Kicker magnets are an important part of the LHC accelerator complex. Installed at the intersection of the LHC ring and the SPS transfer lines, they give each injected beam a “kick” at the right time to put it into orbit in the LHC.
The higher luminosity of the HL-LHC will pose a challenge for these magnets, as increased heat load could result in a miskick of the injected beam. To avoid this, engineers in CERN’s Systems department have developed a new version of the kicker magnet for the HL-LHC, called an “MKI-Cool”. One such magnet was installed in the LHC one year ago, replacing a standard kicker magnet. Measurements during its first year of operation, with high-intensity beam, show that the temperature rise of the MKI-Cool is less than one-fifth of that of the other seven kicker magnets in the LHC. This confirms that no heating issues should occur for the MKI-Cool kicker magnets with HL-LHC beams.
“Based on this excellent result, all the kickers will be sequentially upgraded to MKI-Cools,” says Mike Barnes, senior engineer in CERN’s Systems department. “The full upgrade of the MKIs will be completed during Long Shutdown 3.”*
Unlike other magnets in the accelerator, kicker magnets cannot be fully shielded from the beam. Shielding would interfere with the fast magnetic field pulse that they provide to kick the beam. In addition, the high-voltage pulse required prevents the magnets from being water-cooled, which is a serious hurdle as the ferrite they are made from loses its magnetic properties above the temperature of 125 °C. Under these conditions, the MKIs would miskick the injected beams, causing the downstream magnets to lose their superconductivity.
Following years of research and development, the team came up with a new design. The MKI-Cool design works by moving most of the beam-induced heating from the ferrite yoke to a so-called RF damper, which contains a ferrite cylinder and is mounted just upstream of the magnet. The beam-induced heat is then removed from the RF damper using a water-cooling circuit.
“The concept of moving the heat was demonstrated in computer simulations, but it was very challenging to prove this in lab-based measurements,” Barnes continues. “Hence, to fully prove the concept, a prototype with an RF damper, which was not cooled, was installed in the LHC in 2018 and measurements of temperature, with circulating LHC beam, proved that the RF damper concept worked effectively.”
To measure the difference in the beam-induced heat load between the MKI-Cool and the old kicker magnets, the team used temperature sensors attached to a nearby metal side plate. It was not possible to directly measure the temperature of the ferrite because it is pulsed to a very high voltage during the beam injection.
“During 2023 LHC operation with the MKI, the measured temperature of both the RF damper and the side plate remained relatively low,” Barnes continues. “Based on these temperature measurements and simulations, no heating issues are expected for the MKI-Cool in the HL-LHC era.”
*Mike Barnes retired at the end of 2023: Giorgia Favia is now responsible for the MKI kicker magnets and will oversee the full upgrade to MKI-Cools.