LHC Report: 1,033 bunches per beam and counting

Following the second technical stop, the first beams were injected into the LHC in the early evening of Saturday, 5 September. About ten days later, the machine was operated with around 1,000 bunches per beam. 

Evolution of the stored energy per LHC beam, over time.
 

The first step after a technical stop consists of running through a full LHC cycle, from injection to collisions and beam dump, with a low-intensity bunch (“probe”) to check all machine settings and equipment. This is followed by a series of collimation and absorber validation tests at different points in the LHC cycle. Low-intensity beams – typically the equivalent of three nominal bunches (3 x 10¹¹ protons) – are expanded transversely or longitudinally, or de-bunched to verify that the collimators and absorbers are correctly intercepting lost particles. The techniques for those validations have been progressively improved, and they can now be performed within 24 hours in a few machine cycles.

As soon as the protection systems were validated, the intensity of the beam was ramped up in three steps to 459 bunches per beam – the level that had been reached before the stop. Since mid-August, the LHC bunches used for physics operation have been spaced by 25 ns, which is the LHC design bunch spacing. The goal of the current operation period up to the end of October is to step up the number of bunches as much as possible.

The intensity of the LHC beams is ramped up stepwise: at each step the LHC must be operated for at least three periods of collisions (“stable beams”) integrating at least 20 hours of operation before the next intensity step is authorised. The data collected across many systems, in particular those related to machine protection, are analysed carefully. The green light for an intensity step is only given when all system experts are satisfied with their systems’ performance.

Following this procedure, on Wednesday, 9 September, after two long fills with 459 bunches, beam operation received the green light to ramp up to 745 bunches per beam. The step was achieved the same day and the remainder of the week was spent operating at that level.

The electron cloud activity is still significant, depositing considerable power onto the vacuum chamber beam screen. The beam screen temperature should remain below 30 K; this is achieved by managing the heat load transients. This operation is particularly delicate for the cryogenic system operation team in the CCC when the beams are injected and ramped.

On Monday, sufficient beam time was accumulated to step up to 899 bunches with a push to above 1,000 bunches per beam later in the week. At this level, the stored energy in each beam will have exceeded 100 megajoules for the first time in 2015.

by Jorg Wenninger for the LHC team