7 Design and Principles of Linear Accelerators and Colliders
325
also provides an additional environmental advantage, as the total stored energy
in the system is deposited at very low (injection) energy, providing mitigation of
serious environmental/safety issues. Finally, ERLs, like linacs, offer a flexible time
structure, allowing operation with single bunches, CW bunch trains, and virtually
every combination between these options. As in other linac-based systems, ERLs
can easily manipulate various portions of phase space independently of other
portions; they are fully six-dimensional systems, supporting transverse matching
to desired spot sizes, longitudinal matching to desired bunch length/energy spread
ratios (via transverse/longitudinal coupling), and any (or all) of horizontal/vertical
transverse/longitudinal phase space exchanges.
Next-generation light source or collider applications requiring the following
elements should generally be well-suited to deploying a recirculated and/or energyrecovered linac: CW or other high duty factor operation, high beam average current,
low delivered beam energy spread, and low delivered beam emittance. CW beam
acceleration with high accelerating gradients (>10–20 MV/m) generally requires
deploying a multi-pass RLA consisting of superconducting accelerator structures.
GeV-scale RLAs at 100 mA average current would ordinarily require at least
100 MW of installed RF power merely to accelerate the beam load. Beam energy
recovery allows substantial reduction of the RF beam loading of the cavities.
In applying this idea with a back-to-front beam recirculation, as illustrated in Fig.
7.9, the beam recirculation path length is chosen to be an integral number of RF
wavelengths, plus approximately one-half of the RF wavelength. Because the beam
sees accelerating phase on the lower accelerating beam passes through the linac,
after a phase shift of 180 degrees, energy is delivered back to the (S)RF cavities
by higher beam passes, and transferred directly to the accelerating beams without
the need for additional power from other RF sources [111]. To the extent that the
average beam load from the accelerating passes completely cancels the beam load
from the decelerating beam passes, there is no limit to the average current that may
be accelerated due to RF source capacity. Because the beam transit time through the
recirculated linac is much smaller than the radiation-induced emittance growth times
in the bending arcs, the beam longitudinal and transverse emittances can be much
smaller in energy-recovered linacs than in storage ring accelerators that operate at
the same energy. It should be noted that energy recovery is also an important element
in the design of high average current electrostatic accelerators.
Beam energy recovery was first proposed as a way to construct high-luminosity
colliders for high energy physics [112]. Although never realized in this application,
energy-recovered accelerators have been built as electron cooling drivers and highpower free-electron laser drivers [113–115].
Many proposed applications benefit from the advantages of energy-recovered
linacs. For example, Cornell University is investigating the energy-recovered linac
as an undulator driver yielding superior, high average brilliance X-ray sources as an
upgrade to their conventional synchrotron light facility [116]. Similar programs exist
at Argonne and Daresbury Laboratories [117], and in Japan [118, 119]. Brookhaven
National Laboratory and CERN are investigating the use of high average current
325
also provides an additional environmental advantage, as the total stored energy
in the system is deposited at very low (injection) energy, providing mitigation of
serious environmental/safety issues. Finally, ERLs, like linacs, offer a flexible time
structure, allowing operation with single bunches, CW bunch trains, and virtually
every combination between these options. As in other linac-based systems, ERLs
can easily manipulate various portions of phase space independently of other
portions; they are fully six-dimensional systems, supporting transverse matching
to desired spot sizes, longitudinal matching to desired bunch length/energy spread
ratios (via transverse/longitudinal coupling), and any (or all) of horizontal/vertical
transverse/longitudinal phase space exchanges.
Next-generation light source or collider applications requiring the following
elements should generally be well-suited to deploying a recirculated and/or energyrecovered linac: CW or other high duty factor operation, high beam average current,
low delivered beam energy spread, and low delivered beam emittance. CW beam
acceleration with high accelerating gradients (>10–20 MV/m) generally requires
deploying a multi-pass RLA consisting of superconducting accelerator structures.
GeV-scale RLAs at 100 mA average current would ordinarily require at least
100 MW of installed RF power merely to accelerate the beam load. Beam energy
recovery allows substantial reduction of the RF beam loading of the cavities.
In applying this idea with a back-to-front beam recirculation, as illustrated in Fig.
7.9, the beam recirculation path length is chosen to be an integral number of RF
wavelengths, plus approximately one-half of the RF wavelength. Because the beam
sees accelerating phase on the lower accelerating beam passes through the linac,
after a phase shift of 180 degrees, energy is delivered back to the (S)RF cavities
by higher beam passes, and transferred directly to the accelerating beams without
the need for additional power from other RF sources [111]. To the extent that the
average beam load from the accelerating passes completely cancels the beam load
from the decelerating beam passes, there is no limit to the average current that may
be accelerated due to RF source capacity. Because the beam transit time through the
recirculated linac is much smaller than the radiation-induced emittance growth times
in the bending arcs, the beam longitudinal and transverse emittances can be much
smaller in energy-recovered linacs than in storage ring accelerators that operate at
the same energy. It should be noted that energy recovery is also an important element
in the design of high average current electrostatic accelerators.
Beam energy recovery was first proposed as a way to construct high-luminosity
colliders for high energy physics [112]. Although never realized in this application,
energy-recovered accelerators have been built as electron cooling drivers and highpower free-electron laser drivers [113–115].
Many proposed applications benefit from the advantages of energy-recovered
linacs. For example, Cornell University is investigating the energy-recovered linac
as an undulator driver yielding superior, high average brilliance X-ray sources as an
upgrade to their conventional synchrotron light facility [116]. Similar programs exist
at Argonne and Daresbury Laboratories [117], and in Japan [118, 119]. Brookhaven
National Laboratory and CERN are investigating the use of high average current