Post on 30-Jan-2016
description
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
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
Link between magnetic modes and turbulence
Interplay between magnetic boundary and turbulence in the density limit
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Introduction
RFX-mod
R = 2 m
a = 0.46m
Ip = 0.3 – 1.5 MA
In the Reversed Field Pinch configuration q exhibits a decreasing profile
In the edge region q = 0 (reversal surface)
In the reversal surface all the m = 0 modes resonate
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
/ 0.1Gn n
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
The edge region plays a fundamental role in the plasma confinement
/ 0.1Gn n / 0.3Gn n
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
t=0 t=0.4 s t=0.8 s t=1.2 s
r [mm] r [mm]r [mm]r [mm]
[m
m]
[m
m]
[m
m]
[m
m]
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
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 Br
m=1
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Magnetic Field & Edge Properties
ISIS
HeI light emission in the plasma edge is modulated by the radial magnetic field
The maximum of emission is correlated with Br
m=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
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
Temperature profile flattens in the region inside the m = 0 island
Similar behaviour observed in tokamaks during NTM
Temperature is constant on conserved magnetic lines
F= -0.2 #20367
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 ExBV rr
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
Higher velocity on conserved surface around the island
( )ExB ExBV rr
Flow velocity and radial shear vary during the discharge evolution
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Islands & Profiles
Lower velocity on X-point where magnetic field lines intercept the wall
( )ExB ExBV rr
Flow velocity and radial shear vary during the discharge evolution
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
1. The wall locking is the local source of density
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
1. The wall locking is the local source of density
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
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
1. The wall locking is the local source of density
2. m = 0 island charges the wall with electrons
3. Accumulation point where v= 0
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
Density Limit
n/nG > 0.4
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
1. The wall locking is the local source of density
2. m = 0 island charges the wall with electrons
3. Accumulation point where v= 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
Br > 0 Br < 0
Density limit is due to the interplay between edge toroidal flow and magnetic topology
1. The wall locking is the local source of density
2. m = 0 island charges the wall with electrons
3. Accumulation point where v= 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
TS 0.78 < r/a < 0.87
TS 0.78 < r/a < 0.87
1/ 2152
1,8sec
10
m nn
m
b
bB
Temperature gradient at the edge become steeper
Electron thermal conductivity decreases
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
m = 1 & Turbulence
THB 0.9 < r/a < 0.98
The decrease of the secondary modes is correlated with an improvement of the pressure gradient at the far edge
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
THB 0.9 < r/a < 0.98
m = 1 & Turbulence
THB 0.9 < r/a < 0.98
The decrease of the secondary modes is correlated with an improvement of the pressure gradient at the far edge
Lb = toroidal scale length of edge blobs
Lp = pressure radial scale length
M. Agostini, RFX-mod Programme Workshop 2009 Padova, 20 - 22 January 2009
THB 0.9 < r/a < 0.98
Link With Other Devices
Correlation between edge blobs and pressure gradient is not a peculiar characteristic of RFX-mod
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
Modify the edge profiles
Play a role in the density limit
Interact with the edge blobs
Modify the edge transport and confinement
Study the mechanism of interaction between islands and edge turbulence
Clarify the role of islands and turbulence in the density limit
Open issues:
Magnetic islands are found to exist around the reversal surface interacting with the edge plasma
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
Alcator C-mod
NSTX
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
~ 4% of the total B(a)
SHAx