  Direct laser acceleration of electrons in near-critical-density (NCD) carbon
nanotube foams (CNFs) has its advantages in the high-efficiency generation of
relativistic electrons and broadband X-rays. Here, we report the first
simultaneous measurement on the spectra of laser-driven electrons and X-rays
from CNFs at moderately relativistic intensities of around 5\times{10}^{19}\
W/cm^2.\ The density and thickness of the CNFs were scanned in the experiments,
indicating the optimized electrons temperature of 5.5 MeV and X-ray critical
energy of 5 keV. Two-dimensional (2D) particle-in-cell (PIC) simulations
confirm that the electrons, with a temperature significantly higher than the
pondermotive scale, are directly accelerated by the laser along the NCD plasma
channel, while the bright X-rays are emitted by these electrons through
betatron radiation or Thomson backscattering inside the channel. The
simultaneously generated electrons and X-rays, automatically synchronized with
the femtosecond laser driver, are suitable for applications such as bi-modal
radiography.
