The Recycler Electron Cooler

The Recycler Electron cooler was the first (and so far, the only) cooler working at a relativistic energy ({\gamma} = 9.5). It was successfully developed in 1995-2004 and was in operation at Fermilab in 2005-2011, providing cooling of antiprotons in the Recycler ring. This paper describes the cooler, difficulties in achieving the required electron beam parameters and the ways to overcome them, cooling measurements, and details of operation.