XP915: Error field effects on tearing b limits

1. XP915: Error field effects on tearing b limits by Richard Buttery1 with Stefan Garhardt2, Rob La Haye3, Steve Sabbagh4, Francesco Volpe5 Presented to the 2009 NSTX Results Review 1General Atomics, USA (formerly EURATOM/UKAEA). 2Princeton Plasma Physics Laboratory, NJ. 3General Atomics, USA. 4Columbia University, NY. 5University of Wisconsin, Maddison. Work conducted under the European Fusion Development Agreement and jointly funded by EURATOM, the UK EPSRC, and US DOE.

2. Goals: Understand how error fields interact with plasma to change tearing stability Error field can act through two mechanisms: DIII-D • Locked modes • Error field amplification at high beta? • Proximity to • Role of rotation? • Rotating modes • Perturbing classical / neoclassical stability? • Action through rotation or rotation shear? • Influences delta prime? Directly error field driven Error field perturbing NTM stability How do b limits manifest in low torque plasmas?

3. Early NSTX studies had pointed to rotation effect and shown scope for further studies NSTX: n=3 braking perturbs rotating 2/1 NTM beta limit • n=3 braking showed 2/1 NTM thresholds fell with rotation

4. Early NSTX studies had pointed to rotation effect and shown scope for further studies • n=3 braking showed 2/1 NTM thresholds fell with rotation • Gerhardt database analysis favored action through rotationshear… Aim for controlled study with reproducible conditions: • Decouple rotation roles further with n=1 and n=3 fields • Extend with increased error fields – learn about interaction • Achieved by tuning H mode: shape gas, lithium and error field ramps

5. Wide scan achieved – raw data… • n=1 and n=3 fields varied up to locked mode limits () • See variation in mode onset bN (red bars) • And underlying rotation (blue bars) • Very reproducible plasma thanks to H mode tuning and lithium • Some trends emerging…

6. Wide scan achieved • Varied rotation and rotation shear in controlled conditions • Plotted at q=2 • Locked modes much slower rotation than rotating mode onsets • But not at zero rotation in CER • Bulk plasma takes timeto slow down dF/dRq=2 CER Hz/m

7. Differences in CER q=2 and 2/1 mode rotation • Mode forms locked whileCER shows plasma rotation • Actual mode onset rotationis lower than CER • Coupling to ELMs? • Locked mode stops MHD fluid while plasma still flows • Stops later? bN F2/1 Hz Fq=2 CER Hz F2/1 Hz

8. Mode forms at lower beta when locked • Locked mode thresholdis 0.5 lower in bN • May be partly confinement reduction • stripped out for rest of this analysis (JKP analysing as extension of locked mode scans (XP903)) • Rotating mode shows no rotation dependence! • But need to look at NTM drivesin local parameters & understandwhat we really varied • How does braking impact rotation and mode drives? bN Fq=2 CER Hz

9. Braking analysis • Both n=1 and n=3 brake plasma • Best fit is combination of similar levels of n=1 and n=3 currents: • F21=7500 – (2.26In=1 + 2.52In=3) • Gives good correlation for braking: half the original mode rotation F2/1 Hz

10. Braking trends similar in rotation shear, but more noisy • No one form of braking dominant • Trends far less clear • dF21/dR= –54000 – (14.5In=1 + 11 In=3) • Scope for scan on rotation vs rotation shear dFdR(q=2)

11. NTM bootstrap threshold favors rotation shear role • No measureable trend vs rotation! • Weak positive correlation with normalised rotation • Lowest thresholds at low rotation shear • Highest thresholds at high rotation shear • Best ‘fit’ is power law • No correlations if y-plot bN • Also 2d fit vs rotation & rotation shear offers little improvement m0LqdjBS,Sauter/Bq F2/1 Hz m0LqdjBS,Sauter/Bq -2p(dF/dr) tALS

12. Conclusions • Experiment successfully scanned n=1 and n=3 error fields up to their maximum effect on rotation and tearing mode b limits • Connected with high bN locked mode data: shows w/2 rotation criteria for mode and lower bN limits when rotation stopped • Accessed clear braking from n=1 and n=3 field and determined relative effects – both brake plasma significantly • Led to range of q=2 rotations and rotation shears, with loose correlation between the two • Although measurements of rotation shear are more noisy, a distinct trend is observed for bN 2/1NTM with rotation shear,while rotation itself offer no significant correlations • This reinforces the idea that torque influences TM thresholds through rotation shear at the mode and therefore through modifications to underlying tearing stability

13. Further analysis • Compare extremum cases as anecdotal examples • Need to check out triggers – ELM triggered at all? • Work through locked mode analyses – JKP doing this… • Publish at APS

14. RESERVE SLIDES AND OLDER REFERENCES

15. For reference trends in bN • There are no trends!

16. Optimal 2d fit shows little improvement • Plotted vs optimal combined rotation+rotation shear

17. First report XP915: Rotation and error field influence on tearing mode beta limits New techniques (inc. li) led to broad and reliable scan: • Wide data set scan with n=1 & n=3 braking • Large new regime identified of rotating mode onset with decreased bN onsetwhen braking applied • Great data to study rotation profile and error field roles • Next steps: • Trend exploration in detail • MSE FITS, local parameter trends • Great thanks to the NSTX team for their excellent, generous and expert support

18. Experiment goals on NSTX NSTX: n=3 braking perturbs rotating 2/1 NTM beta limit • NSTX can: • Access 2/1 NTM beta limit with strong beam heating • Perturb rotation with n=3 fields • Perturb rotation and couple to modes with n=1 fields • Key questions we can get at: • How does n=1 error sensitivity vary with beta and rotation? • Action of rotation vs rotation shear  Nature of interaction, NTM physics • Is ultimate limit an NTM (rotating) or error field mode (locked)?  What governs this transition? Goal: What governs error correction and rotation requirements for next step devices?

19. NSTX can distinguish rotation vs rotation shear 2007 data set appears to indicate a trend: • Rotation shear correlates with NTM onset Use n=1 & n=3 RMP tools to understand controlling physics: • Different braking profiles • Resonant / non-resonant • Decouple rotation vs rotation shear further in TM physics • Test action of error field – through rotation perturbation or direct coupling to q=2? [S. Gerhardt, NF letter] 7,

20. First attempt to look at EF role in Feb 08 indicates possible trend in raw data Experiment in 2008 used fixed fields and b ramps: • n=3 brakes plasma • n=1 lowers beta thresholdsfurther • Does n=1 have bigger effect at lower rotation? • Weak trend with rotation • Not really consistent with applied RMPs • What was intrinsic error? • Variability in machine conditions (1st day ops)

21. Analysed data is less clear • No clear trends • Points moved about as EFITs optimised • Shot to shot variability? • bN – FT’ works best?

22. Possible trends in raw June data Data looks promising with different RMPs: Reference shots Pure n=3:lowered bN Pure n=1:lowered bN Mixed n=3 and n=1: lowest bN • Though considerable variability: • Particularly in rotation values

23. June data ‘raw’ trends in magnetic perturbations do not carry over into rotation trends

24. Issues in 2008 …and 2009 resolutions • Variability in machine conditions • Day 1 after restart, then June day after lithium • Seem to get shot to shat variation in behaviour • Sensitive to things like intershot time and glow length? • Avoid lithium, establish large ELMing H mode (Rajesh to advise on tweaking scenario): • Near DN, low trian, low X pt height • Error bars in LRDFIT profile analysis  q=2 location & local params • Not all data was available in all cases. • Get all data and do further checks with MSE EFIT • Knowing n=1 & n=3 intrinsic error • For discussion – usual strategies, dynamic correction, etc? • Getting more reliable modes? • n=1 error field ramps may be more robust

25. 2009 Proposal: Measure n=1 thresholds in better conditions (& piggy back to rampdown studies?) • Do simple scan in bN & n=3 scan: • Ramp n=1 field at various n=3 and beta • Repeats at low and high beta & min/high n=3 fields • Possibilities to fill in at intermediate beta values • Always ramp n=3 field so get mode during n=1 ramp (the goal) • Could do beta ramps with four corners in n=1 and n=3… • Does n=1 threshold change with rotation and/or beta? • Effect of approach towards tearing beta limit? • Variation of effect with rotation • Action of fields • Changes to rotation / rotation shear  NTM stability? • Direct inducing of islands (locked) or changes to TM trigger?