BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//CERN//INDICO//EN
BEGIN:VEVENT
SUMMARY:Cross-scale interactions between trapped-electron-mode and electro
n-temperature-gradient-mode turbulence
DTSTART;VALUE=DATE-TIME:20210513T101000Z
DTEND;VALUE=DATE-TIME:20210513T103000Z
DTSTAMP;VALUE=DATE-TIME:20211027T105646Z
UID:indico-contribution-17497@conferences.iaea.org
DESCRIPTION:Speakers: Shinya Maeyama (Nagoya University)\nWe have analysed
cross-scale interactions between trapped-electron-mode (TEM) and electron
-temperature-gradient (ETG) turbulence by means of gyrokinetic simulations
. We find that (i) TEM turbulence suppresses ETG turbulence and dominates
electron heat transport\, and (ii) the suppression of ETG by TEM can ubiqu
itously happen even when TEM-driven zonal flows are subdominant. Additiona
lly\, from the comparison with hydrogen and deuterium plasmas\, TEM-stabil
ized deuterium plasma is affected by ETG\, resulting in simultaneous enhan
cement of TEM and ETG fluctuations. This brings a new perspective on the i
sotope effects of anomalous electron heat transport via cross-scale intera
ctions between TEM and ETG turbulence. \n\nCross-scale interactions betwe
en electron- and ion-scale fluctuations are one of unresolved issues in tu
rbulent transport. Our previous studies carried out direct numerical simul
ations of ion-temperature-gradient-mode (ITG) or micro-tearing-mode (MTM)
and ETG turbulence\, and revealed the mechanism of cross-scale interaction
s: e.g.\, ITG turbulent eddies suppress ETG streamers\, and ETG turbulence
damps short-wavelength zonal flows or localized current sheet of MTM (Mae
yama 2017). Besides the theoretical studies\, a possible impact of cross-s
cale interactions in ITER is explored via Tokamak experiments such as Alca
tor C-Mod\, DIII-D (Holland 2017)\, and JET (Mantica 2020). On the other h
and\, TEM\, which is known as one of important ion-scale instabilities\, i
s not well analyzed in the context of the cross-scale interactions with ET
G turbulence. A simulation study for a low magnetic shear plasma reported
that TEM-driven zonal flows suppressed ETG turbulence (Asahi 2014). Howeve
r\, it is known that TEM turbulence have a variety of saturation mechanism
s depending on plasma parameters where zonal flows are not necessarily dom
inant. Therefore\, one of our key questions is whether TEM coexists with o
r suppresses ETG\, if there is no dominant zonal flow. \n\nTo deal with t
his problem\, we set up local flux-tube gyrokinetic simulations resolving
electron and ion scales simultaneously. Magnetic field is a circular torus
geometry with $q=1.4$\, $s=0.8$\, $r/R=0.18$. Plasma parameters are $R/L_
{Ti}=1$\, $R/L_{Te}=9.3$\, $R/L_n=3$\, $T_e/T_i=3$ and $m_i/m_e=1836$. In
these parameters\, ITG modes are stable since the ion temperature gradient
is low. From the linear analysis\, ion-scale fluctuations at $k_y\\rho_{t
i}<1$ are dominated by TEM\, while the electron-scale fluctuations are at
$k_y\\rho_{ti}>1$ are ETG. Most unstable wavenumbers of these instabilitie
s are roughly separated by the ratio of ion and electron gyro radii. \n\n
Figure 1 plots the comparison of electron energy flux spectra. At the earl
y phase of nonlinear simulation ($t=9-12R/v_{ti}$)\, ETG modes rapidly gro
w up and saturate. Then ETG driven streamers peaks around $k_y\\rho_{ti}=3
$ and dominate electron energy flux. Simultaneously\, long-wavelength TEM
slowly grows up. After the growth of TEM ($t=20-40R/v_{ti}$)\, TEM turbule
nce around $k_y\\rho_{ti}=0.3$ dominates electron energy flux. One finds t
hat the peak of ETG at electron scale disappears in the presence of TEM. T
he energy flux level is comparable to that in a low-resolution (single-sca
le TEM turbulence) simulation plotted in Fig. 1.\n\n![Electron energy flux
as a function of poloidal wavenumber. Green and blue lines are at early a
nd final saturation phases of multi-scale TEM/ETG turbulence simulation. R
ed line plots a single-scale TEM turbulence simulation as a reference.][1]
\n\nFigure 2 plots the time evolution of electrostatic potential fluctuati
on of zonal flows ($k_y\\rho_{ti} = 0$)\, TEM ($0 < k_y\\rho_{ti} < 1$) an
d ETG ($1 < k_y\\rho_{ti}$). Linear growth of ETG is observed before $t<7R
/v_{ti}$. After the saturation of ETG\, long wavelength TEM grows with sma
ller growth rate than the maximum linear growth rate of TEM (plotted by a
slope in Fig. 2). The growth of TEM also saturates around $t=16R/v_{ti}$.
At the time\, both of zonal flows ($k_y\\rho_{ti}=0$) and high-wavenumber
fluctuations ($1 < k_y\\rho_{ti}$) are generated via inverse and forward c
ascades. It should be noted that the suppression of ETG appears at the sam
e time of TEM growth ($8 < t v_{ti}/R < 14$)\, but not at the zonal flow g
eneration ($16 < t v_{ti}/R$). Additionally\, zonal flow is not the domina
nt fluctuation of TEM turbulence in this case (e.g.\, $\\langle |\\varphi_
{k_y}|^2 \\rangle =46$ for $k_y\\rho_{ti}=0$ and $\\langle |\\varphi_{k_y}
|^2 \\rangle = 251$ for $k_y\\rho_{ti}=0.3$). This is in contrast with the
previous study (Asahi 2014) employing low magnetic shear $s=0.4$ and $T_e
/T_i=1$\, where ETG is suppressed by TEM-driven strong zonal flows. Our st
udy reveals that strong zonal flows is not necessarily required for the su
ppression of ETG by TEM. It resembles the suppression of ETG by ITG (Maeya
ma 2017)\, where ITG-driven turbulent eddies distort ETG streamers and sup
press them.\n\n![Time evolution of electrostatic potential fluctuations of
zonal flow (red)\, TEM (green)\, and ETG (blue) in multi-scale TEM/ETG tu
rbulence. Maximum linear growth rate of ETG and TEM are also plotted by so
lid slopes as a reference.][2]\n\nWe have also carried out a multi-scale T
EM/ETG turbulence simulation for the deuterium plasma ($m_i/m_e=3672$)\, w
here linear growth rates of TEM are reduced by the collisional isotope eff
ect (Nakata 2017). Simultaneous enhancement of TEM and ETG fluctuations wi
th stronger electron heat transport than that in the ion-scale TEM turbule
nce simulation is observed\, suggesting cross-scale interactions of TEM an
d ETG turbulence.\n\nOur study expands the understanding of cross-scale in
teractions between ion- and electron-scale turbulence. It is found that ET
G is suppressed by TEM turbulence even when zonal flows are subdominant. C
omparison with hydrogen and deuterium plasmas brings a new insight into is
otope effects via cross-scale interactions. Experimental measurement of el
ectron-scale fluctuations will be desired future in context of isotope eff
ects.\n\n# References\n(Maeyama 2017) S. Maeyama et al.\, Nucl. Fusion 57
(2017) 066036\; Phys. Rev. Lett. 119 (2017) 195002.\n(Holland 2017) C. Hol
land et al.\, Nucl. Fusion 57 (2017) 066043.\n(Mantica 2020) P. Mantica et
al.\, Plasma Phys. Control. Fusion 62 (2020) 014021.\n(Asahi 2014) Y. Asa
hi et al.\, Phys. Plasmas 21 (2014) 052306.\n(Nakata 2017) M. Nakata et al
.\, Phys. Rev. Lett. 118 (2017) 165002.\n\n\n [1]: https://workshop.nifs.
ac.jp/fec2020/image/9-Maeyama-image-fig_1.png\n [2]: https://workshop.nif
s.ac.jp/fec2020/image/9-Maeyama-image-fig_2.png\n\nhttps://conferences.iae
a.org/event/214/contributions/17497/
LOCATION:Virtual Event
URL:https://conferences.iaea.org/event/214/contributions/17497/
END:VEVENT
END:VCALENDAR