In order to achieve high luminosities in high-energy e+e－ colliding experiments, positron sources must be reinforced, especially towards the next generation of high-energy e+e－ colliders. In a conventional positron source, positrons are produced by high-energy electrons hitting a heavy-metal target, such as a tungsten plate. However, the allowable heat load on the target limits the beam power of the incident electrons. This result gives the limit of the positron intensity. One promising approach to improve the positron-production efficiency and to reduce the heat load on the target is to use a crystal-assisted positron source because it is expected that channelling radiation and coherent bremsstrahlung contribute to the enhancement of the positron intensity. A new tungsten single-crystal target was successfully employed at the positron source of the KEKB injector linac in September 2006. The previously-used conventional tungsten plate was replaced by the tungsten crystal target without any significant modification of the positron source. The tungsten crystal target increased the positron intensity by ～25% compared to that for the conventional tungsten plate. On the contrary, although the positron intensity increased, the heat load on the crystal target decreased by ～20%. We believe that these results could stimulate new interest in the development and application to high-intensity positron sources in the next-generation of high-energy electron/positron linear accelerators.
