Cosmic rays (CRs)—the most energetic particles in the Universe—have recently emerged as an essential agent shaping diverse astrophysical systems. Interactions between CRs and plasma exhibit parallels across the interstellar (ISM), circumgalactic (CGM), and intracluster (ICM) media. Advancing our understanding of these key but incompletely understood processes, particularly CR transport, is essential for assessing the impact of CRs across these environments.

This program will strengthen connections between distinct scientific communities studying CRs across scales—from the solar wind to galaxy clusters—with an emphasis on CR-plasma interactions. Although these communities are grounded in shared physical principles, they have largely evolved in parallel. The program will promote dialogue between these groups to deepen understanding of CRs' role in shaping diverse astrophysical systems.

The program will examine the following themes:

• CRs in turbulent plasmas: How do CR-driven instabilities, wave damping, and intermittent turbulence regulate CR transport?
• CRs in the ISM: How do supernovae accelerate CRs, and how do CRs shape ISM chemistry and star formation?
• CR feedback in galaxies and clusters: How do CRs from supernovae and black hole jets drive galactic winds and shape the CGM and ICM?
• Astrophysical tests of CR transport: How can modeling of individual objects (supernovae, star clusters, radio filaments, galaxies) constrain CR physics?
• Solar energetic particles: How do we model and constrain particle acceleration and solar wind-ISM interactions?
• Laboratory tests: How can laser plasma experiments inform CR propagation and acceleration models?