After declaring stable beams, now it’s the period of scrubbing and intensity ramp-up, until 2556 bunches per beam will circulate in the machine

According to the official LHC schedule first stable beams were scheduled for 12 June, following a 5-week beam recommissioning period and a 7-day scrubbing run.

The excellent availability of the LHC and its injector chain, together with the dedication of the many specialists made it possible to be much ahead of this schedule, as first stable beams could already be declared on Tuesday, 23 May, only 25 days after the first beam was injected. From that moment onwards the remaining commissioning activities were interleaved with longer stable beam periods for physics.

This also triggered the so-called intensity ramp-up period during which a well-defined scheme of increases in the number of bunches and thus beam intensity is rolled out and formal checks for each intensity step are made until 2556 bunches per beam are reached, which is the objective for 2017. This scheme starts with 3 bunches per beam and then goes up to 12, 72, 300, 600, 900, 1200, 1800, 2400 and ends with 2556 bunches per beam. For each step the requirement is a minimum of 20 hours of stable beam in total, but also that the machine is filled three times. The goal of this controlled and step-wise intensity increase is to ensure that all systems work well with many bunches and a high total beam intensity.

Like every year, but even more importantly this year because of the magnet exchange in sector 1-2, there is also a scrubbing run, which aims at conditioning the vacuum chamber in order to reduce the so-called Secondary Electron Yield (SEY) emission, that is the number of secondary electrons produced on average per incident electron on the inner walls of the vacuum chamber. Reducing the SEY lessens or avoids the build-up of electron clouds in the vacuum chamber that can lead to beam instabilities and an increase in the demand for cryogenic cooling power. In an unconditioned machine the electron cloud build-up becomes more important when the bunch trains get longer and reduces when the bunch trains are further apart along the circumference of the machine. Therefore, the scrubbing run initially starts with bunch trains of 72 bunches well-spaced. Once the scrubbing shows its effect (reduced SEY measured by the reduction in heat load on the cryogenics system) the spacing between the bunch trains is reduced.

Since the intensity ramp up was well-advanced and longer bunch trains were required, 24 hours of the planned 7-days scrubbing run were advanced by one week to Monday, 29 May with the aim to perform an initial conditioning and allow the injection of longer bunch trains in the process of the intensity ramp-up.

At present, the remaining 6-day long scrubbing run is in full swing and the LHC is being prepared to receive the full number of 2556 bunches per ring with 144 bunches per injection from the injector chain.