Stepped beam pipes and helical baffles for scattered light absorption in future gravitational wave detectors

The next generation interferometric gravitational wave detectors require arm lengths measured in tens of kilometers, with each cavity storing megawatts of optical power. The beams are contained in ultrahigh vacuum pipes. Scattered interferometer light in the pipes may reenter the cavities and inject extra noise. The pipes are, therefore, provided with optical baffles necessary to eliminate the scattered light. The design of the vacuum pipes and of the optical baffles is tightly intertwined. We present a thorough discussion that opens the door to the design of an optimized stepped-diameter vacuum pipe system using novel helical baffles. Our analysis suggests that a more efficient pipe design (with special reference to scattered light) may use spiral baffles and sectioned stepped tubes.