Vacuum systems for the ILC helical undulator

The International Linear Collider (ILC) positron source uses a helical undulator to generate polarized photons of $∼10MeV$ at the first harmonic. Unlike many undulators used in synchrotron radiation sources, the ILC helical undulator vacuum chamber will be bombarded by photons, generated by the undulator, with energies mostly below that of the first harmonic. Achieving the vacuum specification of $∼100nTorr$ in a narrow chamber of $4–6mm$ inner diameter, with a long length of $100–200m$⁠, makes the design of the vacuum system challenging. This article describes the vacuum specifications and calculations of the flux and energy of photons irradiating the undulator vacuum chamber and considers possible vacuum system design solutions for two cases: cryogenic and room temperature.