plasma-facing components including the divertor targets and main chamber wall.
These issues are crucial for both the feasibility of fusion reactor design and the
reactor lifetime. We also notice that cross-field plasma transport in the SOL and
divertor regions plays an important role in divertor plasma detachment which seems
to be the only plausible solution for the mitigation of large divertor heat load in the
future reactors. In particular, today it is not clear if the plasma turbulence observed in
the SOL is driven by the SOL plasma itself or originates inside the separatrix and
then spreads into the SOL. It is also not clear, through what mechanisms the divertor
plasma conditions (e.g. the attached or detached states) affect edge plasma turbulence, as seems to be observed in experiments.
Another issue, which is closely related to the study of edge plasma turbulence, is
how to incorporate it into the codes simulating the edge plasma. It is clear that
without the use of comprehensive numerical simulations, no progress in edge plasma
studies is possible. However, today, edge plasma transport and turbulence are mostly
simulated with, correspondingly, 2D and 3D codes (the limitations associated with
such an approach were discussed in Chap. 8). Taking into account the multifaceted
nature of the physics of the edge plasma and strong synergy between the different
processes in it, we believe that both edge plasma turbulence and transport should be
described simultaneously by 3D codes. This is a very challenging task combining
different ranges of the timescales, which requires incorporation into the turbulence
codes of both the physics of multispecies plasma and the atomic physics effects.
Obviously, the development and real application of such codes require close collaboration of physicists and computer scientists as well as a new generation of
computers.
A further issue in the edge plasma physics is related to the interactions of the
plasma and neutral particles with the materials of the plasma-facing components. In
the past, such interactions were described by some particle and energy reflection
coefficients for the impinging species and sputtering coefficients for the surface
material. However, nowadays it is well recognized that these interactions are more
complex. They involve not only reflection of the impinging species and erosion and
re-deposition of the wall material, but also the modification of subsurface layers of
the wall material due to complex phenomena associated, in particular, with the
penetration of the impinging particles into the lattice of the wall material (see
Chap. 3). As a result, the wall response to the impact of the charged and neutral
particles of the edge plasma becomes very complex and nonlinear. Since all these
processes determine such crucial parameters of a magnetic fusion reactor as the
lifetime of the plasma-facing components and tritium retention, the topic of the
plasma-material interactions becomes one of the top priorities in the edge plasma
physics. We notice that the neutron damage of the lattice of the plasma-facing
materials, inevitable in fusion reactors, brings additional complication to this issue.
Because of the synergy among edge plasma transport, plasma recycling, impurity
radiation and transport, plasma-material interactions, physics of strongly modified
subsurface layers and wall material erosion, etc., it seems that it is inevitable that
more and more integrated models and codes of different sophistication, describing
the edge plasma in fusion devices, will be developed in the future. As a matter of
fact, such a development is already underway.
10 Conclusions and Outlook
261
These issues are crucial for both the feasibility of fusion reactor design and the
reactor lifetime. We also notice that cross-field plasma transport in the SOL and
divertor regions plays an important role in divertor plasma detachment which seems
to be the only plausible solution for the mitigation of large divertor heat load in the
future reactors. In particular, today it is not clear if the plasma turbulence observed in
the SOL is driven by the SOL plasma itself or originates inside the separatrix and
then spreads into the SOL. It is also not clear, through what mechanisms the divertor
plasma conditions (e.g. the attached or detached states) affect edge plasma turbulence, as seems to be observed in experiments.
Another issue, which is closely related to the study of edge plasma turbulence, is
how to incorporate it into the codes simulating the edge plasma. It is clear that
without the use of comprehensive numerical simulations, no progress in edge plasma
studies is possible. However, today, edge plasma transport and turbulence are mostly
simulated with, correspondingly, 2D and 3D codes (the limitations associated with
such an approach were discussed in Chap. 8). Taking into account the multifaceted
nature of the physics of the edge plasma and strong synergy between the different
processes in it, we believe that both edge plasma turbulence and transport should be
described simultaneously by 3D codes. This is a very challenging task combining
different ranges of the timescales, which requires incorporation into the turbulence
codes of both the physics of multispecies plasma and the atomic physics effects.
Obviously, the development and real application of such codes require close collaboration of physicists and computer scientists as well as a new generation of
computers.
A further issue in the edge plasma physics is related to the interactions of the
plasma and neutral particles with the materials of the plasma-facing components. In
the past, such interactions were described by some particle and energy reflection
coefficients for the impinging species and sputtering coefficients for the surface
material. However, nowadays it is well recognized that these interactions are more
complex. They involve not only reflection of the impinging species and erosion and
re-deposition of the wall material, but also the modification of subsurface layers of
the wall material due to complex phenomena associated, in particular, with the
penetration of the impinging particles into the lattice of the wall material (see
Chap. 3). As a result, the wall response to the impact of the charged and neutral
particles of the edge plasma becomes very complex and nonlinear. Since all these
processes determine such crucial parameters of a magnetic fusion reactor as the
lifetime of the plasma-facing components and tritium retention, the topic of the
plasma-material interactions becomes one of the top priorities in the edge plasma
physics. We notice that the neutron damage of the lattice of the plasma-facing
materials, inevitable in fusion reactors, brings additional complication to this issue.
Because of the synergy among edge plasma transport, plasma recycling, impurity
radiation and transport, plasma-material interactions, physics of strongly modified
subsurface layers and wall material erosion, etc., it seems that it is inevitable that
more and more integrated models and codes of different sophistication, describing
the edge plasma in fusion devices, will be developed in the future. As a matter of
fact, such a development is already underway.
10 Conclusions and Outlook
261
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