Particle kickers
These devices are designed to provide a current pulse of 5000 Amps which will in turn generate a fast magnetic pulse that steers the incoming beam into the LHC. Today, the comprehensive upgrade of the LHC injection kicker system is entering its final stages. The upgraded system will ensure the LHC can be refilled without needing to wait for the kicker magnets to cool, thus enhancing the performance of the whole accelerator.
The LHC is equipped with two kicker systems installed at the injection points (near points 2 and 8, see schematic diagram) where the particle beams coming from the SPS are injected into the accelerator’s orbit. Each system comprises four magnets and four pulse generators in which the field rises to 0.12 Tesla in less than 900 nanoseconds and for a duration of approximately 8 microseconds. Although the injection kickers only pulse 12 times to fill the LHC up with beam, the LHC beam circulates through them constantly.
Kickers are hugely complex magnets that need to be protected from the electromagnetic effects induced by the beams. “Each magnet is equipped with a beam screen placed in the aperture,” explains Mike Barnes, member of the Accelerator Beam Transfer (ABT) group in the Technology department and leader of the upgrade programme of the LHC injection kicker magnets. “The initially implemented beam screen worked well at low-intensity beams. However, as the operation of the LHC moved towards increasingly higher intensity beams, stable for many hours at a time, some of the kickers started to over-heat.”
Indeed, when the temperature of the magnet yoke exceeds 120˚C, the strength of the kick diminishes and the mis-kicked injected beam can cause quenches of several superconducting magnets. Hence, the use of an interlock which inhibits injection if the measured yoke temperature is above specified thresholds. On about ten occasions over the course of 2012, after a series of long fills, it was necessary to wait longer than an hour before the LHC could be refilled to allow the cool-down of a kicker magnet. Therefore, in order to mitigate this effect, the experts of the ABT group upgraded the kicker beam screen by increasing the number of screen conductors. This resulted in reduced beam-induced heating of the magnet yoke while still preserving the fast performance of the whole system. “The studies for the upgrades, implementation of the results and the re-installation of the magnets have required good collaboration of many CERN teams, including BE-ABP, BE-BI, BE-RF, EN-HE, EN-MEF, EN-MME, TE-VSC, TE-ABT-EC and TE-ABT-FPS,” says Barnes. “During LS1, the eight kicker magnets have been removed from the tunnel and upgraded. Following the upgrade work, each magnet is carefully aligned, in a clean room, baked out to 300˚C, and high voltage conditioned.”
The first six kicker magnets have now been reinstalled in the tunnel, four at point 2. The work there will now move into a new phase, requiring bake-out of the interconnects, commissioning of the kicker magnet controls, high-voltage testing of the kicker systems, and then tests from the CCC. The installation of the remaining two kicker magnets at point 8 will be completed at the end of August.