  KAGRA is a cryogenic interferometric gravitational wave detector being
constructed at the underground site of Kamioka mine in Gifu prefecture, Japan.
We performed an optimization of the interferomter design, to achieve the best
sensitivity and a stable operation, with boundary conditions of classical
noises and under various practical constraints, such as the size of the tunnel
or the mirror cooling capacity. Length and alignment sensing schemes for the
robust control of the interferometer are developed. In this paper, we describe
the detailed design of the KAGRA interferometer as well as the reasoning behind
design choices.
