Chapter 9
Physics of Some Edge Plasma Phenomena
Abstract The physics of two phenomena, comprising virtually all processes
discussed in the book, so-called Multifaceted Asymmetric Radiation From the
Edge (MARFE) and divertor plasma detachment are considered in this chapter.
Divertor plasma detachment phenomenon is considered as one of the most important
in edge plasma physics, since it allows the solution of the “heat removal” problem,
the fundamental technical issue of fusion reactors.
In this chapter, we will discuss the physics of some macroscopic phenomena, which
are distinctive for edge plasma. In particular, we will consider (i) MARFE (Multifaceted Asymmetric Radiation From the Edge [1]) and poloidaly symmetric plasma
detachment, (ii) self-sustained divertor plasma oscillations and (iii) divertor plasma
detachment.
9.1 MARFE and Poloidaly Symmetric Plasma Detachment
Due to high plasma temperature in the core region of magnetic confinement devices,
which ensures a high heat conductivity along the magnetic field lines, κ k / T
5/2 ,
plasma temperature can be considered constant on closed magnetic flux surfaces.
However, in the edge region, where temperature and, therefore, plasma heat conduction are relatively low, strongly localized radiation losses can result in an
inhomogeneous distribution of both plasma density and temperature over closed
magnetic flux surfaces, even for the case where they have no direct contact with the
materials of plasma-facing components. Then, obviously, plasma temperature will
experience some depression in the region of high radiation losses. It is interesting,
however, that the localization of enhanced radiation loss can be related to temperature depression itself. Indeed, recalling Fig. 2.14 we can see that the cooling rate
L imp (T e ) for low-Z (e.g. carbon) impurity has non-monotonic dependence on electron temperature, and in the temperature range ~ 8–20 eV, L imp (T e ) for carbon is
increasing with decreasing electron temperature. This feature of L imp (T e ) provides
positive feedback for the thermal plasma instability, which in the simplest case, can
© 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_9
229
Physics of Some Edge Plasma Phenomena
Abstract The physics of two phenomena, comprising virtually all processes
discussed in the book, so-called Multifaceted Asymmetric Radiation From the
Edge (MARFE) and divertor plasma detachment are considered in this chapter.
Divertor plasma detachment phenomenon is considered as one of the most important
in edge plasma physics, since it allows the solution of the “heat removal” problem,
the fundamental technical issue of fusion reactors.
In this chapter, we will discuss the physics of some macroscopic phenomena, which
are distinctive for edge plasma. In particular, we will consider (i) MARFE (Multifaceted Asymmetric Radiation From the Edge [1]) and poloidaly symmetric plasma
detachment, (ii) self-sustained divertor plasma oscillations and (iii) divertor plasma
detachment.
9.1 MARFE and Poloidaly Symmetric Plasma Detachment
Due to high plasma temperature in the core region of magnetic confinement devices,
which ensures a high heat conductivity along the magnetic field lines, κ k / T
5/2 ,
plasma temperature can be considered constant on closed magnetic flux surfaces.
However, in the edge region, where temperature and, therefore, plasma heat conduction are relatively low, strongly localized radiation losses can result in an
inhomogeneous distribution of both plasma density and temperature over closed
magnetic flux surfaces, even for the case where they have no direct contact with the
materials of plasma-facing components. Then, obviously, plasma temperature will
experience some depression in the region of high radiation losses. It is interesting,
however, that the localization of enhanced radiation loss can be related to temperature depression itself. Indeed, recalling Fig. 2.14 we can see that the cooling rate
L imp (T e ) for low-Z (e.g. carbon) impurity has non-monotonic dependence on electron temperature, and in the temperature range ~ 8–20 eV, L imp (T e ) for carbon is
increasing with decreasing electron temperature. This feature of L imp (T e ) provides
positive feedback for the thermal plasma instability, which in the simplest case, can
© 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_9
229