Increasing the intensity and brightness of hadron accelerators is an increasing need to meet the demands accelerator-based nuclear and particle research, as well as applications in the fields of, for example, medical and material sciences.
At CERN, the most notable project requiring an increase of beam brightness (i.e., the ratio of the beam intensity to the beam size) is the High Luminosity Large Hadron Collider (HL-LHC) project. For this, the established target is to double the beam intensity while reducing the beam size at the collision point, yielding significant challenges. To provide this increased beam intensity, the whole LHC injector complex underwent a complete upgrade to address limitations due to the increased beam intensity.
The need to increase beam intensity also applies to other accelerator facilities, such as the European Spallation Source (ESS), the ISIS Neutron and Muon Source, the Spallation Neutron Source or the Japan Proton Accelerator Research Complex. This also concerns heavy ion beams, for which the Facility for Antiproton and Ion Research facility is also being built.
But there are many challenges to increasing beam intensity and the only real way to increase performance is to tackle many (if not all) of these at the same time.
A series of challenges to increasing beam intensity
The requirement for high beam intensities implies that the beams generate large electromagnetic fields, which affect the particles within the same beam, or other beams, or even the beam surroundings (e.g. the vacuum chamber). The result of this interaction with strong electromagnetic fields can lead to growth of the beam size, or rapid beam instabilities creating beam losses (i.e. drop of the intensity), or even hardware damage which thereby impacts accelerator performance.
Typical forces due to high intensity are direct space charge effects, i.e., self-generated fields acting back on the beam, which are important in particular at lower energies in linear accelerators and rings.
A similar effect can be due to scattering of particles within the same beam or particles present in the vacuum pipe.
In addition, image charges created from the beam in the metallic walls of the vacuum pipe, can also act back to the same particle bunch or to consecutive ones. If the underlined equipment is not properly designed to avoid this interaction with the beam (defined as impedance), this can lead to beam instabilities but also to beam induced heating of the vacuum chamber, which can also lead to damage.
Beam instabilities can be cured with the choice of accelerator optics, the use of non-linear magnetic elements, higher harmonic radio frequency cavities or feedback systems. The interaction of the beam with another one when colliding (beam-beam effect) is also strongly non-linear and limits the performance (luminosity) of colliders.
Other two-beam effects involve different particle species, e.g. photoelectrons produced in the vacuum chamber by photons emitted by the particle bunches. Depending on the intensity and how closely bunches are spaced, a cloud of electrons can be created, which can again act back to the hadron beam creating instabilities and losses.
Collaboration the necessary solution
These topics represent the cutting-edge of the knowledge in terms of beam physics and accelerator design, requiring the exploitation of state-of-the-art methods to study collective effects and engineering solutions to mitigate the issues.
The complexity is that to tackle these challenges simultaneously, an overview is necessary, but that relies on deep expertise in the hands of a few specialists. That is why it is vital to collaborate widely to share knowledge across specialities.
The CERN Accelerator School (CAS) is holding a two-week course on intensity limitations in hadron beams this June in Bulgaria.
This CAS course provides an overview of a wide range of beam physics and dynamics effects emerging due to high intensity, their impact and possible mitigations for various types of accelerators, including linear and circular ones.
More information can be found here.