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.

Displacement-mediated bound states in the continuum in all-dielectric superlattice metasurfaces

Bound states in the continuum (BICs) are localized states coexisting with extended waves inside the continuous spectrum range, which have infinite lifetimes without any radiation. To extract high-Q quasi-BIC resonances from the symmetry-protected BIC for practical applications, symmetry-breaking approaches are usually exploited, either by slightly breaking the excitation field symmetry or structure symmetry. Here, we introduce an all-dielectric superlattice metasurface that can symmetry-compatibly convert BIC states into high-Q quasi-BIC modes based on the guided-mode resonance coupling by relative displacement tuning. The metasurface is composed of a superlattice of multiple nanobeams, supporting both magnetic mode and toroidal mode with large tunability. Both modes can interact with the incident continuum by mediating the displacement between nanobeams, which empowers dual asymmetric Fano resonances with high Q-factors. The bandwidth of the toroidal mode under y-polarized incidences and that of the magnetic mode under x-polarized incidences can be readily tuned by the local displacement between nanobeams in each unit cell. Such displacement-mediated BIC resonance is promising for various applications such as bio-molecule sensing and low threshold lasing.

Displacement-mediated bound states in the continuum in all-dielectric superlattice metasurfaces

Bound states in the continuum (BICs) are localized states coexisting with extended waves inside the continuous spectrum range, which have infinite lifetimes without any radiation. To extract high-Q quasi-BIC resonances from the symmetry-protected BIC for practical applications, symmetry-breaking approaches are usually exploited, either by slightly breaking the excitation field symmetry or structure symmetry. Here, we introduce an all-dielectric superlattice metasurface that can symmetry-compatibly convert BIC states into high-Q quasi-BIC modes based on the guided-mode resonance coupling by relative displacement tuning. The metasurface is composed of a superlattice of multiple nanobeams, supporting both magnetic mode and toroidal mode with large tunability. Both modes can interact with the incident continuum by mediating the displacement between nanobeams, which empowers dual asymmetric Fano resonances with high Q-factors. The bandwidth of the toroidal mode under TE-polarized incidences and that of the magnetic mode under TM-polarized incidences can be readily tuned by the local displacement between nanobeams in each unit cell. Such displacement-mediated BIC resonance is promising for various applications such as bio-molecule sensing and low threshold lasing.

Active Switching of Toroidal Resonances by Using a Dirac Semimetal for Terahertz Communication

The dynamical switching of a toroidal dipole resonance channel is demonstrated by tuning the Fermi level of a Dirac semimetal film sandwiched between the back substrates. As the Fermi level is increased from 30 to 150 meV, the resonance frequency is switched from 0.283 to 0.201 THz because of the transition from toroidal mode to hybrid mode. The hybrid mode is formed by the interaction between the toroidal mode and the plasmonic mode (induced by a Dirac semimetal film with metallic properties). The influence of the sandwiched dielectric layer (between the toroidal metasurface pattern and the Dirac semimetal film) on the switching effect was also investigated. This active dual-channel terahertz switching may have potential applications in advanced terahertz communication.

A new catalogue of toroidal-mode overtone splitting function measurements

Spectra of whole Earth oscillations or normal modes provide important constraints on Earth’s large scale structure. The most convenient way to include normal mode constraints in global tomographic models is by using splitting functions or structure coefficients, which describe how the frequency of a specific mode varies regionally. Splitting functions constrain 3D variations in velocity, density structure and boundary topography. They may also constrain anisotropy, especially when combining information from spheroidal modes, which are mainly sensitive to P-SV structure, with toroidal modes, mainly sensitive to SH structure. Spheroidal modes have been measured extensively, but toroidal modes have proven to be much more difficult and as a result only a limited number of toroidal modes have been measured so far. Here we expand the splitting function studies by Resovsky and Ritzwoller (1998) and Deuss et al. (2013), by focusing specifically on toroidal mode overtone observations. We present splitting function measurements for 19 self-coupled toroidal modes of which 13 modes have not been measured before. They are derived from radial and transverse horizontal component normal mode spectra up to 5 mHz for 91 events with MW ≥ 7.4 from the years 1983-2018. Our data include the Tohoku event of 2011 (9.1MW), the Okhotsk event of 2013 (8.3MW) and the Fiji Island event from 2018 (8.2MW). Our measurements provide new constraints on upper and lower mantle shear wave velocity structure and in combination with existing spheroidal mode measurements can be used in future inversions for anisotropic mantle structure. Our new splitting function coefficient data set will be available online.