Attosecond coherent Cherenkov radiation from a flying-focus-driven superluminal wakefield

We present a scheme for attosecond coherent Cherenkov radiation from a superluminal wake driven by a flying-focus pulse. Varying the flying-focus velocity allows the wake phase velocity to cross from subluminal to superluminal, providing direct control over radiation coherence and directivity. In the superluminal regime, the divergence angle follows the Fresnel diffraction limit for a finite source, reducing from tens of milliradians to below 9 mrad. Transverse electron oscillations are found to narrow the angular spread with increasing amplitude. The plasma density serves as a key tuning parameter, leading to a superlinear growth in absolute bandwidth with density, which is expected to further expand the range of photon energy. These results lay a theoretical foundation for realizing tunable attosecond coherent light sources via the flying-focus technique.