Speaker
Lothar Schmitz
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/O2.109.pdf
L-H Transition Dynamics with Applied n=3 Resonant Magnetic
Perturbations*
L. Schmitz,1 M. Kriete,2 R. Wilcox,3 Z. Yan,2 T.L. Rhodes,1 C. Paz-Soldan,4 A. Marinoni,5
G.R. McKee,2 P.Gohil,4 L. Zeng,1 and C.C. Petty.4
1
University of California-Los Angeles, Los Angeles, CA 90095-7799, USA
2
University of Wisconsin-Madison, Madison, WI 53706, USA
3
Oak Ridge National Laboratory, Oak Ridge, TN 37831-0117, USA
4
General Atomics, PO Box 85608, San Diego, CA 92186-5608, USA
5
PFSC, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
In ITER-similar plasmas in DIII-D (=1.5-
5x1019m-3, Bt=1.9-2T, Ip=1.5MA, q95~3.6), the L-H
threshold power PLH with n=3 Resonant Magnetic
Perturbations (RMP) is found to increase strongly
with decreasing collisionality, a concern for H-mode
access in primarily ECH-heated ITER plasmas since
RMP may be applied before the L-H transition in
ITER to safely suppress the first ELM. Low edge
collisionality is thought to lead to substantial
additional thermal losses across the last closed flux
surface in ITER, potentially increasing the L-H power
threshold [1]. Figure 1 clearly shows that PLH
increases at low collisionality, and that the
collisionality dependence of PLH is much more Fig. 1: L-H transition power threshold PLH
pronounced with applied RMP [PLH~(ν*)-0.3] vs. collisionality ν* (ρ = 0.95) without and
compared to non-RMP plasmas [PLH~(ν*)-0.1]. with applied n=3 RMP (3.3×10-4 ≤ δB/B ≤
-4
Pronounced non-axisymmetric modifications of the 4.6×10 The expected ITER L-mode edge
collisionality range is shaded.
L-mode shear layer with RMP include a substantial
local reduction of the Er well and ExB shear [in particular the outboard ExB shear layer is
locally “eroded” for flux tubes connecting to high I-coil perturbation field]. Two-fluid
modeling with the M3D-C1 code [2] shows that the normalized radial density gradient a/Ln is
toroidally modulated and periodically increased on the outboard midplane with RMP. Low-
wave-number turbulence is spatially modulated with RMP and increases substantially in
amplitude on field lines connecting to high RMP perturbation field. We conjecture that the
increase in PLH with RMP results from the combined effects on locally enhanced instability
drive (via increased normalized density gradient) and reduced ExB shear. Theoretically,
increased Reynolds stress would be required to initiate the L-H transition with RMP active
[3], as the Reynolds stress [4] is counteracted by radial forces related to the RMP field
structure. The observed increase of the L-H power threshold may be consistent with this
picture, as the observed local turbulence increase with applied RMP may not substantially
increase the flux-surface-averaged Reynolds stress. Non-resonant n=3 perturbations do not
affect PLH significantly, and the modifications in turbulence level and ExB shear are minimal.
[1] F. Ryter et al. Nucl. Fusion 54 083003 (2014).
[2] R.S. Wilcox et al. Nucl. Fusion 57 116003 (2017).
[3] M. Leconte et al, Nucl. Fusion 54 013004 (2014); G. Choi and T.S. Hahm, Nucl. Fusion 58 026001 (2018).
[4] Z.Yan et al. Nucl. Fusion, 57, 126015 (2017); Z. Yan, et al. Phys Rev. Lett. 107, 055004 (2014).
*This work was supported by the US Department of Energy under DE-FG02-08ER54984, DE-FG02-
89ER53296, DE-FG02-08ER 54999, and DE-FC02-04ER54698.
