Why Radio Frequency?
Charged particles are accelerated by electric fields – the energy they can gain in the process is given by the product voltage times charge. To reach energies above some MeV, one needs fields varying in time at a high (radio) frequency and the particle beam in bunches rather than continuous. This is why Radio Frequency is the technique of choice for particle accelerators.
How does RF interact with the beam
The keyword here is “cavity”: a cavity (an evacuated metallic hollow) can support an electromagnetic field at a given (radio) frequency. An accelerating cavity is constructed such that beam can pass through it (beam pipes upstream and downstream) and that the electric field points in the direction of particle motion (beam axis). In this way, a passing particle beam bunch is accelerated and gains energy. This energy has to be resupplied to the cavity from a powerful RF amplifier via a power coupler. The phase of the cavity field has to be precisely controlled such that the accelerating voltage has the right direction when the bunch passes.
The priorities of the Radio Frequency Group
Supply beams for Physics
The highest priority for the RF group is to contribute to a successful physics run of the LHC. This comprises operation of the superconducting 400 MHz accelerating system (ACS) and the transverse damper (ADT) of the LHC itself, but also all the individual links of the injector chain upstream of the LHC – Linac2, the PSB, the PS and the SPS – don’t forget that it is RF in all these accelerators that truly accelerates! A large variety of RF systems has to operate reliably, often near their limit. Again and again, new tricks had to be found and implemented to go beyond existing performance limits; not to forget the equally demanding operation with Pb ions using in addition Linac3 and LEIR.
Other users also require the full attention of the RF group: fixed target beams to produce secondary beams for physics or for tests. AWAKE for example requires RF settings quite different from those for LHC beams (very short bunches). The Antiproton Decelerator (AD) requires reliable deceleration and cooling of anti-protons, Rex-ISOLDE and since 2015 also HIE-ISOLDE the post-acceleration of radioactive isotopes.
Projects and Studies
In addition to the supply of beams for physics, the RF group has a number of projects and studies to look after: first to mention here is the LIU project aiming at an upgrade of the whole LHC Injector chain towards larger beam intensities, where the RF group is deeply involved in all injector synchrotrons and with Linac4, the new H- injector into the PSB. Modern, more sophisticated and more powerful RF systems were conceived and are in construction for the PSB, the PS and the SPS. Next in line is the HL-LHC project to increase luminosity in the LHC, which has to cope with the increased intensities from the injector chain and to make best use of it for more physics. The main RF topic in the HL-LHC project are the superconducting crab cavities, which allow a significant luminosity increase for small beam sizes. Since the proton bunches have to cross at an angle, luminosity would be lost for small beam sizes since the needle-shaped bunches would not fully overlap when colliding – this can be fixed with the crab cavities that deflect heads and tails of the bunches sideways in opposite directions such that this geometric aberration is exactly compensated. The CLIC study investigates the feasibility of a normal conducting linear collider in the multi-TeV range and probes feasibility of extreme high-gradient X-band acceleration as well as a number of novel concepts. For the FCC study, the RF group looks into the very challenging RF systems of a possible Future Circular Collider with a circumference of about 100 km for both hadrons and leptons.
Many of the CERN accelerators are very old – older than the people operating them; to keep them running reliably requires substantial maintenance and consolidation work, which has to be taken very seriously. Taking good care of these old machines and teaching them new tricks, adapting them to a changing environment and concepts nicely fits between operation and maintenance on one side, upgrades and new projects on the other.