Parallel and Poloidal Sheared Flows close to Instability Threshold

1

• Parallel and Poloidal Sheared Flows close to
  Instability Threshold
• in the TJ-II Stellarator
• M. A. Pedrosa, C. Hidalgo, B. Gonçalves, E.
  Ascasibar, T. Estrada, J. A. Jiménez, A.
  López-Fraguas
• I. 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

2
M. Angeles
INTRODUCTION

• 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.

3
Experimental Set-up
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.
ltRgt 1.5 m ltagt 0.22 m B0 1.2 T ltnegt (0.5 -
1.2) x 1019 m-3 PECRH 600 kW fECRH 53.2
GHz Pulse length 250 ms
4
Langmuir and Mach Probes
Measurements of edge plasma profiles in one
shot. Data obtained in two separated radial
positions. Measurements of Mach Number profiles.
Ion saturation current Is ? nTe1/2 Floating
potential Vf Vp - ?Te Mach Number ? v??/cs
5
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 (t 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.

6
Parallel and Poloidal Flows
7
Radial Correlation Reduction
The radial correlation decreases in the proximity
of the velocity shear layer. Velocity shear layer
suppresses low frequency fluctuations.
radially inwards
Shear Layer
8
Parallel Flows
(100_40_90)
Shear of parallel flow appears close to the
poloidal velocity shear layer.
4 104 m/s
-2 104 m/s
The absolute value of the Mach Number (M
0.4xln(IsatCt / IsatCo) could be affected by
differences in the probe areas as an offset.
9
Magnetic Configurations
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.
10
TJ-II Flexibility
Vacuum Magnetic Configurations with different
magnetic well
0.6
0.2
2.4
11
Fluctuations Changes
Strong change in fluctuations is obtained for the
most unstable configuration (reduced magnetic
well).
12
Edge Instabilities
The 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.
13
Radial Velocity
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).
14
Magnetic Well Effects in Profiles
Electron density profiles measured by
reflectometry show differences in shape depending
on the magnetic well value.
15
Parallel Flows and Instability

• Although no changes have been observed in the
  poloidal velocity shear, parallel flows are
  affected by the degree of instability in the
  plasma boundary region.

16
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.

17
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.

18
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.