  We present an estimation of the noise induced by scattered light inside the
main arms of the Einstein Telescope (ET) gravitational wave detector. Both ET
configurations for high- and low-frequency interferometers are considered, for
which we propose baffle layouts. The level of scattered light and the ET laser
beam clipping losses are intimately related to the baffle inner aperture. We
discuss how this translates into minimum requirements on the vacuum pipe
radius, a critical parameter in the ET design. The noise estimations are
computed using analytical calculations complemented with numerical tools, and
depend on a number of baseline parameters we use as input in the calculations.
We conclude that the scattered light noise can be maintained at acceptable
levels such that does not compromise the ET performance, provided some
requirements are met.

General Relativity and Quantum Cosmology;Instrumentation and Methods for Astrophysics