Title font size to cover width

1
Title font size to cover width R J Buttery1, D
F Howell1, R J La Haye2, T Scoville2, JET-EFDA
contributors 1EURATOM/UKAEA Fusion Association,
Culham Science Centre, Oxon. OX14 3DB.
UK. 2Association General Atomics, San Diego, USA.
See annex of J. Pamela et al, Overview of JET
Results , (Proc. 20th IAEA Fusion Energy
Conference, Vilamoura, Portugal (2004).
ABSTRACT Neoclassical tearing modes (NTMs) are
performance limiting instabilities likely to
require control or avoidance strategies in ITER
baseline scenarios. Inter-device identity
experiments on the JET, ASDEX Upgrade and DIII-D
tokamaks have explored the threshold for NTM
metastability, and how it scales. Coupled with
advances in numerical modelling, these allow the
quantification of the underlying physics terms
and extrapolation towards ITER. Detailed matches
between theory and experiments can be obtained,
constraining the key parameters governing NTM
behaviour. Results suggest ITER baseline
scenarios will operate well above the threshold
for metastability of NTMs, and so highly
susceptible to NTM triggering events. Further, to
remove the modes, ECCD systems will have to drive
islands sizes down to levels of a few cm, similar
to those required for mode removal in present
devices.

• SOME NOTES ON THIS TEMPLATE
• This template has been carefully matched to the
  standard drawing office output for official JET
  EPS posters
• It is set up for single piece of A0 size there
  is another template that print out to the full
  size 1m by 2m poster size on 2 sheets
• Font sizes have been carefully matched to the JET
  standard poster template
• Use guides installed in this template to align
  text boxes and figures
• SOME NOTES ON THIS TEMPLATE
• This template has been carefully matched to the
  standard drawing office output for official JET
  EPS posters
• It is set up for single piece of A0 size there
  is another template that print out to the full
  size 1m by 2m poster size on 2 sheets
• Font sizes have been carefully matched to the JET
  standard poster template
• Use guides installed in this template to align
  text boxes and figures

MOTIVATION AND UNDERLYING THEORY NTM behaviour is
governed by the modified Rutherford equation
1,2. This predicts the growth rate of an
island with full-width, w

• Further data shows 3/2 NTM thresholds generally
  higher than 2/1 NTM thresholds
• for more information on the 2/1 NTM see
  Maraschek paper for corresponding studies just
  started on this 10

• ISSUES FOR ITER
• Two key aspects for ITER NTMs
• How readily and which NTMs are triggered - this
  depends on
• seed size required for neoclassical growth ?
  governed by wd, apol, wb - quantify these
• how large seeding events will be in ITER ? large
  fast particle populations may make triggering
  instabilities worse
• How much is required of ECRH current drive
  systems for NTM control
• must drive island sizes down to avoid confinement
  ? where wd, apol, wb play key roledegradation
  and/or remove the NTMs and setting the level for
  self-stabilisation of the NTM
• The main concept for these experiments is to
  measure the terms governing NTM behaviour, and
  their scaling, to provide a basis for empirical
  extrapolation to ITER, as well as test for
  theory.

Figure Comparison of 3/2 and 2/1 NTMs in
separate DIII-D discharges
Figure Comparison of 3/2 and 2/1 NTMs in a
single dual mode JET discharges

• CONCLUSIONS and IMPLICATIONS FOR ITER
• Conclusion 1
• Results indicate a clear trend of metastable b
  threshold falling with r, that is carries over
  well between the machines
• This indicates that the standard ELMy H-mode ITER
  baseline scenario will operate well above the NTM
  metastability threshold
• Suitable seeds (comparable to those in present
  devices) will excite 3/2 NTMs in ITER
• Preliminary results obtained for DIII-D alone
  indicate a weak dependence of small island size
  terms with r
• Suggests that for complete removal of an NTM by
  ECCD systems in ITER, islands sizes will have to
  be reduced to levels similar to those required on
  present devices (if borne out by JET and ASDEX
  Upgrade)
• However, small islands may also be tolerable in
  ITER, but require continuous ECCD correction for
  each mode
• but amount of ECCD required will be critically
  dependent on small island terms being measured
  here
• ? NEXT STEPS...
• Next step is to fold in ramp-down analyses for
  JET and ASDEX Upgrade
• Explore ion polarisation model for small island
  size effects
• Correct for slight variations in island size due
  to motions of q3/2 surface
• Finalise ITER extrapolations for marginal beta
  and marginal island size
• Work is also progressing on use of similar
  techniques to explore (1) ECCD stabilisation
  physics for NTMs, (2) the physics of the 2/1
  NTM 10

• EXPERIMENTAL RESULTS
• Most shots where a steady power ramp-down is
  applied exhibit a clear marginal b point
• where island breaks b scaling and starts to
  decay more rapidly
• This can be measured and plotted against
  underlyingplasma parameters at the marginal b
  point
• good range of parameters accessed
• Clear trend observed across devices
• Thresholds fall with r
• Good consistency between three devices
• Regression fit gives bPe-marg 8.79
  ri_pol1.160.11 n0.06 0.06
• zero n dependence within error bars

Figure Marginal beta point in cross machine
identity and similarity discharges, plotted in
terms of local parameters calculated at the q3/2
surface
ASIDE COMPARISON of 3/2 and 2/1 NTMs

• ISSUES FOR ITER
• Two key aspects for ITER NTMs
• How readily and which NTMs are triggered - this
  depends on
• seed size required for neoclassical growth ?
  governed by wd, apol, wb - quantify these
• how large seeding events will be in ITER ? large
  fast particle populations may make triggering
  instabilities worse
• How much is required of ECRH current drive
  systems for NTM control
• must drive island sizes down to avoid confinement
  ? where wd, apol, wb play key roledegradation
  and/or remove the NTMs and setting the level for
  self-stabilisation of the NTM
• ISSUES FOR ITER
• Two key aspects for ITER NTMs
• How readily and which NTMs are triggered - this
  depends on
• seed size required for neoclassical growth ?
  governed by wd, apol, wb - quantify these
• how large seeding events will be in ITER ? large
  fast particle populations may make triggering
  instabilities worse

ACKNOWLEDGEMENTS "This work, carried out under
the European Fusion Development Agreement,
supported by the European Communities and
Instituto Superior Técnico, has been carried
out within the Contract of Association between
EURATOM and IST. Financial support was also
received from Fundação para a Ciência e
Tecnologia in the frame of the Contract of
Associated Laboratory. The views and opinions
expressed herein do not necessarily reflect those
of the European Commission, IST or
FCT. Association Specific General
REFERENCES 1 R. Carrera, R. D. Hazeltine, and
M. Kotschenreuther, Phys., Fluids 29, 899
(1986). 2 O. Sauter et al., Phys. Plasmas 4,
1654 (1997). 3 O. Sauter, C. Angioni, and Y.
R. Lin-Liu, Phys. Plas. 6, 2834 (1999), 9 5140
(2002). 4 M. Kotschenreuther, R. D.
Hazeltine, and P. J. Morrison, Phys. Fluids 28,
294 (1985).