  A superconducting (SC) linac is expected to lead to outstanding discoveries
in various scientific fields because its beam current is a few orders of
magnitude larger than in a normal-conducting linac. However, the widespread use
of SC linac is limited by the high construction and operation costs. To resolve
this problem, we propose a continuous wave (CW) operation of a SC linac shared
by electron/positron beams for effective multi-purposes utilization. A high
current positron/electron beam is required for high-energy physics projects
such as linear collider and muon collider while high-current and high-quality
electron beams is expected to realize the next generation X-ray light sources.
As an example, we discuss the injector of the International Linear Collider, an
X-ray free-electron laser and an energy-recovery linac light source. We found a
feasible solution for the proposed multibeam operation despite the high-quality
beam requirements and complicated opertion: control of mixed beams without
pulsed magnets, lower beam loss and heat load in the cavity, high stability of
beam energy, and operation at high average current.
