Prevention of ECCD triggering explosive bursts in reversed magnetic shear tokamak plasmas for disruption avoidance

The explosive burst excited by neoclassical tearing mode (NTM) is one of the possible candidates of disruptive terminations in reversed magnetic shear (RMS) tokamak plasmas. For the purpose of disruption avoidance, numerical investigations have been implemented on the prevention of explosive burst triggered by the ill-advised application of electron cyclotron current drive (ECCD) in RMS configuration. Under the situation of controlling NTMs by ECCD in RMS tokamak plasmas, a threshold in EC driven current has been found. Below the threshold, not only are the NTM islands not effectively suppressed, but also a deleterious explosive burst could be triggered, which might contribute to the major disruption of tokamak plasmas. In order to prevent this ECCD triggering explosive burst, three control strategies have been attempted in this work and two of them have been recognized to be effective. One is to apply differential poloidal plasma rotation in the proximity of outer rational surface during the ECCD control process; The other is to apply two ECCDs to control NTM islands on both rational surfaces at the same time. In the former strategy, the threshold is diminished due to the modification of classical TM index. In the latter strategy, the prevention is accomplished as a consequence of the reduction of the coupling strength between the two rational surfaces via the stabilization of inner islands. Moreover, the physical mechanism behind the excitation of the explosive burst and the control processes by different control strategies have all been discussed in detail.

Secondary Magnetic Reconnection at Earth’s Flank Magnetopause

We report local secondary magnetic reconnection at Earth’s flank magnetopause by using the Magnetospheric Multiscale observations. This reconnection is found at the magnetopause boundary with a large magnetic shear between closed magnetospheric field lines and the open field lines generated by the primary magnetopause reconnection at large scales. Evidence of this secondary reconnection are presented, which include a secondary ion jet and the encounter of the electron diffusion region. Thus the observed secondary reconnection indicates a cross-scale process from a global scale to an electron scale. As the aurora brightening is also observed at the morning ionosphere, the present secondary reconnection suggests a new pathway for the entry of the solar wind into geospace, providing an important modification to the classic Dungey cycle.

TEM in toroidal plasmas with anisotropic electron temperature

Trapped electron modes (TEMs) in tokamak plasmas with anisotropies of electron temperature and its gradient are studied by solving the gyrokinetic integral eigenmode equation. Detailed numerical analyses indicate that, in comparison with that in plasmas of isotropic electron temperature, TEMs are enhanced (weakened) by the anisotropy with temperature in the direction perpendicular to magnetic field higher (lower) than that in the direction parallel to the magnetic field when the latter is kept constant. However, the enhancement is limited such that TEMs are weakened rapidly and even stabilized when the anisotropy is higher than a critic value owing to an effective reduction of bounce movement of the trapped electrons. In addition, it is found that the gradients of perpendicular and parallel temperatures of electrons have driving and suppressing effects on the TEMs, respectively. The overall effects of the temperature gradients of electrons and ions, magnetic shear, safety factor, density gradient on TEMs in the presence of the anisotropies are presented in detail.

Reflection and Evolution of Torsional Alfvén Pulses in Zero-beta Flux Tubes

We model the behavior of a torsional Alfvén pulse, assumed to propagate through the chromosphere. Building on our existing model, we utilize the zero-beta approximation appropriate for plasma in an intense magnetic flux tube, e.g., a magnetic bright point. The model is adapted to investigate the connection between these features and chromospheric spicules. A pulse is introduced at the lower, photospheric boundary of the tube as a magnetic shear perturbation, and the resulting propagating Alfvén waves are reflected from an upper boundary, representing the change in density found at the transition region. The induced upward mass flux is followed by the reversal of the flux that may be identified with the rising and falling behavior of certain lower solar atmospheric jets. The ratio of the transmitted and reflected mass flux is estimated and compared with the relative total mass of spicules and the solar wind. An example is used to study the properties of the pulse. We also find that the interaction between the initial and reflected waves may create a localized flow that persists independently from the pulse itself.

A novel measurement of marginal Alfvén Eigenmode stability during high power auxiliary heating in JET

The interaction of Alfvén Eigenmodes (AEs) and energetic particles is one of many important factors determining the success of future tokamaks. In JET, eight in-vessel antennas were installed to actively probe stable AEs with frequencies ranging 25-250 kHz and toroidal mode numbers |n| < 20. During the 2019-2020 deuterium campaign, almost 7500 resonances and their frequencies f, net damping rates γ < 0, and toroidal mode numbers were measured in almost 800 plasma discharges. From a statistical analysis of this database, continuum and radiative damping are inferred to increase with edge safety factor, edge magnetic shear, and when including non-ideal effects. Both stable AE observations and their associated damping rates are found to decrease with |n|. Active antenna excitation is also found to be ineffective in H-mode as opposed to L-mode; this is likely due to the increased edge density gradient's effect on accessibility and ELM-related noise's impact on mode identification. A novel measurement is reported of a marginally stable, edge-localized Ellipticity-induced AE probed by the antennas during high-power auxiliary heating (ICRH and NBI) up to 25 MW. NOVA-K kinetic-MHD simulations show good agreement with experimental measurements of f, γ, and n, indicating the dominance of continuum and electron Landau damping in this case. Similar experimental and computational studies are planned for the recent hydrogen and ongoing tritium campaigns, in preparation for the upcoming DT campaign.

Kinetic-ballooning-mode turbulence in low-average-magnetic-shear equilibria

Kinetic-ballooning-mode (KBM) turbulence is studied via gyrokinetic flux-tube simulations in three magnetic equilibria that exhibit small average magnetic shear: the Helically Symmetric eXperiment (HSX), the helical-axis Heliotron-J and a circular tokamak geometry. For HSX, the onset of KBM being the dominant instability at low wavenumber occurs at a critical value of normalized plasma pressure $\beta ^{\rm KBM}_{\rm crit}$ that is an order of magnitude smaller than the magnetohydrodynamic (MHD) ballooning limit $\beta ^{\rm MHD}_{\rm crit}$ when a strong ion temperature gradient (ITG) is present. However, $\beta ^{\rm KBM}_{\rm crit}$ increases and approaches the MHD ballooning limit as the ITG tends to zero. For these configurations, $\beta ^{\rm KBM}_{\rm crit}$ also increases as the magnitude of the average magnetic shear increases, regardless of the sign of the normalized magnetic shear. Simulations of Heliotron-J and a circular axisymmetric geometry display behaviour similar to HSX with respect to $\beta ^{\rm KBM}_{\rm crit}$ . Despite large KBM growth rates at long wavelengths in HSX, saturation of KBM turbulence with $\beta > \beta _{\rm crit}^{\rm KBM}$ is achievable in HSX and results in lower heat transport relative to the electrostatic limit by a factor of roughly five. Nonlinear simulations also show that KBM transport dominates the dynamics when KBMs are destabilized linearly, even if KBM growth rates are subdominant to ITG growth rates.