Topological issues and relationships with turbulence in the edge region

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


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


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


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


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


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


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?


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


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


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


Islands & Profiles

Flow velocity and radial shear vary during the discharge evolution

( )ExB ExBV rr


Islands & Profiles

Higher velocity on conserved surface around the island

( )ExB ExBV rr



Islands & Profiles

Lower velocity on X-point where magnetic field lines intercept the wall

( )ExB ExBV rr



Density Limit

n/nG > 0.4

Br > 0 Br < 0

Density limit is due to the interplay between edge toroidal flow and magnetic topology


Density Limit

n/nG > 0.4

Br > 0 Br < 0


1. The wall locking is the local source of density


Density Limit

n/nG > 0.4

Br > 0 Br < 0



2. m = 0 island charges the wall with electrons


Density Limit

n/nG > 0.4

Br > 0 Br < 0




3. Accumulation point where v= 0


Density Limit

n/nG > 0.4

Br > 0 Br < 0





4. Formation of high density toroidally localised region


Density Limit

n/nG > 0.4

Br > 0 Br < 0





4. Formation of high density toroidally localised region

5. Edge cooling and soft landing of the plasma current


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 = 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


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


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


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


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



Edge Parameters

THB 0.9 < r/a < 0.98 THB 0.9 < r/a < 0.98


Turbulence Scaling


Common Properties

Alcator C-mod

NSTX


Density Limit

Radiation pattern in the poloidal plane


Density Limit


~ 4% of the total B(a)

SHAx

Documents

Topological issues and relationships with turbulence in the edge region