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1. Dynamics of rapidly spinning blob-filaments: fluid theory with a parallel kinetic extension

Author

Myra, J. R., Cheng, J., and Parker, S. E.

Subjects
Physics - Plasma Physics
Abstract

Blob-filaments (or simply 'blobs') are coherent structures formed by turbulence and sustained by nonlinear processes in the edge and scrape-off layer (SOL) of tokamaks and other magnetically confined plasmas. The dynamics of these blob-filaments, in particular their radial motion, can influence the scrape-off layer width and plasma interactions with both the divertor target and with the main chamber walls. Motivated by recent results from the XGC1 gyrokinetic simulation code reported on elsewhere [J. Cheng et al. submitted to Nucl. Fusion and available at arXiv:2302.02877v1], a theory of rapidly spinning blob-filaments has been developed. The theory treats blob filaments in the closed flux surface region or the region that is disconnected from sheaths in the SOL. It extends previous work by treating blob spin, arising from partially or fully adiabatic electrons, as the leading order effect and retaining inertial (ion charge polarization) physics in next order. Spin helps to maintain blob coherency and affects the blob's propagation speed. Dipole charge polarization, treated perturbatively, gives rise to blob-filaments with relatively slow radial velocity, comparable to that observed in the simulations. The theory also treats the interaction of rapidly spinning blob filaments with a zonal flow layer. It is shown analytically that the flow layer can act like a transport barrier for these structures. Finally parallel electron kinetic effects are incorporated into the theory. Various asymptotic parameter regimes are discussed and asymptotic expressions for the radial and poloidal motion of the blob-filaments are obtained., Comment: 31 pages, 2 figures, accepted in the journal Physics of Plasmas 30, 072302 (2023)

Published
2023
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2. Comparison of edge turbulence characteristics between DIII-D and C-Mod simulations with XGC1

Author

Charidakos, I. Keramidas, Myra, J. R., Ku, S., Churchill, R. M., Hager, R., Chang, C. S., and Parker, S.

Subjects
Physics - Plasma Physics
Abstract

The physical processes taking place at the edge region are crucial for the operation of tokamaks as they govern the interaction of hot plasma with the vessel walls. Numerical modeling of the edge with state-of-the-art codes attempts to elucidate interactions between neoclassical drifts, turbulence, poloidal and parallel flows that control the physical set-up of the SOL region. Here, we present post-processing analysis of simulations from the gyrokinetic code XGC1, comparing edge turbulence characteristics from a simulation of DIII-D against one of C-Mod. We find that the equilibrium $E \times B$ flux across the separatrix has a similar poloidal pattern in both discharges which can be explained by magnetic drifts and trapped ion excursions. However, collisionality is noted to play a major role in that it prevents local charge accumulations from having global effects in C-Mod. In both cases, turbulent electron heat flux is higher than the ion one. This seems to be a universal characteristic of the tokamak edge. We identify turbulent frequencies and growth rates of the dominant mode in both simulations. In C-Mod, these numbers point to the presence of a drift wave. In DIII-D, linear simulations with Gene reveal a trapped electron mode. Furthermore, we present the amplitude and size distributions of the blobs from both simulations. Amplitude distributions are in qualitative agreement with experimental observations while size distributions are consistent with the fact that most blobs are not connecting to the divertor plates and suggest that they are generated by the shearing of the turbulent modes., Comment: 14 pages, 11 figures

Published
2020
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3. Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation

Author

Charidakos, I. Keramidas, Myra, J. R., Parker, S., Ku, S., Churchill, R. M., Hager, R., and Chang, C. S.

Subjects
Physics - Plasma Physics
Abstract

The SOL width is a parameter of paramount importance in modern tokamaks as it controls the power density deposited at the divertor plates, critical for plasma-facing material survivability. An understanding of the parameters controlling it has consequently long been sought (Connor et al. 1999 NF 39 2). Prior to Chang et al.(2017 NF 57 11), studies of the tokamak edge have been mostly confined to reduced fluid models and simplified geometries, leaving out important pieces of physics. Here, we analyze the results of a DIII-D simulation performed with the full-f gyrokinetic code XGC1 which includes both turbulence and neoclassical effects in realistic divertor geometry. More specifically, we calculate the particle and heat ExB fluxes along the separatrix, discriminating between equilibrium and turbulent contributions. We find that the density SOL width is impacted almost exclusively by the turbulent electron flux. In this simulation, the level of edge turbulence is regulated by a mechanism we are only beginning to understand: $\nabla B$-drifts and ion X-point losses at the top and bottom of the machine, along with ion banana orbits at the low field side (LFS), result in a complex poloidal potential structure at the separatrix which is the cause of the ExB drift pattern that we observe. Turbulence is being suppressed by the shear flows that this potential generates. At the same time, turbulence, along with increased edge collisionality and electron inertia, can influence the shape of the potential structure by making the electrons non-adiabatic. Moreover, being the only means through which the electrons can lose confinement, it needs to be in a balance with the original direct ion orbit losses to maintain charge neutrality., Comment: 14 pages, Submitted for publication to POP

Published
2018
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5. Divertor turbulent transport in the single null and snowflake in the TCV tokamak.

Author

Tsui, C. K., Boedo, J. A., Myra, J. R., Galassi, D., and Wüthrich, C.

Subjects
FUSION reactor divertors, TOKAMAKS, EDDY flux, SNOWFLAKES, ELECTRIC fields, PLASMA turbulence
Abstract

The relative importance of divertor radial turbulent particle fluxes is considered by comparing it against the fluxes in the main-chamber outer midplane (OMP) in a variety of conditions and divertor geometries in the tokamak à configuration variable. Within the first power falloff length, the radial turbulent fluxes in the leg of the outer divertor are consistently found to be small, and about 1/5th the magnitude measured at the OMP. In a low-density single null divertor, magnetic shear is found to play a strong role in isolating the main-chamber turbulence from the divertor. The snowflake divertor is purported to have turbulence-enhancing properties in the volume between the two X-points but was instead found to further reduce the divertor turbulent fluxes compared to the single null. Depending on the collisionality, the electric field fluctuations and radial turbulent fluxes were higher near the X-point than at the outer midplane, which is likely due to the binormal compression of the flux bundles consistent with analytical models of the resistive X-point mode. Density and potential fluctuation amplitudes decrease monotonically with distance from the OMP with a slope that depends on collisionality. [ABSTRACT FROM AUTHOR]

Published
2024
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7. Validity condition for the local sheath impedance boundary condition and a nonlocal generalization.

Author

Myra, J. R. and Kohno, H.

Subjects
*STIMULUS generalization, *ELECTRIC potential, *PLASMA sheaths, *MAGNETIC devices, *MAGNETIC fields, *SURFACES (Technology)
Abstract

ICRF sheaths can cause unwanted interactions of high-power RF waves with material surfaces in magnetic fusion devices. In previous work, a local RF sheath impedance boundary condition (BC) was derived for use in ICRF codes together with a microscale (i.e., Debye or sheath width scale) model for obtaining the sheath impedance used in that BC. This local RF sheath BC matches the normal component of current and electrostatic potential across the sheath-plasma interface. Collapsing the matching conditions at the sheath-plasma interface to a BC depends on the assumption of scale separation, which can be violated when conditions along the local radius of curvature of the surface vary sufficiently rapidly. The validity condition is explored in this contribution, with special attention to the case where the magnetic field approaches being tangent to the surface. When the local sheath BC no longer applies, a nonlocal sheath BC is developed under the assumption of a more relaxed scale separation assumption. It is shown that the nonlocal sheath BC reduces to the previous local sheath BC under appropriate conditions. A surface-integrated sheath admittance parameter [H. Kohno and J. R. Myra, this conference] describes the 2D physics in the new BC. [ABSTRACT FROM AUTHOR]

Published
2023
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11. Resonant waveâ€"filament interactions as a loss mechanism for HHFW heating and current drive.

Author

Tierens, W, Myra, J R, Bilato, R, and Colas, L

Subjects
FIBERS, PLASMA waves, SURFACE waves (Seismic waves), TORUS
Abstract

Perkins et al (2012 Phys. Rev. Lett. 109 045001) reported unexpected power losses during high harmonic fast wave (HHFW) heating and current drive in the National Spherical Torus Experiment (NSTX). Recently, Tierens et al (2020 Phys. Plasmas 27 010702) proposed that these losses may be attributable to surface waves on field-aligned plasma filaments, which carry power along the filaments, to be lost at the endpoints where the filaments intersect the limiters. In this work, we show that there is indeed a resonant loss mechanism associated with the excitation of these surface waves, and derive an analytic expression for the power lost to surface wave modes at each filament. [ABSTRACT FROM AUTHOR]

Published
2022
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29. Radio frequency wave interactions with a plasma sheath: the role of wave and plasma sheath impedances

Author

Myra, J. R. and Kohno, H.

Subjects
ICRF, sheath, Physics::Plasma Physics, Physics::Space Physics, impedance, radio frequency, tokamak
Abstract

The accompanying files contain digital data for figures in the article "Radio frequency wave interactions with a plasma sheath: the role of wave and plasma sheath impedances" by J.R. Myra and H. Kohno, to be submitted to the journal Physics of Plasmas. Abstract: RF sheaths form near surfaces where plasma and strong RF fields coexist. The effect of these RF sheaths on wave propagation near the boundary can be characterized by an effective sheath impedance that includes both resistive and capacitive contributions describing RF sheath rectification and RF power absorption in the sheath [J. R. Myra and D. A. D’Ippolito, Phys. Plasmas 22, 062507 (2015)]. Here we define a dimensionless parameter, the ratio of incoming wave impedance to the sheath impedance, which determines the characteristics of the interaction, ranging from quasi-conducting to quasi-insulating, or in the case of matched impedances, to a sheath-plasma resonance. A semi-analytical analysis is carried out for electrostatic slow waves in the ion cyclotron range of frequencies (ICRF). For the propagating slow wave case, where the incident wave is partially reflected, the fraction of power dissipated in the sheath is calculated. For the evanescent slow wave case, which admits a sheath-plasma resonance, an amplification factor is calculated. Using the impedance ratio approach, RF sheath interactions are characterized for a range of RF wave and plasma parameters including plasma density, magnetic field angle with respect to the surface, wave frequency and wave-vector components tangent to the surface. For a particularly interesting example case, results are compared with the rfSOL code [H. Kohno and J. R. Myra, Comput. Phys. Commun. 220, 129 (2017)]. Finally electromagnetic effects, absent from the semi-analytical analysis, are assessed.

Published
2018
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30. Calculation of RF sheath properties from surface wave-fields: a post-processing method

Author

Myra, J R, Kohno, H, Myra, J R, and Kohno, H

Abstract

In ion cyclotron range of frequency (ICRF) experiments in fusion research devices, radio frequency (RF) sheaths form where plasma, strong RF wave fields and material surfaces coexist. These RF sheaths affect plasma material interactions such as sputtering and localized power deposition, as well as the global RF wave fields themselves. RF sheaths may be modeled by employing a sheath boundary condition (BC) in place of the more customary conducting wall (CW) BC; however, there are still many ICRF computer codes that do not implement the sheath BC. In this paper we present a method for post-processing results obtained with the CW-BC. The post-processing method produces results that are equivalent to those that would have been obtained with the RF sheath BC, under certain assumptions. The post-processing method is also useful for verification of sheath BC implementations and as a guide to interpretation and understanding of the role of RF sheaths and their interactions with the waves that drive them.

Published
2019

31. Radio frequency wave interactions with a plasma sheath: The role of wave and plasma sheath impedances

Author

Lodestar Research Corporation, 2400 Central Avenue, Boulder, Colorado 80301, USA, Department of Mechanical Information Science and Technology, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan, Myra, J. R., Kohno, H., Lodestar Research Corporation, 2400 Central Avenue, Boulder, Colorado 80301, USA, Department of Mechanical Information Science and Technology, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan, Myra, J. R., and Kohno, H.

Abstract

type:Journal Article, Radio frequency (RF) sheaths form near surfaces where plasma and strong RF fields coexist. The effect of these RF sheaths on wave propagation near the boundary can be characterized by an effective sheath impedance that includes both resistive and capacitive contributions describing RF sheath rectification and RF power absorption in the sheath [J. R. Myra and D. A. D'Ippolito, Phys. Plasmas 22, 062507 (2015)]. Here, we define a dimensionless parameter, the ratio of incoming wave impedance to the sheath impedance, which determines the characteristics of the interaction, ranging from quasi-conducting to quasi-insulating, or in the case of matched impedances, to either perfect absorption or a sheath-plasma resonance. A semi-analytical analysis is carried out for electrostatic slow waves in the ion cyclotron range of frequencies. For the propagating slow wave case, where the incident wave is partially reflected, the fraction of power dissipated in the sheath is calculated. For the evanescent slow wave case, which admits a sheath-plasma resonance, an amplification factor is calculated. Using the impedance ratio approach, RF sheath interactions are characterized for a range of RF wave and plasma parameters including plasma density, magnetic field angle with respect to the surface, wave frequency, and wave-vector components tangent to the surface. For a particularly interesting example case, results are compared with the rfSOL code [H. Kohno and J. R. Myra, Comput. Phys. Commun. 220, 129 (2017)]. Finally, electromagnetic effects, absent from the semi-analytical analysis, are assessed.

Published
2019

32. Radio-frequency wave interactions with a plasma sheath in oblique-angle magnetic fields using a sheath impedance model

Author

Department of Mechanical Information Science and Technology, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan, Lodestar Research Corporation, 2400 Central Avenue P-5, Boulder, Colorado 80301, USA, Kohno, H., Myra, J. R., Department of Mechanical Information Science and Technology, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan, Lodestar Research Corporation, 2400 Central Avenue P-5, Boulder, Colorado 80301, USA, Kohno, H., and Myra, J. R.

Abstract

type:Journal Article, The physics of interactions between waves in plasmas and sheaths for background magnetic fields which make oblique angles with sheath surfaces is studied with the use of the self-consistent finite element code rfSOL incorporating the recently developed sheath impedance model [J. R. Myra, Phys. Plasmas 24, 072507 (2017)]. The calculation based on this model employs the generalized sheath boundary condition (sheath BC), which surpasses the previously used capacitive sheath BC in reliability by taking into account the contributions of the ion and electron currents in the sheath and the displacement current. A series of numerical simulations is carried out in two-dimensional slab geometry with a flat or curved sheath surface as part of the boundary. It is shown that the sheath–plasma wave appears when the equilibrium magnetic field line angle with respect to the sheath surface is small, the absolute value of the radio-frequency (RF) sheath voltage is large, and the plasma density is slightly higher than the lower hybrid resonance density (LHR density), all of which bring the sheath property closer to being capacitive. It is also shown that the sharp variation of the magnetic field line angle along the sheath surface can sensitively affect the maximum absolute value of the RF sheath voltage at a plasma density slightly lower than the LHR density.

Published
2019

33. Radio-frequency wave interactions with a plasma sheath in oblique-angle magnetic fields using a sheath impedance model

Author

Kohno, H. and Myra, J. R.

Subjects
radio-frequency, sheath, ICRF, Physics::Plasma Physics, Condensed Matter::Superconductivity, finite element method, Physics::Space Physics, tokamak
Abstract

The physics of interactions between waves in plasmas and sheaths for background magnetic fields which make oblique angles with the sheath surfaces is studied with the use of the self-consistent finite element code rfSOL incorporating the recently developed sheath impedance model [J. R. Myra, Phys. Plasmas 24, 072507 (2017)]. The calculation based on this model employs the generalized sheath boundary condition (sheath BC), which surpasses the previously used capacitive sheath BC in reliability by taking into account the contributions of the ion and electron currents in the sheath as well as the displacement current. A series of numerical simulations is carried out in two-dimensional slab geometry with a flat or curved sheath surface as part of the boundary. For the plasma density higher than the lower hybrid resonance density (LHR density), it is shown that the sheath–plasma wave appears when the equilibrium magnetic field line angle with respect to the sheath surface is small, the absolute value of the radio-frequency (RF) sheath voltage is large, and the plasma density is low within the allowable range, all of which bring the sheath property closer to being capacitive. It is also shown that the sharp variation of the magnetic field line angle along the sheath surface can sensitively affect the maximum absolute value of the RF sheath voltage at a plasma density slightly lower than the LHR density.

Published
2018
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36. Blob wakes in NSTX

Author

Zweben, S. J., primary, Myra, J. R., additional, Diallo, A., additional, Russell, D. A., additional, Scotti, F., additional, and Stotler, D. P., additional

Published
2019
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41. Mean flows and blob velocities in scrape-off layer (SOLT) simulations of an L-mode discharge on Alcator C-Mod

Author

Massachusetts Institute of Technology. Plasma Science and Fusion Center, Labombard, Brian, Hughes Jr, Jerry, Terry, James L, Russell, D. A., Myra, J. R., D'Ippolito, D. A., Zweben, S. J., Massachusetts Institute of Technology. Plasma Science and Fusion Center, Labombard, Brian, Hughes Jr, Jerry, Terry, James L, Russell, D. A., Myra, J. R., D'Ippolito, D. A., and Zweben, S. J.

Abstract

Two-dimensional scrape-off layer turbulence (SOLT) code simulations are compared with an L-mode discharge on the Alcator C-Mod tokamak [Greenwald et al., Phys. Plasmas 21, 110501 (2014)]. Density and temperature profiles for the simulations were obtained by smoothly fitting Thomson scattering and mirror Langmuir probe (MLP) data from the shot. Simulations differing in turbulence intensity were obtained by varying a dissipation parameter. Mean flow profiles and density fluctuation amplitudes are consistent with those measured by MLP in the experiment and with a Fourier space diagnostic designed to measure poloidal phase velocity. Blob velocities in the simulations were determined from the correlation function for density fluctuations, as in the analysis of gas-puff-imaging (GPI) blobs in the experiment. In the simulations, it was found that larger blobs moved poloidally with the E × B flow velocity, v E , in the near-SOL, while smaller fluctuations moved with the group velocity of the dominant linear (interchange) mode, v[subscript E] + 1/2 v[subscript di] , where v [subscript di] is the ion diamagnetic drift velocity. Comparisons are made with the measured GPI correlation velocity for the discharge. The saturation mechanisms operative in the simulation of the discharge are also discussed. It is found that neither sheared flow nor pressure gradient modification can be excluded as saturation mechanisms., United States. Department of Energy. Office of Fusion Energy Sciences (Grant DE-FG02-97ER54392), United States. Department of Energy. Office of Fusion Energy Sciences (Agreement DE-FC02-99ER54512), United States. Department of Energy. Office of Fusion Energy Sciences (Contract DE-AC02-09CH11466)

Published
2018

42. A post-processing method to simulate the generalized RF sheath boundary condition

Author

Myra, J. R. and Kohno, H.

Subjects
radio-frequency sheath, ICRF, tokamak
Abstract

The accompanying file contains digital data for Fig. 2 in the article "A post-processing method to simulate the generalized RF sheath boundary condition" by J.R. Myra and H. Kohno, to be submitted to the European Physical Journal: EPJ Web of Conferences in conjunction with the 22nd Topical Conference on RF Power in Plasma, Aix-en-provence (FRANCE) from 30th May - 2nd June 2017. Data is given as ascii text in csv format. The file may be open and plotted by common spreadsheet programs and may be easily read by procedural programs. The first row gives the column headers. Abstract: For applications of ion cyclotron range of frequencies (ICRF) power in fusion devices, control of RF sheath interactions is of great importance. A sheath boundary condition (SBC) was previously developed to provide an effective surface impedance for the interaction of the RF sheath with the waves. The SBC enables the surface power flux and rectified potential energy available for sputtering to be calculated. For legacy codes which cannot easily implement the SBC, or to speed convergence in codes which do implement it, we consider here an approximate method to simulate SBCs by post-processing results obtained using other, e.g. conducting wall, boundary conditions. The basic approximation is that the modifications resulting from the generalized SBC are driven by a fixed incoming wave which could be either a fast wave or a slow wave. The method is illustrated in slab geometry and compared with exact numerical solutions; it is shown to work very well.

Published
2017
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43. Filament-assisted mode conversion in magnetized plasmas.

Author

Tierens, W., Zhang, W., and Myra, J. R.

Subjects
WAVEGUIDES, FIBERS
Abstract

At density filaments in magnetized plasmas, electromagnetic waves are guided along the filament ("wave-filament bound states"). Several wave-filament bound states exist beyond those predicted by Myra and D'Ippolito [Phys. Plasmas 17, 102510 (2010)]. The new bound states occur under experimentally relevant conditions and are especially common under typical NSTX conditions, where they are a plausible mechanism for the experimentally observed power losses in the edge plasma. [ABSTRACT FROM AUTHOR]

Published
2020
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49. Modeling far-field radio-frequency sheaths in Alcator C-Mod.

Author

D'Ippolito, D. A., Myra, J. R., Ochoukov, R., and Whyte, D. G.

Subjects
RADIO frequency, FRAUNHOFER region (Electromagnetism), PLASMA sheaths, NUCLEAR fusion, ELECTRIC waves, ELECTRIC fields
Abstract

This paper is motivated by the recent measurement of large (>100 V) plasma potentials in Alcator C-Mod during ion cyclotron range of frequency (ICRF) heating. The plasma potential is measured on field lines that intersect a limiter but do not pass near a powered ICRF antenna. The measured potential correlates with the local ICRF fast wave electric field and is a prime candidate to cause increased Mo sputtering from the limiter surface. In this paper, it is shown that a theory of 'far-field' radio-frequency (rf) sheaths can qualitatively explain this experimental observation. The theory describes rf-sheath formation when unabsorbed fast ICRF waves are incident on a conducting boundary far from the antenna. It is shown that the rf-sheath drive is sensitive to the angle between the surface normal and the equilibrium magnetic field. The main conclusion of this work is that the rapid tangential variation in the B field-limiter geometry near the tip of the limiter promotes the formation of large sheath potentials of the same order as the measured ones. [ABSTRACT FROM AUTHOR]

Published
2013
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50. Analytical and numerical study of the transverse Kelvin-Helmholtz instability in tokamak edge plasmas.

Author

Myra, J. R., D'Ippolito, D. A., Russell, D. A., Umansky, M. V., and Baver, D. A.

Subjects
REYNOLDS number, MAGNETIC fields, ELECTROMAGNETIC theory, FIELD theory (Physics), AERODYNAMICS
Abstract

Sheared flows perpendicular to the magnetic field can be driven by the Reynolds stress or ion pressure gradient effects and can potentially influence the stability and turbulent saturation level of edge plasma modes. On the other hand, such flows are subject to the transverse Kelvin-Helmholtz (KH) instability. Here, the linear theory of KH instabilities is first addressed with an analytic model in the asymptotic limit of long wavelengths compared with the flow scale length. The analytic model treats sheared E × B flows, ion diamagnetism (including gyro-viscous terms), density gradients and parallel currents in a slab geometry, enabling a unified summary that encompasses and extends previous results. In particular, while ion diamagnetism, density gradients and parallel currents each individually reduce KH growth rates, the combined effect of density and ion pressure gradients is more complicated and partially counteracting. Secondly, the important role of realistic toroidal geometry is explored numerically using an invariant scaling analysis together with the 2DX eigenvalue code to examine KH modes in both closed and open field line regions. For a typical spherical torus magnetic geometry, it is found that KH modes are more unstable at, and just outside of, the separatrix as a result of the distribution of magnetic shear. Finally implications for reduced edge turbulence modelling codes are discussed. [ABSTRACT FROM AUTHOR]

Published
2016
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