combination of the discrete (e.g. complex 3D3V neutral transport Monte-Carlo
codes treating a vast body of atomic physics) and continuous (e.g. plasma transport
based on the fluid approximation) parts, which complicates convergence and forces
implementation of different algorithms for their numerical solution (see Chap. 8).
Various experimental tools are used in the edge plasma studies, including different Langmuir probes, Thomson scattering, visible and UV spectroscopy, postmortem analysis of the plasma-facing components (including the nuclear reaction
analysis) and in situ surface temperature measurements with infrared radiation, etc.
However, the spatiotemporal resolution of all these experimental techniques is rather
limited and the raw data obtained in the experiments do often require postprocessing (e.g. conversion of the spectroscopic data obtained by integration of the
signal along the viewing chords into the distributions over the magnetic flux
coordinates).
Nonetheless, in spite of all these issues with both the experimental and theoretical/computational studies, substantial progress has been made in the last two
decades in our understanding of the physics of the edge plasma phenomena. In
particular, (i) The main ingredients governing the divertor plasma detachment
process have been identified and confirmed by both numerical simulations and
experimental data. (ii) The impact of the sheared plasma flow on suppression of
anomalous cross-field plasma transport has been clearly shown theoretically and
confirmed by both the results of numerical simulations and the experimental data.
Moreover, it was demonstrated that such a flow can be generated by the plasma
turbulence itself. (iii) It was shown that anomalous plasma cross-field transport can
not always be described by diffusion type equations and in many cases, it can have
more complex and nonlocal nature. In particular, it can be associated with large
intermittent bursts of the plasma particle and energy fluxes. (iv) Dust particles, under
some conditions, can play an important role in plasma contamination with impurity.
(v) Erosion and re-deposition of the materials of the plasma-facing components can
result in the formation of rather thick layers of the co-deposited material. Such loose
co-deposits can cause the formation of hot spots and emission of dust particles and
impurity atoms/molecules, severely limiting the operational window of fusion
devices. In addition, they can retain a large amount of hazardous tritium. (vi) Both
the fluid-based and gyro-kinetics-based codes capable of describing edge plasma
turbulence have been developed. They are more and more frequently used for the
understanding of anomalous transport of the edge plasma and in many cases produce
the experiment-relevant results.
However, there are still many things that can be improved. Here we outline only
the most important gaps in our understanding of the edge plasma phenomena.
Obviously, this list reflects just the view of the authors and it is pretty much possible
that other researches working in the field of the physics of edge plasma in magnetic
confinement devices would alter it.
We think that one of the most important showstoppers on the way to a better
understanding of the edge plasma phenomena is related to our poor comprehension
of edge plasma turbulence. Edge plasma turbulence has an overarching impact on
the processes in the edge plasma. It can govern the heat loading on and erosion of the
260
10 Conclusions and Outlook
codes treating a vast body of atomic physics) and continuous (e.g. plasma transport
based on the fluid approximation) parts, which complicates convergence and forces
implementation of different algorithms for their numerical solution (see Chap. 8).
Various experimental tools are used in the edge plasma studies, including different Langmuir probes, Thomson scattering, visible and UV spectroscopy, postmortem analysis of the plasma-facing components (including the nuclear reaction
analysis) and in situ surface temperature measurements with infrared radiation, etc.
However, the spatiotemporal resolution of all these experimental techniques is rather
limited and the raw data obtained in the experiments do often require postprocessing (e.g. conversion of the spectroscopic data obtained by integration of the
signal along the viewing chords into the distributions over the magnetic flux
coordinates).
Nonetheless, in spite of all these issues with both the experimental and theoretical/computational studies, substantial progress has been made in the last two
decades in our understanding of the physics of the edge plasma phenomena. In
particular, (i) The main ingredients governing the divertor plasma detachment
process have been identified and confirmed by both numerical simulations and
experimental data. (ii) The impact of the sheared plasma flow on suppression of
anomalous cross-field plasma transport has been clearly shown theoretically and
confirmed by both the results of numerical simulations and the experimental data.
Moreover, it was demonstrated that such a flow can be generated by the plasma
turbulence itself. (iii) It was shown that anomalous plasma cross-field transport can
not always be described by diffusion type equations and in many cases, it can have
more complex and nonlocal nature. In particular, it can be associated with large
intermittent bursts of the plasma particle and energy fluxes. (iv) Dust particles, under
some conditions, can play an important role in plasma contamination with impurity.
(v) Erosion and re-deposition of the materials of the plasma-facing components can
result in the formation of rather thick layers of the co-deposited material. Such loose
co-deposits can cause the formation of hot spots and emission of dust particles and
impurity atoms/molecules, severely limiting the operational window of fusion
devices. In addition, they can retain a large amount of hazardous tritium. (vi) Both
the fluid-based and gyro-kinetics-based codes capable of describing edge plasma
turbulence have been developed. They are more and more frequently used for the
understanding of anomalous transport of the edge plasma and in many cases produce
the experiment-relevant results.
However, there are still many things that can be improved. Here we outline only
the most important gaps in our understanding of the edge plasma phenomena.
Obviously, this list reflects just the view of the authors and it is pretty much possible
that other researches working in the field of the physics of edge plasma in magnetic
confinement devices would alter it.
We think that one of the most important showstoppers on the way to a better
understanding of the edge plasma phenomena is related to our poor comprehension
of edge plasma turbulence. Edge plasma turbulence has an overarching impact on
the processes in the edge plasma. It can govern the heat loading on and erosion of the
260
10 Conclusions and Outlook