Around age six, we start learning how to tie our shoelaces, making knots that look like ribbons — or possibly more complex forms, if we are a little clumsy. We use knots every day, but the type of knots we generally use is associated with physical objects, things we can touch. In a recent study, published in two papers, joint collaborations by ICREA/ICFO researchers demonstrated multicolored light twists in new knotted ways. They have broken theoretical and experimental ground in this new field, uncovering new types of knots for twisted light and a new type of angular momentum.

In the first paper, published in Nature Photonics, ICFO researchers Emilio Pisanty, Gerard Jiménez Machado, Veronica Vicuña-Hernández, Antonio Picón, and Alessio Celi, led by ICREA Prof. at ICFO Maciej Lewenstein and UPC Prof. at ICFO Juan P. Torres, have designed a beam of light with a polarization state that forms three-lobed trefoils at each point, by combining light of different frequencies, and making the trefoils connect to each other in a way such that the light beam, as a whole, has the shape of a knot. These beams also exhibit a new kind of angular momentum, associated with the unusual symmetry of the beams, which remain unchanged when they’re rotated — but only when the polarization is rotated by a specific portion of the rotation in space. This new quantity is termed the torus-knot angular momentum, because of the type of knot in the beams, and the researchers were able to experimentally confirm its presence.

In the second paper, published in Physical Review Letters, ICFO researchers Emilio Pisanty and Antonio Picón, led by ICREA Professor at ICFO Maciej Lewenstein, in collaboration with researchers from the University of Salamanca and from CU Boulder, show that the new type of angular momentum is conserved in interactions. They show, via theoretical simulations, that at extremely high intensities, many photons of light can be combined into single photons with high energy, and that these new, bigger photons carry the combined torus-knot angular momentum of the original, smaller photons.

Shear Alfvén instabilities are of considerable interest in fusion plasma research. These instabilities can be driven by energetic particles such as fusion products. Not only can the Alfvén instabilities degrade the confinement and heating efficiency of energetic particles by channeling them but they can also cause severe damage to the plasma-facing components of the plasma confining vessel through high heat fluxes. Thus, investigations of their control, better performance and comprehensive understanding are required.

We have carried out detailed modelling and analysis of experimentally observed energetic particle-driven Alfvénic instabilities in tokamak and stellarator plasmas using three dimensional (3D) numerical tools based on the reduced magnetohydrodynamics (MHD) model.

To explain experimental observations of Alfvén instabilities in TJ-II flexible heliac plasmas, we have compared experimental results with the results obtained through theoretical model of shear Alfvén waves in a 3D toroidal geometry and obtained qualitative agreement.

We have also modelled bifurcated MHD equilibria with 3D helical core surrounded by an axisymmetric 2D mantle for the ASDEX Upgrade tokamak plasmas and compared them to an axisymmetrical 2D case (cf. figure). Our modelling results reveal interesting new physics effects due to 3D helical core such as continuum splitting and variation of mode frequency around the frequency accumulation point as well as new frequency gaps due to helical distortion.