1Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group P.C. de Vries JET-EFDA Culham Science Centre Abingdon OX14 3DB UK Internal Transport Barriers and Rotational Shear

2Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory GroupIntroduction Why Internal Transport Barriers? –ITBs may play a role in advanced tokamak scenario for ITER 1. –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

3Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 ‘Temperature Gradient’ ‘Heat Flux’ Neo-classical  T crit  eff

4Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 minor radius plasma pressure ITB H-mode pedestal  T crit

5Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 m/n=5/2 2/1

6Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group How to make an ITB In plasmas with negative magnetic shear a specific class of ITBs are ‘triggered’ when q min reaches an integer value 1,2,3 –Confirmed by the onset of an Grand-Cascade of Alfven waves 4. 1 JOFFRIN, E., Nucl. Fusion 43 (2003) AUSTIN, M.E., Phys. Plasmas, 13 (2006) WALTZ, R.E., Phys. Plasmas 13 (2006) SHARAPOV, S.E. Nuclear Fusion 46 (2006) S868 GC

7Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 heating 1,2 Not easy to keep the heat flux unchanged –Applying smaller or larger toroidal field ripples 2,3 Change rotation independent from heat flux 1 HAWKES, N.C., et al., Contribution to the th EPS Conference (Warsaw) DE VRIES, P.C., et al., Nucl. Fusion 49 (2009) DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 50 (2008)

8Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group TF ripple and Plasma Rotation JET has the unique capability to alter its toroidal field ripple. –This has a significant effect on the plasma rotation 1. –But less on the heat deposition by NBI and ICRH 1 DE VRIES, P.C., et al., Nucl. Fusion 48 (2008)

9Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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! 2 DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 50 (2008) DE VRIES, P.C., et al., Nucl. Fusion 49 (2009)  BT =1.0% + P ABS =14.5MW GC  BT =0.08% + P ABS =14.5MW

10Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group ITB and Rotation ITB growth is limited in plasmas with a larger TF ripple, i.e. a smaller rotation/less rotational shear

11Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 1 DE VRIES, P.C., et al., Nucl. Fusion 49 (2009)

12Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group Rotational shear and ITG turbulence At the time the transport barrier forms/triggers: –for high TF ripple or a larger ICRH fractions:  ExB ~1-2·10 4 [s -1 ] almost one order of magnitude below the ITG growth rate  ITG –for low TF ripple and high NBI fractions:  ExB ~6·10 4 [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 models 1,2 1 BARANOV, Y.F., et al., Plasma Phys. Control. Fusion 46 (2004) TALA, T, et al., Nucl. Fusion 46 (2006) 548.

13Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 modelling 1,2 : –Without rotation to  ITG = s -1 During growth phase Before triggering 1 DE VRIES, P.C., et al., Nucl. Fusion 49 (2009) CANDY, J., and WALTZ, R.E, Phys. Rev. Lett. (2003) ITG fully stabilised ITG growth rate reduced to  ITG = s -1

14Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group Other devices Similar/near identical results have been obtained in other devices –JT-60U using NBI balancing 1 –DIII-D using NBI balancing 3 1 SAKAMOTO, Y., et al., Nucl. Fusion 41 (2001) DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 51 (2009) SHAFER, M.W, et al., Phys. Rev. Lett. 103 (2009)

15Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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 differences 1 1 DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 51 (2009) JT60U:   at time of strongest ITB o at time of ITB triggering JET:   at time of strongest ITB o at time of ITB triggering

16Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group Turbulence and Profile Stiffness Profile stiffness locally broken in the presence of and Internal transport barrier (ITB). Normalised Gradient R/L T ‘Normalised Heat Flux, q i Neo-classical  T crit  eff ITB growth

17Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group Turbulence and Profile Stiffness But what about plasmas without ITBs? Does the rotation affect turbulence too? Normalised Gradient R/L T ‘Normalised Heat Flux, q i Neo-classical  T crit  eff  T crit usually set by ITG growth rate Stiffness factor:  s  T crit

18Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group Turbulence and Profile Stiffness But what about plasmas without ITBs? Does the rotation affect turbulence too? Normalised Gradient R/L T ‘Normalised Heat Flux, q i Neo-classical  T crit  eff  T crit usually set by ITG growth rate Stiffness factor:  s ss ss

19Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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)   

20Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group 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

21Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory GroupConclusions/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?

22Peter de Vries – ITBs and Rotational Shear – 18 February 2010 – Oxford Plasma Theory Group List of Publications Experimental observations of ITBs –CONNOR, J.W.,et al Nucl. Fusion 44 R1 –WOLF, R.C., Plasma Phys. Control. Fusion 45 (2003) R1-R91 –CHALLIS, C.D.Plasma Phys. Control Fusion (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) –WALTZ, R.E., Phys. Plasmas 13 (2006) –... 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) –DE VRIES, P.C., et al., Nucl. Fusion 49 (2009) –SAKAMOTO, Y., et al., Nucl. Fusion 41 (2001) 865 –DE VRIES, P.C., et al., Plasma Phys. Control. Fusion 51 (2009) –SHAFER, M.W, et al., Phys. Rev. Lett. 103 (2009) Gyro code –CANDY, J., and WALTZ, R.E, Phys. Rev. Lett. 91(2003) (and refs. Therein) Profile Stiffness and Plasma Rotation –MANTICA, P., et al., Phys. Rev. Lett. 102 (2009)