Chapter 4
Sheath Physics
Abstract Although the physics of the “sheath”, a thin layer of plasma just in front of
the material surface, has been studied for more than 100 years, the peculiarities of
fusion devices, such as strong magnetic field, a shallow angle at which the magnetic
field lines intersect the material surface, and inhomogeneity of the plasma parameters, bring some new and important features in this topic, which are discussed in this
chapter.
One of the most distinct processes at the edge of plasma devices is the plasma flow
along the magnetic field lines to the material surface (target) where plasma is
neutralized. Since for the edge plasma conditions, the electron thermal speed greatly
exceeds the ion one, to maintain ambipolarity of plasma flow (for simplicity, here we
assume no electric current), such plasma flow is accompanied by the formation of
the so-called sheath region in the vicinity of the target. This region is characterized
by a rather strong electrostatic electric field, which, usually, repels most of the free
streaming electron flux and, as a result, establishes ambipolarity of the plasma flow
(see Fig. 4.1).
Therefore, in the absence of strong electron emission from the material surface, a
monotonic electrostatic potential, φ(z), is built up between the plasma interior and
the material surface (here the z coordinate goes perpendicular to the surface that is
considered to be flat). The magnitude of the sheath potential drop φ sh (see Fig. 4.1) is
determined either from the condition of ambipolarity of the plasma flow or by the
value of the electric current flowing through the plasma-surface interface. The
electric field in the sheath causes energy exchange between the electrons and ions
and for the target potential negative with respect to the plasma, the energy of the ions
impinging on the surface can significantly exceed the thermal ion energy at the
entrance to the sheath. This effect can boost erosion and degradation of the plasmafacing components and cause unwanted plasma contamination with impurities.
The structure of the sheath depends on the angle, α, between the magnetic field, B
!
,
and the target (see Fig. 4.2; we notice that the direction of B
!
is arbitrary, whereas the
“parallel” coordinate, ℓ, goes toward the material surface). For the case of no magnetic
field or the magnetic field being perpendicular to the surface, the thickness of the
© Springer Nature Switzerland AG 2020
S. Krasheninnikov et al., On the Edge of Magnetic Fusion Devices, Springer Series in
Plasma Science and Technology, https://doi.org/10.1007/978-3-030-49594-7_4
73
Sheath Physics
Abstract Although the physics of the “sheath”, a thin layer of plasma just in front of
the material surface, has been studied for more than 100 years, the peculiarities of
fusion devices, such as strong magnetic field, a shallow angle at which the magnetic
field lines intersect the material surface, and inhomogeneity of the plasma parameters, bring some new and important features in this topic, which are discussed in this
chapter.
One of the most distinct processes at the edge of plasma devices is the plasma flow
along the magnetic field lines to the material surface (target) where plasma is
neutralized. Since for the edge plasma conditions, the electron thermal speed greatly
exceeds the ion one, to maintain ambipolarity of plasma flow (for simplicity, here we
assume no electric current), such plasma flow is accompanied by the formation of
the so-called sheath region in the vicinity of the target. This region is characterized
by a rather strong electrostatic electric field, which, usually, repels most of the free
streaming electron flux and, as a result, establishes ambipolarity of the plasma flow
(see Fig. 4.1).
Therefore, in the absence of strong electron emission from the material surface, a
monotonic electrostatic potential, φ(z), is built up between the plasma interior and
the material surface (here the z coordinate goes perpendicular to the surface that is
considered to be flat). The magnitude of the sheath potential drop φ sh (see Fig. 4.1) is
determined either from the condition of ambipolarity of the plasma flow or by the
value of the electric current flowing through the plasma-surface interface. The
electric field in the sheath causes energy exchange between the electrons and ions
and for the target potential negative with respect to the plasma, the energy of the ions
impinging on the surface can significantly exceed the thermal ion energy at the
entrance to the sheath. This effect can boost erosion and degradation of the plasmafacing components and cause unwanted plasma contamination with impurities.
The structure of the sheath depends on the angle, α, between the magnetic field, B
!
,
and the target (see Fig. 4.2; we notice that the direction of B
!
is arbitrary, whereas the
“parallel” coordinate, ℓ, goes toward the material surface). For the case of no magnetic
field or the magnetic field being perpendicular to the surface, the thickness of the
© Springer Nature Switzerland AG 2020
S. Krasheninnikov et al., On the Edge of Magnetic Fusion Devices, Springer Series in
Plasma Science and Technology, https://doi.org/10.1007/978-3-030-49594-7_4
73