Speaker
Jinyong Kim
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1091.pdf
Strong safety-factor (q) profile dependence of the linear electromagnetic
stabilizing effect and its role in the internal transport barrier formation
J.Y. Kim and H.S. Han
National Fusion Research Institute, Daejeon 34133, Korea
A well-known feature of the internal transport barrier (ITB) is that its formation condition strongly
depends on the safety-factor (q) profile. For example, up to now most ITBs (except the hot-ions,
super-shot, PEP modes) were obtained only in some specific q-profiles with the reverse or weak-
positive shear. The threshold beta for the ITB formation also appears to have a substantial
difference, depending on the magnitude of q-min, even with nearly the same magnetic shear (as
can be seen from comparison between the optimized-shear [1] and hybrid [2] modes). While
numerous theoretical models have been proposed for the ITB trigger mechanism [3], the physics
origin of this strong q-profile dependence is still not so clear. Here, as an effort to find a possible
model we present a re-assessment of the linear electromagnetic effect on the toroidal ion
temperature gradient (ITG) mode, which is widely believed to be the main background turbulence
in high temperature core plasmas where the ITB is formed. Noting that the linear electromagnetic
stabilization of the toroidal ITG, which arises from the coupling with the shear-Alfven ballooning
branch, becomes stronger as plasma beta approaches the ballooning threshold [4], a similar q-
profile dependence is first expected between the ITB formation (through the ITG stabilization) and
the ballooning threshold. With the well-known ballooning threshold property, which becomes
smaller as q increase or magnetic shear (s) decreases through the reduction of the field-line bending
force, it is then shown that a strong q-profile dependence can arise in the ITB formation condition.
Furthermore, in this case the ITB formation is found to occur through not only the increase of ITG
threshold but also the reduction of profile stiffness. With the stabilization of the ITG, the kinetic
ballooning mode can be then excited and an estimate is given on its effect in relation to the further
development of the ITB. In addition, the effect of trapped elections, which can reduce the
electromagnetic stabilization degree, is briefly estimated by using a non-local code in the 1-D
ballooning space, which was upgraded from our previous one in Ref. 4 to include trapped electrons.
[1] C. Gormezano et al., 1997 Proc. 16th Int. Conf. on Fusion Energy vol I (Montreal: IAEA) p 487
[2] O. Gruber et al., Phys. Rev. Lett. 83, 1787 (1999).
[3] for example, see J. Citrin et al., Nucl. Fusion 54, 023008 (2014).
[4] J.Y. Kim et al., Phys. Fluids B 5, 4030 (1993).
