Synchrotron and Storage Ring
1. Brief Overview and Policy
2. Research Activities and Achievements
a) Booster and Beam Transportation
b) Magnet
c) Vacuum
d) RF Acceleration
e) Beam Diagnostics
3. Publication List
1. Brief Overview and Policy
The users of synchrotron radiation have strong interests in its stability, which is determined by the stability of the electron beam stored in SPring-8 storage ring. We take charge of the stable operation of circular accelerators, the storage ring and the SPring-8 booster synchrotron, and run every equipments of the ring smoothly for this purpose. Recently, the top-up beam-operation has been put into action in the user time, and a time of one operation cycle becomes longer. Maintenance of every accelerator elements has become important, and the line of the maintenance has been changed to condition-based maintenance from error-based one, to realize the long-term beam-operation.
2. Research Activities and Achievements
a) Booster and Beam Transportation
We maintain and improve the booster synchrotron and a beam transport line between the booster and the storage ring called SSBT (Synchrotron to Storage ring Beam Transport line).
Booster Synchrotron and SSBT
- Maintenance, improvement and beam studies of the booster synchrotron and SSBT: The booster synchrotron is for accelerating the electron beam of 1GeV from the linac up to 8GeV, and the beam transport line SSBT is for injecting the accelerated electron beam into the storage ring and has a length of about 300 m. There is also a beam dump for operating the booster synchrotron independently of the storage ring. We maintain and improve these accelerator components for providing a beam of high quality and high stability to the storage ring.
Activities and Achievements
- Development and improvement of RF knock-out system: A highly purified single-bunch beam is produced by an RF knock-out (RFKO) system in the booster synchrotron and injected into the storage. The impurity of the order of 10-10, the highest level in the world, is routinely achieved. The top-up operation of the storage ring in a hybrid filling mode which is composed of highly purified single-bunches and a bunch-train of some length has now been realized with the RFKO system. Since this system is essential for top-up operation, it is planned to double it by introducing the same system in the booster.
- Development and improvement of the timing system with high accuracy: The timing system with high accuracy is necessary to inject the beam into an aimed RF bucket of the storage ring and to keep the high purity of a single bunch in a several-bunch filling mode of user operation.
- Development of a non-destructive beam position monitor: Since the long-term stability of an injection beam orbit is required, we are developing the OTR (Optical Transition Radiation) monitor. Some prototype monitors are already installed in SSBT and the beam dump.
- Development of a beam-collimation system: To suppress the injection beam loss in top-up operation we developed a collimation system in SSBT for reducing the horizontal beam size and angular divergence (emittance).
- Orbit stabilization of the injection beam passing through SSBT: To stabilize the injection beam orbit we are investigating a correlation between the orbit drift in SSBT and the stability of relating magnet power supplies.
- Measurement of demagnetization of the undulator magnet at the beam dump line: For effective use of the booster synchrotron except in the beam injection into the storage ring, demagnetization of the magnet of undulators under a high radioactive environment has been investigated. This study is done at the beam dump line to expose the target to high-energy electrons.
- Beam storage operation of the booster synchrotron: Beam storage operation of the booster synchrotron has been performed for beam studies in detail to optimize machine parameters.
- Further details on the activities of beam transportation:
b) Magnet
Main tasks are the maintenance, improvement and beam studies of the storage ring and booster magnet system.
Storage Ring Magnet System
- Maintenance and improvement of the system: The storage ring is composed of 36 normal cells and 4 matching cells including a long straight section. Its circumference is 1436m. The unit cell structure of the storage ring is of the so-called Chasman-Green type and consists of 2 dipole, 10 quadrupole and 7 sextupole magnets (Fig. 1). The parameters of magnets are listed in Table 1. There are four power-supply (PS) rooms (Fig.2). The power supplies of main magnets are in the PS Room-A. In the PS Room-B, PS Room -C and PS Room-D there are power supplies of steering magnets, auxiliary power supplies for quadrupole magnets and power supplies of magnets in the long-straight sections. In addition to these magnets there are septum and bump magnets for beam injection and vertical kicker magnets for beam aborting.
Fig.1: Arrangement of lattice main magnets and other elements in unit cell of the storage ring.
Table 1: Parameters of magnets.
Fig.2: Layout of the four power supply rooms and four pump rooms of cooling water.
Activities and Achievements
- Development and maintenance of control programs of magnet power supplies: The magnet power supplies are controlled by the program so-called EM on the VMEs with the RIO system. The program EM is operated through GUI. The EM and GUI are made and maintained by ourselves. Figure 3 shows an example of GUI for main magnets.
Fig.3: GUI example for main magnets.
- Modification of four long-straight sections: To install a very long undulator and other advanced devices we rearranged magnets in the long-straight sections in 2000 and realized a magnet-free space of about 30m.
- Improvement of DC-septum magnets: To reduce the effect of the leakage magnetic field on the stored beam we designed new septum magnets and replaced.
- Stabilization of power supplies: The stability of the power supply for quadrupole magnets has been improved from 10-4 to 10-6. A high precision current monitoring system with precise DCCT's has been introduced for main magnets.
- Development of steering magnets for orbit correction with high resolution and low hysteresis.
- Development of skew quadrupole magnets for correcting residual vertical dispersion and coupling.
- Improvement of the resolution of steering magnets: The resolution of the steering magnet has been increased from 16 bits to around 21 bits, by using a double DACs control system.
- Addition of the auxiliary power supplies for the quadrupole magnets to perform optics distortion correction.
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Improvement of a power supply of pulsed-septum magnet: A small power supply of high voltage was added to the one of pulsed-septum magnet to stabilize the current at its first firing.
- Development of a method to replace quadrupole and sextupole magnet without removing the vacuum chamber.
- Development of a Hydro Leveling System: We introduced a Hydro Leveling System (HLS) to measure the small shift of the floor level. As shown in Fig. 4, it is found that the floor level around the underpath shows a seasonal change and the maximum amount of the change becomes 2.5µm/day, though it depends on the season.
Fig.4: Measured annual variation of floor level at underpath.
- Survey and alignment: Survey of the magnet position on the horizontal plane is done by the laser tracker. Level survey of the girder is done by the digital level. Straightness check of the quadrupole and sextupole magnets on a girder is done by the laser and CCD camera system. Mechanical check of the straightness is also done.
- Investigation of beam fluctuation sources for beam stability improvement:
Frequency 10~200Hz; This vibration is due to the vacuum chamber.
1~10 Hz; Vibration sources are unknown.
Period ~5 s; Ocean swell causes change of the ring circumference of the order of 10-10.
~45 minutes; Vibration source is temperature change of air from air-conditioners.
12 hours; Earth tide.
1 day; Fluctuation of voltage of 7.7 kV power line.
half month; Vibration sources are unknown
6 months; Seasonal change.
Other identified vibration sources:
rain; Tunnel floor level is elevated by rainfall at some points in the ring.
earthquake
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Ground motion measurements and development of its monitoring system: We set tilt meter, seismometer and strain meter under the 160m below the ground level. We also measured vibration on the tunnel floor in the whole ring. We developed a monitoring system of ground motion for checking it in the control room.
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Measurement of the environment: air temperature, air pressure, water level, rainfall, sun shine and wind (anemoscope)
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Investigation of crack of the floor
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Protection of rubber tubes for cooling water against radiation: Since we experienced radiation damage of rubber tubes for cooling water, we measured radiation level around the tube. We then protected the tube by using lead plates with sufficient thickness.
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Improvement of temperature fluctuation of cooling water: To reduce the effect of temperature fluctuation of cooling water we improved the equipment and achieved the stability better than +0.2 degree.
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Investigation of floor vibration: The chillers for the magnet cooling water are in the four pump rooms in the ring (see Fig.2) and pipes of cooling water are on the tunnel. We measured correlation between the vibration of the floor and the change of the electron orbit. The vibration of the floor is strong near large pumps, and it is planned to improve the pump system to reduce vibration level.
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Further details on the power supplies:
c) Vacuum
We take care of vacuum chambers of the storage ring about 1.4km in circumference and of the booster. Their maintenance, improvement and related beam studies are in our scope.
Vacuum System
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Maintenance and improvement of the vacuum system of the storage ring: Since the vacuum level inside the chamber is directly related to the beam lifetime, it is important to maintain and monitor the ultra high vacuum system. We are making continuous effort for further upgrading the system. We are also improving and developing vacuum components such as photon absorbers and chambers to absorb higher heat load in a possible high-current operation in the future and/or to prevent damage in the low-emittance operation. Suppression of the chamber vibration is another important task for realizing stable beam orbit. Protection of accelerator components against radiation is also in progress.
Activities and Achievements
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Correction of the BA gauge readout: The normalized pressure rise (ΔP/I) for straight chamber is in proportion to D-x with x of about 2/3. However, as shown in Fig. 5, the pressure value for photon absorber is saturated in the range above 1~10 Ahr dose. This phenomenon is well explained by assuming misreading of BA gauge due to the scattered X-ray. After shielding the gauge with Pb against the X-ray, we observed that the pressure value for the absorber shows the expected D-x behavior.
Fig. 5: Normalized pressure vs. radiation dose.
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Development of photon absorber for 200mA operation: Increasing the stored beam current is one of ways to increase the brilliance of synchrotron light. It is then important to develop new absorber for more heat load due to higher current. Figure 6 shows the temperature distribution in the absorber under development corresponding to the heat load by a stored current of 200mA.
Fig. 6: Temperature distribution in the absorber.
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Investigation and cure of damage of injection chamber due to aborted electrons in low emittance operation: The chamber in injection section has very thin wall made of stainless steel to avoid the intervention with septum magnets. The wall location is very near to the beam orbit and aborted electrons hit it in low emittance operation. The localized heat load then made the wall melt down, and the leakage has been occurred. Figure 7 shows the cross sectional picture of the melted down chamber wall. Currently the new chamber with Al dumper has been installed to avoid such damage.
Fig. 7: Cross-sectional picture of the melted down chamber.
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Suppression of chamber vibration due to cooling water for further stability of the beam orbit: Vacuum chamber is vibrating due to flow of cooling water. The chamber vibration makes disturbance of magnetic field at quadrupole magnets due to eddy current and the beam orbit is fluctuated. After suppressing the chamber vibration, the beam orbit has become more stable. Figure 8 shows the comparison of the beam spectra before and after the countermeasure.
Fig. 8: Comparison between the beam spectra before and after the countermeasure.
d) RF Acceleration
We maintain and improve the RF acceleration system and relating equipments.
RF Acceleration System
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Maintenance of RF acceleration system: The RF system is essential to compensate for energy loss of the electron beam caused by synchrotron radiation (sum of 9MeV by bending magnets and the loss by insertion devices), to keep the longitudinal stability, to determine the circumference of the ring, etc. There are four RF stations distributed along the storage ring, and each station has eight accelerating cavities. Maintenance of klystron power supplies, RF low level systems, etc. is regularly carried out.
Activities and Achievements
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Improvement of the timing system which is essential for generating a highly purified single-bunch beam
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High power test of spare cavities and couplers at test stand
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Development of Higher Order Modes free accelerating cavity
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Suppression of coherent synchrotron oscillation caused by RF noises by feedback to the reference frequency
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Further details on the RF system:
http://acc-web.spring8.or.jp/~ohshima/acc/srrf/introduction/
e) Beam Diagnostics
We are in charge of installation, operation and improvement of the beam instrumentation of the rings.
Beam Diagnostics System
In order to operate the light source, monitoring the status of the circulating beam (beam diagnostics) itself is essential as well as that of the status of apparatus such as an RF power-source and magnet. Furthermore, accuracy of beam diagnostics limits the potentiality of the X-ray emitted from the electron beam.
The instrumentation which we take care of observes beam signals by the electro-magnetic interactions between the pickup devices and the circulating electron beams. In the case of beam position measurement, the required resolution is approaching to the region of the thermal noise limited by physics. Nevertheless, the observation of beam behavior by much higher precision reaching to the physics limit, is indispensable for the further understanding of the beam dynamics. To fulfill the requirements, it is necessary to develop the instrumentation based on an innovative idea that removes the obstacles for reaching the limit.
Thus, ceaseless efforts for upgrading the instrumentation for the beam diagnostics are necessary, because improvements of the beam diagnostics greatly contribute to achieve better beam quality of the Storage Ring.
Activities and Achievements
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Fabrication, installation, and operation of the devices for measuring stored beam current: The DC component of the stored beam current is measured with DCCT (Direct Current Current Transformer) in a non-destructive way. The stored current is the most basic and important parameter to the operation of the storage ring. The second DCCT was installed to improve the reliability. Efforts are undertaken to improve the resolution up to 10-5 (1µA / 100 mA).
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Fabrication, installation, and operation of the devices for the tune measurement: The quantity tune is defined by the focusing conditions of the storage ring, one of the most important conditions to determine the stability of the stored beam. The tune measurement system consists of the beam shaker that make a very small disturbance, and the measurement devices to observe the response of the beam to the disturbance. Efforts to develop a system are undertaken to measure very small vibration with good signal to noise ratio for monitoring three normal modes of the beam oscillation (horizontal, vertical, and longitudinal oscillations).
> Further details on the measurement of stored beam current and tune:
Measuring the stored current and the tune
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Fabrication, installation, and operation of the devices for measuring beam positions - Improvement of the beam position monitor system: Beam positions are measured over 250 points along the storage ring. The measured values determine the position stability of the radiated lights. The beam size is a few µm in vertical and a few hundreds µm in horizontal. Precise experiments are utilizing the X-rays which are emitted from the electron beam of the very small size described above. It is very important to control the position and the angle of these emitted X-ray beams. These are determined by the position and angle of the electron beam, and the performance of the beam position monitor system. Through the improvements of the signal processing electronics and the averaging the data, the resolution of the beam position measurement reached to the sub-µm order. However, the time for measurement of the beam positions of whole the ring was over 20 s, and only the slow orbit drift with the time constant longer than several tens seconds could be precisely controlled. To improve the stability of the emitted X-ray beams, a beam position measurement system with much faster and with better resolution is required, and we have developed the new system. The beam positions of the ring can now be measured in 4 s with sub-µm order accuracy.
> Further details on the measurement of beam positions:
Original beam position monitor system
Newly developed beam position monitor system
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Fabrication and operation of the system for controlling the lost beam quantity during the top up injection: From the radiation safety point of view, the amount of lost electrons during the top up injection must be monitored and controlled. In the top up operation, electrons are refilled in short intervals, e.g. 1 min. or 5 min., to keep the stored current constant. The amount of lost electrons are calculated by subtracting the amount of captured electrons in the storage ring from the amount of injected electrons. These values are obtained for each injection continuously during the whole period of the top up operation. An interlock signal will be issued if the amount of lost electrons exceeds a certain limit defined by the radiation safety regulation. Since the system is crucial to abide the radiation safety regulation, very high reliability is required to count the amount of lost electrons. Many efforts are taken to improve the reliability, e.g. improvement of algorithms in the interlock logic, the timing signals, etc.
> Further details on the measurement of lost electrons:
Sytem for controlling the lost beam quantity during the top up injection