The magnetorotational instability (MRI) is a crucial mechanism of angular momentum transport in several astrophysical scenarios, like accretion disks around black holes. The MRI has been widely studied using MHD models and simulations, to understand the behavior of astrophysical fluids in a state of differential rotation. In particular, the MRI plays a crucial role in the transport of angular momentum and turbulence generation in the late nonlinear evolution of the system. In radiatively inefficient accretion flow (RIAF) models for accretion onto compact objects, the accretion proceeds via a hot, low-density plasma with the proton temperature larger than the electron temperature. To maintain the two-temperature flow characteristic of RIAF models, the typical collision rate must be much smaller than the accretion rate. This suggests that the standard MHD approach may be insufficient, and a kinetic description is required instead. Leveraging on our recent implementation of the shearing co-rotating framework in OSIRIS 4.0, we present our recent studies on collisionless MRI in high-beta plasma. On sufficient large simulation domains, we observe the development of an MRI-induced not-relativistic turbulent regime during the late nonlinear stage of the evolution. Increasing the mass ratio of our simulations, we show the development of enhanced ion heating during the early nonlinear phase of collisionless MRI (channel flows regime). We will explore the mechanism responsible for these effects. The development of a drift-kink instability on large domains, combined with the magnetic reconnection, is responsible for the generation of the turbulent regime observed. The rise of an electric field during the early nonlinear regime of the MRI acts differently on the two plasma species, inducing the temperature difference observed in our study. We support our assumptions with a theoretical model for kinetic compression of current sheets, giving a quantitative prediction on the trapped particle acceleration during the compression phase.

There will be an opportunity to interact with Giannandrea's art outreach project Turbulence: Voice of Space, in which the results presented during the seminar are explorable in virtual reality.

Stellarators offer an inherently steady state reactor concept with low recirculating power. Because stellarators do not rely on plasma current for confinement, they are not susceptible to current driven disruptions. Stellarators are also capable of operating at high density, and can perform stably beyond ideal MHD stability limits. Because stellarator configurations have magnetic fields imposed mainly by external coils, there is significant freedom to tailor the confinement properties to the device needs. Only in the last few decades has theoretical knowledge of stellarator confinement advanced so as to produce optimized configurations. This talk will focus on how these devices are optimized, both for current experiments that exist today, and for future experiments, pilot plants, and reactor concepts. Four topical areas, identified as key physics gaps for stellarators, are discussed: turbulent transport optimization by design, energetic particle transport, divertor performance, and coil design.