H) minority heating scenario is also discussed. This work shows that twodimensional effects (included heuristically) are essential in determining whether farfield Sheath potentials are strong enough to cause significant edge interactions, such as impurity generation and reduced heating efficiency.

far field Sheaths due to fast waves incident on material boundaries
RADIO FREQUENCY POWER IN PLASMAS: 17th Topical Conference on Radio Frequency Power in Plasmas, 2007CoAuthors: D A Dippolito, J. R. Myra, E F Jaeger, L A BerryAbstract:The problem of far‐field Sheath formation is studied with a new quantitative 1D model. These Sheaths occur when unabsorbed fast waves (FW) are incident on a conducting surface not aligned with a flux surface. Use of a nonlinear Sheath BC gives self‐consistent solutions for the wave fields and Sheath, and incorporates a Sheath plasma wave (SPW) resonance which enhances the Sheath potential. The model is applied to edge fields computed by the AORSA‐1D full‐wave code for a typical D(H) minority heating scenario. This work indicates the conditions under which far‐field Sheaths can explain some of the “missing power” (low heating efficiency) and rf‐specific impurity generation in ICRF experiments.

a radio frequency Sheath boundary condition and its effect on slow wave propagation
Physics of Plasmas, 2006CoAuthors: D A Dippolito, J. R. MyraAbstract:Predictive modeling of radiofrequency wave propagation in highpower fusion experiments requires accounting for nonlinear losses of wave energy in the plasma edge and at the wall. An important mechanism of “anomalous” power losses is the acceleration of ions into the walls by rf Sheath potentials. Previous work computed the “Sheath power dissipation” nonselfconsistently by postprocessing fields obtained as the solution of models which did not retain Sheaths. Here, a method is proposed for a selfconsistent quantitative calculation of Sheath losses by incorporating a Sheath boundary condition (SBC) in antenna coupling and wave propagation codes. It obtains the selfconsistent Sheath potentials and spatial distribution of the timeaveraged power loss in the solution for the linear rf fields. It can be applied for ion cyclotron and (in some cases) lower hybrid waves. The use of the SBC is illustrated by applying it to the problem of an electron plasma wave propagating in a waveguide. This model problem is...
Michael J Grapperhaus  One of the best experts on this subject based on the ideXlab platform.

a semianalytic radio frequency Sheath model integrated into a two dimensional hybrid model for plasma processing reactors
Journal of Applied Physics, 1997CoAuthors: Michael J Grapperhaus, Mark J KushnerAbstract:In high plasma density ([e]>1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often <100 s μm while the reactor dimensions are 10 s cm. Resolving the Sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the Sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency (rf) biased Sheaths which has been integrated into a twodimensional model for plasma etching reactors. The basis of the Sheath model is to track the charging and discharging of the Sheath in time, and use a onedimensional analytical model to obtain the instantaneous Sheath voltage drop based on the Sheath charge and the plasma conditions at the Sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200–800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl2 will ...

a semianalytic radio frequency Sheath model integrated into a two dimensional hybrid model for plasma processing reactors
Journal of Applied Physics, 1997CoAuthors: Michael J Grapperhaus, Mark J KushnerAbstract:In high plasma density ([e]>1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often 1011–1012 cm−3) reactors for materials processing, the Sheath thickness is often <100 s μm while the reactor dimensions are 10 s cm. Resolving the Sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the Sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency (rf) biased Sheaths which has been integrated into a twodimensional model for plasma etching reactors. The basis of the Sheath model is to track the charging and discharging of the Sheath in time, and use a onedimensional analytical model to obtain the instantaneous Sheath voltage drop based on the Sheath charge and the plasma conditions at the Sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200–800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl2 will ...