Radiation generation from plasma-based accelerators

Abstract

Summary form only given. Laser wakefield accelerators (LWFAs), in which an intense laser pulse is used to drive a large amplitude plasma wave, have demonstrated the production of relativistic electron bunches. In the self-modulated regime of the LWFA, the bunch contains several nC of charge with a broad electron energy distribution, the tail of which exceeds 100 MeV. By a variety of methods, these electron bunches can be used to generate short-pulse radiation from the THz to the hard X-ray regimes. Coherent transition radiation is generated when the electron bunch crosses the plasma-vacuum boundary. Since the bunch is short (on the order of the laser pulse length), the bunch radiates coherently for wavelengths longer than the bunch length. This results in coherent radiation in the THz regime in which the radiated energy scales as the square of the number of electrons in the bunch. Other mechanisms exist for generating radiation in the X-ray regime. As the electrons are accelerated through the plasma, they undergo transverse oscillations (betatron motion) caused by the transverse electric field of the plasma wave (wake). The electrons then produce incoherent synchrotron radiation at a wavelength given by /spl lambda//sub /spl beta///2/spl gamma//sup 2/, where /spl lambda//sub /spl beta// is the betatron wavelength (which can be on the order of 1 mm or less) and /spl gamma/ is the relativistic factor of the electron. Alternatively, an external, counter-propagating laser pulse can be used to scatter off the relativistic electron bunch. In this case, photons are generated by Thomson backscattering at a wavelength given by /spl lambda//sub 0//4/spl gamma//sup 2/, where /spl lambda//sub 0/ is the laser wavelength (on the order of a micron). The theory underlying these mechanism will be described, as well as comparisons to recent results at LBNL and elsewhere.