Thermionic Cooling of the Target Plasma to a Sub-eV Temperature

Campanell, Michael; Johnson, Grant

doi:10.1103/physrevlett.122.015003

Cited by 24 publications

(25 citation statements)

References 37 publications

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“…Here, we show the first results of UEDGE modeling of a plasma equilibrium under conditions emulating both the "standard" (or previously accepted, "traditional" sheath model) and inverse sheath at the plasma edge. While plasma dynamics shown in kinetic simulations indicate the possibility of an improved transition to detachment via the inverse sheath regime [3] , such favorable results are not seen in our simulations. In this paper, we will present and discuss our plasma model and results for "standard" and "inverse" sheath regimes.…”

Section: Introductioncontrasting

confidence: 65%

“…Strong electron thermionic emission from material surfaces has been proposed to cause the formation of an "inverse sheath", a regime at the plasma edge where electron emission (in conjunction with ion and electron cooling due to charge exchange and other collisions) prevents the flow of cold ions to the surface [1][2][3] . This inverse sheath (different than the space-charge limited sheath) features a monotonic, positive potential at the material surface that forms when the emission of electrons from the surface exceeds the influx of electrons from the plasma.…”

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations

Masline

Smirnov

Krasheninnikov

2019

Contributions to Plasma Physics

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Recently, it was shown that strong electron thermionic emission from material walls could result in the formation of an "inverse sheath", which prevents the flow of cold ions to the wall [1][2][3] . Such regimes look very favorably from the point of view of plasma-material interactions at the edge of magnetic fusion devices, where the problem of the erosion of plasma facing components under ion irradiation is one of the key issues for developing of future magnetic fusion reactors. However, it is not clear whether such regimes are compatible with edge plasma parameters and heat removal requirements in fusion reactors.To address the issue of practicality of the "inverse sheath" regime for edge tokamak plasma conditions, we perform a set of numerical simulations with 2D edge plasma transport code UEDGE [4] for a DIII-D-like geometry and magnetic configuration.To describe both "standard" and "inverse sheath" conditions within the framework of the UEDGE code (which does not consider the sheath region per se), at the material surfaces, we apply effective boundary conditions which emulate both "standard" and "inverse sheath" regimes. We demonstrate that for the same input parameters, spatial distributions of edge plasma parameters corresponding to detached divertor and "inverse sheath" regimes are similar, with only a few minor differences. We discuss the compatibility of "inverse sheath" regimes with core plasma parameters.

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Section: Introductioncontrasting

confidence: 65%

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations

Masline

Smirnov

Krasheninnikov

2019

Contributions to Plasma Physics

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“…This approach, that is, choosing a significant breakdown of charge neutrality, was originally adopted by Tonks and Langmuir, while recent study of Chabert confirmed that such choice makes DC sheath size nearly independent of plasma properties, thus facilitating comparison under various conditions. A value of

δ_{se} = 1 %

was chosen in Campanell's recent work . The influence of such choice on sheath size is shown in Figure .…”

Section: Theoretical Modeling Of Emissive Rf Sheathmentioning

confidence: 99%

On the role of secondary electron emission in capacitively coupled radio‐frequency plasma sheath: A theoretical ground

Sun

et al. 2019

Plasma Processes & Polymers

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We propose a theoretical ground for emissive capacitively coupled radio-frequency (rf) plasma sheath under low pressure. The rf sheath is assumed to be collisionless and oscillates with external source. A known sinusoidal voltage instead of current is taken as prerequisite to derive sheath dynamics. Kinetic studies are performed to determine mean wall potential as a function of secondary emission coefficient and applied voltage amplitude, with which the complete mean direct current sheath is resolved. Analytical analyses under homogeneous model and numerical analyses under inhomogeneous model are conducted to deduce real-time sheath properties including space potential, sheath capacitance, and stochastic heating. Obtained results are validated by a continuum kinetic simulation without ionization. The influences of collisionality and ionization induced by secondary electrons are elucidated with a particle-in-cell simulation, which further formalizes proposed theories and inspires future works. K E Y W O R D S capacitively coupled plasma, kinetic theory, modeling, secondary electron emission, surfaces

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“…Sinusoidal source is fixed as Dirichlet-type condition at boundaries. Adopted simulation parameters are listed below to help replicate simulation results: source frequency 13.56 MHz, bulk plasma density 0 = 5 × 10 14 The simulation produces noise-free data for better understanding of sheath physics, similar methods were used in several previous works related to plasma sheath [28][29][30][31] . More detailed algorithm was shown in our previous works of bounded plasma 15,32 .…”

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confidence: 99%

Intense boundary emission destroys normal radio-frequency plasma sheath

Sun

Zhang

2020

Phys. Rev. E

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Current models of capacitively coupled plasma indicate that external bias is mainly consumed by oscillating sheathes which shield external field. We report first evidence that strong boundary emission destroys normal radio-frequency (RF) sheath and establishes a new RF plasma where external bias is consumed by bulk plasma instead of sheathes. This produces ion confinement and intense RF current in bulk plasma, combined with unique particle and energy balance. Proposed model offers new method for ion erosion mitigation, wave mode conversion and a reaction rate control technic in low pressure plasma processing.A capacitively coupled plasma (CCP) is naturally formed by applying radio-frequency (RF) voltage to electrodes. Understanding RF plasma properties is of fundamental interests and is essential in numerous applications 1-7 . Contemporary models of CCP discharge assume that the bulk plasma is well isolated from the boundaries by oscillating sheathes and applied bias mainly rests on sheathes instead of bulk plasma. Conduction current prevails displacement current in bulk plasma and field in bulk plasma is weak. But in this letter we will show that the bias can be primarily consumed by bulk plasma and electric field in plasma center needs not to be shielded by sheathes, due to intense boundary emission.Numerous studies have been done regarding boundary emission in CCP, but its influences are mostly assumed to be unessential 4,[8][9][10][11][12][13][14][15] . The steady flux of ion produces a surface emission flux = due to ion-induced secondary electron emission (SEE), with the emission coefficient. Under low pressure, secondary electrons (SEs) are lost before remarkable ionizations occur and their impact is less significant than that of hot electrons generated by stochastic heating 16 . Higher pressure incurs transition to γ mode where particle balance is modified by ionizations of SEs 10 . Yet the structure of plasma, i.e. bulk plasma connected by Bohm presheath and Child-law sheath, is supposed to be untouched and mean wall potential remains negative relative to plasma 17-21 .In this letter, we show that boundary emission can bring very strong disturbance and restructure entire RF plasma, which happens when is greater than averaged plasma electron flux 〈 〉 . It is well known that a space-charge limited (SCL) sheath is formed if > near floating boundary, such as thermionic emitter, emissive probe, Hall thruster, etc [22][23][24] . In this case, normal Child-law sheath still presents between plasma and the potential dip called virtual cathode (VC) near boundary, so dynamics of plasma are not essentially modified 25 . Recent works reported that a floating SCL sheath can become unstable due to cold ion trapping 26,27 . But no one has studied a RF plasma with intense boundary emission. Below we shall first show with simulation how RF plasma properties behave *

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Thermionic Cooling of the Target Plasma to a Sub-eV Temperature

Cited by 24 publications

References 37 publications

Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations

Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations

On the role of secondary electron emission in capacitively coupled radio‐frequency plasma sheath: A theoretical ground

Intense boundary emission destroys normal radio-frequency plasma sheath

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