Description
Coherent Transition Radiation (CTR) is a primary mechanism for generating high-energy electromagnetic pulses (EMP) in ultra-high intensity laser-plasma interactions [1], reaching sub-Joule to Joule-level energies in the THz range [2,3]. Its characteristics are highly sensitive to the longitudinal and transverse properties of the laser-accelerated electron bunches as they cross the plasma-vacuum interface and, therefore, to the underlying acceleration mechanism.
In this work, we employ two-dimensional particle-in-cell (PIC) simulations using the code CALDER [4], which solves the relativistic Vlasov-Maxwell equations, to perform a parametric study on CTR emission as a function of the target density ranging from underdense ($< 0.01 \, n_{cr}$) to overdense ($\sim 30 \, n_{cr}$) plasma densities. Focusing on an experimentally feasible design with high-repetition-rate gas targets and ultra-short (few- to multi-cycle), Joule-class laser pulses [5], we identify through angular and spectral features that the radiation carries the imprint of the underlying acceleration mechanism — e.g. wakefield acceleration in the blowout regime [6] or electron acceleration in laser-solid interactions [7]. We characterize the transition between the different acceleration mechanisms and find good agreement with theoretical predictions on the impact on the THz yield and spectral distribution.
[1] A. Poyé et al., Phys. Rev. E 91, 043106 (2015).
[2] G. Liao et al., Phys. Rev. X 10, 031062 (2020).
[3] G. Bruhaug et al., Opt. Lett. 49, 1737 (2024).
[4] E. Lefebvre et al., Nucl. Fusion 43, 629 (2003).
[5] M. O. Cernaianu et al., Matter Radiat. Extremes 10, 027204 (2025).
[6] J. Déchard et al., Phys. Rev. Lett. 120, 144801 (2018).
[7] E. Denoual et al., Phys. Rev. E 108, 065211 (2023).