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SUMMARY:Kinetic ion dynamics in the electron-scale turbulent transport: a
key ingredient of multi-scale interactions in turbulence
DTSTART;VALUE=DATE-TIME:20210514T101000Z
DTEND;VALUE=DATE-TIME:20210514T103000Z
DTSTAMP;VALUE=DATE-TIME:20210417T012849Z
UID:indico-contribution-17614@conferences.iaea.org
DESCRIPTION:Speakers: Tomo-Hiko Watanabe (Nagoya University)\nThe present
gyrokinetic simulation study reveals roles of kinetic ion dynamics on the
electron temperature gradient (ETG) turbulence and transport in magnetic f
usion plasma\, and provides new insights into a fundamental process in cro
ss-scale interactions in plasma turbulence. The polarization effect due to
finite ion gyroradius $\\rho_i$ turns out to play essential roles not on
ly in enhancement of the linear instabilities of the slab and toroidal ETG
modes but also in the nonlinear saturation of the ETG turbulent transport
.\n\n The anomalous electron heat transport has long been a key issue in
physics of burning plasma confinement. Recent large-scale gyrokinetic sim
ulations have also demonstrated contributions of electron-scale turbulence
to the total heat transport through cross-scale interactions of turbulenc
e (REf. 1). Historically\, importance of kinetic ion dynamics on the ETG t
urbulence was pointed out by gyrokinetic simulations (Ref. 2)\, while the
physical mechanism leading to a lower transport level has long been an ope
n issue. We have clarified the ion polarization effects on destabilizing a
nd stabilizing the ETG turbulence by means of comprehensive gyrokinetic si
mulations for slab (Ref. 3) and toroidal configurations.\n\n In the slab
ETG turbulence\, which is supposed to drive the electron heat transport n
ear the $q$-minimum surface\, we found dominance of the long wave length f
luctuations after the initial saturation of the instability growth in case
with “kinetic” ions and in the electrostatic (ES) limit. The observed
heat transport was quite strong and an order of magnitude higher than tha
t of the “adiabatic” ions (where the ion density perturbation is appro
ximated in proportion to the ES potential fluctuation) or the electromagne
tic (EM) cases (Fig. 1). Detailed analysis of the linear stability has rev
ealed enhancement of the ETG growth rate $\\gamma$ in a long wavelength re
gime ($k_\\bot \\rho_i \\sim 1$) by the ion polarization effect which was
absent in the “adiabatic” ion model. In spite of the tiny enhancement
of $\\gamma$\, the high saturation amplitude of the low $k_\\bot$ mode bri
ngs a great impact on the transport. Interestingly\, the long wavelength m
odes can be almost stabilized by the magnetic flutter in the EM turbulence
\, and lead to less impact on the transport in a finite $\\beta_e$ (EM) re
gime of $\\beta_e > (m_e/m_i)^{1/2}$.\n![Time histories of the electron he
at diffusivity obtained by gyrokinetic simulations of electrostatic/electr
omagnetic slab ETG turbulence for “adiabatic” and “kinetic” ion ca
ses. The long wavelength mode dominates the nonlinear saturation with “k
inetic” ions in the electrostatic limit of $\\beta_e = 0$.][1]\n\n Rol
e of the ion dynamics on the toroidal ETG turbulence is\, however\, more p
uzzling. It is known that the linear growth rate $\\gamma$ of the toroidal
ETG modes are increased in case with “kinetic” ions with respect to t
he “adiabatic” ion case\, which turns out to be explained by the ion p
olarization effect (Ref. 3). However\, it is widely known that the ETG tur
bulence is saturated at lower amplitudes (an order of magnitude smaller in
some cases) in case with “kinetic” ions even with no ion scale turbul
ence (Ref. 2). The present gyrokinetic simulation confirms the physical sa
turation of the “adiabatic” ion case even with the moderate magnetic s
hear ($\\hat{s} = 0.8$) as well as the lower transport level of the toroid
al ETG turbulence in case with “kinetic” ions (Fig. 2 (green)). It als
o shows that an ion response modeled with the polarization effect (Fig. 2
(cyan)) results in the lower saturation level. The clear difference in non
linear behaviors of the ETG turbulence with and without the ion polarizati
on effect is attributed to enhancement of the nonlinear Kelvin- Helmholtz
type instability of ETG streamers (Ref. 4). Contours of potential fluctuat
ions (Figs. 3 and 4) confirms strong deformation of the ETG streamers in c
ase with “kinetic” ions (Fig. 4) by growth of vortices in scale of $\\
rho_i$\, showing an impact of cross-scale interaction on ETG turbulence.\n
![Time histories of the electron heat flux in toroidal ETG turbulence with
different ion models. The ion response model with the polarization effect
results in the lower saturation level][2]\n\n![Snapshot of the electrosta
tic potential obverted in the nonlinear saturation phase of the toroidal E
TG turbulence in case with “adiabatic” ions.][3]\n\n![Snapshot of the
electrostatic potential obverted in the nonlinear saturation phase of the
toroidal ETG turbulence in case with “kinetic” ions. Here\, finite rad
ial wavenumber modes strongly modulate the ETG streamers.][4]\n\n\nReferen
ces\n1) S. Maeyama et al.\, Phys. Rev. Lett. 114\, 255002 (2015)\; N. Howa
rd et al.\, Phys. Plasmas 21\, 112510 (2014)\; S. Maeyama\, T.-H. Watanabe
\, and A. Ishizawa\, Phys. Rev. Lett. 119\, 195002 (2017).\n2) F. Jenko et
al.\, Phys. Plasmas 7\, 1904 (2000)\; J. Candy et al.\, Plasma Phys. Cont
rol. Fusion 49\, 1209 (2007).\n3) S. Kusaka\, T.-H. Watanabe\, and S. Maey
ama\, to be submitted to Phys. Plasmas.\n4) G. Plunk\, Phys. Plasmas 14\,
112308 (2007).\n\n\n [1]: https://workshop.nifs.ac.jp/fec2020/image/69-Wa
tanabe-image-fig_1.png\n [2]: https://workshop.nifs.ac.jp/fec2020/image/6
9-Watanabe-image-fig_2.png\n [3]: https://workshop.nifs.ac.jp/fec2020/ima
ge/69-Watanabe-image-fig_3.png\n [4]: https://workshop.nifs.ac.jp/fec2020
/image/69-Watanabe-image-fig_4.png\n\nhttps://conferences.iaea.org/event/2
14/contributions/17614/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17614/
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