The results of flux-driven, two-fluid simulations in single-null configurations are used to investigate the processes determining the turbulent transport in the tokamak edge. Three turbulent transport regimes are identified: (i) a developed transport regime with turbulence driven by an interchange instability, which shares a number of features with the standard L-mode of tokamak operation; (ii) a suppressed transport regime, characterized by a higher value of the energy confinement time, low-amplitude relative fluctuations driven by a Kelvin–Helmholtz instability, a strong $\boldsymbol {E}\times \boldsymbol {B}$ sheared flow and the formation of a transport barrier, which recalls the H-mode; and (iii) a degraded confinement regime, characterized by a catastrophically large interchange-driven turbulent transport, which recalls the crossing of the Greenwald density limit. We derive an analytical expression of the pressure gradient length in the three regimes. The transition from the developed transport regime to the suppressed transport regime is obtained by increasing the heat source or decreasing the collisionality and vice versa for the transition from the developed transport regime to the degraded confinement regime. An analytical expression of the power threshold to access the suppressed transport regime, linked to the power threshold for H-mode access, as well as the maximum density achievable before entering the degraded confinement regime, related to the Greenwald density, are also derived. The experimental dependencies of the power threshold for H-mode access on density, tokamak major radius and isotope mass are retrieved. The analytical estimate of the density limit contains the correct dependence on the plasma current and on the tokamak minor radius.

中文翻译：

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使用二流体模拟研究托卡马克边缘的湍流输运机制

单零配置中通量驱动的两流体模拟的结果用于研究确定托卡马克边缘湍流传输的过程。确定了三种湍流输运机制： (i) 由立交不稳定性驱动的发达输运机制，它与托卡马克操作的标准 L 模式具有许多共同特征；(ii) 抑制传输状态，其特征是能量限制时间值较高，由开尔文-亥姆霍兹不稳定性驱动的低幅度相对波动，强 $\boldsymbol {E}\times \boldsymbol {B}$ 剪切流和传输势垒的形成，这让人想起 H 模式；(iii) 退化的限制机制，其特征是灾难性的大互换驱动的湍流传输，这让人想起格林沃尔德密度极限的跨越。我们推导出三种状态下压力梯度长度的解析表达式。通过增加热源或降低碰撞性来获得从发达输运机制到抑制输运机制的转变，反之亦然，从发达输运机制到退化限制机制的转变。访问抑制传输状态的功率阈值的解析表达式，与 H 模式访问的功率阈值以及进入降级约束状态之前可实现的最大密度相关联，与格林沃尔德密度有关，也可以推导出来。检索了 H 模式访问的功率阈值对密度、托卡马克主半径和同位素质量的实验依赖性。密度极限的分析估计包含对等离子体电流和托卡马克小半径的正确依赖性。