Design of Beam Loss Monitor System for an 800 MeV Synchrotron Facility
Yonggang Cui1, Beibei Shao2
Department of Engineering Physics, Tsinghua UniversityYuxiong Li3
NSRL, University of Science and Technology of China
Abstract
The PIN photodiode with the coincidence
technology for Beam Loss Monitor System (BLMs),
was developed at DESY and has been used on HERA
ring. It has been decided to install in the Beam LossMonitor System for National Synchrotron Radiation
Laboratory (NSRL) in Hefei, China. Such a system
will help to tune machine and find the vacuum
leakage. Detailed design of the BLMs is described inthe paper.
It is the first time for some new technologies, suchas FPGA, CAN bus, to be used for accelerator control
in China. The design of the high-speed FPGA countercan be seen in this paper. The system also uses a PC
for data acquisition and data analysis. It implements
field control (here, CAN Bus) with a CAN Bus
network adapter. Compression between this method
and VME plus CAN IP is also described here.
Compared with the 920GeV-p and 30GeV-e in
DESY HERA, there will be some new physics
problem under the lower energy condition, when thedetector is applied in NSRL. All of them arementioned in this paper.

photons will not give out signal at the coincidence
gate.

Figure 1. Principle of beam loss detector
24 detectors will be distributed at 12 places along
the 65.88m HLS ring (figure 2). CAN BUS will be
used to connect these detectors to a PC machine,which is the console of the system.

1. INTRODUCTIONNSRL in USTC has an 800Mev e-storage ring
(HLS – Hefei Light Source). During the upgrading ofaccelerator in USTC, a BLM system will be installed
to detect beam loss along the storage ring, tune themachine, and study the beam lifetime.
The beam loss detectors, originally developed byDESY, will be used to detect shower electrons
induced by lost beam particles. Because of its special
principle (figure 1), it is powerfully to be used inintense synchrotron radiation environment.
One detector has two PIN photodiodes, face toface. Electrons, with enough high energy, can go
through these two diodes. Signals from two diodes
generate output signal at the coincidence gate. As forthe photons, it will be absorbed at one of the twodiodes due to Compton effect, pair effect, andphotoelectron effect. Charged particles generated
from here can be stopped by the copper inlay between
the two diodes. The copper inlay is thick enough tostop this kind of particles. Because of this reason,
1

cuirr@dns.ep.tsinghua.edu.cn
bbshao@dns.ep.tsinghua.edu.cn
3
lyx@ustc.edu.cn
2

Figure 2. Distribution of Beam Loss Detectors
along the HLS ring in NSRL
Because such detectors were originally developedfor HERA ring in DESY, and there are manydifferences between HERA and HLS, there must besome physics problems in the installation and runningof such a system, such as the position of installation,
the detecting efficiency. It is worth studying anddiscussing these problems in such an 800Mev low
energy condition.

2. STRUCTURE OF BEAM LOSSMONITORING SYSTEM
The whole beam loss monitor system uses CAN
BUS to connect all the frontends and console, a PC
machine (figure 3).
The console has a CAN BUS network adapter sothat it can communicate with CAN BUS. We develop
software for BLM system with LabVIEW. It candisplay and print count rates of all the channels, setup
the parameters for the system, and calculate the beam
lifetime. As for the performance, it is enough to use aPC as the console of the system. And also, it is cheap.
As for the frontend, MCUs are used as main
controllers, one chip for one node. Every MCU
controls two detectors. Count rates of the twodetectors are collected by a high-speed counter, which
is designed with FPGA. The count rates are alsotransmitted to PC through CAN BUS. All these
operations are implemented by a MCU –MC68HC11A1.

The PIN photodiodes beam loss detectors wereoriginally developed for DESY HERA rings. There
are many differences between HERA and HLS.
The first one is the energy. HERA has a 920GeV
p-ring, and a 30GeV e-ring. They are all at very highenergies. HLS is at only 800MeV. This will affect thedistribution of shower electrons, induced by lostparticles, on the surface of the vacuum chamber.
The second one is the circumference. HERA ringis 6.3km long, and HLS is only 66.13m long. The
different length induces the different Lattice structure,
the functions, the functions, and also the pathwayof particles in vacuum chamber, which lose energy.
Because of these reasons, there must be many
physics problems related to the installation andrunning of BLM system on HLS.
The First problem is the location of showerelectrons. In another word, where the BLM detector
should be located to detect the shower electrons
induced by lost particles?

Figure 4. [HERA] Path of electrons with anenergy loss of ∆ E E = -1% to –10% though a
FODO structure. X is the horizontal
displacement of the electrons. The pipe wallstarts at x= ± 4 cm.
Figure 3. Structure of Beam Loss Monitor System
The counters have the following performance:•
Two channels, one channel for one detector;•16bits counter with 8bits buffer for eachchannel;•Counting rate can reach 10MHz;•
Each channel can be enabled or disabledseparately;•
Each channel can give interrupt signal;•Interrupt can be enabled or disabledseparately.

3. PHYSICS PROBLEM IN BLMSYSTEM

The deviation of the electron orbit (for thosewhich lose energy due to some reasons, for example,
inelastic scattering on the nuclei of the residual gasmolecules.) from the normal orbit depends on thefunction and the lost energy ∆E . The electrons may
be lost at the next maximum of function. In anotherword, the maximum of function is most sensitive tothose electrons. Figure 4 (Ref.1) is the path calculatedfor HERA e-ring. For that ring, the maximum iswhere the horizontal focusing quadrupoles stand in
the arcs. In such a long ring, the defocusing ability of
dipole is much weaker that that of quadrupoles. The
most possible place of detecting shower electrons isjust behind the horizontal quadrupoles.
For the ring of NSRL, it is very different. Figure 5
is the same calculation based on NSRL ring. This ringis much shorter than HERA e-ring. Trajectories of

electrons will bend 30° at each dipole. Thedefocusing ability of dipoles here is much stronger
than that in lager rings. It is better to detect showerelectrons just behind the dipoles, not behind thehorizontal quadrupoles.

Figure 5. [NSRL] Path of electrons with anenergy loss of ∆ E E = -1% to –10% though aFODO structure. X is the horizontal
displacement of the electrons. The pipe wall
starts at x= ± 4 cm.
Another problem is the efficiency of detecting thebeam loss. For a certain beam loss rate, how many
count rate of shower electrons can be seen in BLM
system?
To solve this problem, the distribution of showerelectrons must be calculated once again for NSRL
ring. It will be different from that of HERA e-ring due
to the much different energy. This is an importantfactor in calculation of the distribution.
Another thing must be mentioned here is about thelifetime of beam.

For high energy machines like HERA e-ring,inelastic scattering with residual gas molecules isusually the dominant factor that determines beamlifetime. It is not the case for low energy rings like
HLS, in which Touschek effect can not be neglected.
Beam loss rate due to Touschek inner-bunch
scattering may be at the same magnitude as that due toinelastic scattering. When inner-bunch scattering
happens, if an electron loses a certain amount of
longitudinal momentum, which implies energy loss inthis case, there must be another electron that gains thesame energy. If the former will hit inside wall of thevacuum chamber and be lost, the latter will reach theoutside wal at almost the same azimuthal location.
This is why we would like to place two detectors at
each position, in the hope that the ratio of outside wallloss to inside wall loss can tell something about the
proportion of Touschek beam loss. Machine physics
studies with different bunch density and RF voltage
will be interesting.
Mentioned above is some aspects of physics
problems. During the installation and running of BLM
system in NSRL, all these problems will be studied
carefully. Some papers about this can be expected in
the near future.

REFERENCE
1.

2.

3.

W.Bialowons,
F.Ridoutt,
K.Wittenburg,“Electron Beam Loss Monitors for HERA”,
DESY
K. Wittenburg, “Reduction of the sensitivity of
the PIN Diode Beam Loss Monitors toSynchrotron Radiation by Use of a Copper
Inlay”
K. Wittenburg, “Beam Loss Detection”