Proceedings of IBIC2012, Tsukuba, JapanTUPB81DESIGN OF THE BEAM PROFILE MONITORSFOR THE SXFEL FACILITY
L.Y.Yu, J.Chen, W.M.Zhou, K.R.Ye, Y.B.Leng
SINAP, Jiading, Shanghai 201800, P.R.China
The Shanghai X-ray Free Electron Laser Facility will
begin construction at next year. The linac electron beamenergy is 0.84 GeV. Over 50 beam profile monitors withOTR and YaG screen will be installed along the linac andundulators. The profile monitor system design is achallenging task, since the system has to measuretransverse electron beam sizes from millimeter down to
40μm scale with a 20μm resolution and 50μm repeat
positioning accuracy. This paper describes the design ofthe mechanical detector, the integrated step-servo motor
controlling system, the beam imaging system, as well as
the software system.

INTRODUCTIONSXFEL proposal was approved in February 2011 by
China government, the construction is expected to start at
the SSRF campus in early 2012. This test facility, basedon an 840MeV electron linac, aims at generating 8.8nm
FEL radiation with two-stage cascaded HGHG scheme[1].
Figure 1 shows the layout of the SXFEL linac.

Figure 1: Layout of the SXFEL accelerator & undulator.
SXFEL injector have 3 beam diagnostic stations,require 6 beam profile monitors designed by Tsinghua
University, linac part requires about 31 beam profile
monitors or so, undulator part require 21 beam profilemonitors.
Table 1 list the physical specification of the beamprofile monitors[2].
We are going to use Yag/OTR screens and wire canners
as the beam profile monitors originally. Considering thecost, reliability and we have not any experiences on wire
scanners, so we decide to improve the design of Yag/OTR
profile monitor to satisfy the specification. In addition thesystem has to be robust, remote controlled and easy tomaintain, we adopt a new type integrated step servo

motor to redesign the mechanical detector and select ahigh resolution CCD, the whole profile system is morecompact.
Table 1: Specification of the beam profile monitors
Parameter

Linac

Undulator

Quantity

3121210mm

150mm

Monitor length
Repeat positioning
accuracy

50 μm

Resolution

20 μm

Charge range

0.02~1nC

RMS beam length

0.015~4 psMeasuring range

15*15/5*5 mm2

MECHANICAL DETECTOR
The beam profile monitor will be a main diagnostic
tool during the commissioning of SXFEL experiments. It
has three position screens for test. One is the calibration
screen, The beam profile monitor is not only used tomeasure the electron beam transverse shape and size, butalso to synchronize the seeding laser and electron beamon spatial domain and temporal domain respectively. So
all beam profile monitor is designed as shown as Figure2
with four stages pneumatics screen monitor. One screen
which material is YAG:Ce crystal plate is used at theinitial commissioning and at low beam charge situation,
another screen which material is made from 100nm
aluminium deposited on a polished silicon wafer[3],generating optical transition radiation(OTR) when
Electron hits on it, is used to measure beam energy spreadand beam emittance respectively, because the OTR
represents a linear radiation source while YAG screen has
the saturation problem. The last screen is calibration
screen where holes array radius is 1mm and spacing is
5mm. It is used for calibrate the optical relay and CCD
image acquisition system. The light of beam can beextracted from both viewport, at the undulator location,
the energy of seeding laser is very huge to damage theCCD camera, so the beam image can be obtained frombackside viewport. The camera is located below beam
pipe 1.1 meter through planar - mirror optical relay toreduce the radiation damage. The viewport flange can be
replaced conveniently with crystal viewport flangeISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

TUPB81Proceedings of IBIC2012, Tsukuba, Japan

Figure 2: Design of the mechanical detector.

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○where the ultraviolet light is need to extract from thescreen[3].
To ensure the angle between screen and beam line is
45degrees, we add a new design named tune on themechanical detector, it can be fine rotated the axis of thescreen. Combined with the optical setup, we can calibrate
the 45degree accurately. Figure 3 is the finished prototype
mechanical detector.

High stiffness due to the nature of the stepper motor
combined with the highly responsive servo control, space
vector current control with 5000 line high resolution
encoder, gives smooth and quiet operation, especially at
low speeds. The SSM24Q motor integrates the motor,
driver, controller, encoder and IO, so the movercontrolling system is very concise, the motor can start towork just providing DC24V&DC48V and RS485 bus,
DC24V is used to supply the IO, DC48V is used tosupply the motor, serial commands directly to the driverto communicate with the motor by RS485 serial bus. Theoutput torque is 2.4N.m, it can easily pull out the screen
quickly off the beam line without the help of deceleration
mechanism. To avoid screen stopping the beam line, themover works smoothly and stably with very low noise.
Figure 5 shows the block diagram of SSM24Q[5].

Figure 3: Prototype mechanical detector.

MOVER CONTROLLING SYSTEM
We select a new type motor for the design of beamprofile monitor SXFEL, The type is SSM24Q-3RG
produced by China. Figure 4 shows the overview of theintegrated step servo motor.

Figure4: Integrated step-servo motor.

Figure 5: Block diagram of SSM24Q.
We can use the In connector of the motor as the screen
state display function , use the Out connector of themotor as the illuminate function for the optical part. Use
the encoder of the motor to position the screens without
extra using grating ruler which is used at the beam profile
monitor of SDUV-FEL. All the function realization
implement with serial commands.
This motor has a high positioning accuracy, encoder
resolution is 20000 counts/rev, the positioning error isonly±0.018°, so the beam profile monitor can satisfy thephysical requirement: repeat positioning accuracy<50 μm.

ISBN 978-3-95450-119-92Transverse Profiles, Screens & Wires Monitors

Proceedings of IBIC2012, Tsukuba, JapanThe optical system used to image the YaG/OTR light to
the CCD camera is almost the same with the design ofSDUV-FEL, except the CCD type, it consists of a mirror
deflecting the OTR light downwards, one focus lens, anda CCD camera, providing nominal magnification is 1. The
components are mounted on two rails on a stainless steel
plate. The plate will be fixed to the support structure ofthe machine. The optical system is protected against thestray light, and the CCD camera is shielded with lead toreduce radiation damage. The camera is a digital CCD
camera (Basler scA1000-30) with Ethernet interface. The
sensor size of the camera is 1034 x 779 pixels with a pixel
dimension of 4.65 µm x 4.65 µm.
The initial adjustment of the optical set-up will be doneon an optical table. The optical axis is defined with a helpof a He-Ne laser. The correct position of the lens issearched by manually measuring the contrast on acalibration target. When the correct position of the lensand the camera is ok, their position is fixed. On the beamline, the distance of the whole system from the screen isadjusted with the help of the marks on the calibration
screen. The same marks are used to calibrate themagnification.
The GigE Vision camera is adopted based on following
two main advantages: First, digital signals can betransmitted much more stably than analog signals overlong distance. Second, The GigE CCD camera gain
parameter can be adjusted remotely according to the beam
intensity during commissioning[4]. Basler scA1000-30
CCD cameras were chosen as the major devices in image
acquisition subsystem. With a 1:1 lens, the spatial
resolution could be 10 microns. The range of gray scale
could be 0 to 65535 because the camera has a 16 bits
ADC. External trigger mode is important for the system,
because the action of capturing the beam images must
synchronize with the timing signal of the entire system.

decision of the final solution for the connection scheme
will be made within a few months. We will do somesimulation and test of the screen materials, to select moresuitable one. The whole YaG/OTR beam profile monitor
system is expected to be finished 5 month later.

ACKNOWLEDGMENT
The authors would like to thank colleagues at FELGroup in SINAP for the valuable discussions. Special
thanks are due to H.B.Pan for many mechanical designs
support.

REFERENCES[1] Z. T. Zhao, S.Y. Chen, L.H. Yu, C.X. Tang , L.X.
Yin, D. Wang, Q.Gu, “Shanghai Soft X-Ray Free
Electron Laser Test Facility”,THPC053 Proceedingsof IPAC2011, San Sebastián, Spain.
[2] Z. T. Zhao, L.H. Yu, C.X. Tang , L.X. Yin, D. Wang,Q.Gu et al., “CDR of SXFEL”, 2011.
[3] Y.Z.Chen, Y.C Xu, L.F.Han, W.M.Zhou, L.Y.Yu etal., “Beam Diagnostics at SDUV-FEL”, FROA3,
Proceedings of FEL2010, Malmö, Sweden .
[4] Y. C. Xu, Y. Z. Chen, L. F. Han, et al., “A New
Beam Profile Diagnostic System Based on theIndustrial Ethernet”, MOPE033, Proceedings ofIPAC’10, Kyoto, Japan.
[5] “Specification of the SSM24Q”, P2, 2011.

SOFTWARE
The beam profile monitors control system is simple andflexible using two filed buses: The industrial Ethernet bus
for communication between control room and cameras;
The 485 serial bus for communication between control
room and the integrated step-servo motor.
Use the MOXA serial servers can easily convert theRS485 signals to Ethernet signals. The beam profile
monitors control system software is based on EPICS,
every beam profile monitor control system can be
designed as a standard software IOC.

STATUS AND OUTLOOKThe prototype mechanical detector of the beam profile
is ready and the mover control software is ok, the screen
moving control is very smooth and stable. The prototypeof the optical set-up will be installed in lab on November
this year, and the first table test will be done soon. The
test of the digital camera system continues and theISBN 978-3-95450-119-9
Transverse Profiles, Screens & Wires Monitors

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○OPTICAL SET-UP

TUPB81