A better beam quality

Progress has been made on two fronts, providing physics data and preparing for higher intensities.

Over the Whitsun weekend of May 22 to 24, 5 fills for physics provided almost 30 hours of stable colliding beams, all with bunch intensities around 2x1010 protons and at a β* of 2m. The first three of these fills were with 6 bunches per beam, giving 3 pairs of collisions in all experiments. For the other two fills, the number of bunches per beam was increased to 13, giving 8 pairs of colliding bunches, and for the first time luminosities were pushed above 1029 cm-2s-1, 2 orders of magnitude higher than first collisions in March.

In between and after these physics fills, nominal bunches of 1011 protons were successfully ramped and brought into collision in ATLAS and CMS for the first time (not in stable beam conditions and without squeeze). Event rates seen by the experiments were in the expected range for these conditions. In the middle of this work, a short fill with beams of 7 nominal bunches was made at injection energy.
A number of technical problems then slowed down the beam commissioning programme, before a major power cut late on the evening of Friday May 28 halted the whole accelerator complex. LHC was still on the road to recovery going into a scheduled three-day technical stop on Monday May 31.

The last part of the week was mainly dedicated to accumulating more experience with collisions with 13 bunches per beam with bunch intensities of 2x1010 and at a β* of 2m and to progress with the commissioning of the transverse feedback which is vital to maintaining a good beam quality when increasing the bunch intensity or the number of bunches.

What is a β*?

β* describes the beam size at the interaction point. More mathematically, it is a measure of the distance from the interaction point at which the beam is twice the size of that at the interaction point. The lower the beta, the smaller the beam at the interaction point, therefore the better for the physics. Before beam squeezing, beta is typically 11 m at ATLAS and CMS. Today, we’re running with a beta of 2 m, and the ultimate goal is to reduce it to 0.55 m.

by CERN Bulletin