the surface, reflection and desorption of hydrogenic species, erosion and
re-deposition of the PFC materials; etc.
In addition, the distribution of the plasma and neutral gas parameters in the edge
plasma is very non-uniform. Whereas at the midplane of the SOL, the plasma density
and temperature are ~10
13–14 cm
À3 and ~100 eV, in the divertor region the plasma
density can reach ~10
15 cm
À3 (and even higher) and the temperature drops to ~10 eV
in the attached and even to sub-eV in detached regimes. Whereas the neutral density
at the midplane is well below the plasma density, in the divertor volume the neutral
gas density can be comparable to the plasma one. The characteristic cross-field scale
of plasma parameter variation at the midplane of the SOL is ~few mm, whereas the
effective distance between the SOL midplane and divertor targets along the magnetic
field (the connection length) can be ~100 m.
All of these make quantitative theoretical/computational description of edge
plasma phenomena very difficult. Moreover, as of today, it is not feasible to describe
all processes in the edge plasma with a single “super code”. Therefore, the
researchers usually separate the fast “micro” turbulence in the plasma and the slow
macroscopic transport of the plasma and neutral gas and describe them with different
and very complex codes. We notice that the applicability of such splitting of the
processes into the turbulent and “mean-field” parts in the edge plasmas is often
questionable. From the experimental side, the situation is not any simpler because of
the strong non-uniformity of the plasma parameters and often a limited plasma
parameter range accessible to some diagnostics (e.g. for the Langmuir probes). In
addition, due to the geometrical complexity, it is often difficult to post-process the
available experimental data, and incompleteness of the experimental data complicates its interpretation.
Nonetheless, by interconnecting many bits of the information coming from the
experimental data, simplified theoretical models, and the results of numerical simulations, the edge plasma community has been able to build up a rather complete
physical picture of many important edge plasma phenomena.
It is obvious that today it is not possible to describe in detail all aspects of the edge
plasma physics because some topics are still poorly understood (e.g. modification of
Fig. 1.10 Magnetic
configuration and divertor
geometry in ITER
10
1 Edge Plasma Issues in Magnetic Fusion Devices
re-deposition of the PFC materials; etc.
In addition, the distribution of the plasma and neutral gas parameters in the edge
plasma is very non-uniform. Whereas at the midplane of the SOL, the plasma density
and temperature are ~10
13–14 cm
À3 and ~100 eV, in the divertor region the plasma
density can reach ~10
15 cm
À3 (and even higher) and the temperature drops to ~10 eV
in the attached and even to sub-eV in detached regimes. Whereas the neutral density
at the midplane is well below the plasma density, in the divertor volume the neutral
gas density can be comparable to the plasma one. The characteristic cross-field scale
of plasma parameter variation at the midplane of the SOL is ~few mm, whereas the
effective distance between the SOL midplane and divertor targets along the magnetic
field (the connection length) can be ~100 m.
All of these make quantitative theoretical/computational description of edge
plasma phenomena very difficult. Moreover, as of today, it is not feasible to describe
all processes in the edge plasma with a single “super code”. Therefore, the
researchers usually separate the fast “micro” turbulence in the plasma and the slow
macroscopic transport of the plasma and neutral gas and describe them with different
and very complex codes. We notice that the applicability of such splitting of the
processes into the turbulent and “mean-field” parts in the edge plasmas is often
questionable. From the experimental side, the situation is not any simpler because of
the strong non-uniformity of the plasma parameters and often a limited plasma
parameter range accessible to some diagnostics (e.g. for the Langmuir probes). In
addition, due to the geometrical complexity, it is often difficult to post-process the
available experimental data, and incompleteness of the experimental data complicates its interpretation.
Nonetheless, by interconnecting many bits of the information coming from the
experimental data, simplified theoretical models, and the results of numerical simulations, the edge plasma community has been able to build up a rather complete
physical picture of many important edge plasma phenomena.
It is obvious that today it is not possible to describe in detail all aspects of the edge
plasma physics because some topics are still poorly understood (e.g. modification of
Fig. 1.10 Magnetic
configuration and divertor
geometry in ITER
10
1 Edge Plasma Issues in Magnetic Fusion Devices