  Modern particle accelerators and their applications increasingly rely on
precisely coordinated interactions of intense charged particle and laser beams.
Femtosecond-scale synchronization alongside micrometre-scale spatial precision
are essential e.g. for pump-probe experiments, seeding and diagnostics of
advanced light sources and for plasma-based accelerators. State-of-the-art
temporal or spatial diagnostics typically operate with low-intensity beams to
avoid material damage at high intensity. As such, we present a plasma-based
approach, which allows measurement of both temporal and spatial overlap of
high-intensity beams directly at their interaction point. It exploits
amplification of plasma afterglow arising from the passage of an electron beam
through a laser-generated plasma filament. The corresponding photon yield
carries the spatiotemporal signature of the femtosecond-scale dynamics, yet can
be observed as a visible light signal on microsecond-millimetre scales.
