averaging can introduce significant errors in the neutral penetration through the SOL
[32]. Besides this, the blobs perturb the distribution of the electric potential in the
edge plasma and can affect the E
! Â B
!
-driven flows significantly [32]. The second
drawback is that applying a prescribed outward velocity to all the species of the
multi-component plasma, one over-estimates the impurity screening since the impurities are flushed away with the convection. Moreover, applying any prescribed
convection velocity to the impurity ion transport can be physically wrong. Indeed,
given the apparent interchange nature of the blobs, their propagation outwards must
cause correspondingly enhanced inward transport of the background plasma. One
can easily assume that the bulk, hydrogenic ions are more abundant closer to the
separatrix, so their net “convective” flow is directed outwards. However, for the
impurities, this is not always the case. Therefore, for physically correct modeling of
intermittent, “blobby” transport in 2D transport models, one needs to take into
account both the heterogeneity of the plasma parameters and the presence of the
effective backflow of the background plasma.
A first step to producing a model of this kind was made in [33]. An approach of
including the blob dynamics in a transport model by averaging over the ensemble of
the blobs was developed there; however, the size and velocity of the blobs, as well as
their starting location are still external parameters in the model. The blobs are 3D
structures that occupy the full poloidal extent of the outboard SOL when mapped
onto the (R, Z) plane. Combining multiple single blobs (which can be interpreted as
time averaging) into a single “macro-blob” that travels across the SOL plasma
without interaction allows one to solve the equations describing the propagation of
the 3D filament in the framework of the 2D plasma solver (UEDGE in [33]). Such an
approach provides a description of the intermittent wall loading and introduces to
some extent a heterogeneous plasma background for interaction with neutrals, as
well as the plasma backflow that is mimicked by the “bypass” that transfers the
plasma from the blob front immediately to its wake. However, the effect of this
approach on the electric fields – and hence on the E
! Â B
!
drifts – is not yet clear.
Indeed, in this approach, a single macroscale circulation around the macro-blob
replaces a set of fluctuating mesoscale circulations around every single blob. A
similar concern arises regarding the heterogeneity of the plasma background in this
model. Here the macro-blob acts like a piston effectively screening the neutrals that
recycle off the sidewalls, whereas the ensemble of the single, poloidally localized
blobs leaves a gap for neutral penetration.
8.2 Neutral Transport Models
The neutral particles play an important role in the processes occurring in the edge
plasma [34]. They are not affected by the magnetic field, so their transport is
different from that of the charged components of the plasma. Eqs. (8.1) contain
the source terms describing also the interactions between the charged and neutral
8.2 Neutral Transport Models
207
[32]. Besides this, the blobs perturb the distribution of the electric potential in the
edge plasma and can affect the E
! Â B
!
-driven flows significantly [32]. The second
drawback is that applying a prescribed outward velocity to all the species of the
multi-component plasma, one over-estimates the impurity screening since the impurities are flushed away with the convection. Moreover, applying any prescribed
convection velocity to the impurity ion transport can be physically wrong. Indeed,
given the apparent interchange nature of the blobs, their propagation outwards must
cause correspondingly enhanced inward transport of the background plasma. One
can easily assume that the bulk, hydrogenic ions are more abundant closer to the
separatrix, so their net “convective” flow is directed outwards. However, for the
impurities, this is not always the case. Therefore, for physically correct modeling of
intermittent, “blobby” transport in 2D transport models, one needs to take into
account both the heterogeneity of the plasma parameters and the presence of the
effective backflow of the background plasma.
A first step to producing a model of this kind was made in [33]. An approach of
including the blob dynamics in a transport model by averaging over the ensemble of
the blobs was developed there; however, the size and velocity of the blobs, as well as
their starting location are still external parameters in the model. The blobs are 3D
structures that occupy the full poloidal extent of the outboard SOL when mapped
onto the (R, Z) plane. Combining multiple single blobs (which can be interpreted as
time averaging) into a single “macro-blob” that travels across the SOL plasma
without interaction allows one to solve the equations describing the propagation of
the 3D filament in the framework of the 2D plasma solver (UEDGE in [33]). Such an
approach provides a description of the intermittent wall loading and introduces to
some extent a heterogeneous plasma background for interaction with neutrals, as
well as the plasma backflow that is mimicked by the “bypass” that transfers the
plasma from the blob front immediately to its wake. However, the effect of this
approach on the electric fields – and hence on the E
! Â B
!
drifts – is not yet clear.
Indeed, in this approach, a single macroscale circulation around the macro-blob
replaces a set of fluctuating mesoscale circulations around every single blob. A
similar concern arises regarding the heterogeneity of the plasma background in this
model. Here the macro-blob acts like a piston effectively screening the neutrals that
recycle off the sidewalls, whereas the ensemble of the single, poloidally localized
blobs leaves a gap for neutral penetration.
8.2 Neutral Transport Models
The neutral particles play an important role in the processes occurring in the edge
plasma [34]. They are not affected by the magnetic field, so their transport is
different from that of the charged components of the plasma. Eqs. (8.1) contain
the source terms describing also the interactions between the charged and neutral
8.2 Neutral Transport Models
207