3D particle-in-cell simulations with OSIRIS reproduced the experiment and showed why: the finite divergence and energy spread of the pairs stabilize the plasma. Perfectly collimated beams would easily filament, but realistic ones don’t, matching what we measured at CERN.

Under realistic, nonideal conditions, pair-beam instabilities are strongly suppressed, meaning they likely don’t affect (or less than we thought) blazar pair cascades. This supports the idea that the IGM contains a magnetic field of unknown origin that may well be a relic of the early Universe.

This paper is a major step forward:
✅ Starts from realistic kinetic distributions
✅ Observes the actual coherent radiation emerge
✅ Shows it persists, thanks to cooling
Big thanks to our team @golp-ist.bsky.social

We explore the parameter space where this mechanism operates.
Turns out, synchrotron-cooled pair plasmas can support this instability over astrophysically and experimentally relevant conditions.

Even more striking—cooling sustains the instability!
Instead of damping it out, radiation feeds the population inversion.
That means long-lived emission, which is rare in standard ECMI setups.
Could this help explain coherent emission from pulsars and magnetars?

Using massive particle-in-cell simulations, we show that these rings don’t just form—they radiate.
What comes out is linearly polarized, coherent, narrowband radiation, driven by the electron cyclotron maser instability (ECMI).

In our earlier PRL journals.aps.org/prl/abstract..., we showed how radiation reaction sculpts relativistic plasmas into ring-shaped momentum distributions, with inverted Landau populations.
These are kinetically unstable and can drive emission.
The new work? We watched that emission emerge.

We explore the parameter space where this mechanism operates.
Turns out, synchrotron-cooled pair plasmas can support this instability over a wide range of conditions, among them some relevant to astrophysics and (maybe) experiments.

What really surprised us was that cooling can sustains the instability!
Instead of reaching saturation like usual, synchrotron radiation sustains the population inversion.
That means long-lived emission.
Could this help explain coherent emission from pulsars and magnetars?

Using massive particle-in-cell simulations (some of the larges on this kind of plasma), we show that
what comes out is linearly polarized, coherent, narrow-band radiation, driven by the electron cyclotron maser instability (ECMI).

In earlier journals.aps.org/prl/abstract..., we showed how radiation reaction shapes relativistic plasmas into ring-shaped momentum distributions, with inverted Landau populations.
These are kinetically unstable and can drive emission.
The new work? We watched that emission emerge self-consistently.