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Title 156: ECCD control of sawteeth and monster sawteeth to avoid NTMs
Name:Richard Buttery () Affiliation:UKAEA
Research Area:NTM Stabilization Presentation time: Requested
Co-Author(s): I Chapman, J Graves, R J La Haye, O Sauter
Description: The goal of this experiment is to explore the effects of control and variation in sawtooth period on NTM thresholds in general, with particular emphasis on the most deleterious 2/1 NTM in particular. It is proposed to use ICRH to simulate fast particle effects, partially stabilising sawteeth to lengthen their periods. The impact of this on NTM thresholds should be tested in medium betan (~2) plasmas. ECCD q=1 control (destabilisation) of sawteeth should then be deployed to identify its benefits in raising NTM thresholds. In this way we hope to move towards the control schemes required in baseline and "advanced inductive" (or low q hybrid) scenarios that will be required in future fusion devices such as ITER. The experiments will also provide key insights and tests into the physics of sawteeth and NTM triggering. Extensions could also include the application of this technique on top of ECRH pre-emptive current drive at the NTM resonant surface to understand how this might ease the job for the system. [Note: The author is happy if these ideas are followed up as part of the ongoing and very interesting work being performed in 2006-7 programmes on fast particle sawtooth stabilisation and control, and does not necessarily seek/expect a leading role in these studies. ]
Experimental Approach/Plan: These experiments will be quite challenging, but benefit from DIII-D's unique ECRH and control capabilities. The most challenging element will be to use real time system to keep ECCD targeted on q=1 (or just outside/inside). The approach at this stage is largely proof of principal, so does not require many successful shots, just a step-wise progression through the sequence: (i) establish H modes with beta ramps to trigger 3/2 and 2/1 NTMs; (ii) repeat, applying ICRH to stabilise sawteeth and observe NTM threshold changes; (iii) repeat, with ECCD at q=1 to reduce sawtooth size in presence of ICRH (and scan ECCD power levels needed). Additional shots at fixed betan values may be required to optimise coupling or deposition at various points. An alternative approach, which may prove fruitful should q=1 ECCD tracking be difficult, would be to operate each shot at fixed betan, and then apply ICRH power ramps to vary sawtooth period within a shot; pre-programmed levels/positions of ECCD could then be added in repeat shots to test the effectiveness of sawtooth control. Conditions: Any reliable baseline scenario, q95~4.3 would provide better disruption avoidance, but lower q95 may help access mode more easily.
Background: The ITER Q=10 baseline scenario is expected to suffer from large, long period, fast particle stabilised sawteeth [Porcelli, F., et al., Plas. Phys. Con. Fus. 38 (1996) 2163]. Such events can readily trigger serious, performance terminating, 2/1 Neoclassical Tearing Modes (NTMs) with large amplitude at low betan values, which otherwise appear to be confined to higher beta plasmas [Buttery 20th IAEA 2004]. They also lead to much lower than expected beta thresholds for 3/2 NTMs [Sauter PRL 2002, Buttery NF 2004], which might be expected to reduce fusion power by ~30-40% if left unchecked.
Control of fast particle stabilised sawteeth was demonstrated on JET [Eriksson PRL 2004] using two "flavours" of ICRH: core heating to induce fast particles to stabilise the sawteeth; and q=1 current drive to destabilise them again. However the use of the ICRH for the current drive element may also have helped eject fast particles from the core. ICRH also provides a relatively imprecise control, with a broader deposition that might be desirable. For future devices, we need to demonstrate ECCD control of sawteeth (which can be much more localised and efficient) in realistic medium to high betan scenarios.
Thus this study will help specify requirements for sawtooth control in ITER, and the benefits in terms of requirements for pre-emptive / post-onset NTM control via current drive at the resonant surface. It will also provide key data on the underlying physics, including that of sawtooth stability, and NTM seeding physics. [ITPA MDC5 and MDC8].
Resource Requirements: 4-5 co beams. ICRH. As many gyrotrons as possible (2 min). I coils for error correction.
Diagnostic Requirements: CER (all channels needed), ECE, magnetics, TS, MSE is key.
Analysis Requirements: Results should be largely self evident, although provide an ongoing basis for more detailed physics investigation into sawtooth physics and NTM triggering. Detailed cutting edge sawtooth triggering MHD modelling will be performed as part of this project.
Other Requirements: None.