Magnetization dynamics approaching an equilibrium vortex state from an initial nonequilibrium state under zero magnetic field has been studied in a circular-shaped Fe disk with a thickness of 5nm and a diameter of 1200nm using micromagnetic simulations. Upon starting from the initial random configuration of in-plane magnetizations, a great number of vortex (V)–antivortex (V¯) pairs are generated at a lot of nucleation sites where both types of V and V¯ are energetically favorable to form. The V and V¯ are propagated and then annihilated by their attractive interactions during the relaxation dynamic process. These results reveal that temporal magnetization evolutions can be dominated by the nucleation of V–V¯ pairs, followed by their propagation and annihilation. The dynamic process driven by V–V¯ pairs can play a significant role in various magnetization reversals.

It is shown that electron drift waves can be destabilized by trapped fast ions produced by a fusion reaction because a significant fraction of those particles reverses their precession direction in reversed shear plasmas toward the electron diamagnetic direction and can resonate with the electron drift wave. We perform a local stability analysis and calculate the consequent quasi-linear transport caused by this new instability using bounce-averaged gyrokinetic equations in toroidal geometry and under fusion reactor conditions. We consider the equilibrium distribution function of fast ions as the slowing down distribution and compare with the equivalent Maxwellian case for illustration. The instability occurs when the temperature profile of fast ions peaks sufficiently compared to the density profile, and the magnetic shear is strongly negative. The resulting particle flux of fast ions is outward, while the particle flux of main hydrogenic ions can be inward.

A new electron drift instability driven by resonance with precession reversed fast trapped ions in reversed magnetic shear tokamak plasmas is identified from gyrokinetic simulations. Results from the initial value GKW code and eigenmode analysis in the ballooning space are in broad agreement with predictions from the local analytic theory [B. J. Kang and T. S. Hahm, Phys. Plasmas 26, 042501 (2019)].

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IntroductionThe so-called magnetic vortex structure in micron (or less) sized elements of ullrathin magnetic films has attracted much attention because of its fundamental interest and technological applications to ultrahigh-density memory and other spintronics devices. The vortex structure in confincd systems has thus been intensively studied both theoretically and