  The performance of a particle accelerator can be limited by the build-up of
an electron cloud (EC) in the vacuum chamber. Secondary electron emission from
the chamber walls can contribute to EC growth. An apparatus for in-situ
measurements of the secondary electron yield (SEY) in the Cornell Electron
Storage Ring (CESR) was developed in connection with EC studies for the CESR
Test Accelerator program. The CESR in-situ system, in operation since 2010,
allows for SEY measurements as a function of incident electron energy and angle
on samples that are exposed to the accelerator environment, typically 5.3 GeV
counter-rotating beams of electrons and positrons. The system was designed for
periodic measurements to observe beam conditioning of the SEY with
discrimination between exposure to direct photons from synchrotron radiation
versus scattered photons and cloud electrons. The samples can be exchanged
without venting the CESR vacuum chamber. Measurements have been done on metal
surfaces and EC-mitigation coatings. The in-situ SEY apparatus and improvements
to the measurement tools and techniques are described.
