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利用 TRANSP code 模拟 HL-2A 托卡马克的反磁剪切运行,将2.5 MW 的中性束注入到密度分布逐渐峰化的靶等离子体中,随着等离子体 β 值的增高,形成了持续的反磁剪切位形。这类反磁剪切位形的极小 q 值(qmin)位于 rmin/a≈0.3 处并逐步从 qmin>3.0 向 qmin<2.0 演化,它们的特征是异常峰化的压强分布和非常强的中心负磁剪切。在建立了自洽的反磁剪切位形后,分析了它们的磁流体力学稳定性,这些分析包括理想的低 n 模 MHD,气球模和电阻交换模不稳定性。研究结果表明,在剪切反转点附近的低磁剪切区域产生了不稳定的低 n 模,说明这类不稳定模是压强驱动的。对本征扰动位移进行了 Fourier 分析,在 nqmin>4.0 的情况下,径向扰动位移的 Fourier 谐量是 m=1, 2, 3, 4。而且不稳定性增长率随环向模 n 的变化是振荡的,显示具有 infernal mode的特征。对于电阻交换模不稳定性,我们在中心负磁剪切区对不稳定性判据进行ε(反环径比)展开,结果显示该区域相当大的压强梯度驱动电阻交换模不稳定,但不稳定窗口只扩展到 rmin/a≈0.2,并不覆盖发生 low-n modes 不稳定性的低磁剪切区域。
The transcantile shear mode of HL-2A tokamak was modeled with TRANSP code. A 2.5 MW neutral beam was injected into the target plasma with a gradually increasing density distribution. As plasma β increased, a continuous Diamagnetic shear shape. The minimum q value (qmin) of this diamagnetic shear configuration is located at rmin / a≈0.3 and gradually evolves from qmin> 3.0 to qmin <2.0, and they are characterized by anomalous peaking pressure distribution and a very strong center Negative magnetic shear. After establishing self-consistent diamagnetic shear configurations, their MHD stability is analyzed. These analyzes include ideal low-n-mode MHD, Balloon modes, and resistance-mode excursions. The results show that the unstable low n mode is generated in the low magnetic shear region near the shear reversal point, which indicates that this type of unstable mode is pressure-driven. The Fourier analysis of the intrinsic perturbation displacement is carried out. When nqmin> 4.0, the Fourier harmonic of the radial perturbation displacement is m = 1, 2, 3, 4. Moreover, the rate of instability growth oscillates with the change of the ring mode n, showing the feature of infernal mode. For the instability of the resistance-exchange module, we expand the instability criterion ε (anti-ring-radius ratio) in the center negative-shear region. The results show that the rather large pressure gradient-driven resistance exchange mode in this region is unstable, The stability window only extends to rmin / a≈0.2 and does not cover areas with low magnetic shear where low-n modes are unstable.