LHC pushes limits of performance

The LHC tunnel. The machine is surpassing all performance expectations (Image: Jacques Fichet/ CERN)

The Large Hadron Collider’s (LHC) performance continued to surpass expectations, when this week it achieved 2220 proton bunches in each of its counter-rotating beams – the most it will achieve this year.

This is not the maximum the machine is capable of holding (at full intensity the beam will have nearly 2800 bunches) but it is currently limited by a technical issue in the Super Proton Synchrotron (SPS).

“Performance is excellent, given this limitation,” says Mike Lamont, head of the Operations team. “We’re 10% above design luminosity (which we surpassed in June), we have these really long fills (where the beam is circulating for up to 20 hours or so) and very good collision rates. 2220 bunches is just us squeezing as much in as we can, given the restrictions, to maximize delivery to the experiments.”

As an example of the machine’s brilliant performance, with almost two months left in this year’s run it has already reached an integrated luminosity of 22fb-1 – very close to the goal for 2016 of 25fb-1 (up from 4fb-1 last year.)

Luminosity is an essential indicator of the performance of an accelerator, measuring the potential number of collisions that can occur in a given amount of time, and integrated luminosity (measured in inverse femtobarns, fb-1) is the accumulated number of potential collisions. At its peak, the LHC’s proton-proton collision rate reaches about 1 billion collisions per second giving a chance that even the rarest processes at the highest energy could occur.

The SPS is currently experiencing a small fault that could be exacerbated by high beam intensity – hence the number of proton bunches sent to the LHC per injection is limited to 96, compared to the normal 288.

“Once this issue is fixed in the coming year-end technical stop, we’ll be able to push up the number of bunches even further. Next year we should be able to go to new record levels,” says Lamont with a wry grin.