ELENA: from the first ideas to the project

Successful commissioning of the CERN Antiproton Decelerator (AD) in 2000 was followed by significant progress in the creation of anti-hydrogen atoms. The extraction energy of the decelerated antiprotons is nevertheless very high compared to that required by the experiments and results in a trapping efficiency of only 0.1% to 3%. To improve this value by an order of magnitude the study of an Extra Low ENergy Antiproton ring (ELENA) started in 2003 and was approved as a CERN construction project in 2011. During these years the choice of the main machine parameters such as the beam extraction energy, emittance and bunch length were defined, taking into account requests from the physics community. The main challenges were also identified, such as dealing with the large space charge tune, the ultra- high vacuum required and the tight requirements for the electron cooler. Housing the ELENA ring within the AD hall significantly reduced the project cost as well as simplifying the beam transfer from AD to ELENA and from ELENA to the existing experimental areas. This contribution will follow ELENA from its beginnings to the final, approved project proposal.