LHC Report: Now it’s full speed ahead (still with probe beam)
Since the last report, the commissioning with beam was delayed after a short to ground appeared in the cold mass of the main dipole chain in sector 3-4. After a remarkable team effort coordinated by the Machine Protection group, a procedure to burn away the small piece of metallic debris that was causing the earth fault was conceived, prototyped, tested and deployed. The intervention was successfully completed on the afternoon of 31 March and the first beams circulated in the LHC on Sunday, 5 April. Just a few days later, at just past midnight on Friday, 10 April, beam was ramped up to 6.5 TeV.
"LHC page 1" shows the status of the LHC last night. The black line shows the beam energy increasing to 6.5 TeV.
The intervention successfully conducted in sector 3-4 opened the way for the completion of quench training in the sector and the final qualification of the circuit. This marked the end of a long and arduous powering test campaign that has fully qualified all circuits for 6.5 TeV operation.
There then followed a series of machine protection tests designed to test the critical beam interlock system (BIS) and its links to other key protection related systems such as the beam dump and injection.
Beam was finally taken down the two SPS-LHC transfer lines on Sunday morning. With the injection regions having already been in action during the sector test, progress was rapid. One at a time, both beams were steered sector by sector around the ring. Progress around the ring was staged by stopping the beam on collimators, with the beam trajectory being corrected at each step. When the final collimators were retracted, the beam immediately performed many turns. During this initial phase of beam commissioning, the LHC is working with a “probe beam” – a single low-intensity bunch (~5 x 109 protons).
The Radio Frequency (RF) team working from point 4 were able to make the necessary adjustments and the beams were soon “captured”, meaning that the single bunch was injected cleanly into the chosen RF bucket and then remained nicely bunched. This allowed preliminary commissioning of the key beam instrumentation, which soon gave measurements of the closed orbit, tune and chromaticity. After correction of these parameters, the beams circulated happily with good lifetimes. At first glance, the 450 GeV machine is similar to that of Run 1. More detailed measurements will be performed in the coming days.
For the moment, all beam-based work has been done at injection energy. The main thrust has been the beam-based system commissioning of: RF; beam instrumentation; feedbacks; collimation; machine protection; and the beam-dump system.
On Tuesday evening, “splashes” were delivered to ATLAS and CMS. This is where the incoming bunch is intercepted by a tertiary collimator just upstream of the experiment, resulting in a spectacular shower of particles in the detector. The splashes are used by the experiments to time in their sub-detectors and to produce some very nice images.
A first ramp attempt was made at around 1.00 a.m. on Friday, 10 April, again with a single probe bunch in each beam. Beam 1 was lost shortly after the start of the ramp due to large tune excursion. Beam 2, however, went happily to 6.5 TeV, where a few preliminary measurements were performed.
Over the coming days, system commissioning continues combined with ramp commissioning. A first look at the beam squeezing is scheduled for some time next week.
| The LHC Restart in the news  Top: Number of online press clippings during recent months. Bottom: Number of television retransmissions via Eurovision of the images broadcast by CERN. Not only was the number of press clippings significantly higher than average, but the information also reached more major print media. The television stations also covered the event on their news channels and numerous national channels. We were able to identify no fewer than 555 retransmissions via Eurovision of the images broadcast by CERN. The images were also broadcast by other large press agencies. |
by Mike Lamont
