LHC report: Ions cross protons

The first stable beams were achieved on 20 January with 13 individual bunches per beam. In the next fill, the first bunch-trains were injected and stable beams were achieved with 96 proton on 120 ion bunches.  This fill was very important because we were able to study the so-called moving long-range beam-beam encounters. Long-range encounters, which are also seen in proton-proton runs, occur when the bunches in the two beams “see” each other as they travel in the same vacuum chamber at either side of the experiments.  The situation becomes more complicated with proton-lead ions because the two species have different revolution times (until the frequencies are locked at top energy- see “Cogging exercises”) and thus these encounters move. We found that this effect does not cause significant beam losses or emittance blow-up. This clarification was long-awaited.

The full filling scheme with 338 on 338 bunches was injected and successfully ramped on 21 January. In addition, a record lead bunch intensity in the LHC was achieved. The performance of the injectors that provide the bunches of both particle species has been a great success and has been key to the excellent performance of the proton-ion run.

Since 24 January, we have been routinely achieving stable beams with the full filling scheme. Currently, protons are injected in ring 1 (clockwise) and lead ions in ring 2. But in a few days the beams will be swapped so that ALICE, inherently an asymmetric detector, can take data in both directions.

On a good day, there are two physics fills with a peak luminosity at the beginning of the collisions of around 1029 s-1cm-2 in ALICE, ATLAS and CMS. The LHC is producing an integrated luminosity well above expectations: around 2 inverse nanobarns per experiment per day. The LHC efficiency over the past week has also been remarkably good: 45% of the time the machine has been in stable beams.

Did you know?
Currently, the proton and ion bunches have an average intensity of 1.5x1010 charges at the start of a fill. However the ion bunch current (expressed in proton charges) decreases faster than it does for protons: each time an inelastic collision takes place, one proton is lost in the proton beam, but one ion representing 82 charges (protons) is lost from the lead ion beam (the lead ion is fully stripped of electrons and it has a nucleus with 82 protons). This is the expected luminosity burn-off. The lead bunches also suffer from increased transverse blow-up due to intra-beam scattering.
Cogging excercises
When two different types of particles are circulating in the LHC, the locking of the two radio frequency (RF) systems (known as “cogging”) can be a challenge. Along with other activities, cogging was one of the many delicate tasks addressed during the proton-lead beam-commissioning phase.
During the first cogging exercises the beam dump was triggered due to high beam-losses. This was later found to be caused by an improper synchronization of the two RF frequencies. The de-synchronization provoked an overshoot of frequency during adjustment and thus an overshoot of the position of beam 2 in the horizontal plane. As a consequence, the beam 2 was scraped and some ions lost at the collimators at point 7. Careful fine-tuning was needed for the cogging process to overcome the problem.