Multi-turn H- charge exchange is a popular injection system into many
of the world’s leading synchrotrons. H- beam from the linear
accelerator is stripped of two electrons, leaving only the protons, by a
thin stripping foil, before entry into the synchrotron ring. The
advantage of the charge exchange injection method is that it enables
avoidance of satisfying the Liouville Theorem. This states that the
local density of particles in phase space must remain constant in the
absence of collisions and dissipation. However, this only holds for
particles of like charge, so by injecting in a different charge state
negates the required fulfilment of this theorem.
A high stripping efficiency is required, to minimise beam loss caused by unwanted un-stripped (H-) or partially stripped (H0) present after the foil. Incident beam energy, foil thickness and foil material all determine the overall stripping efficiency and temperature rises in the foil. Electron-loss cross sections vary between different materials and as the beam energy increases the thickness is increased to maintain efficiency. However, using thicker foils increases the scattering interactions of the beam in the foil, leading to losses which consequently limit operational intensity. As thin foils are bombarded by high energy beam their temperature increases. If these temperatures become excessive they can cause slight evaporation of the foil surface. This is evidenced in a reduction of foil thickness which alters the stripping efficiency and ultimately shortens the foil lifetime. At the heart of any efficient charge exchange injection system is a stripping foil with carefully designed characteristics.
Possible injection upgrade schemes to ISIS, replacing the existing 70 MeV linear accelerator with one of 180 MeV, have been studied. Currently ISIS operates with a thin, 0.3 µm Aluminium Oxide stripping foil, which is around 97% efficient. Increase in the beam energy from 70 – 180 MeV is incompatible with the current foil system.
An in house computer simulation code was developed to investigate electron stripping, scattering events and temperature rises in thin foils. Modelling these foil interactions enables greater optimisation of foil parameters for possible future ISIS injection scheme upgrades. The code was benchmarked against current ISIS operations and experimental results and simulations from J-PARC, the SNS at ORNL and Linac4 at CERN.
The following specification for an electron-stripping foil for 180 MeV injection into the ISIS proton synchrotron has been developed:
Foil Material: Amorphous Carbon
Foil Thickness: 200µg/cm2
Identification of suitable stripping foil properties continues during various iterations of the current 180 MeV injection upgrade design project. Foil studies will also be imperative when research begins into possible multi-turn H- injection schemes from an 800 MeV linear accelerator into a proposed 3.3 GeV RCS, including associated foil exchange mechanisms and diagnostic tools. Work is also ongoing in investigating the temperature effects in double-layered Hybrid-Boron-doped-Carbon (HBC) foils at KEK and J-PARC in Japan.