LHC Report: The beam is back at the LHC

A series of sector beam tests paved the way for the start-up of the LHC in 2008 and 2009. These tests and the follow-up of the issues that arose were part of the process that led to a smooth start-up with beam.

Given this experience, sector tests were scheduled to take place several weeks before the 2015 start-up. On the weekend of 6-9 March, beam from the SPS was injected into both LHC injection regions, followed by a first pass through the downstream LHC sectors. For the clockwise LHC beam (called “beam 1”) this meant passing through ALICE and into Sector 2-3, while the anticlockwise beam (called “beam 2”) was threaded through LHCb and all the way from Point 8 to Point 6, where it was extracted by the beam dump kickers onto the beam dump block.

The dry runs in the previous weeks were mainly targeted at preparation for the sector tests. The systems tested included: injection, timing, synchronisation and beam instrumentation. The beam interlock system was switched to a special configuration for the sector tests to allow arming and firing of the beam dump kickers, while still monitoring other critical systems. A low intensity beam was used, i.e. single bunches with a “probe” bunch population of 5 x 10⁹ protons - compared with 1.15 x 10¹¹ protons in one nominal bunch.

On Thursday, one day before the sector test, the final global LHC Departmental Safety Officer (DSO) tests took place. This series of tests ensures the proper functioning of all systems related to the protection of personnel. The access system is obviously heavily involved, along with a number of magnet circuits that can only be powered if the machine is in closed mode and ready for beam. Only with all the required signatures in place following the DSO tests could beam be injected into the LHC.

On the afternoon of Friday, 6 March, beam 1 and beam 2 were extracted from the SPS into the transfer lines between the SPS and the LHC and stopped on dumps a few hundred metres before the LHC injection points. When the dump for the injection line of beam 1 was retracted, however, the beam appeared not to have made it into the LHC. It was lost in the injection septum magnets, a series of special magnets that guide the incoming beam into the LHC. The injection of beam 2 was also problematic. The extraction interlock system of the SPS did not allow the extraction of beam 2 without the transfer line dump being in the line. The extraction interlock issue was solved during Friday night and beam 2 was successfully injected into the LHC at 4.50 a.m. onto the injection protection device situated upstream of the LHCb experiment.

In the course of Saturday morning, beam 2 was steered all the way to the LHC beam dump at Point 6. The beam dump kickers were synchronised with the injection kicker and fired at the moment of the beam passage to kick the beam into the beam dump channel and onto the 7-metre-long graphite beam dump block.

After some investigations using beam, it was decided to switch the polarity of the beam 1 injection septa. At 5 p.m. on Saturday, beam 1 was successfully injected and made it all the way to Point 3, where it was stopped as planned on the collimators in that region.

Next, a programme of measurements was carried out until 6 a.m. on Monday. Among these measurements were: injection kicker waveform scans; shots on the dumps in the transfer lines for the LHCb and ALICE experiments – where the secondary particles produced allowed them to perform the alignment of the detectors and to test their trigger systems; aperture measurements; beam dump synchronisation measurements; and beam loss monitor and beam position monitor response checks.

Several non-conformities were discovered and will be resolved in the coming weeks before the start-up of the LHC. The sector tests of 6-9 March 2015 proved to be a very useful exercise on many fronts. LHC start-up with beam at the end of March will certainly be made easier by the progress made last weekend.

by Reyes Alemany, Verena Kain