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  1. Island size (cm) JET#59046 b arb scale 2/1 mode 3/2 mode b arb scale marginal point Spectrogram 3/2 2/1 55 65 Time (s) ion polarisation model finite island transport model D3D: 114781114782 Island width (cm) 2/1 NTM 3/2 NTM bN 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, wbplay 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 q=3/2 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_pol*1.16±0.11n0.06 ±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 q=3/2 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, wbplay 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).