I was having a serious case of Deja vu of this plot point from a book I was sure I had read in the past. It wasn’t Forever Peace by Haldeman, was it? Great book, and enjoying the discussion here related to space-based particle accelerators. Let’s hope our attempts at an orbital accelerator doesn’t have the same potential side effect as in that story! 
So you’re going to seal the tunnel against dust? Then you’ll need vacuum pumps to keep the tunnel/tube at better vacuum & dust-free. There goes one of the claimed advantages of building on the moon.
You’ll also have major difficulty getting rid of waste heat.
Well, aside from the fact that it is being done on the Moon instead of Earth, which means transporting, assembling, operating, and maintaining heavy equipment which can operate in a vacuum environment and tolerate the abrasive feldspar produced by boring through the lunar substrate, notwithstanding all of the difficulties in operating on the lunar surface.
Really, if you can transport this much material into Earth orbit, there is no reason to land it on the lunar surface and bore tunnels; it would be much easier to simply construct an accelerator in orbit of any arbitrary size. Doing that would still be vastly beyond the state of the art but at least it does not introduce an additional and persistent contaminant and hazard to cope with in addition to boring hundreds of kilometers of tunnels through abrasive substrate and dealing with the lunar regolith.
Stranger
SLAC pretty much is already built directly on the San Andreas fault.
After a major earthquake it was renamed the SPLAC:
[spoiler]Stanford Piecewise-Linear Accelerator Center.
:D[/spoiler]
You could put the accelerator components in the same sort of wagon-wheel orbit that was intended for the LISA gravitational wave detector and its kin. LISA itself would only have been a “ring” of three vertices, but it works the same way no matter how many vertices you have.
It’s a tunnel, remember? Any dust from within the tunnel is going to be different from the dust outside. We know how to deal with dust from terrestrial mining.
Even in the Sea of Thirst?
We don’t know how to dig a tunnel and turn it into a 10^-10 torr vacuum chamber. Not even remotely close to it.
Sure we do: we do it already with the LHC.
I meant we can’t turn the tunnel itself into a vacuum chamber, which you seemed to be suggesting. The LHC vacuum system is a polished stainless-steel tube inside the tunnel, with massive cryogenic vacuum pumps attached. Sure you can do that in a tunnel on the Moon, but then you lose one of the advantages of building on the Moon that you suggested.
It wasn’t I who suggested the Moon, it was ftg, and I don’t see how you lose the vacuum as long as you keep the tunnel well-vented to allow any stray gasses from leaking spacesuits etc to escape.
Nope, toward the very end of “Death’s End” by Cixin Liu.
I think I didn’t explain what the book was saying very well- it was an orbital accelerator- the circular path was basically solar orbit (I don’t remember what the radius was off the top of my head). So all the magnets, etc… had the same orbital radius.
LISA isn’t a model for a particle beam. The LISA satellites have large velocities relative to one another, which needn’t be a problem for the interferometric measurement but is a showstopper for a circulating particle beam.
The science in Cixin Liu’s writing is really, really, really, really, really, really bad. (Judging from reading 3BP and a summary of The Wandering Earth.)