LHC Report: playing with angles

Ready (after a machine development period), steady (running), go (for a special run)!


The crossing angles are an essential feature of the machine set-up. They have to be big enough to reduce the long-range beam-beam effect.

The LHC has recently enjoyed a period of steady running and managed to set a new record for “Maximum Stable Luminosity Delivered in 7 days” of 3.29 fb-1 between 29 August and 4 September. The number of bunches per beam remains pegged at 2220 because of the limitations imposed by the SPS beam dump. The bunch population is also somewhat reduced due to outgassing near one of the injection kickers at point 8. Both limitations will be addressed during the year-end technical stop, opening the way for increased performance in 2017.

On 10 and 11 September, a two day machine development (MD) period took place. The MD programme included a look at the possibility of reducing the crossing angle at the high-luminosity interaction points. The crossing angles are an essential feature of the machine set-up, intended to avoid undesired collisions of bunches either side of the interaction point when the beams are travelling in a single beam pipe. Despite crossing at an angle, the bunches still interact with each other via long-range electromagnetic interactions. The crossing angles therefore have to be big enough to ensure sufficient separation, reducing this long-range beam-beam effect to an acceptable level.

However, a distinct drawback of the increase in the crossing angle is a decrease in luminosity, with the reduction factor depending on the crossing angle, bunch length and beam size. Under the present LHC parameters, the high luminosity experiments see around 60% of what they would if we could operate without a crossing angle.

Two measures are being explored to claw back some of this reduction. One is a reduction of the crossing angle at the start of a fill. We can consider this because of the smaller than nominal beams being delivered by the injectors (smaller beams means better separation at the long range encounters). The aim is to restart physics in the last few months of the year with the crossing angle reduced from 370 to 280 microradians. This should increase the peak luminosity by around 15%. The second approach, targeted by the MD, is a stepwise reduction of the crossing angle during a fill as the bunch population decreases. This will not be implemented this year but could be used after appropriate tests in 2017.

A five-day technical stop followed the MD period. One of the major interventions performed during the stop was the replacement of the bushings on the transformer damaged by the “weasel” earlier this year.

Finally, this week a four-day special physics run for the forward experiments will start. It will be aimed at measuring small angle elastic proton–proton scattering. You can read more about this special physics run here.

by Mike Lamont for the LHC team