LS1 Report: Thank you magnetic horn!

Experiments at the Antimatter Decelerator (AD) have been receiving beams since the beginning of this week. There is a crucial element at the heart of the chain that prepares the antiproton beam: the so-called magnetic horn, a delicate piece of equipment that had to be refurbished during LS1 and that is now showing just how well it can perform.


View from the top of the target and horn trolley, along the direction of the beam.

Antiprotons for the AD are produced by smashing a beam of protons from the PS onto an iridium target. However, the particles produced by the nuclear interactions are emitted at very wide angles; without a focussing element, all these precious particles would be lost. “A magnetic horn is placed at the exit of the target to focus back a large fraction of the negative particles, including antiprotons, parallel to the beam line and with the right momentum,” explains Marco Calviani, physicist in the EN Department and the expert in charge of the AD target area. “Its performance is of vital importance for the AD physics programme because experiments need a good antiproton yield in order to carry out their research programmes on antimatter. Without the horn, the number of antiprotons for the experiments would be reduced by a factor of 50.”

At the end of 2013, the magnetic horn that had been in operation for 20 years was inspected and found to be severely damaged by electric arcs. Luckily, it had not yet stopped working and no further damage was done to the surrounding structures. However, given the state of the electrical contacts, the horn current transmission system had to be replaced in order to ensure reliable AD operation after LS1. “The horn assembly is a magnetic system composed of three main parts,” describes Calviani. "The first is the horn itself, constituted of two concentric aluminium conductors, in which the internal one has a double parabolic shape. When a 400 kA current pulse flows through the conductors, a 13 Tesla magnetic field is generated in between the conductors. This very high field allows the particles entering the horn inner volume to be focussed. The second part is a six-metre-long aluminium strip line that carries the current from the generators to the horn. The third part is a movable clamping system between the first two, which ensures the electrical continuity of the device."

Given the critical situation, the experts decided to replace all three components. They had only six months to re-assemble and test the over 20-year-old spares. “We found the old spare of the strip line, but it needed a lot of work in order to be made usable,” explains Calviani. “At the same time we launched the construction of additional pieces that were created in record time thanks to very good collaboration between the EN, TE and BE Departments and the HSE unit.”

The consolidated system was first assembled on the surface, tested at full current in a dedicated test bench far enough from the radioactive environment of the target area and then installed underground in the target area. Now that the beam is back after LS1, the experts can measure the system's performance. The verdict is: no unconformities.

Did you know?

The magnetic horn was invented at CERN in the 60s by Simon Van der Meer. The original application of the magnetic horn was for neutrino physics. Since its invention, the magnetic horn has found many applications all around the world, in both neutrino physics and the production of antiprotons.

Meanwhile, elsewhere...

On Friday 12 September, the SPS accelerated its first proton beam of the LHC’s second run. Prior to startup, a leak was found in one of the accelerator's main water-cooled cables, which date back to the construction of the machine. Its antiquated design allows water to run through the cable itself, rather than surrounding it. As no backups of such cables were kept, the cable was "cannibalised" and only the leaky section of the cable was removed. A simple non-water-cooled bus bar was put in place in this short section, providing a quick but effective solution to the issue.

Over at the LHC, the first powering tests started in Sector 6-7 on Monday 15 September. Meanwhile, the cool down of the machine continues in five sectors, with Sector 6-7 at the nominal temperature.

Next week, LS1 coordination leaders will be heading to the Chamonix workshop to discuss their final plans before the machine restarts in week 11 of 2015. Plans for LS2 will also be on the table, a full four years in advance!


by Antonella Del Rosso & Katarina Anthony