3.1 Design Considerations and Characteristics of TPS
3.1 Design considerations and characteristics of TPS
3.1.1 Competition on functions
From first-generation sources of synchrotron radiation, constructed mainly for high-energy
physics and parasitically available to research with synchrotron radiation, synchrotrons have further
progressed to second-generation dedicated sources utilizing dipole magnetic fields, and finally to
third-generation sources that use mainly insertion devices. There has been significant development
of the brightness, stability and reliability of beams. During almost two decades of development and
improvement of third-generation synchrotron light sources, machines are classifiable according to
their energy: low energy – less than 2 GeV, medium energy – 2~4 GeV, and high energy– 6~8 GeV.
From the point of view of scientific research, many advanced projects demand X-rays of high
brightness. A low-energy synchrotron light source can not meet the requirements for an X-ray beam
of high quality. In the 1990s, Europe (ESRF), USA (APS) and Japan (SPring-8) have established
third-generation synchrotron light sources with both high energy and high brightness, but such a
high-energy source of synchrotron light not only is extremely expensive but also involves great
production of excessive thermal loading in the beamlines components. With the major
developments of insertion devices and accelerator techniques, a synchrotron light source at medium
energy but great brightness becomes the preferred radiation source. At the beginning of the
twenty-first century, many new constructions are in progress around the world; other exiting
mid-energy synchrotron light sources are upgrading their facilities to improve from original
specifications. Three new facilities in Australia (Australian Synchrotron), England (DIAMOND)
and France (SOLEIL) have just been commissioned, while the latest source in Spain (ALBA) is
nearing completion.
In 1993, Taiwan completed construction of a 1.3-GeV synchrotron light source, known as the
Taiwan Light Source (TLS), which became the first third-generation synchrotron radiation facility
in Asia. In 1996, the operation energy was increased to 1.5 GeV through ramping in the storage ring.
There are series upgrade projects making the full energy injection from booster possible in 2000.
Since that time, the reliability and stability of the light source has been significantly improved and
many insertion devices have been developed. However, the TLS is limited by its lowest energy
among the world’s third-generation synchrotron light sources. Because of limitations of straight
sections and space constraints in the storage ring, the TLS is losing its ability to respond to the
demands of present and future advanced scientific research.
Other than conventional circular third-generation synchrotron light source, there are also light
sources of two other types: Energy-Recovery Linac (ERL) and X-ray Free-Electron Laser (FEL).
Although each of the latter can provide an instant brightness much greater than a conventional
third-generation synchrotron light source by several orders of magnitude, a conventional
third-generation synchrotron light source provides greater stability and superior technical
development at lower operation and construction cost. Europe, USA and Japan have invested much
effort and money in the development of these ERL and X-FEL, but the practicability of the former
is still distant. Based on the demand for a light source of greater brightness in higher photon energy
from Taiwan’s advanced researchers, the optimum solution is to build a mid-energy third-generation
synchrotron light source.
At present, more than ten mid-energy synchrotron accelerators around the world are either in
operation or under construction. Their technical specifications are being continuously upgraded. To
have a more competitive leading position in the future, TPS must thus have outstanding features.
From considerations of the design of such a light source, TPS is designed to emphasize on electron
beam of small emittance (Fig 3.1.1.1) and great brightness, stability and reliability. The subsystems
of the TPS will rely the most advanced and reliable techniques, such as superconducting devices,
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NSRRC - TPS Design Handbook - January 2009
topping-up injection, ultra-high vacuum, advanced beam monitoring and control, precision
mechanical system, small-ripple power supplies, a low-noise electrical grounding system, etc. The
NSRRC is strongly capable of handling all these techniques and has already implemented many of
these technical advances in the TLS.
Fig. 3.1.1.1 Chart of
emittance and circumference
of light sources.
3.1.2 Emittance of the electron beam and brightness of the photon beam
Based on the existing land available, the circumference of the TPS storage ring is 518 m. The
new lattice of the TPS storage ring has a double-bend achromat (DBA) structure to minimize the
emittance of the electron beam; it is composed of 24 cells with six long-straight sections of 12 m in
length and 18 standard-straight sections of 7m in length that can accommodate several insertion
devices to enhance the brightness of the photon beam.
Besides conventional undulators, the designs include in-vacuum undulators, superconducting
undulators and wigglers to enhance the energy, flux and brightness of the photon beam. According
to TPS specifications, the photon energy that this mid-energy synchrotron accelerator provides can
reach 30 keV; the maximum brightness can achieve 1021 photons/s/0.1%BW /mm2/mr2 at 10 keV.
Compared with current existing high-energy synchrotron light sources, the TPS can provide ten
times the brightness and become one of the brightest synchrotron light sources in the world.
3.1.3 Stability of the electron beam
After the magnetic lattice has been decided, the design of each accelerator subsystem must be
evaluated carefully to maintain the small emittance of the electron beam and high brightness of the
photon beam. Moreover, the main goal is to ensure high stability in each subsystem. Figure 3.1.3.1
indicates possible factors, and their frequency ranges, that can affect beam stability. According to
this figure, the considering factors are due to ground settlement, effects of heat, mechanical
vibration and beam instability effects. This section describes briefly the stability concerns related to
ground and building, thermal deformation, mechanical vibration, electric power and grounding
system, the suppression of an excitation source on the electron-beam fluctuation, and a system for
beam monitoring and feedback. The first four items pertain mostly to infrastructure, whereas the
latter two are essential systems for an accelerator.
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3.1 Design Considerations and Characteristics of TPS
Fig. 3.1.3.1 Factors
contributing to the beam
stability
3.1.3.1 Building groundwork and its structural stability
The quality of the ground and the structure of a building can affect the stability of all devices
in the accelerator. The motion and settlement of the ground, structural resonance and vibration
propagation must be treated carefully. To diminish the relative movement of the accelerator
components due to external excitations, the main body of the TPS will be built on a monolithic
concrete base, which will use an expansion-joint method to isolate the groundwork of adjacent
buildings. This method has been proven to decrease the transfer of vibrations and deformation.
Because thermal effects due to solar exposure might cause movement of devices in the accelerator,
the external wall and ceiling of the building should include layers of thermal insulation to diminish
the thermal effects on the building structure and on the electron beam.
3.1.3.2 Thermal deformation
Thermal deformation is a principal factor affecting the stability of an electron beam. In this
regard, the first concern is the prevention of structural thermal deformation, then the control of
thermal stability of thermal sources. For the former, the material selection and structural design of
major devices in the accelerator and their supporting designs (including supports for magnets, beam
position monitor etc.) must be considered to minimize the influence of thermal deformation.
Choosing a material with a small coefficient of thermal expansion, using isolating materials and
increasing the heat capacity of the material are possible solutions to suppress thermal deformation.
Three aspects of control of thermal sources are the water temperature, air-conditioning, and
dissipation of heat from electric power. Most devices in the accelerator use cooling water to remove
thermal energy. A change in the temperature of the cooling water causes a corresponding variation
of the dimensions and positions of devices in the accelerator; this effect causes instability and
movement of the electron-beam flux. There is an analogous effect due to variations of ambient
temperature. The TPS design will devote much attention to these issues. The design target of
cooling water and ambient air is the control within a range of 0.1°C. Because dissipation of thermal
energy into the air is difficult to control, most devices will use cooling water to remove their heat.
If the voltage of the electrical line power or the operating conditions in the accelerator (such as
changes of the current in the magnets or changes of electron beam strength) varies, they cause a
temperature variation in devices. The most strident examples are the variations in strength of
electromagnets and the decay of the intensity of the electron beam with time, when the electron
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NSRRC - TPS Design Handbook - January 2009
beam is injected. Because these two issues are disadvantageous to the accelerator’s stability, TPS
will use a full energy injector and top-up injection to diminish the influence of the above issues. As
the top-up mode can maintain the stability of the electron beam and the beam strength in the
synchrotron accelerator, it also contributes to maintain the stability of optical devices in the photon
beam lines under extreme photon beam power loads.
3.1.3.3 Mechanical vibration
Vibrations of components, especially magnets, can cause greatly magnified variation of the
beam position. For an advanced synchrotron light source like the TPS, the variation of the electron
beam position must be controlled to within 0.2 μm; the vibration of devices in the accelerator
should thus be decreased to several tens of nanometers. Because the instability of supports of
devices can magnify this issue, the transfer of vibrations from the ground must be seriously
restricted. To resolve or to diminish the influence on mechanical vibrations, three aspects must be
considered: increasing the resonance frequency of the mechanical structure, using damping
materials, and isolating vibrating sources. To achieve several tens of nanometers stability or less, the
engineering details must be evaluated and reviewed carefully. Subsystems, such as magnets,
vacuum systems and electron-beam position monitors of TPS, will use independent supporting
mechanisms to avoid coupled interference. Among these, the deformation and vibration of the
vacuum chambers must be separate from magnets. Furthermore, the pipelines for water cooling and
air-conditioning are the main media for vibration transfer; careful solution involves methods of
vibration damping. For the surrounding experimental equipment, such as vacuum pumps and
motors, in TPS, the issues of the management will also be considered
3.1.3.4 Stability of electrical power and grounding system
According to strict requirements for a synchrotron light source, although the DC current in a
magnet has fine regulation, the voltage variation in the AC electrical system still influence the
stabilities of power supplies for magnets and many other devices in an accelerator. Besides a
superior standard of the stability of magnet power supplies in the TPS, sensitive devices will use
harmonic filters and precise AC voltage regulators, which provide satisfactory AC quality and
improve the output stability of power supplies. Moreover, for main sub-systems or others that can
readily cause enhanced noise, there will be independent electrical feeders to prevent interference.
As for the electrical grounding system, a grounding network of small resistance (< 0.2 Ω) will
be installed and the grounding current will be evaluated carefully for power supplies, major devices
and supporting structures in the accelerator. Current guiding techniques – isolation or conduction –
will be used when necessary. This practice will prevent ground current from moving everywhere
and causing noise interference. With regard to issues of electro-magnetic noise from electric power
and grounding system, special shielding covers will be installed for interfering sources, cables, and
possible devices that can cause interference. In addition, there will be special standards and
regulations for each connection’s impedance in both electric power and the grounding system, to
ensure that electric currents flows smoothly and predictably.
3.1.3.5 Suppression of sources of instability of the electron beam
In the storage ring, issues that perturb the stability of the electron beam are broad. In the TPS,
there are three major topics involved in treating instabilities: enhancing the instability-related
monitoring, signal processing and feedback systems; enhancing theoretical study and developing
related application software; and enhancing the design and construction of the subsystems, such as
vacuum and radio-frequency systems, which are potential sources closely related to instability.
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3.1 Design Considerations and Characteristics of TPS
The TPS storage ring will use superconducting RF cavities to provide energy to the electron
beam. The high-order modes (HOM) generated from a superconducting RF cavity are well damped
comparing to a conventional RF cavity. The design of vacuum systems should avoid discontinuous
or abrupt changes in cross sections in the vacuum chambers that cause cavities’ effects and excited
instability of the electron beam when it passes discontinuous cross sections. The vacuum system
will maintain the ultimate performance to increase the lifetime of the electron beam, which also
helps in decreasing the possibility of exciting the effects of ions trapping or fast ion, and diminishes
the interaction of the electron beam and ions in the beam duct.
3.1.3.6 Beam monitor and feedback system
To increase the stability of the electron beam, feedback systems are needed to compensate for
variations under diverse operational and environmental conditions. The main function for the beam
monitor and feedback system is to measure/feedback the beam position and beam profiles of the
electron and photon beams. The data will be collected and analyzed, and active components will
then be driven to tune the electron beam orbit and beam size to achieve control within steady
values.
The diagnostic and feedback system of the TPS will include precise beam monitors, reliable
electrical feedback circuits and a high-performance feedback algorithm. The monitors include
electron-beam position monitors, beam-profile monitors, photon beam-position monitors etc. In
active components, they consist of an excitation electrode, correction magnet, fast kicker etc.
Feedback systems will use a rapid feedback (loop bandwidth ~100 Hz) mechanism, which is an
important key in TPS. The variations of the electron-beam orbit and beam size must be at
sub-micrometer level even during the dynamic tuning of insertion devices. Strict requirements of
the beam monitoring and feedback system will cause the TPS to achieve the most advanced and
reliable design. Methods for photon-beam monitoring and feedback will be established to improve
the stability of the photon beam.
3.1.4 Operational reliability of the accelerator
The operational reliability of the TPS can be considered according to three main aspects:
• reliability of infrastructure, such as electricity, water and cryogenic systems;
• reliability of sub-systems in the accelerator;
• reliability of signal processing and alarm system.
To ensure reliability at the highest level, all systems will be designed to be either maintenance free
or easily and rapidly maintained. For related major equipment, not only a high-standard
maintenance procedure but also redundant design will be considered to increase the flexibility of
maintenance and to diminish the possibility of failure of the accelerator.
Utilities provided to the TPS are water, electricity, gas, air conditioning, and liquid nitrogen
and helium in the cryogenics system. To facilities like a synchrotron accelerator, these are daily
essentials and must be supplied with high stability. To diminish corrosion and other issues from
cooling water, the quality of water must be controlled, through control of pH, content of oxygen and
impurities etc. In accelerator sub-systems, there are reliable detection and independent, interlock
safe systems to protect each sub-system. Besides, the reliability of the injection system – Linac,
booster ring, transport line, impulse-magnet system etc. – will be highly reinforced. While the
injection system is in operation, it not only can generates pulsing interference to other systems but
also sometimes influences the performance and reliability of devices, when these high-power
pulsing devices are operating continually. Furthermore, establishing a complete system of signals
and alarms assists to improve the overall stability of an accelerator. There will hence be a method of
analysis of parameters in the accelerator system to indicate malfunctioning devices and abnormal
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NSRRC - TPS Design Handbook - January 2009
states. To improve the operating stability of the accelerator, for signal transfer, attention will be
devoted to prevention of noise, such as in methods of grounding connection and of shielding of
electromagnetic waves.
Reliability concerns should also should include consideration of radiation safety, especially as
the TPS will use a top-up injection method. There is a close relation between the efficiency of
transfer and injection, and radiation dosage; TPS will use a high standard to request the
improvement of injection efficiency and the reliability of the injection system. About the system for
personal safety, an interlock system will be used with a ‘fail-safe’ logic; it will use also a
‘hard-wired’ method to cope with signals to prevent any possibility of fault action.
3.1.5 Controllability of the accelerator
When an accelerator is at the stage of commissioning or in normal operation, the parameters of
the equipment should be adjusted to cooperate with alterations that have been made in some
conditions. This approach helps to attain an optimum solution. To adjust the operating conditions
easily and to improve the efficiency of commissioning of an accelerator, many data-acquisition
systems have been developed and many application programs available from the world’s accelerator
facilities. Coupled with the operation of the hardware system, it can improve the accelerator’s
controllability.
The accelerator’s controllability is classifiable into three parts:
• real-time recording of operating data from each equipment and device,
• analysis of the operating data and post-process command output,
• automatic sequential control on related programs.
The TPS design will first incorporate a complete data-acquisition system, and use or refer to
the current application software used at the TLS or other advanced accelerator facilities around the
world.. The software can analyze and process the sequence of command output, and build a routine
operating program. Building a complicated expert system is an enduring objective.
The design of TPS controllability will assist each sub-system in indicating and analyzing
related data efficiently; it also provides for online inquiry from the internet. Moreover, for analyzing
each subsystem and indicating related data easily, this part will cooperate with subsystems to
develop a platform for individual application software. It can also develop a related and cooperative
hardware system.
3.1.6 Technical capability of NSRRC
To the improvement in the accelerator, the staff of NSRRC has been accumulating much
technical capability from early design stages for construction, installation, commissioning,
operation and maintenance of the TLS. These techniques include beam dynamics, electromagnetic
devices, superconducting insertion devices, ultra-high vacuum systems, instrumentation and control
systems, magnet power-supply systems, radio-frequency systems, mechanical positioning systems,
cryogenic systems for liquid helium, utility systems, and radiation-safety system. These design and
constructions have been developed successfully. TLS attains several important targets that will be of
great help in building the new light source with high brightness.
3.1.6.1 Photon beam flux and brightness
Since 1993, the NSRRC has been aggressively developing insertion devices. In addition to the
commercial devices, a U10 undulator and the EPU5.6 elliptically polarized undulator were designed
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3.1 Design Considerations and Characteristics of TPS
and constructed in-house. The first SW6 superconducting wiggler was constructed in cooperation
with a foreign company. These insertion devices improve the flux of the light source, extend photon
energy and its brightness. In the vacuum-ultraviolet range, the brightness has been increased by
about a factor of 10,000; in the X-ray range, the brightness is increased about 1000 times. At the
same time, NSRRC also developed several techniques to cope with the operation of insertion
devices, such as systems for instrument control, design of vacuum chambers, beam tuning etc. This
valuable experience is an important key factor for successful design and construction of undulators
for the TPS.
With regard to the improvement in photon flux, energy span and brightness, superconducting
technologies are worthy of mention. These include a superconducting RF cavity, superconducting
insertion devices, and a cryogenics system for liquid helium. NSRRC has cooperated with Cornell
University and foreign industry to establish the technologies in the superconducting cavity to
improve the beam stability and increasing the photon flux of the storage ring. As to superconducting
magnets, a superconducting (SC) wavelength shifter and a SC superconducting wiggler have been
developed in succession; there are three SC wigglers have been installed in the TLS and function in
satisfactory condition. A cryogenic system to supply liquid helium with the largest capacity in South
Asia has been built successfully in Taiwan; it supplies the liquid Helium to the superconducting RF
cavity and superconducting magnets. These superconducting technologies have been successfully
established domestically, and are key factors in building a high-brightness TPS.
3.1.6.2 Stability of the electron beam
Building and mechanical structure
The entire TLS storage ring is located on one 90cm thick concrete base. To minimize vibration
transfer, expansion-joints are used to connect these concrete base plates to each other. For any
vibration external to TLS, all devices in the storage ring maintain constant relative positions, so as
to decrease the impact on operation of the electron beam. After a severe earthquake measuring 7.2
on the Richter magnitude scale on 21 September 1999 in central Taiwan, only few devices suffered
slight displacements, without influencing normal operation of the entire accelerator. This incident
proves the reliability of the design of such a concrete base. Wear to one underpass, designed for
cables in the storage ring, the structure is weaker; Measurements indicated that the vibrational was
three times as great as in other locations; this phenomenon is a valuable reference in designing and
building the TPS.
About the magnet support, TLS has developed a method to increase the resonance frequency
and to diminish the vibrational magnification. A measurement analysis has clarified the method of
vibrational transport from cooling-water pipelines; this information will help to select an
appropriate method to decrease future vibrations from of cooling water pipelines. In the early stage
of TLS operation, the vibration from the air-handling unit had a great influence on the electron
beam. After improving the bearings in the air conditioner and using shock-absorber springs with an
appropriate damping coefficient, the vibrational magnitude decreased to a tenth its former value.
This experience likewise provides a valuable reference for future designs in the TPS.
Control of the electron beam orbit
The TLS utilizes many ways to improve the stability of the electron-beam orbit, including
improvements in temperature variations of water and ambient air. The systems for water supply, air
conditioning and control have been improved. The temperature variation has been improved locally
from 1 °C to less than 0.1 °C. For the magnet girder, material for thermal insulation diminishes the
influence of variations of ambient temperature.
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NSRRC - TPS Design Handbook - January 2009
Furthermore, TLS has improved the supply of electric power to magnets to diminish output
noise, and uses a global orbit-feedback system and a digital electron-beam position monitor system
to improve control of the electron-beam orbit. As a result, orbit fluctuation decreased significantly:
fluctuation is less than 1 μm rms short time, and ~5 μm for a long period of 8 h. Many photon-beam
position monitors have been recently installed; their resolution can be less than 0.3 μm. This
condition is useful for steady operation of the electron beam. Figure 3.1.6.1 shows temperature
fluctuation and electron orbit stability as well as the duration of storage failure in the past few years;
it shows that stability is increased about five times in the past few years.
Fig. 3.1.6.1 Temperature
fluctuation and electron orbit
stability, the duration of
storage failure in the TLS.
Control of the size of the electron beam
Besides the electron orbit, the stability of the size of the electron beam is an important factor
for accelerator performance and also for the users. TLS utilized skew quadrupoles to reduce the
coupling effect significantly. Collective effects, such as from ions, and longitudinal instabilities that
affect the beam size, were investigated. Beyond the partial-filling method to solve ion instabilities,
the TLS has developed several schemes before the implementation of SC RF cavity: e.g.: a precise
control of temperature of the RF cavity, of RF voltage modulation, of a dual RF tuner, of a
longitudinal-feedback system, and of a superconducting cavity.
A beam-size monitor (BSM) by an optical interference method was developed at TLS. The
BSM measurement showed that the size of the electron beam is < 30 μm vertical and 150 μm
horizontal. To monitor beam stability, including the combined effect of electron-beam orbit and
beam size, TLS has built three diagnostic beam lines that utilize focusing lenses and pinholes with
de-trend algorisms to index the variation of beam intensity. Figure 3.1.6.2 shows measurement
results from one beam line. After decreasing thermal variations of the pinholes’ monitoring system
and the noise from electric-power supplies and electron beam-position monitor (E-BPM), the
measured stability of the light beam reaches about 0.06 % of its size. Hence the variation of the
electron-beam size can be controlled to under 0.5 μm, which is consistent with the same as the BSM
measuring results. These data show that the TLS photon beam stability has attained an optimal level
in a synchrotron light source.
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3.1 Design Considerations and Characteristics of TPS
Fig. 3.1.6.2 Measurement of
the electron beam of a selected
beamline over the years in the
TLS
3.1.6.3 Reliability of the accelerator
From the experience in the past few years, utilities are important factors that influence the
reliability of the TLS accelerator. Water and electric-power systems are the most important: they
have significant influence on the reliability of subsystems in the accelerator. The variation of water
temperature has been improved from ±1 °C at the initial stage of operation to under ±0.1 °C. The
pH value was controlled to 7±1.5 at the initial stage and now to 7±0.5. The oxygen content in the
water is improved from no regulation to less than 10 ppb. The control of water pressure, flow rate
and resistance is improved substantially. At the initial operating stage, unstable parameters in the
water system, which frequently activated the safety interlock system of other subsystems and
caused the accelerator to trip, have been obviously improved, as shown in Figure 3.1.6.3 (a).
Furthermore, although the numbers of cessations were slightly excessive at the early stage due to
unstable electricity system, after improving these systems, the number of trip has decreased
substantially, as shown in Figure 3.1.6.3 (b). These improvements include establishing a new
grounding network to decrease the grounding resistance to <0.2 Ω in the area of the storage ring. To
avoid interference noise, there is an independent power-supply system for each individual facility
and instrument. Systems to monitor the quality of electricity and grounding are installed in many
locations.
(a)
(b)
Fig. 3.1.6.3 Numbers of accelerator trip reduced due to the stability improvement of (a) water
system (b) electricity system.
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NSRRC - TPS Design Handbook - January 2009
Besides utilities, the reliability of other accelerator subsystems is of great importance. The TLS
considered not only reliability issues for all subsystems in the accelerator at the design and
construction stage, but most designs and construction were made domestically, yielding a
time-efficient advantage for maintenance. As data archive and display systems are currently applied
to most subsystems, a real-time assessment whether the operation is in a normal state is practicable.
Preventive maintenance with the statistics of mean-time-between- failure is arranged in advance to
replace components before it fails. In addition, there are routine test items for all systems to ensure
best performance for the total accelerator system. After improvements in the TLS over several years,
more than 5000 of beam time hours per year are provided to users, the availability is better than 97
% of the scheduled user beam time. The average frequency for unexpected beam failure is about
one per week. These data indicate that the operating reliability in the TLS has reached beyond
global standard.
3.1.6.4 Controllability of the accelerator
Most equipment and devices in the TLS are connected to real-time data-acquisition systems.
Most operating data, for magnets, electric power, mechanical vibration, electron-beam parameters,
vacuum system, water system, air temperature in the storage ring, RF system and cryogenic system,
can be found in real time with data acquisition and application programs via the internet. With
regard to on-line control and analysis of operating data and post-process command output, such as
software for dynamic tuning of insertion devices, orbit correction, and operations of all feedback
systems, TLS has established a computer network for parameter tuning since the initial stages of
operation. As for integration of the batch control programs, these programs are integrated from the
electron gun, booster ring, transport line to the storage ring to help the operation in control room.
An automatic injection and control program has been developed to fulfill the beam current
requirements for top-up injection.
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