Influence of plasma flow shear on tearing in DIII-D hybrids
Influence of plasma flow shear on tearing in DIII-D hybrids
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DOI：
10.1088/0029-5515/51/5/053013
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发表时间：
2011-04
期刊：
影响因子：
3.3
通讯作者：
R. L. La Haye;C. Petty;P. Politzer
中科院分区：
文献类型：
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作者：
R. L. La Haye;C. Petty;P. Politzer
等离子体流剪切对撕裂稳定性有稳定作用，这是在DIII-D、JET和NSTX上的实验中发现的新见解。高阶(m/n=4/3或3/2)撕裂模式在混合场景中是一个关键的有益特征，其作用是调节Q分布以在没有大的m/n=1/1锯齿的情况下保持Q(0)∼1。低阶(m/n=2/1)撕裂的失稳是可实现的β的极限。在具有4/3撕裂模式的DIII-D中，在大的co(To Ip)中性束扭矩下，混合放电可以稳定地运行在略低于2/1撕裂测试值的极限。然而，减小扭矩会对现有的4/3撕裂模式幅值和2/1撕裂模式失稳时的β值产生影响。结果表明，合理表面处的流动剪切与4/3模振幅的减小和较高的β2/1模起始值有很好的相关性。工作的物理模型是，流动剪切是经典的稳定化，即使撕裂稳定性指数Δ‘更负；这既降低了新经典撕裂模式的幅度，又使模式失稳变得更加困难(需要更高的β)。然而，对流动剪切对撕裂稳定性的影响的详细了解仍然是理论和模拟的挑战。在DIII-D(甚至未来更大的托卡马克)中，流动剪切的经典稳定效应是在大磁普朗特和非常大的Lundquist数的区域内；这对于区分哪些流动和流动剪切的物理效应是相关的，可能是稳定的，甚至是不稳定的，这是重要的。在4/3模“调节”的DIII-D混合方案中，分析了施加扭矩变化的实验数据，包括4/3模振幅、2/1起始和2/1模锁定的临界性。对于现有的m/n=4/3模和m/n=2/1模的起始点，约数量级的局部流动剪切对撕裂有显著的稳定化作用。此外，为了解释阻性壁的锁模行为，提出了一种增加局部粘性的流动剪切效应。
That plasma flow shear has a stabilizing effect on tearing stability is a new insight found from experiments on DIII-D, JET and NSTX. High-order (m/n = 4/3 or 3/2) tearing modes are a key beneficial feature in hybrid scenarios that act to regulate the q-profile to keep q(0) ∼ 1 with the absence of large m/n = 1/1 sawteeth. The destabilization of low-order (m/n = 2/1) tearing acts as the limit on achievable beta. Hybrid discharges in DIII-D with 4/3 tearing modes under large co- (to Ip) neutral-beam torque can be run steadily just below the 2/1 tearing beta limit. However, reducing the torque has consequences on both the existing 4/3 tearing mode amplitude and the beta at which the 2/1 tearing mode destabilizes. It is found that flow shear at a rational surface is well correlated with both decreased 4/3 mode amplitude and higher beta 2/1 mode onset. The working physics model is that flow shear is classically stabilizing, i.e. makes the tearing stability index Δ′ more negative; this both reduces the amplitude of neoclassical tearing modes and makes mode destabilization more difficult (requiring higher beta). However, a detailed understanding of the effects of flow shear on tearing stability remains a challenge for theory and modelling. The classically stabilizing effect of flow shear in DIII-D (and indeed future larger tokamaks) is in the regime of large magnetic Prandtl and very large Lundquist numbers; this is significant for sorting out which physical effects of flow and flow shear are relevant and would be stabilizing or even destabilizing. Experimental data with applied torque varied from all co- to near-balanced neutral beams in the DIII-D hybrid scenario with 4/3 mode ‘regulation’ is analysed for 4/3 mode amplitude, 2/1 onset and criticality for 2/1 mode locking. For both existing m/n = 4/3 modes and for the onset of m/n = 2/1 modes, a local flow shear of the order of is found to have a significant stabilizing effect on tearing. In addition, a flow shear effect increasing local viscosity is suggested in order to explain the behaviour of mode locking to the resistive wall.