Models, assumptions, and experimental tests of flows near boundaries in magnetized plasmas

Siddiqui, M. Umair; Thompson, Donald B.; Jackson, Cory; Kim, Justin F.; Hershkowitz, N.; Scime, Earl

doi:10.1063/1.4943523

Cited by 19 publications

(21 citation statements)

References 35 publications

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“…In the direction parallel to the magnetic field sheath boundary conditions are needed. The correct boundary conditions to apply at the entrance to the sheath in magnetically confined plasmas is the subject of a long and ongoing debate [29,30,31,32,33]. Here we adopt relatively simple boundary conditions, leaving the choice of more complex boundary condition to future work.…”

Section: Boundary Conditionsmentioning

confidence: 99%

Hermes: global plasma edge fluid turbulence simulations

Dudson

Leddy

2017

Plasma Phys. Control. Fusion

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Abstract. The transport of heat and particles in the relatively collisional edge regions of magnetically confined plasmas is a scientifically challenging and technologically important problem. Understanding and predicting this transport requires the self-consistent evolution of plasma fluctuations, global profiles and flows, but the numerical tools capable of doing this in realistic (diverted) geometry are only now being developed.Here a 5-field reduced 2-fluid plasma model for the study of instabilities and turbulence in magnetised plasmas is presented, built on the BOUT++ framework. This cold ion model allows the evolution of global profiles, electric fields and flows on transport timescales, with flux-driven cross-field transport determined self-consistently by electromagnetic turbulence. Developments in the model formulation and numerical implementation are described, and simulations are performed in poloidally limited and diverted tokamak configurations.

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Section: Boundary Conditionsmentioning

confidence: 99%

Hermes: global plasma edge fluid turbulence simulations

Dudson

Leddy

2017

Plasma Phys. Control. Fusion

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“…Preliminary results related to the experiments reported in this dissertation were described two years later in Ref. [23]. Early measurements of ion drifts in the direction parallel to E × B were reported for the first time.…”

Section: Weakly Collisional Presheaths In Oblique Magnetic Fieldsmentioning

confidence: 78%

“…Recently, Siddiqui et al argued that magnetic presheath physics might explain anomalous erosion rates observed in Hall thruster channels (see Fig. 1.2 [23]).…”

Section: The Engineeringmentioning

confidence: 99%

“…Since that time, magnetic presheath research has enjoyed broad theoretical progress [11][12][13][14][15][16][17]. Only a few experiments exist in literature to provide data to validate these models [18][19][20][21][22][23]. Of these experiments, the early focus was on mapping the plasma potential structure of the boundary region with electrostatic probes.…”

Section: Weakly Collisional Presheaths In Oblique Magnetic Fieldsmentioning

confidence: 99%

“…Measurements of the background metastable neutral population were unavailable to the authors of Refs. [22] and [23]. Through elastic scattering collisions, ionization, and charge exchange, ion and neutral populations modify each other's distribution functions.…”

Section: Weakly Collisional Presheaths In Oblique Magnetic Fieldsmentioning

confidence: 99%

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Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

Thompson¹

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The physics of a weakly magnetized boundary region are investigated by immersing a 76.2 mm diameter stainless steel disk in a helicon plasma. The velocity distributions of ions and neutral particles are mapped in the boundary region, created in 3.6 mTorr argon gas, 650 W RF power, and magnetic field B = 0.06 T, where the magnetic field lines obliquely intersect the wall (α = 16 •). These distributions are mapped using laser induced fluorescence in three spatial (3D) and three velocity (3V) dimensions, and reveal that models neglecting any one of the velocity components omit important components of the flow field. Electrostatic probe measurements are presented and establish a typical helicon plasma discharge with electron temperature T e ≈ 4.2 eV and density n ≈ 5.5 × 10 17 m −3. In addition to the observed ion temperature, T i ≈ 0.37 eV, these measurements indicate a plasma in which the electrons are highly magnetized (the ratio of the electron cyclotron frequency to the electron collision frequency is ω ce /ν e = 2.9 × 10 3) and the ions are weakly collisional (the ratio of the ion cyclotron frequency to the ion collision frequency is ω ci /ν i = 0.5 ± 0.3). Ion drift velocity measurements are presented in the E × B direction and are compared to predictions from measured plasma potential gradients. Ion flow fields are compared to predictions from collisional fluid simulations and particle-in-cell models. These models require collisions to reproduce the ion drifts well. Neutral particle distributions are observed to be in thermal equilibrium with the chamber walls (T n ≈ 0.028 eV) and essentially non-flowing on average. Charge exchange population fractions are expected to be O(10 −2), and are too small to be resolved, but are excluded to the 5% level. "O me! O life!... of the questions of these recurring; Of the endless trains of the faithless-of cities fill'd with the foolish; Of myself forever reproaching myself, (for who more foolish than I, and who more faithless?) Of eyes that vainly crave the light-of the objects mean-of the struggle ever renew'd; Of the poor results of all-of the plodding and sordid crowds I see around me; Of the empty and useless years of the rest-with the rest me intertwined; The question, O me! so sad, recurring-What good amid these, O me, O life? Answer. That you are here-that life exists, and identity; That the powerful play goes on, and you may contribute a verse." Walt Whitman, Leaves of Grass (1892) v I want to first express my gratitude for the support of my advisor, Earl Scime, who smoothed my way to WVU and who for the last two and half years has trusted me with his lab probably more than he had reason to. He has answered annoying questions at 7 am and after midnight and was available for lunch almost every day I've been in Morgantown. That kind of face-to-face relationship building is exceptional and not lost on me. He is a mentor and a friend.

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Dependence on ion temperature of shallow-angle magnetic presheaths with adiabatic electrons

2019

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The magnetic presheath is a boundary layer occurring when magnetized plasma is in contact with a wall and the magnetic field B makes an oblique angle α with the wall. Here, we consider the fusion-relevant case of a shallow-angle, α 1, electronrepelling sheath, with the electron density given by a Boltzmann distribution, valid for α/ √ τ + 1 m e /m i , where m e is the electron mass, m i is the ion mass, τ = T i /ZT e , T e is the electron temperature, T i is the ion temperature, and Z is the ionic charge state. The thickness of the magnetic presheath is of the order of a few ion sound Larmor radii ρ s = m i (ZT e + T i )/ZeB, where e is the proton charge and B = |B| is the magnitude of the magnetic field. We study the dependence on τ of the electrostatic potential and ion distribution function in the magnetic presheath by using a set of prescribed ion distribution functions at the magnetic presheath entrance, parameterized by τ . The kinetic model is shown to be asymptotically equivalent to Chodura's fluid model at small ion temperature, τ 1, for | ln α| > 3| ln τ | 1. However, in this limit ion gyro-orbits acquire a spatial extent that occupies a large portion of the magnetic presheath, which means that kinetic effects are not negligible. At large ion temperature, τ 1, relevant because T i is measured to be a few times larger than T e near divertor targets of fusion devices, ions reach the Debye sheath entrance (and subsequently the wall) at a shallow angle whose size is given by √ α or 1/ √ τ , depending on which is largest. †

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Models, assumptions, and experimental tests of flows near boundaries in magnetized plasmas

Cited by 19 publications

References 35 publications

Hermes: global plasma edge fluid turbulence simulations

Hermes: global plasma edge fluid turbulence simulations

Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

Dependence on ion temperature of shallow-angle magnetic presheaths with adiabatic electrons

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