162 / 2023-04-14 23:30:13

Brilliant femtosecond-laser-driven hard X-ray flashes from carbon nanotube plasma

laser driven x-ray sources;,direct laser acceleration;,novel targets

极端光下的基础物理学 > 辐射源

Ultra-intense femtosecond lasers pave new ways of generating brilliant ultrashort x-rays flashes by exploiting relativistic electrons that used to be available only in large accelerator facilities. Femtosecond hard x-ray andγ-ray sources as bright but far more compact than conventional synchrotrons have also been demonstrated by wiggling LWFA electrons with a counter-propagating laser or a self-generated space-charge electric field in the plasma. Despite the fascinating accomplishments, the energy conversion efficiency from laser to hard x-rays is, merely 10^-7-10^-5, which seriously undermines the competitiveness of laser-driven x-ray/γ-ray sources against conventional light sources.

Recently, we boost the laser-to-x-ray conversion efficiency to an unprecedented level of 10^-3 by exploiting carbon nanotube plasma instead of widely used gas plasma. Hard x-ray yield exceeding 10^10 photons/J was measured for the first time in the petawatt laser facility of CoReLS. Such a breakthrough is owning to the unique sub-wavelength architectures of the targets: a 3D nano-network constructed by CNT bundles with diameter of 5-10 nm and length of 5-50 μm, in which 99.9% space is vacant. The electron acceleration and x-ray generation mechanism in such CNT plasma is entirely different from the prevailing laser wake field acceleration (LWFA) scheme. Our study indicates that the extension of the current scheme with a 10-PW laser (upcoming in ELI, SIOM...) could produceγ-rays with the average photon energy exceeding 1 MeV and the conversion efficiency of more than 1%, which would promptly broad applications including the transmutation of long-lived radioactive nuclides contained in nuclear waste.