Betatron Cooling and Phase-Space Shaping in Wakefield Accelerators

Relativistic charged-particle beams undergoing betatron oscillations in high-density plasmas can experience extreme radiative losses. In this regime, nonlinear cooling reshapes the beam’s kinetic properties, modifying both its momentum and spatial distribution and driving the formation of structured phase-space beams [1]. We derive the characteristic timescales for this process and validate our results through Particle-in-Cell simulations with OSIRIS [2]. The resulting phase-space beams exhibit pitch-angle anisotropies and population inversions, key ingredients for the onset of kinetic instabilities such as the ion channel laser (ICL) [3]. This mechanism parallels the electron cyclotron maser (ECM) instability in radiative magnetized astrophysical plasmas and provides a viable laboratory analogue [4]. These findings highlight a new pathway for generating coherent betatron emission and for probing maser-like processes driven by radiative cooling in extreme plasma conditions.

[1] P. J. Bilbao et al., in prep. (2025)

[2] R. Fonseca, et al. International Conference on Computational Science, Springer, 342–351 (2002)

[3] D. H. Whittum, et al., PRL 64.21:2511 (1990)

[4] P. J. Bilbao, T. Silva, & L. O. Silva, Sci. Adv. 11 (15), eadt8912 (2025)