Transport Phenomena in Partially Ionized Plasma

Rozhansky, V.; Tsendin, L. D.

doi:10.1201/9781482288094

Cited by 138 publications

(182 citation statements)

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“…A strong electric field called the Debye sheath [1] forms at the boundary, accelerating ions to the surface while reflecting enough approaching electrons to keep the two fluxes equal. Sheath theory is essential for studying plasma-wall interaction, setting boundary conditions in fluid simulation codes [2,3] and measuring plasma properties with Langmuir probes [4].If the bulk plasma has a Maxwellian electron velocity distribution function (EVDF), the electron flux to the surface can be written as Г e = Г e,0 exp(-eΦ/T e ) [ 5 ] where Г e,0 = n(T e /2πm e ) 1/2 is the thermal flux that would strike a surface without the insulating sheath barrier. Here Φ is the sheath potential magnitude, n is the plasma density and T e is the electron temperature.…”

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confidence: 99%

“…The net emission γ net saturates to a critical value γ cr < 1 so that the ion flux in (1) can still be balanced. The "space-charge limited" (SCL) sheath is usually assumed to form under very strong emission in tokamaks [2,9], Hall thrusters [10], emissive probes [3,11] and general plasma-wall systems [4,8].All theories invoking the SCL sheath rely on a sheath structure existing a priori as the SEE intensity increases beyond the threshold for saturation. For instance, the original Hobbs-Wesson paper [8] assumes ions "arrive at the sheath edge" with a velocity related to the Bohm criterion.…”

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Absence of Debye Sheaths due to Secondary Electron Emission

2012

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A bounded plasma where the electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.

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Absence of Debye Sheaths due to Secondary Electron Emission

2012

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“…14. In the general case the field-driven fluxes of the charged particles are of a complex nature, and the electric field must be explicitly retained in the transport equations. It was shown in [15,16] that the linear equation for ambipolar diffusion is strictly valid only in the case of so called simple 5 weakly ionized plasma, which consists of electrons and one species of the positive ions with constant field-independent mobilities and diffusion coefficients. Indeed, in the case of two- and via quasineutrality condition (n e =p-n) depends nonlinearly on the negative ion density too.…”

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confidence: 99%

Negative Ion Density Fronts

Kaganovich

2000

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Negative ions tend to stratify in electronegative plasmas with hot electrons (electron temperature T e much larger than ion temperature T i , T e >>T i ). The boundary separating a plasma containing negative ions, and a plasma, without negative ions, is usually thin, so that the negative ion density falls rapidly to zero -forming a negative ion density front. We review theoretical, experimental and numerical results giving the spatio-temporal evolution of negative ion density fronts during plasma ignition, the steady state, and extinction (afterglow). During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlinear convection. In a steadystate plasma, the fronts are boundary layers with steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nature.Negative ions diffuse freely in the plasma core (no convection), whereas the negative ion front propagates towards the chamber walls with a nearly constant velocity. The concept of fronts turns out to be very effective in analysis of plasma density profile evolution in strongly non-isothermal plasmas.

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“…В этих областях разряда электрическое поле мало, и в зависимости от длины разрядного проме-жутка здесь формируется одна или две точки обращения знака электрического поля (см., например, [2][3][4][5][6][7][8][9]). Фор-мально возможен случай прямого электрического поля во всей плазменной области тлеющего разряда.…”

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“…Поскольку плазма NG и FDS всегда сильно неоднородна, то большой градиент плот-ности электронов приводит к тому, что диффузионная компонента электронного тока там практически всегда превышает разрядный ток (eD e ∇n > j) и возникает обратное поле, тормозящее электроны и ускоряющее ионы по направлению уже не к катоду, а к положи-тельному аноду [1, 2,4]. В рамках гидродинамической модели уточненный критерий обращения по [1,24] поля представлен в [9].…”

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