Subcritical fluctuations and suppression of turbulence in differentially rotating gyrokinetic plasmas
Subcritical fluctuations and suppression of turbulence in differentially rotating gyrokinetic plasmas
复制标题
差速旋转回旋等离子体中的亚临界波动和湍流抑制
DOI：
10.1088/0741-3335/54/5/055011
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发表时间：
2012
影响因子：
2.2
通讯作者：
S. Cowley
中科院分区：
文献类型：
--
作者：
A. Schekochihin;E. Highcock;S. Cowley
众所周知，差动旋转可以抑制聚变等离子体中的线性不稳定性。然而，数值实验表明，即使在没有增长本征模的情况下，瞬时增长的亚临界涨落也会导致持续的湍流，从而限制了速度切变抑制反常输送的能力。本文考虑了在垂直(E×B)速度剪切作用下，由平行速度梯度(PVG)和离子温度梯度(ITG)驱动的静电涨落的瞬时增长。在局部剪切盒近似的框架下，处理了最大简化(但数值模拟表明，最有希望减少输运)的零磁剪切情况。在这种情况下，没有线性增长的本征模，所以所有的激发都是瞬时的。在PVG为主的区域，最大放大因子与N∝q/ϵ(安全系数/反长径比)En有关，垂直于磁场和平行于磁场的最大放大波数与KyρI≈(ϵ/Q(1/3k∥vthi/S)有关，其中ρi为离子拉莫尔半径，vthi为离子热速，S为E×B剪切。在ITG占优势的区域，N与波数和N∝vthi/(Lts)无关，其中LT是离子-温度标度长度。还分析了中间的ITG-PVG区域，并计算了N作为Q/ϵ、LT和S的函数。分析结果得到了线性回转动力数值试验的证实和补充。在我们的模型中，当Q/≲7足够低时，对于所有波数，Nϵ(≲为1的区域是可能的；在这种区域，离子尺度的湍流有望被完全抑制。对于不被抑制的情况，提出了亚临界PVG湍流导致伴随离子热通量与Q、ϵ、S和LT的标度的基本启发式理论，认为输运远不如ITG区的“刚性”。
Differential rotation is known to suppress linear instabilities in fusion plasmas. However, numerical experiments show that even in the absence of growing eigenmodes, subcritical fluctuations that grow transiently can lead to sustained turbulence, limiting the ability of the velocity shear to suppress anomalous transport. Here transient growth of electrostatic fluctuations driven by the parallel velocity gradient (PVG) and the ion temperature gradient (ITG) in the presence of a perpendicular (E × B) velocity shear is considered. The maximally simplified (but, as numerical simulations suggest, most promising for transport reduction) case of zero magnetic shear is treated in the framework of a local shearing box approximation. In this case there are no linearly growing eigenmodes, so all excitations are transient. In the PVG-dominated regime, the maximum amplification factor is found to be eN with N ∝ q/ϵ (safety factor/inverse aspect ratio), the maximally amplified wavenumbers perpendicular and parallel to the magnetic field are related by kyρi ≈ (ϵ/q)1/3k∥vthi/S, where ρi is the ion Larmor radius, vthi the ion thermal speed and S the E × B shear. In the ITG-dominated regime, N is independent of wavenumber and N ∝ vthi/(LTS), where LT is the ion-temperature scale length. Intermediate ITG–PVG regimes are also analysed and N is calculated as a function of q/ϵ, LT and S. Analytical results are corroborated and supplemented by linear gyrokinetic numerical tests. Regimes with N ≲ 1 for all wavenumbers are possible for sufficiently low values of q/ϵ (≲7 in our model); ion-scale turbulence is expected to be fully suppressed in such regimes. For cases when it is not suppressed, an elementary heuristic theory of subcritical PVG turbulence leading to a scaling of the associated ion heat flux with q, ϵ, S and LT is proposed; it is argued that the transport is much less ‘stiff’ than in the ITG regime.
影响因子：
2.2
作者：
I. Abel;Michael Barnes;S. Cowley;W. Dorland;A. Schekochihin
通讯作者：
I. Abel;Michael Barnes;S. Cowley;W. Dorland;A. Schekochihin
影响因子：
8.6
作者：
Barnes, M.;Parra, F. I.;Schekochihin, A. A.
通讯作者：
Schekochihin, A. A.