Large-amplitude electron plasma waves are usually described either by trapping physics or by Landau damping, depending on the regime. We show with Vlasov-Poisson-Fokker-Planck simulations that in the weakly collisional limit — where the bounce frequency far exceeds the electron-electron collision frequency — the wave passes through three distinct phases, and that the long-lived middle phase is governed by neither of the two familiar descriptions.
[Publication in the Journal of Plasma Physics]
A.S. Joglekar and A.G.R. Thomas, J. Plasma Phys. 92, E97 (2026)
The simulations show the wave evolving through:
The dispersion relation is well established during phases I and III. Phase II is the one that has not been well characterized, and it is where the wave spends most of its life.
During the detrapping phase, weak electron-electron collisions compete with strong wave-electron interactions. The result is counterintuitive: rather than relaxing the wave back toward the linear root, the interplay drives the frequency shift further from it, and that shift keeps increasing as the phase proceeds.
The phase ends when the distribution function has been collisionally relaxed to something nearly Maxwellian. At that point the frequency shift rapidly collapses and the wave begins damping at a larger rate, consistent with linear Landau theory.
The detrapping phase dominates the lifetime of these waves, so a model that only captures trapping and Landau damping misses the regime that actually governs the wave’s behavior — including the frequency shift that a Thomson-scattering diagnostic would measure. This is directly relevant to interpreting measurements of large-amplitude waves in inertial fusion plasmas.
[Publication in the Journal of Plasma Physics] · [arXiv:2603.11225]