Description
Ultraintense laser–solid interactions offer a route to the QED–plasma regime, where collective dynamics and strong-field radiation emission become intertwined. A key step is to establish experimental conditions in which synchrotron-like emission from relativistic electrons dominates over collisional bremsstrahlung, which typically masks radiative signatures in solid-density targets at laser intensities achievable at present.
We report an experimental study of hard X-ray generation from ultrathin plastic foils irradiated at normal incidence with a0≈20. Scanning the target thickness reveals a pronounced, reproducible maximum in photon yield for 80–120 nm foils, while thicker targets produce lower yield and a spectral behaviour consistent with a stronger collisional contribution. The use of low-Z foils strongly suppresses bremsstrahlung, isolating the dynamics-driven radiative channel.
To interpret the measurements, we perform particle-in-cell simulations including self-consistent radiation emission. The simulations reproduce the yield optimum and allow the radiative channels to be separated: in the 80–120 nm window synchrotron emission peaks and exceeds bremsstrahlung by a factor of ~6.6. The underlying physics is a competition between (i) enhanced electron energisation and field sampling when the interaction enters relativistic self-induced transparency, and (ii) increased laser reflection and reduced energy coupling when the foil remains opaque. This identifies an experimentally accessible “transparency-threshold” condition that maximises the electron quantum parameter and radiated power.
Our results provide evidence for a transition to a synchrotron-dominated X-ray source governed by fundamental transparency and energy-coupling physics, and they define practical target-design criteria for forthcoming multi-PW campaigns aimed at controlled studies of radiation reaction and, ultimately, coupled QED–plasma phenomena.
[1] J. K. Patel, R. Wilson, M. King, M. O. Cernaianu, R. M. Deas, E. J. Dolier, D. Doria, D. Dorobantu, E. Gerstmayr, R. J. Gray, C. Ingleby, K. L. Lancaster, C. Marin, M. Peat, P. P. Rajeev, C. P. Ridgers, D. Sangwan, G. Sarri, B. Torrance, L. Tudor, C. D. Armstrong and P. McKenna, Under review (2026)