Neoclassical tearing mode onset in NSTX

1
Supported by
Neoclassical tearing mode onset in NSTX
Columbia U Comp-X General Atomics INEL Johns
Hopkins U LANL LLNL Lodestar MIT Nova
Photonics NYU ORNL PPPL PSI SNL UC Davis UC
Irvine UCLA UCSD U Maryland U Rochester U
Washington U Wisconsin Culham Sci Ctr Hiroshima
U HIST Kyushu Tokai U Niigata U Tsukuba U U
Tokyo JAERI Ioffe Inst TRINITI KBSI KAIST ENEA,
Frascati CEA, Cadarache IPP, Jülich IPP,
Garching U Quebec
E D Fredricksona, J Menarda, D. Stutmanb,
K. Tritzb a Princeton Plasma Physics Laboratory,
NJ b Johns Hopkins University, MD 46th Annual
Meeting of the Division of Plasma Physics,
American Physical Society 15 19 November
2004 Savannah, Georgia
2
Sawtooth crashes often trigger modes which appear
similar to NTMs

• Toroidal mode number of n  2 suggests NTM.
• Strong interaction with sawtooth instability,
  drop in amplitude at sawtooth.
• Can amplitude evolution be modeled with extended
  Rutherford equation?
• "Spontaneous" n  2 modes are also seen.

3
Amplitude drop at sawteeth not predicted by
Rutherford equation

• Amplitude evolution of 3/2 modeled with
  Rutherford equation, TRANSP.
• "NTM" coefficients adjusted to match Mirnov
  amplitude.
• Local parameters not affected by sawtooth crash -
  island width should evolve on resistive time
  scale.
• Need internal measurements.

4
Early in sawtooth period n2 is kink-like, strong
n1 mode

• Weakly phase locked with 32 frequency match.
• n2 not triggered through non-linear coupling to
  n1.
• Are core oscillations a chord-integral effect?
• Sawtooth crash triggers n2 kink?

• Insufficient data for tomography, use simulations.

5
Equilibrium J, q profiles are used to simulate
3/2 tearing mode

• Soft x-ray data Abel inverted on EFIT grid to
  find local emissivity.
• Emissivity remapped on island flux plot and chord
  integrated.
• Comparison to fluctuation amplitude in
  experimental data is used to constrain island
  size, eigenmode structure.

6
Simulation suggests 2/2 persists in core, as at
high aspect ratio

• Core oscillation's phase pattern, amplitude
  differs from simulation.
• Coupling to 2/2 mode may reduce classic D' of 3/2
  mode.
• Or is this a 2/2 island?

• q profile is not measured, adjusted to match
  phase inversion.

7
The inferred q1 radius is comparable to the
inferred q1.5 radius

• The q1.5 radius was inferred from simulating the
  island inversion radius.
• Could these n  2 modes be 2/2 kinks evolving to
  2/2 islands?

• Radius of q1 surface is 1.2 times the chord
  integrated inversion radius

8
Summary

• Studies of NTM onset in NSTX raise more questions
  than are answered.
• NTMs interact much more strongly with sawteeth
  than should be expected.
• SXI implies islands near q  1 surface, i.e.,
  islands are not 3/2 modes.
• Routine current profile measurements expected for
  next campaign are key to progress.