Download Document

Topological issues and
relationships with
turbulence in the edge
region
Presented by Matteo Agostini
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Outline
 Edge region: magnetic and kinetic point of view
 Magnetic boundary, edge profiles and confinement properties
 Interplay between magnetic boundary and turbulence in the density
limit
 Link between magnetic modes and turbulence
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Introduction
 In the Reversed Field Pinch
configuration q exhibits a
decreasing profile
 In the edge region q = 0 (reversal
surface)
RFX-mod
 In the reversal surface all the m = 0
modes resonate
R=2m
a = 0.46m
Ip = 0.3 – 1.5 MA
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Edge: Gradients
 The edge region is characterised by strong radial gradients
 They are source of free energy for the development of
instabilities which could degrade the confinement
n / nG  0.1
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Edge: Gradients
 The edge region is characterised by strong radial gradients
 They are source of free energy for the development of
instabilities which could degrade the confinement
n / nG  0.1
n / nG  0.3
The edge region plays
a fundamental role in
the plasma
confinement
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Edge: Turbulence
 In the region outside the reversal surface turbulence is responsible for
the greatest part of the particle transport
 It is caused mainly by coherent structures: density blobs
 They are filaments elongated in the direction of the magnetic field
 Blobs can interact with the magnetic islands
Dt=0.4 ms
r [mm]
r [mm]
Dt=1.2 ms
f [mm]
Dt=0.8 ms
f [mm]
f [mm]
f [mm]
Dt=0
r [mm]
r [mm]
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Edge: Magnetic Topology
Magnetic topology of the edge is dominated by m = 0 islands due to
modes resonating on the reversal surface
Multiple Helicity
Large spectrum of m=1 modes is
destabilised
Quasi Single Helicity
Mode m=1 n = -7 dominates
Do the magnetic islands influence the edge properties and the transport?
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Magnetic Field & Edge Properties
 HeI light emission in the plasma
edge is modulated by the radial
magnetic field
 The maximum of emission is
correlated with Brm=1
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Magnetic Field & Edge Properties
 HeI light emission in the plasma
edge is modulated by the radial
magnetic field
 The maximum of emission is
correlated with Brm=1
 Floating potential measured by
probes array shows the same spatial
periodicity of the radial magnetic field
Indication of the interaction between
plasma edge and local magnetic field
ISIS
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
F= -0.2
#20367
Temperature profile flattens in the region
inside the m = 0 island
Temperature is constant on conserved
magnetic lines
Similar behaviour observed in tokamaks
during NTM
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
 Flow velocity and radial shear
vary during the discharge evolution
 ExB 

VExB (r )
r
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
 Flow velocity and radial shear
vary during the discharge evolution
 ExB 

VExB (r )
r
Higher velocity on conserved surface
around the island
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
 Flow velocity and radial shear
vary during the discharge evolution
 ExB 

VExB (r )
r
Lower velocity on X-point where magnetic
field lines intercept the wall
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
Br > 0
Br < 0
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
1. The wall locking is the local
source of density
Br > 0
Br < 0
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
1. The wall locking is the local
source of density
Br > 0
Br < 0
2. m = 0 island charges the wall
with electrons
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
1. The wall locking is the local
source of density
Br > 0
Br < 0
2. m = 0 island charges the wall
with electrons
3. Accumulation point where vf= 0
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
1. The wall locking is the local
source of density
Br > 0
Br < 0
2. m = 0 island charges the wall
with electrons
3. Accumulation point where vf= 0
4. Formation of high density
toroidally localised region
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Density limit is due to the interplay
between edge toroidal flow and
magnetic topology
1. The wall locking is the local
source of density
Br > 0
Br < 0
2. m = 0 island charges the wall
with electrons
3. Accumulation point where vf= 0
4. Formation of high density
toroidally localised region
5. Edge cooling and soft landing of
the plasma current
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Scaling With m = 1 Modes
Confinement improves with the improvement of the magnetic boundary
control
1/ 2
TS 0.78 < r/a < 0.87
 15 2 
  bm 1,n 

bmsec1   n 8
B0
Temperature gradient at the edge
become steeper
TS 0.78 < r/a < 0.87
Electron thermal conductivity
decreases
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
m = 1 & Turbulence
The decrease of the secondary modes is correlated with an improvement of
the pressure gradient at the far edge
THB 0.9 < r/a < 0.98
Pressure gradient is commonly
recognised as a source of free energy for
the edge turbulence
Lp = pressure radial scale
length
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
m = 1 & Turbulence
The decrease of the secondary modes is correlated with an improvement of
the pressure gradient at the far edge
THB 0.9 < r/a < 0.98
Lp = pressure radial scale
length
THB 0.9 < r/a < 0.98
Lb = toroidal scale length of edge
blobs
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Link With Other Devices
 Correlation between edge blobs and
pressure gradient is not a peculiar
characteristic of RFX-mod
THB 0.9 < r/a < 0.98
Alcator C-mod and NSTX points
reflect the same scaling
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Conclusions
Magnetic islands are found to exist around the reversal surface interacting
with the edge plasma
 Modify the edge profiles
 Play a role in the density limit
 Interact with the edge blobs
 Modify the edge transport and confinement
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Conclusions
Magnetic islands are found to exist around the reversal surface interacting
with the edge plasma
 Modify the edge profiles
 Play a role in the density limit
 Interact with the edge blobs
 Modify the edge transport and confinement
Open issues:
Study the mechanism of interaction between islands and edge turbulence
Clarify the role of islands and turbulence in the density limit
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Edge Parameters
THB 0.9 < r/a < 0.98
THB 0.9 < r/a < 0.98
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Turbulence Scaling
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Common Properties
NSTX
Alcator C-mod
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
Radiation pattern in the poloidal
plane
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
SHAx
~ 4% of the total B(a)
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009