67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P07

S. Chen† , X. X Fu, H. X. Deng, B. Liu, Shanghai Advanced Research Institute, Shanghai, China
B. Y. Yan, University of Chinese Academy of Sciences, Beijing, China
Abstract
For feeding the three parallel undulator lines by the CW
beam of a single SRF linac simultaneously, a beam switchyard between the linac and undulator lines is designed withthe consideration of bunch-by-bunch beam separation andbeam quality per. In this work, the schematic design of thebeam switchyard for bunch-by-bunch beam separation of
CW beam is described, and the current lattice design of thelinac-to-undulator deflection branches and the start-to-end
tracking simulation results are presented.

INTRODUCTION
As a high repetition rate XFEL and extreme light facility,
the SHINE project is now under construction near the SSRF
and ShanghaiTech campus [1,2]. High quality electron beam
is generated by a VHF gun in CW mode with a repetition
rate up to about 1 MHz [3]. A superconducting RF linac
with two bunch compressors then accelerates the electron
beam to about 8 GeV. The electron beam is used to feed
the FEL undulator complex with three parallel undulatorlines, referred to as FEL-I (3-15 keV), FEL-II (0.4-3 keV)and FEL-III (10-25 keV) respectively. In addition, spaces
are reserved for future upgrade of more undulator lines.
For the simultaneous operation of multiple undulator lines
with different parameters and operation modes in SHINE, abeam switchyard is located in between the SRF linac and theundulator lines complex. In the beam switchyard, he 1 MHz
CW electron beam should be separated bunch-by-bunch in anarbitrarily programmable pattern and then delivered to each
undulator line through the corresponding linac-to-undulator(LTU) branch respectively. In this work, we present thephysics design of the beam switchyard for SHINE.

MAIN LAYOUT
A schematic view of the SHINE is seen in Fig. 1. Most of
the beam equipment will be installed in the tunnels and shafts
about 30 m underground. Electron beam starts from theinjector in #1 shaft and is accelerated by the SRF linac in a
1.4 km long tunnel. The three undulator lines are installed in
two of the undulator tunnels: FEL-I and FEL-II in the middle
undulator tunnel and FEL-III in the west undulator tunnel.
All the three undulator lines are parallel to the linac line,in the same vertical plain but not collinear. The horizontal
distance between the three undulator line and the linac isabout +1.85 m for FEL-II, -1.45 m for FEL-I and -8.95 m
for FEL-III, where ’+’(’-’) denotes left(right) to the linac
beam direction. The linac tunnel and the undulator tunnels∗ Worksupported by the Natural Science Foundation of Shanghai(22ZR1470200).
† chens@sari.ac.cn

are connected by the #2 shaft. Some key beam parametersof SHINE are listed in Table 1.
Table 1: SHINE Main Beam Parameters
Parameter
Beam Energy (𝐸0 )Slice Energy Spread (𝜎𝐸 /𝐸0 )RMS Norm. Emittance (𝜀𝑛 )Bunch Frequency (𝑓𝑟𝑒𝑝 )Bunch Charge (𝑄)Bunch Length (𝑙𝑏 )Peak Current (𝐼𝑝𝑘 )ValueUnit

8.0
∼0.01
≤0.45
1000
10∼300∼100≥1500

GeV%µm · radkHzpCfsA

The beam switchyard section starts from the end part ofthe linac tunnel, passes through the #2 shaft and ends at theentrance of the undulator lines. Electron bunches from thelinac should be separated and delivered either to the three
undulator lines or to a 800-kW beam dump in the middleof #2 shaft. Starts from the end of the SRF linac, a linacto-dump (LTD) line brings the undeflected bunches to themain dump. Three LTU deflection branches extract bunchesfrom the LTD line in the sequence of LTU-2, LTU-3, andLTU-1. This arrangement avoids the conflict of the three
LTU branches and reserves spaces for future extension of
more LTU branches for more undulator lines.

KICKER-SEPTUM MODULE
An electron bunch that is wanted by an undulator line
should be firstly extracted from the 1 MHz CW electron
bunch train without affecting the bunches to other directions.
Typically it is realized by pulsed kicker magnets. For maximizing the available beam modes that can be provided tothe user experiments and their flexibility of switching, thekicker magnets should be able to perform a stable bunch-bybunch kick to the electron beam and, what’s more, should
be programmable for arbitrary distribution pattern. Considering the beam parameter and the limited geometry, asingle kicker magnet could not meet the requirements independently. Therefore, the actual scheme is based on a setof small angle vertical kicker magnets combined with a DC
Lamberson septum magnet, which can be a compact schemewith high enough frequency and stability.
The configuration of the kicker-septum module is seen in
Fig. 2. The fast kicker set and the DC Lamberson septum
magnet are inserted in a FODO cell with about 24 m period.
The fast kicker set consists of 8(+2) special designed lumpedinductance kicker magnets with a total deflection angle ofabout 0.8 mrad [4]. The pole of the kicker magnet is out ofvacuum so that ceramic vacuum chamber should be used. To
reduce the wakefields, the ceramic vacuum chamber should
TU4P07

A - Linac-based Light Sources

87

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

DESIGN OF THE BEAM DISTRIBUTION SYSTEM OF SHINE∗

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P07

Figure 1: SHINE Schematic Layout

LINAC-TO-UNDULATOR BRANCHES

Figure 2: Schematic view of a kicker-septum module.

be coated with a conductive material and the alignment along
the kicker set is slightly adjusted to prevent the beam to be
too close to the chamber side. In order to ensure the relative
beam trajectory stability < 0.1𝜎𝑥,𝑦 , it is required that therepetitive stability of the magnetic field of the kicker magnet
be below 100 ppm.

After being separated by the kicker-septum section, theelectron bunches are then delivered by the three LTU
branches to the corresponding undulator lines respectively.
Based on the relative geometric relation between the linac
and the three undulator lines, in all the three LTU branches,
electron bunches should firstly be deflected to the directionof the undulator lines by a horizontal dog-leg. In this process, special concern should be given to various collectiveeffects of high-intensity electron beam being deflected, suchas the emittance growth due to dispersion effect and coherent synchrotron radiation (CSR) effect, the micro-bunching
instability that may spoil the longitudinal phase space. For
preserving the beam quality, several integrated optics measures are adopted for suppressing those collective effects inthe lattice design of the LTU branches. The lattice functionsof the three LTU branches are shown in Fig. 4.

(a) LTU-2

Figure 3: Lattice functions of the kicker-septum modules.
The quadrupoles of the FODO cells are aligned to becentering with the non-kicked straight bunch so that thekicked bunch experiences an extra deflection while passing
them off-axis. The vertical angle of the bunch is eliminatedby a small DC septum and the bunch then enters the DC
Lamberson septum with ∼17 mm vertical offset to the unkicked bunch for further horizontal deflection. Because ofthe limited longitudinal space available for beam switchyard,
the kicker-septum section is designed to be modularized sothat all the three LTU branches share the same kicker-septum
configuration, as is seen in the right figure of Fig. 3. Forradiation safety reason, beam position monitors and beamcollimators should be installed in front and behind the kicker
set in order to protect the kickers and the septa. What’smore, in case of emergency, all the kickers are switched off
immediately to lead all the electron beam to the main dump
until the beam is aborted upstream.

(b) LTU-3(c) LTU-1

Figure 4: Lattics functions of the three LTU branches.
A direct approach of reducing the CSR effect is by reducing the deflection angle and the beam size at the bending magnet. The deflection part of each LTU branch is designed to bea dual-DBA dog-leg, i.e., replace the two reverse angle bendof the dog-leg by two reverse angle double-bend-achromatic
cells for halving the deflection angle of a single bend (1.5∘for LTU-2, 1.8∘ for LTU-3 and 1.0∘ for LTU-1). The opticsof the dog-leg is designed to be mirror symmetrical with aTU4P07
88

A - Linac-based Light Sourcessmall 𝛽𝑥 (∼5 m at the middle of the DC Lamberson septa,
which is determined by the FODO of kicker-septum module)at each bend. In addition, a more effective approach of further suppressing the CSR emittance growth is by matching
the betatron phase advance between two adjacent dipoles
to be an odd multiple of 𝜋 in the deflection direction, i.e.,the optics balance method [5]. To implement this method
more effectively, the lattice function of the whole dog-leg isdesigned to be symmetrical for almost the same CSR kickin all the dipole magnets.
While deflecting the beam in the horizontal direction, thevertical offset and dispersion introduced by the kickers remain. These will be corrected by a small-angle vertical
dog-leg after the horizontal dog-leg. At this point, the beam
is dispersion-free both horizontally and vertically and heads
towards the direction of the undulator line. A series of periodic FODO cells with 16/20 m period and 𝜋/4 phase advancethen brings the beam to the entrance of undulator line. Inthis FODO section, the beam diagnostics instruments, suchas beam profile monitors and wire scanners are inserted foremittance and twiss parameter measurement. Besides, inorder to protect the radiation sensitive equipment such as thepermanent magnet undulators, a final post-linac collimation
should be done in advance of the beam entering the undulator lines. There will be an energy collimator inserted in
the middle of the first DBA cell of the horizontal dog-leg foreliminating the large off-momentum electrons and several
halo collimators inserted in the 𝜋/4-phase-advance FODO
cells for removing the halo electrons that may lost in theundulator sections. All the collimators are designed to haveadjustable gap to meet with different beam conditions.

START-2-END TRACKING
A collaborated start-2-end tracking simulation is done
based on the baseline beam parameter of SHINE. The electron bunch is generated by the VHG gun with optimized
parameters and accelerated by the injector and the SRF linac
to the entrance of the beam switchyard with the longitudinal
phase space distribution shown in Fig. 5. The simulation isdone by the particle tracking code Elegant [6].

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P07

after the beam passing through the beam switchyard withthe maximum emittance growth less than 5%.

Figure 6: Evolution of emittance along the three LTUbranches.
Another important beam dynamic issue of beam switchyard is the micro-bunching instability. For suppressing themicro-bunching growth in the beam switchyard of SHINE, achicane with adjustable 𝑅56 is inserted in the proper positionon each LTU branch. For LTU-1 and LTU-2, it is inserted in
the drift space between the two quadrupoles of the FODO
section after the deflection part. For LTU-3, it is inserted in
the long matching section between the two DBA cells. Theevolution of R56 of the three LTU branches and a comparison of the 𝑡 − 𝑥 phase space between non-isochronous caseand isochronous case of the three LTU branches are shownin Fig. 7. For the non-isochronous case, the maximum 𝑅56of LTU-1, LTU-2, LTU-3 is about 100, 400, and 650 µm,respectively. Obvious micro-bunching gain can be observedin the phase space for this case, especially for the LTU-3
branch. For the isochronous case, the 𝑅56 is almost eliminated by the properly set chicane, micro-bunching gain in
the phase space is barely observable.

Non-isochronous cases

Figure 5: Longitudinal phase space at the exit of linac.
The evolution of the normalized emittance of the three
LTU branches is shown in Fig. 6. With the properly designed
optics for dispersion free and CSR suppression, both the horizontal and vertical normalized emittance are well preserved

Isochronous cases
Figure 7: Comparison of the current profile at the exit ofeach LTU branch.

TU4P07
A - Linac-based Light Sources

89

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

67th ICFA Adv. Beam Dyn. Workshop Future Light Sources
ISBN: 978-3-95450-224-0
ISSN: 2673-7035

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

REFERENCES[1] Z. T. Zhao, D. Wang, Z. H. Yang et al., “SCLF: An 8-GeV CWSCRF linac-based x-Ray FEL facility in Shanghai, in Proc.
FEL’17, Santa Fe, NM, USA, MOP055.
doi:10.18429/JACoW-FEL2017-MOP055[2] C. Feng and H.-X. Deng, “Review of fully coherent freeelectron lasers”, Nucl. Sci. Tech., vol. 29, p. 160, 2018.
doi:10.1007/s41365-018-0490-1[3] H. Chen, L.-M. Zheng, and B. Gao, et al., “Beam dynamics
optimization of very-high-frequency gun photoinjector”, Nucl.
Sci. Tech., vol. 33, p. 116, 2022.
doi:10.1007/s41365-022-01105-y

FLS2023, Luzern, Switzerland

JACoW Publishingdoi:10.18429/JACoW-FLS2023-TU4P07[4] Y. Liu, R. Wang, J. Tong, et al., “Development of a highrepetition-rate lumped inductance kicker magnet prototype forbeam switchyard of SHINE, Nucl. Sci. Tech., to be published.
[5] S. Di Mitri, M. Cornacchia, and S. Spampinati, “Cancellationof coherent synchrotron radiation kicks with optics balance”,
Phys. Rev. Lett., vol. 110, p. 014801, 2013.
doi:10.1103/PhysRevLett.110.014801[6] M. Borland, Elegant: A flexible SDDS-compliant code foraccelerator simulation, Advanced Photon Source, Argonne
National Lab, IL, USA, Rep. LS-287, Sep. 2000.

TU4P0790

A - Linac-based Light Sources