Chapter 1
Edge Plasma Issues in Magnetic Fusion
Devices
Abstract This chapter gives a short overview of the history of edge plasma studies,
defines the terminology used in the rest of the book, and gives some particular
examples of how multifaceted edge plasma physics plays one of the key roles in the
modern magnetic fusion research.
Today, there are two major classes of magnetic confinement devices: stellarators and
tokamaks. The main difference between them is the way to form the helical structure
of magnetic field lines, which is needed to compensate unwanted drift of charged
particles in the curved and inhomogeneous magnetic field and to form nested
magnetic flux surfaces necessary for plasma confinement (see the magnetic field
structure in a tokamak in Fig. 1.1). In a stellarator that has no toroidally symmetric
magnetic field, this is done by a complex shaping of the magnetic coils, whereas in a
toroidally symmetric tokamak by toroidal current flowing through the plasma.
As already mentioned in the Preface, the negative impact of the interaction of the
hot plasma with the plasma-facing components (PFC) of the vacuum chamber on
reactor performance was envisioned at the very beginning of the fusion era. The
main identified issues were (i) contamination of the core plasma with the eroded PFC
material beyond the acceptable level (which actually is very low for high-Z impurities), where the plasma radiation loss due to impurity exceeds the fusion power
released in alpha-particles so that no self-sustained fusion burn becomes possible,
and (ii) strong erosion of the PFC material, which can severely limit the lifetime of
the fusion reactor and make it unfeasible.
L. Spitzer and I.E. Tamm with A.D. Sakharov suggested two conceptually
different solutions to this problem in the 1950th. The Spitzer’s idea (e.g. see [2]
and the references therein) was to isolate as much as possible the region of intense
plasma-wall interaction from the core plasma. For this he suggested to use special
magnetic coils to divert the magnetic field lines at the edge of the magnetic fusion
device into some partially closed volume – the divertor (see Fig. 1.2). As a result, the
magnetic field lines in the core and in the edge become separated by the so-called
separatrix, whereas the impurity flux from the divertor into the core plasma is
suppressed due to both a rather narrow divertor throat and plugging with the plasma
flowing into the divertor.
© 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_1
1
Edge Plasma Issues in Magnetic Fusion
Devices
Abstract This chapter gives a short overview of the history of edge plasma studies,
defines the terminology used in the rest of the book, and gives some particular
examples of how multifaceted edge plasma physics plays one of the key roles in the
modern magnetic fusion research.
Today, there are two major classes of magnetic confinement devices: stellarators and
tokamaks. The main difference between them is the way to form the helical structure
of magnetic field lines, which is needed to compensate unwanted drift of charged
particles in the curved and inhomogeneous magnetic field and to form nested
magnetic flux surfaces necessary for plasma confinement (see the magnetic field
structure in a tokamak in Fig. 1.1). In a stellarator that has no toroidally symmetric
magnetic field, this is done by a complex shaping of the magnetic coils, whereas in a
toroidally symmetric tokamak by toroidal current flowing through the plasma.
As already mentioned in the Preface, the negative impact of the interaction of the
hot plasma with the plasma-facing components (PFC) of the vacuum chamber on
reactor performance was envisioned at the very beginning of the fusion era. The
main identified issues were (i) contamination of the core plasma with the eroded PFC
material beyond the acceptable level (which actually is very low for high-Z impurities), where the plasma radiation loss due to impurity exceeds the fusion power
released in alpha-particles so that no self-sustained fusion burn becomes possible,
and (ii) strong erosion of the PFC material, which can severely limit the lifetime of
the fusion reactor and make it unfeasible.
L. Spitzer and I.E. Tamm with A.D. Sakharov suggested two conceptually
different solutions to this problem in the 1950th. The Spitzer’s idea (e.g. see [2]
and the references therein) was to isolate as much as possible the region of intense
plasma-wall interaction from the core plasma. For this he suggested to use special
magnetic coils to divert the magnetic field lines at the edge of the magnetic fusion
device into some partially closed volume – the divertor (see Fig. 1.2). As a result, the
magnetic field lines in the core and in the edge become separated by the so-called
separatrix, whereas the impurity flux from the divertor into the core plasma is
suppressed due to both a rather narrow divertor throat and plugging with the plasma
flowing into the divertor.
© 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_1
1