incompatible with the diffusive nature of plasma transport [99, 125], the physics of
blobs became one of the central topics of the edge plasma studies.
As of today, the blobs are observed in virtually all magnetic fusion devices
including both tokamaks and stellarators [126–129] and there is vast amount of
literature dedicated to different experimental and theoretical aspects of blobby
transport (e.g. see review papers [96, 98, 126]). As a matter of fact, the physical
reason for the radial advection of blobs (which are the filamentary structures
extended along the magnetic field, see Fig. 7.29) is simple [100].
Consider an isolated plasma filament situated in a vacuum (or in very low density
plasma) sketched in Fig. 7.30. Then magnetic drifts of the electrons and ions will
result in the polarization of the filament and the formation of a vertical (along the
major tokamak axis) electric field (for simplicity, we ignore the effect of the
magnetic shear). Such an electric field will cause outward advection of the filament
(see Fig. 7.24) due to E
! Â B
!
drift.
Therefore, in tokamaks, blobs are mainly observed at the outer side of the tours.
The strength of the vertical electric field is altered by the parallel electric current.
Different theoretical models for such a current, giving different scalings for the blob
speed, were used over the years including the sheath- and X-point-limited current,
and the “inhibited” current (see the review papers [96, 98] for details).
In the latter case, the blobs can reach the highest speed, which, according to
the numerical simulations [130], for n b ) n a can be estimated as U max %
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
2 T e þ T i
ð
Þ =M
p
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
n b =n a
ð
Þ δ b =R
ð
Þ
p
, where n b and n a are plasma density in the blob
and in the ambient plasma, δ b is the initial cross-field size of the blob.
0.4
0.8 I s (A)
1.0
0.0
Fig. 7.29 Time dependence of the ion saturation current on the probe situated in the SOL of the
DIII-D tokamak. (Reproduced with permission from [124], © AIP Publishing 2001)
plasma blob
δ
R
wall
V ∇ B ~ C s
+
–
V E
B
ρ i
Fig. 7.30 Schematic view
of the filament’s
polarization and advection
due to magnetic and E
! Â B
!
drifts. (Reproduced with
permission from [100],
© Elsevier 2001)
188
7 Anomalous Cross-Field Transport in Edge Plasma
blobs became one of the central topics of the edge plasma studies.
As of today, the blobs are observed in virtually all magnetic fusion devices
including both tokamaks and stellarators [126–129] and there is vast amount of
literature dedicated to different experimental and theoretical aspects of blobby
transport (e.g. see review papers [96, 98, 126]). As a matter of fact, the physical
reason for the radial advection of blobs (which are the filamentary structures
extended along the magnetic field, see Fig. 7.29) is simple [100].
Consider an isolated plasma filament situated in a vacuum (or in very low density
plasma) sketched in Fig. 7.30. Then magnetic drifts of the electrons and ions will
result in the polarization of the filament and the formation of a vertical (along the
major tokamak axis) electric field (for simplicity, we ignore the effect of the
magnetic shear). Such an electric field will cause outward advection of the filament
(see Fig. 7.24) due to E
! Â B
!
drift.
Therefore, in tokamaks, blobs are mainly observed at the outer side of the tours.
The strength of the vertical electric field is altered by the parallel electric current.
Different theoretical models for such a current, giving different scalings for the blob
speed, were used over the years including the sheath- and X-point-limited current,
and the “inhibited” current (see the review papers [96, 98] for details).
In the latter case, the blobs can reach the highest speed, which, according to
the numerical simulations [130], for n b ) n a can be estimated as U max %
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
2 T e þ T i
ð
Þ =M
p
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
n b =n a
ð
Þ δ b =R
ð
Þ
p
, where n b and n a are plasma density in the blob
and in the ambient plasma, δ b is the initial cross-field size of the blob.
0.4
0.8 I s (A)
1.0
0.0
Fig. 7.29 Time dependence of the ion saturation current on the probe situated in the SOL of the
DIII-D tokamak. (Reproduced with permission from [124], © AIP Publishing 2001)
plasma blob
δ
R
wall
V ∇ B ~ C s
+
–
V E
B
ρ i
Fig. 7.30 Schematic view
of the filament’s
polarization and advection
due to magnetic and E
! Â B
!
drifts. (Reproduced with
permission from [100],
© Elsevier 2001)
188
7 Anomalous Cross-Field Transport in Edge Plasma