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-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 the 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 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.