7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Gyro-Landau-fluid simulations of impurity effects on ion temperature gradientdriven turbulence transport
Yifei Liu1, Jiquan Li1
1Southwestern Institute of Physics, Chengdu 610041, Chinae-mail (speaker): liuyifei@swip.ac.cn

Microturbulence caused by drift wave instabilities, e.g.,
the ion temperature gradient (ITG) mode, is a leading
candidate to explain the transport [1-4]. Suppressing theturbulence and controlling the transport are ofimportance for achievement of high-performance
plasmas and realizing fusion energy. External impurity
injection is observed to lead to ion thermal diffusivity
reduction and confinement improvement in various
experiments [5-7]. Some theoretical analyses andnumerical simulations found that low or medium-Z
impurities can destabilize or stabilize micro-instabilities
in certain conditions. Therefore, impurity injection is apromising way to control the turbulence transport, andthe impurity effects on the microturbulence deservein-depth study.
In this work, the impurity effects on ITG driven
turbulence transport in tokamak core plasmas areinvestigated numerically via global simulations of
microturbulence with carbon impurities and adiabatic
electrons using extended fluid code (ExFC) based on afour-field gyro Landau-fluid (GLF) model [8]. A numberof simulations with parametric profiles of the cyclone
base case (CBC) and different initial carbon impurity
profiles are performed to investigate the impurity effects
mainly in two aspects.
First, note that the ITG turbulence could be regulated byzonal flows and quenched in low temperature gradient, ahigher critical temperature gradient than the linear
prediction has been observed, i.e., the so-called Dimits
shift. In fact, the impurity effects on the criticaltemperature gradient are also of interest. Simulations inFig. 1(a) show that the critical temperature gradient canbe influenced obviously by impurities with different
profiles. The sign of impurity density gradient parameter
𝑅𝑅/𝐿𝐿𝑛𝑛𝑛𝑛 corresponds to the different peaking directions of
impurity density profile, and 𝑓𝑓𝑧𝑧 represents the impurity
fraction at the reference surface. Without considering theradiation effect, the inwardly peaked impurity density
profile can improve the confinement.
Second, considering that the heat flux mainly resultsfrom bulk ions on account of small fraction of impurity,
we focus on the impurity effects on 𝜒𝜒𝑖𝑖 in the ITG
turbulence near critical temperature gradient. Fig. 1(b)presents a more complete picture of the impurity effects
on the confinement, it shows the improved confinement
by inwardly peaked impurity and deteriorated
confinement by outwardly peaked impurity near thecritical temperature gradient, but the transport level isinstead reduced by impurity at higher temperature
gradient. It may be ascribed to that the steep density
gradient of both ions and impurities have relaxed quickly(within the magnitude of 1ms) to a flatter one by particle
transport induced by impurities. Fast relaxation indicatesthat the destabilization of the outwardly peaked impurity
profile is a transient state response. Moreover, in theregion where turbulence excited by impurity effect, theunquenched turbulence with the thermal diffusivity in2
𝑐𝑐𝑠𝑠0 /𝑎𝑎0 ) is
neoclassical transport level (about 0.1𝜌𝜌𝑠𝑠0
found near the linear critical gradient of ITG without
impurity (𝑅𝑅/𝐿𝐿 𝑇𝑇 ≈ 4).
These results may evidence the plasma confinement
improvement by the impurities probably throughadjusting both heat diffusivity and critical temperature
gradient.
This work was supported by National Natural Science
Foundation of China under Contact Nos. U1967206 and12275071, National Key R & D Program of China underGrant Nos. 2017YFE0301201.

Figure 1: (a) Quenched points and fit lines of criticaltemperature gradient versus impurity fraction 𝑓𝑓𝑧𝑧 withdifferent 𝑅𝑅/𝐿𝐿nz ; (b) Heat diffusivity 𝜒𝜒i of turbulence
without impurity and with impurity near criticaltemperature gradient, lines are labeled by the values ofparameters (𝑓𝑓z , 𝑅𝑅/𝐿𝐿nz ).

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