Speaker
Description
The development of high-repetition-rate, few-terawatt (TW) laser systems enables the generation of high-average-flux few-keV betatron X-rays for coherent diffraction imaging and submicrometer-scale sample characterization. In principle, the betatron radiation power scales as $P_\beta \propto n_e^2 \gamma^2 r_\beta^2$ [1], where $n_e$ is the plasma density, $\gamma$ is the relativistic Lorentz factor of the electron, and $r_\beta$ is the electron oscillation radius. Therefore, tailoring the longitudinal plasma density profile, with an appropriate density up-ramp increasing the electron energy through phase locking [2] and a sharp density down-ramp promoting plasma-bubble expansion [3] and increasing $r_\beta$, provides a potential approach to enhancing betatron emission.
Based on these concepts, we perform particle-in-cell simulations to investigate the feasibility of enhancing betatron emission from laser wakefield acceleration driven by a 2-TW laser pulse interacting with an asymmetric plasma density profile. For a baseline 670-$\mu\text{m}$-wide Gaussian nitrogen target, an optimal peak electron density of $n_{e0} = 4 \times 10^{19}\text{ cm}^{-3}$ induces sufficient self-focusing of the laser pulse to drive LWFA and form well-defined plasma bubbles. Under these conditions, electrons are accelerated to approximately 70 MeV with an average oscillation radius of 1 $\mu\text{m}$, producing betatron X-rays extending to 6 keV. Increasing the front-side density up-ramp to an effective width of $w_f = 475\ \mu\text{m}$ allows the electrons to remain phase-locked with the accelerating field and reach a maximum energy of approximately 83 MeV, resulting in a twofold enhancement of the betatron radiation intensity. Further tailoring the rear-side density down-ramp to a shortened width of $w_b = 150\ \mu\text{m}$ facilitates plasma-bubble expansion and increases the electron oscillation radius to approximately 2 $\mu\text{m}$. This results in a sevenfold enhancement of the betatron emission relative to the baseline Gaussian target and extends the X-ray spectrum to approximately 10 keV. Our results confirm that a sub-mm-scale asymmetric gas target provides a promising approach to increasing electron energy and betatron oscillation amplitude, thereby enhancing betatron X-ray emission from few-TW laser wakefield accelerators.
References
[1] E. Esarey, B. A. Shadwick, P. Catravas, and W. P. Leemans, “Synchrotron radiation from electron beams in plasma-focusing channels,” Phys. Rev. E, 65, 056505 (2002).
[2] W. Rittershofer, C. B. Schroeder, E. Esarey, F. J. Grüner, and W. P. Leemans, “Tapered plasma channels to phase-lock accelerating and focusing forces in laser-plasma accelerators,” Phys. Plasma, 17, 063104 (2010).
[3] C.-H .Yu, J.-S. Liu, W.-T. Wang, W.-T. Li, R. Qi, Z.-J. Zhang, Z.-Y. Qin, J.-Q. Liu, M. Fang, K. Feng, Y. Wu, L.-T. Ke, Y. Chen, C. Wang, Y. Xu, Y. Leng, C.-Q. Xia, R.-X. Li, and Z.-Z. Xu, “Enhanced betatron radiation by steering a laser-driven plasma wakefield with a tilted shock front,” Appl. Phys. Lett. 112, 133503 (2018)