11 Application of Accelerators and Storage Rings
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high power nature of these machines means that one of the largest cost drivers will
be the power sources for the accelerating RF. In order to save cost it is advisable to
take advantage of already-designed components, and thus RF frequencies previously
adopted by other facilities.
Typically this results in acceleration RF whose frequency is an integer multiple
of 176.105 MHz or 201.5 MHz.
Since it is possible to achieve higher acceleration gradients with higher frequencies, the self-force (i.e. space charge) of the beam at the lower energy regions of
the machine mean that the strong focusing that results from high frequencies can
be problematic. It can therefore be optimal to use multiple acceleration frequencies,
with that in later sections of the linac being a multiple of two or four of the low
energy acceleration.
11.4.2.2 Linac Layout
A linac is laid out as a series of consecutive stages of acceleration. The following
sections each provide some detail on the most common technologies.
Proton Source
The ion source for a long pulse spallation source delivers protons rather than Hions, since there is no need to inject into an accumulator ring using charge exchange
injection (see section on synchrotrons). Proton sources can be implemented using
various techniques. For example, compact Electron Cyclotron Resonance (ECR)
sources such as VIS [118] and SILHI [119] are well suited to the task, delivering
continuous proton beams of up to 100 mA. The diverging beam leaving the source
has to be transported and matched to RFQ, usually using Einzel lenses or solenoids
in the Low Energy Beam Transport (LEBT). The LEBT has to be designed carefully
because it defines the beam quality throughout the rest of the linac [120].
Radio Frequency Quadrupole
Modern proton sources typically output a continuous beam of protons, however it
is not possible to directly accelerate such a CW current. The structure immediately
following the proton source must be one that compresses the current into a series
of bunches at the fundamental frequency of the accelerating RF. In the majority of
linacs a Radio Frequency Quadrupole (RFQ) performs this job.
An RFQ is a resonant cavity loaded with four vanes or rods in such a way that
the beam experiences a quadrupolar electrical field that strongly confines the size
of the beam. These vanes or rods are modulated in such a way that the beam
also experiences longitudinal forces. Initially these forces are tuned so that the
continuous current will begin to coalesce into a series of bunches at the frequency
of the accelerating RF.
As the beam proceeds further through the RFQ, the modulation of the vanes/rods
will be adapted so that there will be an acceleration force in addition to the focusing
terms. This will be increased throughout the length of the RFQ in a way that
preserves the quality of the beam, while keeping the structure to a reasonable length.
743
high power nature of these machines means that one of the largest cost drivers will
be the power sources for the accelerating RF. In order to save cost it is advisable to
take advantage of already-designed components, and thus RF frequencies previously
adopted by other facilities.
Typically this results in acceleration RF whose frequency is an integer multiple
of 176.105 MHz or 201.5 MHz.
Since it is possible to achieve higher acceleration gradients with higher frequencies, the self-force (i.e. space charge) of the beam at the lower energy regions of
the machine mean that the strong focusing that results from high frequencies can
be problematic. It can therefore be optimal to use multiple acceleration frequencies,
with that in later sections of the linac being a multiple of two or four of the low
energy acceleration.
11.4.2.2 Linac Layout
A linac is laid out as a series of consecutive stages of acceleration. The following
sections each provide some detail on the most common technologies.
Proton Source
The ion source for a long pulse spallation source delivers protons rather than Hions, since there is no need to inject into an accumulator ring using charge exchange
injection (see section on synchrotrons). Proton sources can be implemented using
various techniques. For example, compact Electron Cyclotron Resonance (ECR)
sources such as VIS [118] and SILHI [119] are well suited to the task, delivering
continuous proton beams of up to 100 mA. The diverging beam leaving the source
has to be transported and matched to RFQ, usually using Einzel lenses or solenoids
in the Low Energy Beam Transport (LEBT). The LEBT has to be designed carefully
because it defines the beam quality throughout the rest of the linac [120].
Radio Frequency Quadrupole
Modern proton sources typically output a continuous beam of protons, however it
is not possible to directly accelerate such a CW current. The structure immediately
following the proton source must be one that compresses the current into a series
of bunches at the fundamental frequency of the accelerating RF. In the majority of
linacs a Radio Frequency Quadrupole (RFQ) performs this job.
An RFQ is a resonant cavity loaded with four vanes or rods in such a way that
the beam experiences a quadrupolar electrical field that strongly confines the size
of the beam. These vanes or rods are modulated in such a way that the beam
also experiences longitudinal forces. Initially these forces are tuned so that the
continuous current will begin to coalesce into a series of bunches at the frequency
of the accelerating RF.
As the beam proceeds further through the RFQ, the modulation of the vanes/rods
will be adapted so that there will be an acceleration force in addition to the focusing
terms. This will be increased throughout the length of the RFQ in a way that
preserves the quality of the beam, while keeping the structure to a reasonable length.