11 Application of Accelerators and Storage Rings
753
An alternative way to realize a clean extraction is charge stripping with a thin
foil. In this case incompletely stripped ions are accelerated in the cyclotron, for
example H − or H 2
+ . At the extraction foil the ions change their charge and are
separated from the circulating beam by the action of the bending field. This scheme
is employed in the TRIUMF cyclotron to extract proton beams up to 520 MeV [143].
A disadvantage of this method is the non-negligible rate of stripping that occurs in
the bending field, causing losses and activation. The H 2
+ molecule has a higher
binding energy in the ground state, resulting in significantly lower dissociation
probability. However, disadvantages are the lower charge to mass ratio requiring
stronger fields, and the complex extraction path across the centre of the cyclotron,
resulting from a reduction of the bending radius by a factor 2 after stripping.
Transverse space charge forces present a limitation for the maximum feasible
beam intensity. Above a certain charge density the repelling space charge forces
exceed the relatively weak vertical focusing forces in a cyclotron. With the PSI
concept the realization of cyclotrons for beam powers up to 10 MW seems feasible.
Classical cyclotron magnets represent rather heavy and complicated devices
since they need to cover a wide radius variation of the beam orbit. To ensure
the condition of constant revolution time the radial field shape must be precisely
controlled. Radially distributed trim coil circuits are used for fine tuning on the basis
of beam phase measurements. Modern cyclotrons like the RIKEN SRC employ
superconducting magnets [144, 145] to obtain higher bending strength. In view
of turn separation and efficient extraction it is generally not desirable to design
very compact high intensity cyclotrons with the help of superconducting magnets.
However, with a large radius the gained space can be used to maximize the installed
RF voltage.
For the production of MW-class beams the energy efficiency of the accelerator systems is important. Cyclotrons with continuous RF generated by tetrode
amplifiers and utilizing copper resonators can reach a relatively high efficiency.
A comparison of different accelerator concepts suited for producing high intensity
proton beams is presented in [146]. At the PSI facility a fraction of 18% of the total
grid power is converted to beam power.
In Table 11.11 selected properties of three cyclotrons are listed.
In summary cyclotrons present an effective and relatively compact solution to
generate high intensity proton beams at energies below 1 GeV and beam powers of
a few MW.
Acknowledgements
Mohammad Eshraqi, ESS
Steve Peggs, ESS and BNL
Romuald Duperrier, CEA
Jim Stowall, Los Alamos
John Galambos, SNS
Ference Mezei, ESS
753
An alternative way to realize a clean extraction is charge stripping with a thin
foil. In this case incompletely stripped ions are accelerated in the cyclotron, for
example H − or H 2
+ . At the extraction foil the ions change their charge and are
separated from the circulating beam by the action of the bending field. This scheme
is employed in the TRIUMF cyclotron to extract proton beams up to 520 MeV [143].
A disadvantage of this method is the non-negligible rate of stripping that occurs in
the bending field, causing losses and activation. The H 2
+ molecule has a higher
binding energy in the ground state, resulting in significantly lower dissociation
probability. However, disadvantages are the lower charge to mass ratio requiring
stronger fields, and the complex extraction path across the centre of the cyclotron,
resulting from a reduction of the bending radius by a factor 2 after stripping.
Transverse space charge forces present a limitation for the maximum feasible
beam intensity. Above a certain charge density the repelling space charge forces
exceed the relatively weak vertical focusing forces in a cyclotron. With the PSI
concept the realization of cyclotrons for beam powers up to 10 MW seems feasible.
Classical cyclotron magnets represent rather heavy and complicated devices
since they need to cover a wide radius variation of the beam orbit. To ensure
the condition of constant revolution time the radial field shape must be precisely
controlled. Radially distributed trim coil circuits are used for fine tuning on the basis
of beam phase measurements. Modern cyclotrons like the RIKEN SRC employ
superconducting magnets [144, 145] to obtain higher bending strength. In view
of turn separation and efficient extraction it is generally not desirable to design
very compact high intensity cyclotrons with the help of superconducting magnets.
However, with a large radius the gained space can be used to maximize the installed
RF voltage.
For the production of MW-class beams the energy efficiency of the accelerator systems is important. Cyclotrons with continuous RF generated by tetrode
amplifiers and utilizing copper resonators can reach a relatively high efficiency.
A comparison of different accelerator concepts suited for producing high intensity
proton beams is presented in [146]. At the PSI facility a fraction of 18% of the total
grid power is converted to beam power.
In Table 11.11 selected properties of three cyclotrons are listed.
In summary cyclotrons present an effective and relatively compact solution to
generate high intensity proton beams at energies below 1 GeV and beam powers of
a few MW.
Acknowledgements
Mohammad Eshraqi, ESS
Steve Peggs, ESS and BNL
Romuald Duperrier, CEA
Jim Stowall, Los Alamos
John Galambos, SNS
Ference Mezei, ESS