Gamma-rays from harmonically resonant betatron oscillations in a plasma wake

An intense laser pulse in a plasma can accelerate electrons(1-4) to GeV energies in centimetres(5-7). Transverse betatron motion(8,9) in the plasma wake results in X-ray photons with an energy that depends on the electron energy, oscillation amplitude and frequency of the betatron motion(10-12). Betatron X-rays from laser-accelerator electrons have hitherto been limited to spectra peaking between 1 and 10 keV (ref. 13). Here we show that the betatron amplitude is resonantly enhanced when electrons interact with the rear of the laser pulse(14,15). At high electron energy, resonance occurs when the laser frequency is a harmonic of the betatron frequency, leading to a significant increase in the photon energy. 108 gammaray photons, with spectra peaking between 20 and 150 keV, and a peak brilliance > 10(23) photons s(-1) mrad(-2) mm(-2) per 0.1% bandwidth, are measured for 700MeV beams, with 10(7) photons emitted between 1 and 7MeV. Femtosecond duration gamma-rays may find uses in imaging, isotope production, probing dense matter, homeland security and nuclear physics(16).