北宋时期的司天监于1054年记录了一次超新星爆发，这一事件形成了一颗半径约10公里的中子星和包裹它的蟹状星云。巧合的是，该中子星的磁轴恰好对准了6500光年外的地球。通过高速自转，中子星沿着磁轴发出的电磁辐射每秒扫过地球30次。1968年，科学家首次探测到来自这颗中子星的巨脉冲（giant pulse）无线电波。2003年，Hankins的观测结果显示这些巨脉冲由众多仅1纳秒宽的子脉冲堆积而成，并且这些子脉冲具有极高的圆偏振度。左图：蟹状星云；右图：中子星辐射示意图2007年，Lorimer首次报道了快射电暴（fast radio burst），可以说快射电暴是近年来天文领域最重要的意外发现。2020年，人们观测到银河系内一颗磁星（磁场更强的中子星）爆发了快射电暴。与巨脉冲相似，部分快射电暴也展现出高度的圆偏振。然而，巨脉冲和快射电暴的圆偏振机制至今尚不明确。等间隔双线分布的巨脉冲信号本研究首次在蟹状星云中子星的一个巨脉冲中发现了等间隔成对分布的子脉冲。每对脉冲中，第一个为左旋圆偏振，第二个为右旋圆偏振。上图展示了9对子脉冲的配对情况，成对子脉冲的时间间隔均在21微秒左右，不确定度仅为0.8微秒。理论上，法拉第效应可以用于解释这一现象。然而，中子星磁层内的等离子体由电子和正电子组成，沿磁场方向观测，它们的回旋运动对左右圆偏振是对称的，因此法拉第效应不起作用。我们大胆假设中子星磁层内的正负电子等离子体存在高度不对称性，即正负电荷没有完全中和，或正负电子能量存在显著差异。在这种不对称等离子体中，法拉第效应会将一个线偏振波分裂成前后延迟的左右圆偏振模式。在磁层参数允许的范围内，只有考虑巨脉冲的相对论强场效应，我们才能计算出符合观测结果的一对圆偏振射电脉冲。该理论还成功解释了巨脉冲的反常色散和随机偏振角等特异现象。此外，已有观测表明某些快射电暴也包含纳秒量级的子脉冲，其圆偏振可以用类似的机制进行解释。鉴于广泛认为中子星磁场方向上存在电场，这些电场可以加速正负电子，并可能诱发所假设的等离子体不对称性。此外，部分磁层粒子模拟结果也显示出不对称等离子体的迹象。关于巨脉冲和快射电暴的产生机制，学术界尚无定论，根本原因在于我们对中子星磁层的认识不足。通过观察中子星的电磁辐射，难以推断出更多有用的磁层信息。本研究发现的纳秒级成对巨脉冲可作为一种诊断磁层的新工具，有望帮助确定中子星磁层的等离子体参数。该研究由浙江大学物理学院武慧春教授完成，近日发表在《Astrophysical Journal Letters》。论文链接：https://doi.org/10.3847/2041-8213/ad8154

Fluid simulations of resistive drift-wave turbulencewith diamagnetic flow in ZPED experimentsCite as: Phys. Plasmas 31, 052108 (2024); Submitted: 3 December 2023 . Accepted: 23 April 2024 .Published Online: 7 May 2024H. J. Zhao, Y. Mao, Z. Y. Wang, W. W. Xiao,a) and Y. Xiaoa)AFFILIATIONSInstitute for Fusion Theory and Simulation, School of Physics, Zhejiang University, Hangzhou 310027, Chinaa)Authors to whom correspondence should be addressed: wwxiao@zju.edu.cn and yxiao@zju.edu.cnABSTRACTWe derive a diamagnetic resistive fluid model (DRF) and develop an associated two-dimensional fluid simulation code (DRF-2D) to explore the dynamics of resistive drift modes within the plasmas of the Zheda Plasma Experiment Device (ZPED). The validation of the linear dispersion relation for the DRF-2D code revealed a harmonious agreement between analytical theory and linear numerical simulations. Leveraging plasma parameters obtained from the ZPED experiments, we conducted a comprehensive series of nonlinear simulations using the DRF-2D code. Our simulations successfully replicate the nonlinear trends in turbulent fluctuations and transport observed in the ZPED experiments, particularly demonstrating a remarkably accurate alignment of the turning point in the magnetic field. Notably, the DRF model sheds light on the observed frequency sign reversal from the electron diamagnetic direction to the ion diamagnetic direction in the ZPED experiments. This is demonstrated through well-matched turning points in the confining magnetic field between the nonlinear simulations and ZPED experiments. The fidelity of our model in capturing these phenomena underscores its efficacy in providing valuable insights and predictive capabilities for the intricate dynamics observed in the ZPED plasmas.  FIG. 4. Density mode structures obtained from nonlinear simulations. The m-spectra are presented in (a)–(c), respectively. Corresponding 2D mode structures are depicted in (d)–(f).Fluid simulations of resistive drift-wave turbulence with diamagnetic flow in ZPED experiments.pdf

J. Plasma Phys. (2023), vol. 89, 905890612 A simple model for internal transport barrierinduced by fishbone in tokamak plasmasZhaoyang Liu 1 and Guoyong Fu 1,†1Institute for Fusion Theory and Simulation and School of Physics, Zhejiang University,Hangzhou 310027, PR China(Received 17 July 2023; revised 24 November 2023; accepted 24 November 2023)Fishbone bursts have been observed to strongly correlate to internal transport barrier (ITB)formation in a number of tokamak devices. A simple model incorporating the fishbonedynamics and ion pressure gradient evolution is proposed in order to investigate the keyphysics parameters assisting the triggering of ITB. The time evolution of fishbone isdescribed by the well-known predator–prey model. For each burst cycle, the energeticparticles (EPs) resonantly interact with fishbone and are radially expelled from innerregion leading to a radial current. A compensating bulk plasma return current and, hence,poloidal flow can be induced if the fishbone cycle frequency is greater than the poloidalflow damping rate. When the shear of the poloidal flow exceeds a critical value, theturbulent fluctuations are suppressed and the bulk ion pressure gradient transits to thehigh-confinement state. It is shown that this process is only sensitive to the depositionrate of the trapped EPs within the q = 1 surface, but not sensitive to other parameters. Aquantitative formula for the shearing rate of poloidal flow induced by fishbone bursts isderived and verified numerically. FIGURE 2. Poloidal flow (a) and flow shear (b) driven by a single burst of fishbone (dashed line) and multiple bursts of fishbone (solid line). A simple model for internal transport barrier induced by fishbone in tokamak plasmas.pdf