Accelerating Cavity Studies for ESS/SPL

European Spallation Source (ESS)

The European Spallation Source (ESS) is a machine planned for construction in Lund, Sweden, designed for the production of an intense beam of neutrons for use in spectroscopy experiments. Funded by multiple European nations, ESS will be the world's most intense source of neutrons, and the premier facility for neutron spectroscopy experiments for many decades.

ESS calls for a beam of 2.5 GeV protons to impact a target in order to generate the beam of neutrons, and a total power of 5 MW. Despite the huge bunch population that this implies, the losses of particles along the length of the machine must be limited to less than 1 W/m. These tight tolerances put tremendous limits on the performance of the accelerating cavities, and it is these that are being studied here.

Superconducting Proton Linac (SPL)

The Superconducting Proton Linac (SPL) is a machine very similar to ESS, but designed for a completely different purpose. SPL will be used to accelerate H- ions to 5 GeV in order to generate a beam of neutrinos for high energy physics studies.

Due to the strong similarities in the parameters required of ESS and SPL, their designs are extremely similar, which means that studies of the accelerating cavities for one machine are often directly applicable to the other design.

Cavity studies

Software

The complexity of these studies requires the use of multiple CPUs (perhaps many hundreds!) acting in parallel in order to reduce the calculation time to a reasonable level. In order to meet these demands, we are using the ACE3P code suite provided by the SLAC Advanced Computations Group. In addition to providing access to their state-of-the-art code, they have also very kindly provided us with a significant amount of computing time at the NERSC supercomputing facility.

Some results

Although the beampipe acts to cut off the flow of HOM power between cavities, this is not a hard-edged cut off, but rather a strong exponential damping. Thus, it is possible for the field to penetrate into a neighbouring cavity and excite an oscillation there. Studies of this effect were recently presented at IPAC10.

Further details of these studies may be found here.

Note that these results were derived from simulations of the SPL cavity design, however work is ongoing to apply this work to the various cavities planned for ESS.