Chapter 7
Anomalous Cross-Field Transport in Edge
Plasma
Abstract So-called “anomalous” plasma transport due to micro- and meso-scale
fluctuations of electromagnetic fields caused by different plasma instabilities is one
of the backbones of plasma physics in magnetic fusion devices. In this chapter the
main mechanisms driving and stabilizing the most typical edge plasma instabilities
are considered and the underlying physics is discussed.
7.1 Introduction
As we discussed in Chap. 6, classical cross-field plasma transport is determined,
roughly speaking, by two main components: charged particle motion in virtually
stationary magnetic and electric fields and Coulomb collisions. Cross-field drift of
charged particles related to inhomogeneity of the magnetic field, the direction of
which depends on the sign of the charge, causes global polarization of the tokamak
plasma column, accompanied by the E
! Â B
!
plasma convection, and, in addition, can
result in a significant departure of the particles from their initial magnetic flux
surfaces. On the one hand, the Coulomb collisions control the electric current
along the magnetic field lines, which balances plasma polarization due to the
magnetic drift and, therefore, settles the intensity of the E
! Â B
!
plasma convection.
On the other hand, the collisions “erase memory” of the charged particles on their
“initial” magnetic flux surface and introduce stochasticity in the charged particle
motion. As a result, even though the classical plasma energy and particle fluxes
through magnetic flux surface are determined by the local plasma parameters and
their gradients on the flux surface, the processes governing these fluxes (e.g. E
! Â B
!
plasma convection) happen on a “global” length-scale of the order of the tokamak
minor radius a.
However, due to inhomogeneity of the density and temperature, the tokamak
plasma, as it is discussed below, is often unstable. These instabilities result in the
formation of electrostatic potential φ r
!
, t
having filamentary structure extended
along the magnetic field lines over some distance λ k and a relatively small characteristic cross-field size, λ ⊥ ( a, λ k (for simplicity, we neglect perturbations 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_7
139
Anomalous Cross-Field Transport in Edge
Plasma
Abstract So-called “anomalous” plasma transport due to micro- and meso-scale
fluctuations of electromagnetic fields caused by different plasma instabilities is one
of the backbones of plasma physics in magnetic fusion devices. In this chapter the
main mechanisms driving and stabilizing the most typical edge plasma instabilities
are considered and the underlying physics is discussed.
7.1 Introduction
As we discussed in Chap. 6, classical cross-field plasma transport is determined,
roughly speaking, by two main components: charged particle motion in virtually
stationary magnetic and electric fields and Coulomb collisions. Cross-field drift of
charged particles related to inhomogeneity of the magnetic field, the direction of
which depends on the sign of the charge, causes global polarization of the tokamak
plasma column, accompanied by the E
! Â B
!
plasma convection, and, in addition, can
result in a significant departure of the particles from their initial magnetic flux
surfaces. On the one hand, the Coulomb collisions control the electric current
along the magnetic field lines, which balances plasma polarization due to the
magnetic drift and, therefore, settles the intensity of the E
! Â B
!
plasma convection.
On the other hand, the collisions “erase memory” of the charged particles on their
“initial” magnetic flux surface and introduce stochasticity in the charged particle
motion. As a result, even though the classical plasma energy and particle fluxes
through magnetic flux surface are determined by the local plasma parameters and
their gradients on the flux surface, the processes governing these fluxes (e.g. E
! Â B
!
plasma convection) happen on a “global” length-scale of the order of the tokamak
minor radius a.
However, due to inhomogeneity of the density and temperature, the tokamak
plasma, as it is discussed below, is often unstable. These instabilities result in the
formation of electrostatic potential φ r
!
, t
having filamentary structure extended
along the magnetic field lines over some distance λ k and a relatively small characteristic cross-field size, λ ⊥ ( a, λ k (for simplicity, we neglect perturbations 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_7
139