High-brilliance X/gamma sources can nowadays be efficiently generated by compact, all-optical, Thomson/Compton processes in linear or nonlinear regime  driven by Laser Wake Field Accelerators (LWFA). High-brilliance sources can be obtained with a combination of large charge, small energy spread and small transverse momentum of the accelerated electron bunch, which rely on the so called high-brightness (i.e. high-quality) e-beams.
The production of high-quality e-beams in LWFA relies on the possibility to inject ultra-low emittance bunches in the plasma wave. A new bunch injection scheme (Resonant Multi-Pulse Ionization, ReMPI) has been conceived and studied in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses . The ReMPI injection scheme relies on currently available laser technology and is being considered for implementation of future compact X-ray free electron laser schemes . Simulations with either a 200TW or a 1PW Ti:Sa laser system show that high-quality electron bunches with energy up to 5 GeV, with normalized emittance below 0.1 mm mrad and energy spread below 1% can be obtained with a single stage.
The LWFA source in the REMPI configuration is flexible in beam energy, charge and duration and is able to generate beams with a remarkable low transverse size. The extremely low transverse size (down to 0.5 m) and duration (down to less-than-1fs) of the e-beams make this accelerating scheme particularly appealing as a driver for Thomson/Compton sources delivering sub-fs X/gamma rays or ultra small-size sources suitable for phase-contrast imaging.
 P. Tomassini et al., Appl. Phys. B 80, 419–436 (2005)
 P. Tomassini et al., The Resonant Multi-Pulse Ionization injection, Physics of Plasmas 24, 103120 (2017)
 EuPRAXIA collaboration, http://www.eupraxia-project.eu/; R. Assman et al., Eur. Phys. J. Spec. Top. 229, 3675–4284 (2020).