Chapter 6
Fluid Description of Edge Plasma Transport
Abstract The comprehensive description of all features of plasma dynamics
requires kinetic consideration, which, however, is extremely complex and currently
non-tractable in its entirety. In many cases, simplified fluid approximation based on
the moments of the distribution functions of plasma particles gives rather accurate
results. The main approach to the derivation of the fluid equations and some
particular results of the so-called “classical” plasma transport, used in the book,
are considered in this chapter.
Tokamak edge plasma is especially difficult for modeling – it is the interface
between the hot core plasma and relatively low-temperature divertor region where
multiple impurity species and neutral gas dynamics are important. The presence of
material surfaces, e.g. divertor plates, with additional surface interactions and
reactions, complex magnetic field geometry, such as separatrix, and strong electric
field and plasma flows make it even more complex. Additional complications are
introduced by the presence of non-neutral regions (boundary sheath) near the
plasma-wall boundaries. In general, the plasma edge is the region where multiple
collision and atomic processes having very different characteristic times and lengths
are equally important and need to be considered self-consistently and simultaneously
with the anomalous turbulent transport phenomena. Clear separation of the time and
length scales, e.g. between the equilibrium and fluctuating quantities, is often
impossible in the edge region, which creates additional challenges.
The plasma turbulence and anomalous transport remain the biggest challenge for
the physics of edge plasmas. Currently, it is not feasible to simulate plasma turbulence in the edge and fully include all possible kinetic effects, neutrals and atomic
physics, sheath, boundaries, etc. Such simulations are still outside the modern
computer capabilities. Therefore, a number of simplifications and reductions of the
problem are usually performed. Presently, one can identify two major directions in
the theoretical description and modeling of the plasma edge. In one approach, which
is conditionally called here the first principle turbulence modeling, the focus is on
formulating adequate physics models, which would include relevant physics at small
scales to describe properly instabilities, turbulence and anomalous transport (see
Chap. 7 for further discussions). The global turbulence codes are also being
© 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_6
115
Fluid Description of Edge Plasma Transport
Abstract The comprehensive description of all features of plasma dynamics
requires kinetic consideration, which, however, is extremely complex and currently
non-tractable in its entirety. In many cases, simplified fluid approximation based on
the moments of the distribution functions of plasma particles gives rather accurate
results. The main approach to the derivation of the fluid equations and some
particular results of the so-called “classical” plasma transport, used in the book,
are considered in this chapter.
Tokamak edge plasma is especially difficult for modeling – it is the interface
between the hot core plasma and relatively low-temperature divertor region where
multiple impurity species and neutral gas dynamics are important. The presence of
material surfaces, e.g. divertor plates, with additional surface interactions and
reactions, complex magnetic field geometry, such as separatrix, and strong electric
field and plasma flows make it even more complex. Additional complications are
introduced by the presence of non-neutral regions (boundary sheath) near the
plasma-wall boundaries. In general, the plasma edge is the region where multiple
collision and atomic processes having very different characteristic times and lengths
are equally important and need to be considered self-consistently and simultaneously
with the anomalous turbulent transport phenomena. Clear separation of the time and
length scales, e.g. between the equilibrium and fluctuating quantities, is often
impossible in the edge region, which creates additional challenges.
The plasma turbulence and anomalous transport remain the biggest challenge for
the physics of edge plasmas. Currently, it is not feasible to simulate plasma turbulence in the edge and fully include all possible kinetic effects, neutrals and atomic
physics, sheath, boundaries, etc. Such simulations are still outside the modern
computer capabilities. Therefore, a number of simplifications and reductions of the
problem are usually performed. Presently, one can identify two major directions in
the theoretical description and modeling of the plasma edge. In one approach, which
is conditionally called here the first principle turbulence modeling, the focus is on
formulating adequate physics models, which would include relevant physics at small
scales to describe properly instabilities, turbulence and anomalous transport (see
Chap. 7 for further discussions). The global turbulence codes are also being
© 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_6
115