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NSTX XP prep meeting: Jan 2008 NTM - error field and rotation role for NSTX piggy back study in 2008 R J Buttery 1 with S Gerhard, R. J. La Haye 2 , S. Sabbagh 3 , E. J. Strait 2. 1 EURATOM/UKAEA Fusion Association, Culham Science Centre, UK.
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NSTX XP prep meeting: Jan 2008 NTM - error field and rotation role for NSTX piggy back study in 2008 R J Buttery1 with S Gerhard, R. J. La Haye2, S. Sabbagh3, E. J. Strait2. 1EURATOM/UKAEA Fusion Association, Culham Science Centre, UK. 2General Atomics, USA 3Columbia University, NY Work conducted under the European Fusion Development Agreement and jointly funded by EURATOM, the UK EPSRC, Swiss NSF, and US DOE.
Shot list for piggy back EF / rotation study 0. Level check: • Apply maximum tolerable n=3 field (as #123874?) with optimal n=1 field error correction at fixed high bn (~3.5). • Ramp additional n=1 field to trigger mode. • Once mode triggered ramp down n=1 error field to try to unlock for second phase. 1. Maximum tolerable n=3 field, and NBI ramps to trigger NTMs • Start with optimal n=1 field error correction (=zero n=-1?) • Repeats with additional n=1 field: 25%, 50%, 75%, 100% of maximum determined in (0) 2. Repeat scan in (1) at zero n=3 field - 4-5 shots ideally (counter Bt and Ip points highly desirable in future campaign)
Technical details • Obviously sit on back of RJL discharges - we are piggy back! • q95 seems high enough to help avoid/unlock modes • Reference shot is 123876 for levels of power, field, etc. • This is from same series as that used by Rob… • Ip~1MA Bt ~0.44T at Rgeo • n=3 typical levels and examples taken from 123786 series (where it was varied somewhat - see: 123866 - highest rotation, 123869, 123870, 123873, 123874 - lowest rotation, 123876) • Do NBI as simple power control under pini modulation • Beta_N values around 3-5 to be explored in power ramps • Diagnostics: magnetics, SXR, TS, MSE, CHERS + other? • Will need n=1 and n=3 fields simultaneously and independently
Points of debate (1) • What is maximum n=1 field current to apply and phasing (to add to intrinsic error)? • Is intrinsic error much? (is an optimal correction point needed)? • Can we have a real time trigger of n=1 mode detection (either locked, or preferably, rotating n=1 mode)? • This needs to trigger a switch to zero n=1 field, RJL-requested n=3, and a step down in power • If we get a locked mode, will a power step down unlock it? • If we can’t unlock mode, then must avoid n=1 fields (or only apply at very low levels) so mode born rotating • Or we could explore use of short pulses of error field, but greatly limits scan data points, as won’t capture full beta ramp
Points of debate (2) • Need to scope out timing of window in power ramp up phase • Some references of Ohmic error field modes would be helpful… • Other diagnostics needed? • Any key points of diag’c specification (time windows, rates)?
2/1 NTM co vs counter rotation dependence DIII-D bN (2/1 NTM onset) sawtoothing ELMy H modes counter co DIII-D: shows strong rotation dependence in 2/1 NTM bN limit • But what is physics? • Does counter rotation stabilise mode? • Is threshold dependent on rotation shear relative to magnetic shear (á la theory) • Need to test and explore this important result…
2/1 NTM co vs counter rotation dependence DIII-D, NSTX n=3 braking study Similar % fall in bN per % Mach bN (2/1 NTM onset) counter-co DIII-D: shows strong rotation dependence in 2/1 NTM bN limit • But what is physics? • Does counter rotation stabilise mode? • Is threshold dependent on rotation shear relative to magnetic shear (á la theory) • Need to test and explore this important result… • NSTX made a start… • Lets extend to strong counter injection ? NSTX: about 0.5 shifts, counter BT and IP Apply ramps in b to trigger 2/1 NTM (ref shot 123876) May need co- comparison, and vary rotation with n=3… (see also next part…)
