The Proton Synchrotron (PS) now boasts two new internal beam dumps. Installed inside the accelerator in June, they are the result of five years of development work in the framework of the LHC Injectors Upgrade (LIU) project. The size of a shoebox, the core of the dump comprises two absorbing elements – isostatic graphite and a copper, chrome and zirconium compound – through which the beam will pass in turn. Each dump is sheathed in layers of steel and concrete shielding, which will help to absorb the beam. “The dump itself absorbs only 8% of the energy of the beam”, explains François-Xavier Nuiry, the project leader from the EN-STI group. “This represents a power of up to 2.2 kW, which will need to be evacuated. We chose the hot isostatic pressing technique for the core of the dump as it allows optimum heat removal.”
The new beam dumps operate differently to all the other beam dumps at CERN: in order to stop the beam, they place themselves across its path in a rapid (300 ms) oscillation movement. This technique, which was first developed back in 1973 and was then redesigned and modernised for the LIU project, provides a major advantage in terms of reliability. Two ball bearings, springs and an electromagnet are all it takes to trigger the oscillation. “Thanks to their specific design and the careful selection of each of the components that make up the oscillation mechanism, these dumps require very little maintenance and have a high resistance to radiation,” adds François-Xavier Nuiry. Each dump will have to perform 200,000 oscillations a year for 15 years.
The two new dumps are already operational and will undergo oscillation tests in situ until the PS is closed in October.
A new injection line has also been installed. The Booster, which has undergone a complete transformation, will henceforth supply the PS with particles accelerated to up to 2 GeV, compared with 1.4 GeV previously. The old magnets of the injection line, in particular the septum magnet, have thus had to be replaced. “In the 'septum magnet' section of the Accelerator Beam Transfer (ABT) group, we have developed a new magnet that’s based on the principle of eddy currents”, explains Michael Hourican, work package leader. “This is the first time that this type of septum magnet has been used at CERN.”
The injection septum magnets will deflect the beams of particles coming from one accelerator and inject them into the next without perturbing the beams that are already circulating. The magnetic field inside the magnet has a tendency to “leak”, which can interfere with the circulating beams, hence the use of eddy currents. “The eddy currents induced in the magnet create a secondary magnetic field that opposes the leaking magnetic field and helps to cancel it out, thus reducing the impact on the circulating beams,” explains Michael Hourican.
In the PS, the septum magnet is accompanied by five bumper magnets and an injection kicker magnet, which together make up the injection system. The septum magnet deflects the trajectory of the beam coming from the PSB and directs it to the PS ring. The bumper magnets modify the orbit inside the PS so that its position and angle correspond to those of the beam coming out of the septum. Finally, the injection kicker magnet, located downstream, places the injected beam onto the normal orbit.
It’s a bit like a car (the beam) taking a slip road (the injection line) to enter a motorway (the PS): on the slipway, the car takes a bend (the septum magnet), then enters a lane of the motorway. The only difference here is that the motorway lane itself physically moves (under the influence of the bumper magnets) in order to “receive” the car.
For reasons of space and geometry, one of the five bumper magnets has been attached to the new septum magnet (see photo). The two magnets are located inside a vacuum tank, which can be positioned remotely from the CERN Control Centre (CCC) in order to optimise the injection. The assembly was installed inside the PS at the end of June and is currently undergoing tests.