1peter de vries – itbs and rotational shear – 18 february 2010 – oxford plasma theory group...

1Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group

P.C. de Vries

JET-EFDA Culham Science CentreAbingdon OX14 3DB UK

Internal Transport Barriers and Rotational ShearInternal Transport Barriers and Rotational Shear


IntroductionIntroduction 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


Turbulence and TransportTurbulence 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

‘Temperature Gradient’

‘Hea

t F

lux’

Neo-classical

Tcrit

eff


Internal Transport BarriersInternal Transport Barriers Profile stiffness locally broken in the presence of and Internal

transport barrier (ITB).– Studying ITBs may improve our understanding of transport physics

minor radius

pla

sma

pre

ssu

re

ITB

H-mode

pedestalTcrit


How to make an ITB 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 … )

GC GC

m/n=5/22/1


How to make an ITBHow 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.

TLT

LT

*

1JOFFRIN, E., Nucl. Fusion 43 (2003) 11672AUSTIN, M.E., Phys. Plasmas, 13 (2006) 082502

3WALTZ, R.E., Phys. Plasmas 13 (2006) 0523014SHARAPOV, S.E. Nuclear Fusion 46 (2006) S868

GC


ITBs and plasma rotationITBs 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.


TF ripple and Plasma RotationTF 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.


ITBs and plasma rotationITBs 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.

BT=1.0% + PABS=14.5MW

GC

BT=0.08% + PABS=14.5MW


ITB and RotationITB and Rotation ITB growth is limited in plasmas with a larger TF ripple, i.e. a

smaller rotation/less rotational shear


Rotational shear and ITBsRotational 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.

RB

E

rB

RB rExB

r

P

ZneBvBvEr

1


Rotational shear and ITG turbulenceRotational shear and ITG turbulence At the time the transport barrier forms/triggers:

– for high TF ripple or a larger ICRH fractions: ExB~1-2·104 [s-1]

almost one order of magnitude below the ITG growth rate ITG

– for low TF ripple and high NBI fractions: ExB~6·104 [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.


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 ITG=6-7 104 s-1

During growth phase

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 fully stabilised

ITG growth rate reduced to ITG=1.5 104 s-1


Other devicesOther 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.


JET and JT-60U comparisonJET 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

1DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 51 (2009) 124050.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 5 10 15 20

ExB (104 s-1)

*T

i0.014

0.023

JT60U:

at time of strongest ITB

o at time of ITB triggering

JET:

at time of strongest ITB

o at time of ITB triggering


Turbulence and Profile StiffnessTurbulence and Profile Stiffness Profile stiffness locally broken in the presence of and Internal

transport barrier (ITB).

Normalised Gradient R/LT

‘No

rmal

ised

Hea

t F

lux,

qi

Neo-classical

Tcrit

eff

ITB

growth


Turbulence and Profile StiffnessTurbulence and Profile Stiffness But what about plasmas without ITBs? Does the rotation affect

turbulence too?


‘No

rmal

ised

Hea

t F

lux,

qi

Neo-classical

Tcrit

eff

qi qires ni q

1.5 s Ti

2ieBR2

R

LTifR

LTiR

LTi crit

H

R

LT iR

LTi crit

Tcrit usually set by ITG growth rate

Stiffness factor: s

Tcrit


Turbulence and Profile StiffnessTurbulence and Profile Stiffness But what about plasmas without ITBs? Does the rotation affect

turbulence too?


‘No

rmal

ised

Hea

t F

lux,

qi

Neo-classical

Tcrit

eff

qi qires ni q

1.5 s Ti

2ieBR2

R

LTifR

LTiR

LTi crit

H

R

LT iR

LTi crit

Tcrit usually set by ITG growth rate

Stiffness factor: s

s

s


Stiffness and rotation(al shear)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


Stiffness and rotation(al shear)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?

Stiffness decreases with rotation Stiffness high for any rotation

Core region (R=3.33 m) = lower q’/q

1 MANTICA, P. , JET Science Meeting (2009)

Outer region (R=3.60 m) = higher q’/q


Conclusions/DiscussionConclusions/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?


List of PublicationsList 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)

Profile Stiffness and Plasma Rotation– MANTICA, P. , et al., Phys. Rev. Lett. 102 (2009) 175002

1peter de vries – itbs and rotational shear – 18 february 2010 – oxford plasma theory group...

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