  A new scheme for bright hard x-ray emission from laser wakefield electron
accelerator is reported, where pure nitrogen gas is adopted. Intense Betatron
x-ray beams are generated from ionization injected K-shell electrons of
nitrogen into the accelerating wave bucket. The x-ray radiation shows
synchrotron-like spectrum with total photon yield 8$\times$10$^8$/shot and
$10^8$ over 110keV. In particular, the betatron hard x-ray photon yield is 10
times higher compared to the case of helium gas under the same laser
parameters. Particle-in-cell simulation suggests that the enhancement of the
x-ray yield results from ionization injection, which enables the electrons to
be quickly accelerated to the driving laser region for subsequent betatron
resonance. Employing the present scheme,the single stage nitrogen gas target
could be used to generate stable high brightness betatron hard x-ray beams.
