microscopic dust particles from the PFM can penetrate into the fusion grade plasma
and cool it down, thus killing the fusion reactions. Therefore, the plasma temperature
in the vicinity of the PFM should be below the erosion threshold (preferably ~ 1 eV)
to avoid such catastrophic scenarios. As we see, we are facing a very challenging
issue: to ensure a descent rate of the fusion reactions, the core plasma temperature
must be ~10
4 eV, whereas the plasma temperature near the plasma-facing components (PFCs) situated at the distance ~ 1 m from the core should be in the range of
1 eV.
An additional complication is that the magnetic confinement of plasma is not
perfect. First, there is so-called “classical” transport of the plasma energy and
particles across the magnetic field associated with collisions of the charged particles.
Secondly, there is also the so-called “anomalous” plasma leakage across the magnetic field, which is related to the turbulent electromagnetic fields driven by different
plasma instabilities, which often exceeds classical transport by orders of magnitude.
As a result, both the energy and particles (in particular, the products of fusion
reactions, the so-called “ash”—helium for the case of the deuterium-tritium reactions) are transported from the core to the peripheral region. Although this can heat
the plasma near the PFCs above the erosion limit, it allows removing the ash and
prevents the “poisoning” of the magnetic fusion device with the ash. Yet, the energy
transport to the PFCs can be strongly non-uniform and cause a local overheating of
the PFCs beyond the acceptable level. All these issues, associated with the exhaust
of fusion energy and ash, are controlled by plasma transport processes in the
peripheral region of magnetic fusion devices close to the PFCs (which is usually
called the “edge plasma”).
Transport processes in edge plasmas are complex and nonlinear and often result
in the formation of the plasma transport barrier and the transition of the entire
magnetic fusion device to a high-confinement operation regime (the so-called “Hmode”). Today, it is assumed that all future magnetic fusion reactors (such as ITER
which is under construction in France, and the first electric power generating device
DEMO which is under discussion) will be operating in H-mode. At the same time,
steep variations of the plasma parameters within the transport barrier can trigger
violent plasma instabilities causing rapid expulsion of hot (~ few keV) plasma
towards the PFCs, which for the reactor-relevant parameters can have catastrophic
consequences for the PFM.
Thus, we see that the edge plasma plays a crucial and multifaceted role in
magnetic fusion devices: on the one hand, it sets the plasma confinement which
defines virtually all major parameters of the magnetic fusion reactor (e.g., the size of
the apparatus and the magnetic field strength), whereas on the other hand, it
determines the energy and ash exhaust as well as the plasma interaction with the
PFCs, which, in addition to the potential threat of core plasma contamination by the
eroded material, control the lifetime of the entire device. As a matter of fact, the latter
issues were recognized by such “founding fathers” of magnetic fusion as A. D.
Sakharov and L. Spitzer already at the dawn of the era of magnetic fusion research.
Today, it is widely accepted that without reliable solutions to the outstanding issues
Preface
ix
and cool it down, thus killing the fusion reactions. Therefore, the plasma temperature
in the vicinity of the PFM should be below the erosion threshold (preferably ~ 1 eV)
to avoid such catastrophic scenarios. As we see, we are facing a very challenging
issue: to ensure a descent rate of the fusion reactions, the core plasma temperature
must be ~10
4 eV, whereas the plasma temperature near the plasma-facing components (PFCs) situated at the distance ~ 1 m from the core should be in the range of
1 eV.
An additional complication is that the magnetic confinement of plasma is not
perfect. First, there is so-called “classical” transport of the plasma energy and
particles across the magnetic field associated with collisions of the charged particles.
Secondly, there is also the so-called “anomalous” plasma leakage across the magnetic field, which is related to the turbulent electromagnetic fields driven by different
plasma instabilities, which often exceeds classical transport by orders of magnitude.
As a result, both the energy and particles (in particular, the products of fusion
reactions, the so-called “ash”—helium for the case of the deuterium-tritium reactions) are transported from the core to the peripheral region. Although this can heat
the plasma near the PFCs above the erosion limit, it allows removing the ash and
prevents the “poisoning” of the magnetic fusion device with the ash. Yet, the energy
transport to the PFCs can be strongly non-uniform and cause a local overheating of
the PFCs beyond the acceptable level. All these issues, associated with the exhaust
of fusion energy and ash, are controlled by plasma transport processes in the
peripheral region of magnetic fusion devices close to the PFCs (which is usually
called the “edge plasma”).
Transport processes in edge plasmas are complex and nonlinear and often result
in the formation of the plasma transport barrier and the transition of the entire
magnetic fusion device to a high-confinement operation regime (the so-called “Hmode”). Today, it is assumed that all future magnetic fusion reactors (such as ITER
which is under construction in France, and the first electric power generating device
DEMO which is under discussion) will be operating in H-mode. At the same time,
steep variations of the plasma parameters within the transport barrier can trigger
violent plasma instabilities causing rapid expulsion of hot (~ few keV) plasma
towards the PFCs, which for the reactor-relevant parameters can have catastrophic
consequences for the PFM.
Thus, we see that the edge plasma plays a crucial and multifaceted role in
magnetic fusion devices: on the one hand, it sets the plasma confinement which
defines virtually all major parameters of the magnetic fusion reactor (e.g., the size of
the apparatus and the magnetic field strength), whereas on the other hand, it
determines the energy and ash exhaust as well as the plasma interaction with the
PFCs, which, in addition to the potential threat of core plasma contamination by the
eroded material, control the lifetime of the entire device. As a matter of fact, the latter
issues were recognized by such “founding fathers” of magnetic fusion as A. D.
Sakharov and L. Spitzer already at the dawn of the era of magnetic fusion research.
Today, it is widely accepted that without reliable solutions to the outstanding issues
Preface
ix