Analysis of MHD in high density ITB discharges

1
Internal Transport Barriers and Rotational Shear
P.C. de Vries JET-EFDA Culham Science
Centre Abingdon OX14 3DB UK
2
Introduction

• Why Internal Transport Barriers?
• ITBs may play a role in advanced tokamak scenario
  for ITER1.
• Studying ITBs may improve our understanding of
  transport physics
• Scope of this presentation
• Present experimental observations on ITBs at JET
• Especially focussing on the role of rotation(al)
  shear
• How do these relate to turbulence and transport
  physics?
• Present results on other recent transport studies
  at JET
• Provide a reference to various experimental
  papers
• Start a discussion

3
Turbulence and Transport

• Transport in Tokamak plasma is predominantly
  driven by turbulence.
• Temperature gradient driven turbulence ? stiff
  profiles
• Non-diffusive behaviour ? ?T does not change with
  Heat Flux

4
Internal Transport Barriers

• Profile stiffness locally broken in the presence
  of and Internal transport barrier (ITB).
• Studying ITBs may improve our understanding of
  transport physics

5
How to make an ITB

• Empirical recipe to form strong ion internal
  transport barriers
• Optimised q-profiles with low or negative
  magnetic shear (q/q)
• Often significant Neutral Beam Injection (NBI)
  heating
• Similar recipe used in various Tokamaks (JT-60U,
  DIII-D, AUG )

6
How to make an ITB

• In plasmas with negative magnetic shear a
  specific class of ITBs are triggered when qmin
  reaches an integer value1,2,3
• Confirmed by the onset of an Grand-Cascade of
  Alfven waves4.

1JOFFRIN, E., Nucl. Fusion 43 (2003)
1167 2AUSTIN, M.E., Phys. Plasmas, 13 (2006)
082502 3WALTZ, R.E., Phys. Plasmas 13 (2006)
052301 4SHARAPOV, S.E. Nuclear Fusion 46 (2006)
S868
7
ITBs and plasma rotation

• How important is the NBI ingredient? ?? rotation?
• Can we make strong ITBs without fast plasma
  rotation?
• Experiments on ITBs at JET were carried out,
  where the plasma rotation was changed by
• Replacing the NBI by ICRH ion heating1,2
• Not easy to keep the heat flux unchanged
• Applying smaller or larger toroidal field
  ripples2,3
• Change rotation independent from heat flux

1HAWKES, N.C., et al., Contribution to the th EPS
Conference (Warsaw) 2008. 2DE VRIES, P.C., et
al., Nucl. Fusion 49 (2009) 075007. 3DE VRIES,
P.C., et al., Plasma Phys. Control. Fusion 50
(2008) 065008.
8
TF ripple and Plasma Rotation

• JET has the unique capability to alter its
  toroidal field ripple.
• This has a significant effect on the plasma
  rotation1.
• But less on the heat deposition by NBI and ICRH

1DE VRIES, P.C., et al., Nucl. Fusion 48 (2008)
035007.
9
ITBs and plasma rotation

• Increasing the TF ripple amplitude and reducing
  the rotational shear
• has a detrimental effect on the growth of the
  ITB.
• Nevertheless, an ITB triggering event is still
  visible!

2DE VRIES, P.C., et al., Plasma Phys. Control.
Fusion 50 (2008) 065008. 3DE VRIES, P.C., et al.,
Nucl. Fusion 49 (2009) 075007.
10
ITB and Rotation

• ITB growth is limited in plasmas with a larger TF
  ripple, i.e. a smaller rotation/less rotational
  shear

11
Rotational shear and ITBs

• The rotational shear or shearing rate ?ExB has
  been calculated under the assumption of
  neo-classical poloidal rotation.
• The rotational shear at the time the ITB is
  triggered varied with TF ripple

1DE VRIES, P.C., et al., Nucl. Fusion 49 (2009)
075007.
12
Rotational shear and ITG turbulence

• At the time the transport barrier forms/triggers
• for high TF ripple or a larger ICRH fractions
  ?ExB1-2104 s-1
• almost one order of magnitude below the ITG
  growth rate ?ITG
• for low TF ripple and high NBI fractions
  ?ExB6104 s-1
• of the order of ITG growth rate ?ITG
• Detailed modelling with the GYRO code showed that
  in the second case the ITG growth rate is
  affected but not yet fully stabilised.
• The triggering of ion ITBs in JET are usually not
  predicted from theory based transport models1,2

1BARANOV, Y.F., et al., Plasma Phys. Control.
Fusion 46 (2004) 1181. 2TALA, T, et al., Nucl.
Fusion 46 (2006) 548.
13
ITB growth

• The ITB will enhance the gradient in toroidal
  rotation
• Thus the ITB itself may be able to push up
  ?ExB/?ITG.
• As long as this ratio is high enough at the time
  of triggering
• GYRO modelling1,2
• Without rotation to gITG6-7 104 s-1

During growth phase
ITG fully stabilised
Before triggering
1DE VRIES, P.C., et al., Nucl. Fusion 49 (2009)
075007. 2 CANDY, J., and WALTZ, R.E, Phys. Rev.
Lett. (2003) 045001
ITG growth rate reduced to gITG1.5 104 s-1
14
Other devices

• Similar/near identical results have been obtained
  in other devices
• JT-60U using NBI balancing1
• DIII-D using NBI balancing3

1SAKAMOTO, Y., et al., Nucl. Fusion 41 (2001) 865
2DE VRIES, P.C., et al., Plasma Phys. Control.
Fusion 51 (2009) 124050. 3SHAFER, M.W, et al.,
Phys. Rev. Lett. 103 (2009) 075004.
15
JET and JT-60U comparison

• JT-60U and JET ITB identity experiments showed
  that differences in ITBs between both devices
  could be explained (partly) by rotation
  differences1

• JT60U ?
• ? at time of strongest ITB
• o at time of ITB triggering

• JET ?
• ? at time of strongest ITB
• o at time of ITB triggering

1DE VRIES, P.C., et al., Plasma Phys. Control.
Fusion 51 (2009) 124050.
16
Turbulence and Profile Stiffness

• Profile stiffness locally broken in the presence
  of and Internal transport barrier (ITB).

ITB growth
17
Turbulence and Profile Stiffness

• But what about plasmas without ITBs? Does the
  rotation affect turbulence too?

?Tcrit usually set by ITG growth rate Stiffness
factor cs

?Tcrit
?Tcrit
Normalised Heat Flux, qi
?eff
Normalised Gradient R/LT
Neo-classical
18
Turbulence and Profile Stiffness

• But what about plasmas without ITBs? Does the
  rotation affect turbulence too?

?Tcrit usually set by ITG growth rate Stiffness
factor cs
cs

cs
19
Stiffness and rotation(al shear)

• Detailed experiments at JET indicate that the
  stiffness is affected by the plasma
  rotation/rotational shear
• from power balance and modulation experiments

1 MANTICA, P. , et al., Phys. Rev. Lett. 102
(2009) 175002
20
Stiffness and rotation(al shear)

• Latest analysis in suggest that the impact of the
  rotation on the profile stiffness may depend on q
  or q/q
• Question Does a flat q profile enables the
  rotation(al shear) to affect the ion stiffness?

Core region (R3.33 m) lower q/q
Outer region (R3.60 m) higher q/q
Stiffness high for any rotation
Stiffness decreases with rotation
1 MANTICA, P. , JET Science Meeting (2009)
21
Conclusions/Discussion

• Ion ITBs are triggered independent of the
  rotation
• Strong player in the triggering process is the q
  profile
• The ITB growth is strongly affected by the
  rotational shear
• ITBs do not grow after triggering if the
  rotational shear is too low
• Note that these results do not excluded other
  mechanisms that aid the growth of ITBs
• such as fast-particles, etc.
• The physics of electron ITBs differ all together
  (q-profile).
• GYRO modelling suggest that ITG turbulence is
  suppressed in strong ITBs
• Rotational shear affects the growth rate/critical
  gradient
• Or is it the stiffness that is affected by the
  rotational and q?

22
List of Publications

• Experimental observations of ITBs
• CONNOR, J.W.,et al. 2004 Nucl. Fusion 44 R1
• WOLF, R.C., Plasma Phys. Control. Fusion 45
  (2003) R1-R91
• CHALLIS, C.D.Plasma Phys. Control Fusion (2004)
  46 2004
• CHALLIS, C.D., et al., Plasma Phys. Control.
  Fusion 43 (2001) 861
• JOFFRIN, E., Nucl. Fusion 43 (2003) 1167
• AUSTIN, M.E., Phys. Plasmas, 13 (2006) 082502
• WALTZ, R.E., Phys. Plasmas 13 (2006) 052301
• ...
• ITBs and Plasma Rotation
• BURRELL, K.H.,et al., Phys. Plasmas 4 (1997) 1499
  
• DE VRIES, P.C., et al., Plasma Phys. Control.
  Fusion 50 (2008) 065008.
• DE VRIES, P.C., et al., Nucl. Fusion 49 (2009)
  075007
• SAKAMOTO, Y., et al., Nucl. Fusion 41 (2001) 865
• DE VRIES, P.C., et al., Plasma Phys. Control.
  Fusion 51 (2009) 124050.
• SHAFER, M.W, et al., Phys. Rev. Lett. 103 (2009)
  075004.
• Gyro code
• CANDY, J., and WALTZ, R.E, Phys. Rev. Lett.
  91(2003) 045001 (and refs. Therein)