  The main `bottleneck' limiting the beam power in circular machines is caused
by space charge effects that produce beam instabilities. To increase maximally
the beam power of a `proton driver', it is proposed to build a facility
consisting solely of a 2.5-GeV injector linac (PI) and a 20-GeV pulsed
superconducting linac (SCL). Such a facility could be constructed using the
existing KEK accelerator infrastructure. The PI, based on the European
Spallation Source (ESS) linac, would serve both as an injector to the SCL and a
source of proton beams that could be used to copiously produce, e.g., neutrons
and muons. Protons accelerated by the SCL would be transferred through the KEK
Tristan ring in order to create neutrino, kaon and muon beams for fixed-target
experiments. At a later stage, a 70-GeV proton synchrotron could be installed
inside the Tristan ring. The SCL, comprising 1.3-GHz ILC-type rf cavities,
could also accelerate polarized or unpolarized electron beams. After
acceleration, electrons could be used to produce polarized positrons, or may
traverse an XFEL undulator.
