An electron beam travelling through air will quickly lose electrons as they collide with air molecules and lose energy. To keep the electron beam circulating in the storage ring, the tube they travel in must be kept at a very high vacuum.
It is never possible to create a perfect vacuum, there will always be some residual molecules left in the system. The electrons accidentally hitting the residual gas molecules will lose energy and will be eventually lost in the chamber wall. However, the vacuum at Diamond is good enough to keep the beam circulating for more than 10 hours before it needs to be refilled!
It is worth noting that usually the mass of electrons is thousands of times less that that of the air molecules. However at very high velocities (close to the speed of light) relativistic effects mean that the mass of the electrons can become greater than the mass of the air molecules!