Chapter 8
Computational Modeling of the Edge
Plasma Transport Phenomena
Abstract Both nonlinear plasma dynamics due to plasma instabilities and macroscopic edge plasma transport, involving different atomic physics processes, impurity
effects and plasma-material interactions are extremely complex. As a result, only
limited cases can be treated analytically, whereas a thorough study requires numerical simulations. The main approaches to computational modeling of different edge
plasma phenomena, their assumptions and limitations are considered in this chapter.
Given the broad variety of the physical processes occurring in the edge plasma on
very different time scales, it would be impractical to attempt producing a single,
comprehensive model describing all the processes in the plasma edge from the first
principles. Due to the large disparity of the characteristic time and length scales
involved, such a model would require computer resources orders of magnitude larger
than available at present or in the near future. Moreover, even if such a model were
developed, it would probably require a special theory describing the detail of the
model itself and the “second-order” modeling of the processes involved in order to
understand the results. Most of the collision processes involving the edge plasma and
neutral gas species occur on sub-microsecond time scales. The small-scale turbulence develops in a fraction of a millisecond. The time scale of equilibration of the
plasma parameters along the magnetic field is determined by the sound speed and
connection length and is typically several tens milliseconds, which is also the time
scale for the cross-field transport outside the separatrix. Finally, the evolution of the
state of the material surfaces surrounding the plasma, which determines the recycling
conditions and impurity production rate, can take 100’s to 1000’s seconds until the
saturation (if any) is achieved. Therefore, there is a more than nine orders of
magnitude span of the time scales involved in the physical processes in the edge
plasma, so resolving all of them in one model looks unrealistic.
Nevertheless, computational modeling plays the crucial role bridging the theoretical understanding and experimental observations. The equations describing the
edge plasma are very complex, so analytical treatment is only possible with introducing strong simplifying assumptions that are not always strictly justified. The
experimental data are difficult to obtain and the measurements are not straightforward to interpret. Despite all limitations and many approximations, computational
© 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_8
201
Computational Modeling of the Edge
Plasma Transport Phenomena
Abstract Both nonlinear plasma dynamics due to plasma instabilities and macroscopic edge plasma transport, involving different atomic physics processes, impurity
effects and plasma-material interactions are extremely complex. As a result, only
limited cases can be treated analytically, whereas a thorough study requires numerical simulations. The main approaches to computational modeling of different edge
plasma phenomena, their assumptions and limitations are considered in this chapter.
Given the broad variety of the physical processes occurring in the edge plasma on
very different time scales, it would be impractical to attempt producing a single,
comprehensive model describing all the processes in the plasma edge from the first
principles. Due to the large disparity of the characteristic time and length scales
involved, such a model would require computer resources orders of magnitude larger
than available at present or in the near future. Moreover, even if such a model were
developed, it would probably require a special theory describing the detail of the
model itself and the “second-order” modeling of the processes involved in order to
understand the results. Most of the collision processes involving the edge plasma and
neutral gas species occur on sub-microsecond time scales. The small-scale turbulence develops in a fraction of a millisecond. The time scale of equilibration of the
plasma parameters along the magnetic field is determined by the sound speed and
connection length and is typically several tens milliseconds, which is also the time
scale for the cross-field transport outside the separatrix. Finally, the evolution of the
state of the material surfaces surrounding the plasma, which determines the recycling
conditions and impurity production rate, can take 100’s to 1000’s seconds until the
saturation (if any) is achieved. Therefore, there is a more than nine orders of
magnitude span of the time scales involved in the physical processes in the edge
plasma, so resolving all of them in one model looks unrealistic.
Nevertheless, computational modeling plays the crucial role bridging the theoretical understanding and experimental observations. The equations describing the
edge plasma are very complex, so analytical treatment is only possible with introducing strong simplifying assumptions that are not always strictly justified. The
experimental data are difficult to obtain and the measurements are not straightforward to interpret. Despite all limitations and many approximations, computational
© 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_8
201