First 13TeV collisions: reporting from the CCC

On Wednesday 20 May at around 10.30 p.m., protons collided in the LHC at the record-breaking energy of 13 TeV for the first time. These test collisions were to set up various systems and, in particular, the collimators. The tests and the technical adjustments will continue in the coming days.


The CCC was abuzz as the LHC experiments saw 13 TeV collisions.

Preparation for the first physics run at 6.5 TeV per beam has continued in the LHC. This included the set-up and verification of the machine protection systems. In addition, precise measurements of the overall focusing properties of the ring – the so-called “optics” – were performed by inducing oscillations of the bunches, and observing the response over many turns with the beam position monitors (BPM).

The transverse beam size in the accelerator changes from the order of a millimetre around most of the circumference down to some tens of microns at the centre of the experiments where the beams collide. Reducing the beam size to the micrometre level while at top energy at the interaction points is called “squeezing”. Quadrupole magnets shape the beam and small imperfections in magnetic field strength can mean that the actual beam sizes don’t exactly match the model. After an in depth analysis of the BPM measurements and after simulating the results with correction models, the operators made small corrections to the magnetic fields. As a result, the beam sizes fit the model to within a few percent. This is remarkable for a 27 km machine!

The preparation for first collisions at beam energies of 6.5 TeV started Wednesday, 20 May in the late evening. Soon after, the first record-breaking collisions were seen in the LHC experiments. On Thursday, 21 May, the operators went on to test the whole machine in collision mode with beams that were "de-squeezed" at the interaction points. During the “de-squeeze”, the beam is made larger at the experiment collision points than those used for standard operation. These large beams are interesting for calibration measurements at the experiments, during which the beams are scanned across each other – the so-called "Van der Meer scans".

The two spots are beam 1 (clockwise) and beam 2 (anti-clockwise) traveling inside the LHC in opposite directions. The images are elaborated from data from the synchrotron light monitors. The beam sizes aren’t exactly the same at the B1 and B2 telescopes as the beam intensity as well as the beam optics setup can differ.

Progress was also made on the beam intensity front. In fact, last week the LHC also broke the intensity record for 2015 by circulating 40 nominal bunches in each of the rings, giving a beam intensity of 4×1012 protons per beam. There were some concerns that the unidentified obstacle in the beam-pipe of a Sector 8-1 dipole could be affected by the higher beam currents. The good news is that this is not the case. No beam losses occurred at the location of the obstacle and, after two hours, the operators dumped the beams in the standard way. Commissioning continues and the LHC is on track for the start of its first high-energy physics run in a couple of weeks.

by Jan Uythoven for the LHC team