We perform global nonlinear simulations of ion temperature gradient (ITG) turbulence in a concentric circular tokamak geometry with the gyrofluid code GF2-BOUT++ and compare the dynamics between flux-driven and gradient-driven global simulations. Both approaches present broad 1/f-type power spectra for the turbulent ion heat flux, indicative of self-organized criticality (SOC)-like transport avalanches. Despite the similar levels of the total ion heat fluxes, the two approaches yield markedly different normalized ITG length (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$R_0/L_{T_i}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>R</mi> <mn>0</mn> </msub> <mo stretchy="false">/</mo> <msub> <mi>L</mi> <msub> <mi>T</mi> <mi>i</mi> </msub> </msub> </mrow> </math></EquationSource> </InlineEquation>). This discrepancy is attributed to differences in turbulence characteristics and low-frequency <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$\varvec{E}\times \varvec{B}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mi mathvariant="bold-italic">E</mi> </mrow> <mo>×</mo> <mrow> <mi mathvariant="bold-italic">B</mi> </mrow> </mrow> </math></EquationSource> </InlineEquation> flow shear. In the flux-driven simulations, mesoscale heat transport is prominent, and the system self-organizes toward a marginally stable state through enhanced mesoscale avalanche-like transport, leading to a lower average temperature gradient than in the gradient-driven case. Strong and persistent <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$\varvec{E}\times \varvec{B}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mi mathvariant="bold-italic">E</mi> </mrow> <mo>×</mo> <mrow> <mi mathvariant="bold-italic">B</mi> </mrow> </mrow> </math></EquationSource> </InlineEquation> shear in the gradient-driven simulations sustains the same heat flux at higher <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$R_0/L_{T_i}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>R</mi> <mn>0</mn> </msub> <mo stretchy="false">/</mo> <msub> <mi>L</mi> <msub> <mi>T</mi> <mi>i</mi> </msub> </msub> </mrow> </math></EquationSource> </InlineEquation> than in the flux-driven ones. The combined effects of these mechanisms cause <InlineEquation ID="IEq5"> <EquationSource Format="TEX">$R_0/L_{T_i}$</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>R</mi> <mn>0</mn> </msub> <mo stretchy="false">/</mo> <msub> <mi>L</mi> <msub> <mi>T</mi> <mi>i</mi> </msub> </msub> </mrow> </math></EquationSource> </InlineEquation> in the flux-driven case to remain near the critical value of approximately 6, which is comparable to full-f gyrokinetic results, and lead to stronger profile stiffness compared to the gradient-driven case.

Comparison of turbulence and transport characteristics between global flux-driven and gradient-driven gyrofluid simulations in tokamak plasmas

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发表日期：

2026年3月23日

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S. H. Ko,
S. S. Kim,
Juhyung Kim

摘要

We perform global nonlinear simulations of ion temperature gradient (ITG) turbulence in a concentric circular tokamak geometry with the gyrofluid code GF2-BOUT++ and compare the dynamics between flux-driven and gradient-driven global simulations. Both approaches present broad 1/f-type power spectra for the turbulent ion heat flux, indicative of self-organized criticality (SOC)-like transport avalanches. Despite the similar levels of the total ion heat fluxes, the two approaches yield markedly different normalized ITG length ( $R_0/L_{T_i}$ $R_{0} / L_{T_{i}}$ ). This discrepancy is attributed to differences in turbulence characteristics and low-frequency $\varvec{E}\times \varvec{B}$ $E \times B$ flow shear. In the flux-driven simulations, mesoscale heat transport is prominent, and the system self-organizes toward a marginally stable state through enhanced mesoscale avalanche-like transport, leading to a lower average temperature gradient than in the gradient-driven case. Strong and persistent $\varvec{E}\times \varvec{B}$ $E \times B$ shear in the gradient-driven simulations sustains the same heat flux at higher $R_0/L_{T_i}$ $R_{0} / L_{T_{i}}$ than in the flux-driven ones. The combined effects of these mechanisms cause $R_0/L_{T_i}$ $R_{0} / L_{T_{i}}$ in the flux-driven case to remain near the critical value of approximately 6, which is comparable to full-f gyrokinetic results, and lead to stronger profile stiffness compared to the gradient-driven case.