Why is the storage ring of a synchrotron a vacuum? I know that when the accelerated particles get to the storage ring they lose energy, releasing synchrotron radiation, but how does the vacuum make them lose energy? Does it slow it down, increase, change to wave or what?
Check synchrotron radiation in the wiki article.
When high-energy particles are in rapid motion, including electrons forced to travel in a curved path by a magnetic field, synchrotron radiation is produced.
Charges when accelerated radiate electromagnetic radiation, and the curved paths in the synchrotron give a continuous angular acceleration to the electron.
The vacuum is absolutely necessary for accelerators , in air the beam would hit the gas molecules and dissipate immediately. The vacuum has to be very good in accelerators in order not to lose the beam and not to generate unnecessary radiation by beam gas interactions.