Parallel and Poloidal Sheared Flows close to Instability Threshold

in the TJ-II StellaratorM. A. Pedrosa, C. Hidalgo, B. Gonçalves*, E. Ascasibar,

T. Estrada, J. A. Jiménez, A. López-FraguasI. Pastor, and the TJ-II team

Laboratorio Nacional de Fusión por Confinamiento Magnético Asociación EURATOM-CIEMAT, 28040-Madrid, SPAIN

*EURATOM-IST, 1049-001 Lisbon, Portugal

The operational flexibility of TJ-II makes it an ideal device to study the onset of fluctuations and related phenomena close to instability thresholds. The absence of magnetic well (i.e. magnetic hill) in TJ-II gives rise to instabilities.

Evidence of sheared parallel flow linked to poloidal sheared flow has been previously found in the proximity of the LCFS in tokamaks. Similar results are reported in the TJ-II stellarator. Parallel flows are modified as the magnetic configuration becomes unstable (i.e. the

magnetic well is reduced).

The transition to improved regimes implies non-monotonic relations between gradients and transport. Strong dynamical coupling between density gradients and turbulent transport has been experimentally found in the boundary region of TJ-II plasmas.

INTRODUCTIONM. Angeles:M. Angeles:

Experimental Set-up

<R> = 1.5 m

<a> ≤ 0.22 m

B0 ≤ 1.2 T

<ne>≈ (0.5 - 1.2) x 1019 m-3

PECRH ≤ 600 kW

fECRH = 53.2 GHz

Pulse length ≤ 250 ms

TJ-II is a four-field-period low-magnetic-shear stellarator.A fast movable Langmuir probe array is inserted into the plasma edge region at a velocity ≈ 1 m/s.

Langmuir and Mach Probes

Ion saturation current Is nTe1/2

Floating potential Vf ≈ Vp - Te

Mach Number v/cs

Measurements of edge plasma profiles in one shot.

Data obtained in two separated radial positions.

Measurements of Mach Number profiles.

Parallel and Poloidal Flows

• A steep gradient in the Ion Saturation Current profile is observed close to the shear layer.

• The Floating Potential changes sign close to the shear layer.

• Shear of parallel flow appears close to the poloidal velocity shear layer.

• The resulting radial gradient dvphase/dr is in the range of 105 s-1, comparable

to the inverse of the correlation time of fluctuations ( ≈ 10 s).

• The presence of sheared flows with shearing rates close to the critical value modify plasma turbulence in the plasma boundary.

• In some plasma conditions a reduction in the level of fluctuations close to the velocity shear layer has been obtained.

-2000

-1000

0

1000

2000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Shear Layer

0

0.01

0.02

0.03

0.04

-80

-60

-40

-20

0

20

40

0

0.01

0.02

0.03

0.04

0.05

0.06

2

6

10

14

18

180 200 220 240 260

Probe Position (mm)

Radially Inwards

Parallel and Poloidal Flows

Radial Correlation Reduction

60 70 80 90 100 110

200

150

100

50

0

Time (ms)

Frequency (kHz)

-9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0Esp__V12_V13_8170

The radial correlation decreases in the proximity of the velocity shear

layer.

Velocity shear layer suppresses low frequency fluctuations.

radially inwards

Shear Layer

Parallel FlowsShear of parallel flow appears close to the poloidal velocity shear layer.

The absolute value of the Mach Number (M= 0.4xln(IsatCt / IsatCo) could be affected by differences in the probe areas as an offset.

≈ 4 104 m/s

≈ -2 104 m/s-3000

-2000

-1000

0

1000

2000

-0.2

0

0.2

0.4

170 190 210 230 250

Poloidal phase velocity (m/s) Mach Number

z(mm)

Shear Layer

(100_40_90)

Magnetic Configurations

1.50

1.54

1.58

1.62

0 5 10 15 20 25

2.4 %0.6 %0.2 %

( )Mean Plasma Radius cm

-0.50

0.0

0.50

1.0

1.5

2.0

2.5

0 5 10 15 20 25

2.4 %0.6 %0.2 %

( )Mean Plasma Radius cm

It is possible in TJ-II to compare different magnetic configurations with a close value of the rotational transform and different magnetic well.

A region having magnetic well in the bulk of the plasma can coexist with a region having magnetic hill in the plasma edge.

TJ-II Flexibility

-0.4

-0.2

0

0.2

0.4

-0.4 -0.2 0 0.2 0.4

100_40_63

R-R0 (m)-0.4 -0.2 0 0.2 0.4

100_40_81

R-R0 (m)-0.4 -0.2 0 0.2 0.4

100_40_90

R-R0 (m)

2.4 % 0.6 % 0.2 %

Vacuum Magnetic Configurations with different magnetic well

Fluctuations Changes

Strong change in fluctuations is obtained for the most unstable configuration (reduced magnetic well).

-0.04

0

0.04

0.08100_40_63 Magnetic Well≈2.4% 100_40_81 Magnetic Well≈0.6% 100_40_90 Magnetic Well≈0.2%

-20

-10

0

10

20

120 125 130 135Time (ms)

130 135 140 145Time (ms)

185 190 195 200Time (ms)

Edge InstabilitiesThe strong events observed in the most unstable configuration are observed simultaneously in the H, ECE and line average density signals, similarly to ELMs events characteristics.

0

1

2

3

0

0.4

0.8

0 100 200 300( )Time ms

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1.61.8

22.22.42.62.8

33.2

0.65

0.66

0.67

0.68

0.69

0.7

0.71

185 190 195 200 205 210 215 220 ( )Time ms

Radial Velocity

0.04

0.08

0.12

0.16

0

0.02

0.04

0.06

195.5 196 196.5 197 197.5 198

Isat (A) Probe IN Isat (A) Probe OUT

Time (ms)

Δ ≈ 20 t s

The measured radial time delay in the propagation of large transport events implies radial velocities up to 500 - 1000 m/s.This velocity seems very similar to the radial propagation of ELMs in tokamaks (e.g. JET).

Magnetic Well Effects in Profiles

Electron density profiles measured by reflectometry show differences in shape depending on the magnetic well value.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

100_40_63 2.4%100_40_63 2.4%100_40_81 0.6%100_40_81 0.6%100_40_90 0.2%

n e (1013cm-3)

ρ

Parallel Flows and InstabilityAlthough no changes have been observed in the poloidal velocity shear, parallel flows are affected by the degree of instability in the plasma boundary region.

-0.2

0

0.2

0.4

0.6

180 200 220 240

Mach Number

z(mm)

Magnetic Well≈2.4 %Magnetic Well≈0.6 %Magnetic Well≈0.2 %

Shear Layer

Transport and Gradients

As the density gradient increases above the most probable gradient,

• the ExB turbulent driven transport increases

• the system perform a relaxation which tends to drive the plasma back to the marginal stable situation which minimized the size of transport events.

0

20

40

60

800.9 %0.9 %1.6 %2.4 %2.4 %

PDF

∇r I S

( . .)a u

-5

0

5

10

15

20

25

30

-0.03 -0.02 -0.01 0 0.01 0.02 0.03

0.9 %0.9 %1.6 %2.4 %2.4 %

E[ΓExB

/<Γ>| ∇

r IS]

∇r I

S( )A

-TJ II

Radial Velocity and Electric Fields

• The radial velocity is close to 20 m/s for small deviations from the averaged gradient but increases up to 1000 m/s for large transport events (reduced magnetic well).

• The effective radial velocity is consistent with the ExB drift velocity.

-200

0

200

400

600

800

1000

-0.03 -0.02 -0.01 0 0.01 0.02 0.03

0.9 %1.6 %2.4 %2.4 %

E[veff

| ∇

r IS] ( / )m s

∇r I

S( )A

-1000

-500

0

500

1000

1500

2000

-0.03 -0.02 -0.01 0 0.01 0.02 0.03

0.9 %1.6 %2.4 %2.4 %

E[Eθ |

∇r IS

] ( / )V m

∇r I

S( )A

CONCLUSIONS

• Comparative studies for different magnetic configurations with different magnetic well show changes in density and potential fluctuations in the plasma edge suggesting the existence of an instability threshold.

• The radial velocity obtained for the large events associated to the magnetic well-induced instability is in the range of 1000 m/s.

• The bursty beaviour of turbulent transport is linked to a departure from

the most probable radial gradient.

• Sheared poloidal end parallel flows are linked in the proximity of the LCFS of TJ-II in agreement with previous results obtained in tokamaks.

• Parallel flow affected by the degree of instability although no relevant modification has been found in the poloidal flow as the magnetic well changes.