742
M. Dohlus et al.
[117]. Synchrotrons and accumulators, alike, have a intensity limits due to space
charge effects that introduce tune spreads and cause beam instabilities, resulting in
significant beam losses.
11.4.2 The Linear Accelerator
For many spallation-based sources, a good choice of technology for the acceleration
of the proton beam is a linear accelerator (linac). These very complex machines are
conceptually quite simple—a beam of H+ ions or protons is generated by a source
and passed through a series of accelerating structures on its way to the spallation
target. Since the spallation process means that the time structure of the neutron
pulses will match that of the proton beam, the requirements of the experiments
directly dictate the time structure of the proton current, and thus the high-level
design of the linac.
11.4.2.1 High-Level Machine Design
Typically the specifications given by the experiments are the following.
• The desired repetition rate of the facility, usually less than 120 Hz.
• The duty factor or the length of the neutron pulses.
• The beam power on target; either average or maximum.
Given these parameters, the accelerator designers then begin sketching out the
specifications of the design.
Kinetic Energy and Current
The maximum beam power on target constrains the maximum energy and current
of the proton beam, and engineering capabilities will place additional limitations on
these quantities. For example, the spallation target may have an energy threshold
beyond which it will no longer survive.
As mentioned previously, the optimal beam energy for a spallation target system
is in the range 1–3 GeV, and so an accelerator designer is likely to pick an energy in
this range. From this, the required beam current is easily derived.
After this, these parameters will be optimised in an iterative way. Engineering
limitations will first be reached in either the power transmission capabilities of the
couplers on the accelerating structures, or the acceleration gradients required within
the cavities. Once these have been determined, the designer can iteratively tweak
the beam energy and current in order to balance the performance requirements of
the cavities.
RF Frequency
Due to unavoidable losses in the generation and transmission of the RF, the installed
capability will need to be more than a factor of two larger than the beam power. The
M. Dohlus et al.
[117]. Synchrotrons and accumulators, alike, have a intensity limits due to space
charge effects that introduce tune spreads and cause beam instabilities, resulting in
significant beam losses.
11.4.2 The Linear Accelerator
For many spallation-based sources, a good choice of technology for the acceleration
of the proton beam is a linear accelerator (linac). These very complex machines are
conceptually quite simple—a beam of H+ ions or protons is generated by a source
and passed through a series of accelerating structures on its way to the spallation
target. Since the spallation process means that the time structure of the neutron
pulses will match that of the proton beam, the requirements of the experiments
directly dictate the time structure of the proton current, and thus the high-level
design of the linac.
11.4.2.1 High-Level Machine Design
Typically the specifications given by the experiments are the following.
• The desired repetition rate of the facility, usually less than 120 Hz.
• The duty factor or the length of the neutron pulses.
• The beam power on target; either average or maximum.
Given these parameters, the accelerator designers then begin sketching out the
specifications of the design.
Kinetic Energy and Current
The maximum beam power on target constrains the maximum energy and current
of the proton beam, and engineering capabilities will place additional limitations on
these quantities. For example, the spallation target may have an energy threshold
beyond which it will no longer survive.
As mentioned previously, the optimal beam energy for a spallation target system
is in the range 1–3 GeV, and so an accelerator designer is likely to pick an energy in
this range. From this, the required beam current is easily derived.
After this, these parameters will be optimised in an iterative way. Engineering
limitations will first be reached in either the power transmission capabilities of the
couplers on the accelerating structures, or the acceleration gradients required within
the cavities. Once these have been determined, the designer can iteratively tweak
the beam energy and current in order to balance the performance requirements of
the cavities.
RF Frequency
Due to unavoidable losses in the generation and transmission of the RF, the installed
capability will need to be more than a factor of two larger than the beam power. The