LHC Report: machine commissioning - drawing to a close

The recommissioning of the LHC is going well: the machine has delivered first pilot Stable Beams collisions.
 

Some of the first collisions recorded by the experiments during the LHC 2016 commissioning with low-intensity stable beams. (Image: CERN)

TOTEM bump
The main goal of the past couple of weeks was to advance with the preparation of collimators settings and protection devices. Over the weekend of 16-17 April, collisions were re-established after setting up a new orbit bump around the Roman Pot locations in IP5 (TOTEM), in order to improve their acceptance. The bump was smoothly incorporated into the machine settings that lead into Stable Beams. The LHC orbit was corrected towards the reference leaving the machine ready for the next steps: aperture measurements and final collimator alignment.

Alignment and aperture at 40 cm
The aperture is the available space in the transverse plane of the machine. Detailed simulations are used to predict the minimum machine aperture. At 40 cm β*, the bottlenecks (the locations in the machine with the smallest aperture) are the triplet magnets either side of the experiments. In order to protect these magnets, the tertiary collimators have to be set up to shadow the triplets and catch and absorb beam losses that would otherwise end up in the magnets. A very precise measurement of the aperture was performed at the end of squeeze and in collisions. About 10 σ is the smallest measured aperture in the machine (here σ is the transverse beam size). This allows for the predicted setup of the tertiary collimators at 9 σ.

Roman Pot alignment and TCT/TCL alignment
The final steps to prepare the machine for collisions are the alignment of the tertiary collimators (TCT) and physics debris collimators (TCL) located near the collision points. For the TCL the alignment is done by moving the collimator in steps of 5 μm towards the beam until both collimator jaws touch the beam halo. This gives a very precise measurement of the beam centre but is a lengthy process. In order to speed the collimator alignment, each tertiary collimator was equipped during LS1 with 4 beam position monitors embedded in the collimator jaws. The alignment for these collimators (16 collimators in total) now takes less than one minute with the additional advantage that the collimator is aligned without touching the beam halo. The alignment of the collimator system was successfully completed and was followed by the alignment of the Roman Pot detectors: TOTEM in IR5 and AFP in IR1.

Impedance and e-cloud
In order to prepare the machine for high beam intensity, the machine impedance needs also to be evaluated, in particular, the contribution from the collimators. This is measured by observing the tune shift while changing the collimator gaps. This was done this week for the main injection protection collimator, which is known as the TDI - a 4 m long graphite collimator that protects the machine in case of an injection failure. The contribution from the ring collimators will be measured during the coming days. Other equipment also needs to be prepared for high intensities; here the transverse damper system is a key player.

A tune scan at injection with 3 nominal bunches circulating in the machine was also performed in order to find the optimal working point for the tunes compatible with machine conditions during the upcoming scrubbing run. This run is aimed at reduction of the electron cloud. The transfer lines between SPS and LHC were successfully set up with nominal bunches. This exercise includes trajectory and collimators, and is now followed by setup and checks with bunch trains of 12 and 72 bunches.

Machine protection
A full campaign of machine protection validation is currently on-going in order to establish the first Stable Beam collisions and allow for higher intensities. Before the declaration of Stable Beams and the ramp-up of beam intensity, the full LHC cycle must be qualified. This is done by analyzing the expected beam loss distributions in the LHC ring in case of a failure scenario at every cycle step: injection, ramp, flat top, squeezed beams and collisions. Reference loss maps are established and thoroughly analyzed. This is done by inducing in an extremely controlled way beam losses with very little intensity in the machine (less than 3 x 10¹¹ protons per beam). The main beam loss locations are evaluated and maximum loss scaled to the final intensities to verify that it is well below the magnets' quench limits.

Exciting moments in the next days, the machine is almost ready to start 2016's physics programme.

by Belen Salvachua Ferrando for the LHC team