Axial channeling of relativistic electrons in crystals as a source for positron production

In the framework of studies of positron sources dedicated to linear colliders, crystals are considered as photon radiators in association with thin amorphous converters or as compact positron sources where photon production and pair creation occur in the same medium. The main features of such sources in comparison with conventional amorphous sources are described. Taking linear collider requirements for such sources, a crystal converter suited for the Japanese Linear Collider (JLC) is considered. Limitations caused by radiation damage are mentioned and indications on critical fluences are given. A proof-of-principle experiment has been undertaken in Orsay, observing radiation enhancement in a tungsten crystal oriented along the [111] axis and submitted to a 2 GeV electron beam. The main results are reported.