By: Nuno Lemos
At: Instituto de Investigação Interdisciplinar, Anfiteatro
The laser-plasma wakefield accelerator concept was born in 1979 when Tajima and Dawson proposed a new mechanism to accelerate electrons to relativistic energies in very short distances using high-power lasers propagating in underdense plasmas. As the laser pulse propagates in a plasma, generates plasma waves and the electrons can surf the electric field wave of a laserâ€™s wake. Todayâ€™s experimental results show that electrons can be accelerated to GeV energies in the centimetre scale leading to large amplitude transverse betatron motion, which produces copious emission of X-ray photons in femtosecond pulses. The photon energy, which depends on the electron energy and betatron amplitude, was limited until now to 1 â€“ 10 keV.
Here we show that the betatron oscillation amplitude is resonantly enhanced when electrons interact with the laser pulse resulting in orders of magnitude increase in the radiation rate and photon energy. For 700 MeV quasi mono-energetic electron beams, we measure 10^8 gamma-ray photons with spectra peaking between 20 and 150 keV, with a peak brilliance of >10^23 photons/(s^-1 mrad^-2 mm^-2 per 0.1%bandwidth), for 30 pC, sub-10 fs electron bunches. The x-ray spectra peaking at 150keV correspond to a critical energy of 450keV where a large fraction of photons above 1 MeV are emitted. This brilliant femtosecond duration gamma ray point source may find use in medical imaging, isotope production, homeland security and potentially probing nuclear phenomena.