7 Design and Principles of Linear Accelerators and Colliders
327
conducting devices have operated with non-isochronous recirculation and microtron
phase stability [123] as an inherent design choice [124]. Recirculator optics based on
two-bend and three-bend achromatic optics have been deployed. With the advent of
beam energies where synchrotron radiation is an important part of the beam quality
from the recirculator, arcs are being suitably designed and implemented to minimize
emittance and energy spread growth, as at synchrotron radiation sources.
An additional multi-bunch beam instability potentially affects recirculated and
energy-recovered linacs: multi-pass beam breakup (BBU) [125, 126]. In this
instability, deflections of the beam generated by high order modes in the accelerating
cavities can drive unstable feedback if the deflections translate after recirculation
into position offsets that act to further excite the high order mode. This instability
was first observed in the first superconducting recirculator at Illinois, and restricted
the operating current in the early Stanford superconducting recirculator. When
the much-larger-scale CEBAF accelerator was built, the solution to the instability
problem was provided by building the linac from cavities that were known to have
good HOM damping. This approach has continued to the present, where cavities
have been designed and tested that are expected to support 100 mA to 1 A currents
in recirculating linac arrangements.
Successful operations of high-power FEL drivers stimulated community interest
in ERL technology, and there followed numerous proposals and a number of actual
systems. These included: FEL drivers: the JLab IR Upgrade FEL [127], ALICE
[128], and the JLab UV Demo FEL [129]; test facilities: CEBAF-ER [130], the KEK
cERL [131], bERLinPro [132], ER@CEBAF [133], and PERLE [134, 135]; and
systems associated with nuclear physics facilities: the BNL test ERL, an electron
cooler concept [136], MESA [137], and most recently, conversion of the Darmstadt
S-DALINAC to an ERL [138].
Having described the motivation for ERLs and how they operate, we briefly
survey the contemporary ERL landscape, review progress to date, and detail
challenges confronting upcoming generations of these machines. We also provide
an overview of applications (servicing FELs, high-energy electron cooling systems,
inverse Compton-driven gamma sources, internal-target experiments, accelerator
science/technology test platforms).
7.8.1 Novosibirsk ERL
The Novosibirsk ERL—developed, built and commissioned at BINP in 2003—was
the first multi-pass ERL operating in CW mode [139]. It initially reached a top
energy of 12 MeV, with high average current of 30 mA [140]. The ERL is fed
by a 300 kV electrostatic gun with a thermionic cathode (Q ∼ 1 nC, τ = 1 ns,
f rep = 10 kHz–50 MHz), followed by one bunching and two accelerating cavities
for effective bunch compression. The facility uses a normal-conducting accelerating
system at 180 MHz, with average power up to 0.5 kW (peak power of about
1 MW). The ERL drives three separate FELs (NovoFEL facility) [141] operating
327
conducting devices have operated with non-isochronous recirculation and microtron
phase stability [123] as an inherent design choice [124]. Recirculator optics based on
two-bend and three-bend achromatic optics have been deployed. With the advent of
beam energies where synchrotron radiation is an important part of the beam quality
from the recirculator, arcs are being suitably designed and implemented to minimize
emittance and energy spread growth, as at synchrotron radiation sources.
An additional multi-bunch beam instability potentially affects recirculated and
energy-recovered linacs: multi-pass beam breakup (BBU) [125, 126]. In this
instability, deflections of the beam generated by high order modes in the accelerating
cavities can drive unstable feedback if the deflections translate after recirculation
into position offsets that act to further excite the high order mode. This instability
was first observed in the first superconducting recirculator at Illinois, and restricted
the operating current in the early Stanford superconducting recirculator. When
the much-larger-scale CEBAF accelerator was built, the solution to the instability
problem was provided by building the linac from cavities that were known to have
good HOM damping. This approach has continued to the present, where cavities
have been designed and tested that are expected to support 100 mA to 1 A currents
in recirculating linac arrangements.
Successful operations of high-power FEL drivers stimulated community interest
in ERL technology, and there followed numerous proposals and a number of actual
systems. These included: FEL drivers: the JLab IR Upgrade FEL [127], ALICE
[128], and the JLab UV Demo FEL [129]; test facilities: CEBAF-ER [130], the KEK
cERL [131], bERLinPro [132], ER@CEBAF [133], and PERLE [134, 135]; and
systems associated with nuclear physics facilities: the BNL test ERL, an electron
cooler concept [136], MESA [137], and most recently, conversion of the Darmstadt
S-DALINAC to an ERL [138].
Having described the motivation for ERLs and how they operate, we briefly
survey the contemporary ERL landscape, review progress to date, and detail
challenges confronting upcoming generations of these machines. We also provide
an overview of applications (servicing FELs, high-energy electron cooling systems,
inverse Compton-driven gamma sources, internal-target experiments, accelerator
science/technology test platforms).
7.8.1 Novosibirsk ERL
The Novosibirsk ERL—developed, built and commissioned at BINP in 2003—was
the first multi-pass ERL operating in CW mode [139]. It initially reached a top
energy of 12 MeV, with high average current of 30 mA [140]. The ERL is fed
by a 300 kV electrostatic gun with a thermionic cathode (Q ∼ 1 nC, τ = 1 ns,
f rep = 10 kHz–50 MHz), followed by one bunching and two accelerating cavities
for effective bunch compression. The facility uses a normal-conducting accelerating
system at 180 MHz, with average power up to 0.5 kW (peak power of about
1 MW). The ERL drives three separate FELs (NovoFEL facility) [141] operating