Speaker
Andreas Wingen
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1045.pdf
Onset conditions of helical cores in tokamaks for extrapolation to ITER
A. Wingen1 , R.S. Wilcox1 , L.F. Delgado-Aparicio2 , M.R Cianciosa1 , S.P. Hirshman1 , S.K. Seal1
1 Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
2 Princeton Plasma Physics Laboratory, Princeton, NJ, USA
Large, spontaneous m/n = 1/1 helical cores are predicted in tokamaks such as ITER with ex-
tended regions of low- or reversed- magnetic shear profiles and q near 1 in the core. Their impact
on ITER has not yet been fully quantified. Beneficial effects can include flux pumping and pre-
venting sawteeth. On the other hand fast ion confinement is predicted to degrade significantly
and the ensuing rotation breaking could be detrimental to tearing mode and microturbulence
suppression. The threshold for the spontaneous symmetry breaking is determined using VMEC
scans, beginning with reconstructed 3D equilibria from DIII-D and Alcator C-Mod based on
observed internal 3-D deformations. The helical core is a saturated internal kink mode; its onset
threshold, shown by the black line in Fig. 1, is proportional to (d p/dρ)/Bt2 around q = 1.
Below the threshold, applied 3-D
fields can drive a helical core to finite
size, as in DIII-D. The helical core size
ζ
thereby depends on the magnitude of
the applied perturbation. Above it, a
small, random 3-D kick causes a bifur-
cation from axisymmetry and excites a
spontaneous helical core, which is in-
dependent of the kick size. The on-
set threshold is very sensitive to the Figure 1: Onset of helical cores for an ITER mock-up dis-
q-shear in the core. Helical cores oc- charge (15 MA, H-mode), compared to 3-D reconstructed
cur frequently in Alcator C-Mod dur- DIII-D and C-Mod discharges. The y-axis is normalized
ing ramp-up when slow current penetra- so that the thresholds for all machines coincide, with:
6
tion results in a reversed shear q-profile, ζDIII−D = 2.545, ζIT ER = 0.5 and ζC−Mod = 0.145 10 Pa.
which is favorable for helical core formation. A comparison of the helical core onset threshold
for discharges from DIII-D, C-Mod and ITER, shown by the markers in Fig. 1, confirms that
while DIII-D is marginally stable, C-Mod and especially ITER are highly susceptible to heli-
cal core formation without externally applied 3-D magnetic fields. This work is supported by
the US Department of Energy under DE-AC05-00OR22725, DE-AC02-09CH11466 and DE-
FC02-04ER54698 and used resources of the Oak Ridge Leadership Computing Facility.
