Build it on the moon.
No high vacuum pumps and pipes required, just set up your magnets on the regolith and send the protons on their way parallel to the ground. There's plenty of solar power available, and you can build it from locally mined materials. You can start off with a modest linear accelerator and gradually expand it until it girdles the moon.
Finally, a really good reason to go back. An experiment that can really be done nowhere else.