occupied by hot fusion grade plasma, from the open ones where the magnetic field
lines intersect the PFC material.
The schematic view of the poloidal cross-section of magnetic configuration in a
tokamak with the simplest poloidal divertor is shown in Fig. 1.3b. Such magnetic
configuration can be formed just by adding a toroidally symmetric magnetic coil
under the divertor targets, which carries the electric current in the same direction as
the electric current in the plasma. These two currents create the magnetic separatrix
that plays the role of the LCFS for the case of the toroidal limiter and separates the
closed and open magnetic flux surfaces. Under the X-point, where the total poloidal
magnetic field vanishes by definition, there is a so-called “private flux” region (PFR)
having a very limited connection to the core plasma. Due to cross-field plasma
transport, the heat from the core comes to the “scrape-off layer” (SOL) plasma where
it can reach divertor targets quickly due to fast plasma transport along the magnetic
field lines. The region between the X-point and the divertor targets is called the
“divertor volume” or just a “divertor” and is often used for the designation of the
whole ensemble of the PFR and the “outer” and “inner” divertors located, respectively at the outer and inner sides of the torus.
The footprints of the heat flux at the targets are determined by the competition of
fast plasma transport along the magnetic field lines and relatively slow cross-field
plasma transport. As a result, the footprints appear to be small and all estimates show
that if in fusion reactor all the power Q SOL coming into the SOL from the core would
reach the targets, the maximum heat load on the targets would greatly exceed the
tolerable level. Therefore, a large fraction of this power should be dissipated on the
way to the target through the impurity and hydrogen radiation losses and this is one
of the main missions of the divertors.
The first poloidal magnetic divertors were implemented in tokamaks only in the
1970s (e.g. see Fig. 1.4). And, like it had been found before in the stellarators, it was
demonstrated that the implementation of a divertor in a tokamak reduces the
impurity content in the core plasma significantly.
Apart from that, at the beginning of the 1980s, it was discovered that the divertor
magnetic configuration promotes transition into new regimes of (i) improved core
plasma confinement, the so-called “H-mode” [7]; and (ii) highly radiative divertor
operation regimes with dense cold plasma and neutral gas cushion formed in the
divertor region, resulting in a strong reduction of the heat loading on the PFCs
(e.g. see [8–11]). Since that, these new regimes became the key ingredients of the
tokamak reactor designs and the main topics in the tokamak research. In some sense,
these divertor operation conditions are the combination of Spitzer’s divertor concept
and Tamm & Sakharov’s idea of the neutral gas cushion in front of the PFCs.
Such divertor regimes, called “high recycling regimes”, are characterized by a
very strong recirculation of neutrals and plasma in the divertor volume via neutral
ionization and plasma neutralization at the divertor targets and through the volumetric recombination processes. As a result, the neutral ionization source in the divertor
region in the high recycling regimes appears to be by orders of magnitude higher
than the neutral puffing and pumping rates.
4
1 Edge Plasma Issues in Magnetic Fusion Devices
lines intersect the PFC material.
The schematic view of the poloidal cross-section of magnetic configuration in a
tokamak with the simplest poloidal divertor is shown in Fig. 1.3b. Such magnetic
configuration can be formed just by adding a toroidally symmetric magnetic coil
under the divertor targets, which carries the electric current in the same direction as
the electric current in the plasma. These two currents create the magnetic separatrix
that plays the role of the LCFS for the case of the toroidal limiter and separates the
closed and open magnetic flux surfaces. Under the X-point, where the total poloidal
magnetic field vanishes by definition, there is a so-called “private flux” region (PFR)
having a very limited connection to the core plasma. Due to cross-field plasma
transport, the heat from the core comes to the “scrape-off layer” (SOL) plasma where
it can reach divertor targets quickly due to fast plasma transport along the magnetic
field lines. The region between the X-point and the divertor targets is called the
“divertor volume” or just a “divertor” and is often used for the designation of the
whole ensemble of the PFR and the “outer” and “inner” divertors located, respectively at the outer and inner sides of the torus.
The footprints of the heat flux at the targets are determined by the competition of
fast plasma transport along the magnetic field lines and relatively slow cross-field
plasma transport. As a result, the footprints appear to be small and all estimates show
that if in fusion reactor all the power Q SOL coming into the SOL from the core would
reach the targets, the maximum heat load on the targets would greatly exceed the
tolerable level. Therefore, a large fraction of this power should be dissipated on the
way to the target through the impurity and hydrogen radiation losses and this is one
of the main missions of the divertors.
The first poloidal magnetic divertors were implemented in tokamaks only in the
1970s (e.g. see Fig. 1.4). And, like it had been found before in the stellarators, it was
demonstrated that the implementation of a divertor in a tokamak reduces the
impurity content in the core plasma significantly.
Apart from that, at the beginning of the 1980s, it was discovered that the divertor
magnetic configuration promotes transition into new regimes of (i) improved core
plasma confinement, the so-called “H-mode” [7]; and (ii) highly radiative divertor
operation regimes with dense cold plasma and neutral gas cushion formed in the
divertor region, resulting in a strong reduction of the heat loading on the PFCs
(e.g. see [8–11]). Since that, these new regimes became the key ingredients of the
tokamak reactor designs and the main topics in the tokamak research. In some sense,
these divertor operation conditions are the combination of Spitzer’s divertor concept
and Tamm & Sakharov’s idea of the neutral gas cushion in front of the PFCs.
Such divertor regimes, called “high recycling regimes”, are characterized by a
very strong recirculation of neutrals and plasma in the divertor volume via neutral
ionization and plasma neutralization at the divertor targets and through the volumetric recombination processes. As a result, the neutral ionization source in the divertor
region in the high recycling regimes appears to be by orders of magnitude higher
than the neutral puffing and pumping rates.
4
1 Edge Plasma Issues in Magnetic Fusion Devices