Safety Considerations For The Top-up Operation Of An 8 GeV Class Synchrotron Radiation Facility

Transcription

1 Safety Considerations For The Top-up Operation Of An 8 GeV Class Synchrotron Radiation Facility Yoshihiro Asano 1, and Tetsuya Takagi 2 1 Synchrotron Radiation Research Center. Japan Atomic Energy Research Institute, SPring-8, Mikazuki, Hyogo, , Japan. asano@spring8.or.jp 2 Safety Office, Japan Synchrotron Radiation Research Institute, SPring-8, Mikazuki, Hyogo, , Japan Abstract. The safety systems are discussed for the electron beam injection to top up the stored beam current under the operation of the synchrotron radiation beamlines of an 8 GeV class synchrotron radiation facility, SPring-8. In order to carry out the top up operation safely, the two policies were made. One is that the electron beam never invades into the beamlines during the operation. Another is that the dose outside the shield tunnel due to the beam loss never exceeds the limitations during the operation including the top up. To assure the two, the guidelines of the accelerator operation were decided and the interlock systems were reconstructed. Under these conditions, the safety analyses were performed and the top up operation was fulfilled without any trouble during the past half year. 1. Introduction In recent years, the third generation synchrotron radiation facilities have been intended to reduce the stored electron beam emittance and increase the number of the electrons per one bunch to obtain the extremely high brilliance photon beam. The life time of the stored electrons shortens more and more in the reasons. The electrons are, therefore, injected into the storage ring in the short interval to supplement the decreased electrons during synchrotron radiation experiments. The so called top up operation is also effective from the view point of the prevention of the change of the heat load to the optical elements so that the operation increases in importance for the third generation facilities, and many efforts have been spent to get the high quality performance such as the beam stability during the injection[1,2]. The safety shutter, so called main beam shutter, of the synchrotron radiation beamline is opened during the top-up operation, which is generally closed during the beam injection to prevent the invasion of the high energy photons and neutrons due to the stored electron beam loss to the beamline. Besides, most of the gaps of the insertion devices have been narrowed to contribute the incrimination of the electron beam loss during the injection. The safety conditions such as the beam loss scenario and the radiation shielding under the top up operation are, therefore, widely different from the conditions of the normal operation. Many facilities such as Advanced Photon Source in USA[3], European Synchrotron Radiation Facility in France[4], and ELLETRA in Italy[5], are now under top up operation or testing. However, there are no common methods of safety analyses for the top up operation under the circumstances, and it is important to discuss the safety analyses for the operation. In addition to the safety analyses of the normal operation, we must consider the two requirements of the radiation safety to fulfill the top up operation of SPring-8. Based on the two fundamental requirements, the safety systems of SPring-8 for the top up operation consist of the methods of the machine operation and associated safety analyses, safety interlock and monitering systems to gurantee and confirm the requirements. 2. An 8 GeV class synchrotron radiation facility, SPring-8 The SPring-8, which is currently the facility with the highest stored electron energy of 8GeV and very low beam emmittance, is composed of an electron linear accelerator (linac), a booster synchrotron injector and a storage ring, as illustrated in Fig.1. The linac is about 140 m long and accelerates electrons up to about 1GeV. The booster synchrotron with a circumference of 396 m accelerates the 1

2 electrons injected from the linac up to 8GeV. The electrons are then running up and injected into the storage ring, which is about 9 m higher altitude than that of the booster and capable of storing circulating currents of 8GeV up to 100mA. The storage ring with a circumference of 1496 m has 44 straight sections, 38 of which are available for insertion devices. Thirty-four sections are standard ones of 19 m long and other 4 sections are the long ones of 40 m long. Further, 23 beamlines can be installed from bending magnets. The electrons emit synchrotron radiation while they are being deflected in the field of the ring bending magnets or insertion devices placed in the straight sections. The emitted synchrotron radiation is introduced to an experimental hall by a beamline through a ratchet shaped bulk shielding wall of the storage ring. The beamline consists of the front end line within the shield tunnel and the beam transport line in the experimental hall. As shown in Fig.1, a safety related typical structure of the synchrotron radiation beamlines of the SPring-8 consists of a main beam shutter (MBS), a down stream shutter (DSS), a gamma stop, a beam stop, an optics hutch for a white beam and an experimental hutch for a monochromatic beam. A monochromator installed within the optics hutch is regarded as a scatterer of the synchrotron radiation for the shielding design purpose. The main beam shutter (MBS) is installed inside the shielding wall of the storage ring to control the synchrotron radiation beam to be transmitted into the beamline and the down stream shutter (DSS) is to control the monochromatic synchrotron radiation beam to be transmitted into the experimental hutch. The gamma stop made of lead is to prevent gas bremsstrahlung from expanding to downstream of the beamline, while the beam stop, usually installed at the most downstream of the beamline, is to prevent the monochromatic synchrotron radiation beam from leaking out the experimental hutch wall. The walls of both the optics and experimental hutches are designed to shield against scattered synchrotron radiation[6]. Beamlines Experimental Hutch Optics Hutch Shield Wall Monochromator Synchrotron Booster 8GeV Storage Ring ID Shield Wall MBS SR DSS Monochromator Beam Stop Experimental Hutch Linac SR MBS Shield Wall Optics Hutch GammaStop Beamline (BL01B1) Electron Gun Fig.11. Illustration of the accelerator system and synchrotron radiation beamlines of SPring-8. ID means insertion devices such as undulators or wigglers. SR indicates the synchrotron radiation beam. MBS and DSS are the main beam shutter and the down stream shutter of the synchrotron radiation beamlines. 3. Safety considerations for top up operation 3.1. Fundamental requirements 2

3 There are no differences between the radiation safety requirements during the top up operation and that of the normal operation at SPring-8. Based on the ALARA principle, both the top up and the normal modes must be operated under the same radiation dose limits that are 1.3µSv/h at the boundary of the controlled area, and 50µSv/y at the nearest access to the general public. The doses outside the shield wall or the hutch never exceed the limit of the radiation workers, and that is 1000µSv/w. Since the injection efficiency is low and the electron beam loss is high during the injection, all the main beam shutters (MBS) are closed during the electron beam injection in normal operation. Besides, the electrons which exit in precarious balance during the injection are impossible to creep into the beamlines. And both the shielding designs of storage ring and the beamlines were performed under the normal operation conditions. In the case of the assumed failure for top up operation, the doses outside the hutch due to invaded electrons with 8 GeV were estimated. The gamma stop made of lead is installed on the synchrotron radiation beam axis in all beamlines at SPring-8 so that the invaded electrons cannot fail to hit the gamma stop with the size of 30cm square and 30cm in thickness. Fig.2 shows the calculation configuration of the dose estimation outside the hutch due to invaded electrons. In this case, lead was assumed for hutch wall with 10mm in thickness, and the distance from the beam axis to the gamma stop is 1m. As shown in Fig.1, the optical element such as monochromator is normally installed in front of the gamma stop so that we estimated the dose with and without the copper scatterer in the assumption of the optical elements by using the convenient code, SHIELD11[7], and the Monte Carlo code, EGS4[8]. Fig.2 Calculation configuration of the leakage dose outside the hutch due to invaded 8 GeV electrons. The results of the calculations are shown in Fig.3 with and without the copper scatterer. In this figure, the horizontal axis shows the distance from the nearest point outside the hutch from the gamma stop as indicated 0 in Fig.2. As shown in Fig.3, the results of the EGS4 calculations indicate that the dose outside the hutch due to gamma ray is growing up with increasing the thickness of the scatterer. In comparison with SHIELD11 calculations, the doses due to the gamma ray without scatterer reasonably agree with the EGS4 calculations. The dose due to photoneutrons that are produced by photo-nuclear reaction of electromagnetic shower is dominant in the case of without scatterer, and the dose due to gamma ray is almost compatible with the dose due to photoneutrons in case of with the scatterer of 5cm. It means that the maximum dose outside the hutch due to the invaded one electron can be estimated to approximately 10-8 Sv, and it is correspondent to 100 Sv if about the 0.3 % of stored electrons inject and invade under top up operation conditions. It is hazardous so that the arrangements to avoid the situation must be required physically. In addition to the main beam shutters (MBS) open during the injection, the top up operation has mainly two characteristics in comparison with normal operation at SPring-8. One is that the high injection efficiency will be required to avoid the disturbance of the synchrotron radiation beam as much as possible, and another is that the electron beam loss will be occurred mainly at the insertion 3

4 devices with narrow gap. Therefore, the injected electrons will be lost at a specific point. The scenario of the electron beam loss must be developed under the two characteristics and the maximum stored beam current of 100 ma. Dose (microsv/8gev electron) 10 8 Pb 30cm(total) Pb 30cm(neutron) 10 9 Pb 30cm(EGS4) Cu5cm+Pb30cm(EGS4) Pb 30cm(gamma) Cu1.0cm+Pb30cm(EGS4) Distance(mm) Fig.3 Dose outside the hutch due to the 8GeV electrons into the beamline. The solid line indicates the total dose and the dotted line and dashed line are the dose due to photoneutron and gamma ray without scatterer by using SHIELD11, respectively. Black circles indicate the doses due to gamma ray without copper scatterer by using EGS4, and green and red circles are the doses due to gamma ray with the copper scatterer of 1.0 and 5.0 cm in thickness by using EGS4, respectively Machine operation and associated safety analyses On the machine design, there are no possibilities that the stored electrons can invade into beamlines. According to the beam dynamics analysis, the condition, which the electrons can creep into beamlines, is that the magnetic field strength of bending magnets mismatches with the electron energy. Furthermore, it is the conditions when the field strength falls down to less than half of the fixed strength [9]. Only the mismatched injection beam, therefore, can exist under such conditions. As illustrated in Fig.1, the BL01B1 beamline of SPring-8 is located at the continuation of the injection line so that we must pay attention on the beamline. In order to conduct the permanent countermeasure, the power supply of the bending magnet at the transport line between the booster and the storage ring was connected directly to that of the storage ring. The transport line is placed under the ground with sufficient shielding. By this countermeasure, the mismatch never occurs physically, and all the electrons abort at the transport line when the extraction beam energy from the booster mismatches the injection energy of the storage ring. The scenario of the beam loss consists of three parts for normal operation. One is for the tuning, one is for the injection and the other is for the user time [10]. Based on the normal operation, the top up is operated under the distinctive confinements such as high injection efficiency and localized beam loss. The scraper of the electron beam was installed into the transport line by the accelerator group to obtain the high injection efficiency during the top up operation. After that, more than 80% of the injection efficiency was secured, and we agreed that 80% was employed for the injection efficiency and the rest was assumed to be lost at any one point of the storage ring, conservatively. The potential amount of the injection is limited to be always less than the stored current of 100mA. The injection efficiency is always monitored by subtracting the outputs of the DC current transformers (DCCT) of the storage ring from the beam charge monitor (BCM) at the transport line. The leakage dose estimation outside the shield wall was performed under these conditions. We assumed conservatively that the stored electron beam current of 100 ma ( stored electrons) is lost linearly within 10 hours. The lost electrons are compensated by top up operation so that the 4

5 maximum amount of the injection by the top up operation is electrons during the working time of 34.1 hours a week. The rest of 40 hours a week is spent for the tuning and injection operation in the worst case. The beam loss rate due to top up operation is s -1 at any point during the user time. The dose estimation due to the stored electron beam loss including the top up operation with 80% injection efficiency is listed up in Tables1and 2 for the representative examples. In Table1, the forward means the dose outside the ratchet shield wall from which the synchrotron radiation beam will be lead, and the lateral is the dose nearest outside the bulk shield wall. For the beamlines in Table 2, we must consider the additional dose due to gas bremsstrahlung and synchrotron radiation. And, the dose due to gas bremsstrahlung is dominant for the insertion device beamlines such as BL45XU. In any case, the total leakage doses never exceed the limits. Consequently, we found that the leakage doses outside the shield wall of storage ring or optics hutch of the beamlines never exceed the limits under the injection efficiency of 80% during the top up operation without the improvement of the shield. Table 1 Leakage dose estimation outside the shield wall of the storage ring including the top up operation for the worst case by using modified Jenkins formula [11] Beam loss Shield Distance Average Beam Dose rate ( Sv/h) point (cm) (m) loss rate(s -1 ) neutron muon total Injection 60(Pb)+ (forward) 40(O.C) < (lateral) 165(O.C) Non-injection (forward) 60(Pb)+ 20(O.C) < < (lateral) 95(O.C) Table 2 Leakage dose estimation due to the stored electron beam loss outside the back wall of the optics hutch during the top up operation. Area Shield Distance Average Beam Dose rate ( Sv/h) (cm) (m) loss rate(s -1 ) neutron Muon total BL01B1 (MBS open) 35.2(Pb) < < BL45XU (MBS open) 32(Pb) < Safety interlock and radiation monitoring systems The safety interlock system for normal operation consists of the systems of individual beamlines and the accelerators, and these are closed by itself from each other so that the operations and safety are fundamentally ensured by itself [11]. In addition to the system, the beam loss integrated system, which is observed the differentials between the beam charge monitor (BCM) of the extraction from the booster and the DC current transformers (DCCT) of the stored electrons was installed for top up operation. As illustrated in Fig.4, the process of the interlock for top up operation is as follows. (1) The request of the top up operation is sent to the radiation safety interlock system first. The status of the main beam shutters (MBS) are checked and the interlock system is turned to top up operation to be allowed to inject beam without closing the main beam shutters. (2) The status of the permission of the top up operation is connected with the accelerator control system, and the beam loss integrator is then started. (3) The status of the top up operation is connected to the safety interlock system and the beam loss integrator monitors the injection efficiency during top up operation. If the injection efficiency is less than the reference that is decided beforehand, the signal is sent to the electron gun to shutdown immediately. The average injection efficiency is checked every 8 hours, and if the cumulative beam loss of every one week is in excess of the preset value, the permission of the top up operation is cancelled. 5

6 The radiation monitoring system of top up operation is fundamentally operated without changing systems for the normal operation [11]. The ion chamber for (X) ray and the helium-3 neutron counter are located at the injection area. The monitors have the function of raising an alarm, and these are connected to the interlock system to shutdown the operation. Some radiation monitors were installed temporally around the shield wall of storage ring in the first stage of the top up operation. Furthermore, glass dosimeters are attached on the place of the possibilities, in which the doses rise, to measure the cumulative dose of every one week. Electron Gun Linac (1GeV) Synchrotron Booster (8GeV) Radiation Monitors BCM StoragRing (8GeV) DCCT Operation Control (Accelerator) Beam Off Beam Loss Integrator for Injection Top up Operation Radiation Safety Interlock MBS (Beamline) Top up Operation (accelerator) Fig.4 Conceptual flow diagram of the interlock system for the top up operation at SPring-8 4. Summary Based on the two fundamental requirements for the top up operation of the 8 GeV class synchrotron radiation facility, which are the operation without the injected electron creeping into the beamlines and without the leakage doses outside the shield wall due to the beam loss exceeding the limitations, the safety systems were discussed and constructed. In order to satisfy the requirements, we devised two countermeasures, which are the direct connection of the power supply of the bending magnets of the transport line with that of the storage ring to avoid the electrons creeping into beamlines physically, and the assurance of the high injection efficiency of more than 80% for top up operation. By keeping the high injection efficiency, we found that the leakage dose outside the shield wall or hutch never exceed the limits, even if the rest of the injected electron was lost at any one point. To assure the high injection efficiency, the interlock system was reconstructed and the beam loss integrator was installed. The radiation monitors were set and connected to interlock system to confirm the dose level and provide against any emergency. The radiation survey was performed before the routine operation of the top up. As the results, the anticipated dose rates outside the shield wall and hutch were detected at several points, and these are almost neutron dose rate of 0.1 Sv/h. The gamma dose due to the beam loss was not proved because the levels were low and buried in the fluctuation of the background. The punctual injection, normally twice a day, has been performed under the top up operation from the September 2003 without any trouble. The routine top up operation to obtain the constant current will be started in the near future. 6

7 ACKNOWLEDGEMENTS We wish to thank the staff of SPring-8, especially the members of the SPring-8 accelerator group for their valuable discussions and their kind help. REFERENCES 1. Ohkuma,H. et al, Beam-performance improvement of the SPring-8 storage ring Proc. on Particle Accelerator conf MPPB025 Portland U.S.A (2003) 2. Kimura, H., Present status of top up operation at SPring-8 storage ring II SPring-8 information Vol.8 No.5 in Japanese (2003) 3. Job, P.K. Experiences in radiation safety during the top-up operation at Advanced Photon Source Proc.of the 2 nd international conf. on Radia. Safety at synchrotron radiation sources P.76 Grenoble France (2002) 4. Berkvens,P., Colomp, P., Bidault, F., Injection with front ends open at the ESRF Proc.of the 2 nd international conf. on Radia. Safety at synchrotron radiation sources P.89 Grenoble France (2002) 5. Casarin, K, Tromba, G., Vascotto, A. The new full- injector for the Elettra light source Proc.of the 2 nd international conf. on Radia. Safety at synchrotron radiation sources P.80 Grenoble France (2002) 6. Asano, Y., Shielding Design Calculation of SPring-8 Beamline using STAC8 Journal of Synchrotron Radiation Vol.5 p.615 (1998) 7. Nelson, W.R., The SHIELD11 computer code Stanford Accelerator Center RP01-01 (2001) 8. Nelson, W.R., Hirayama, H., Rogers, D.W.O., The EGS4 code system Stanford Accelerator Center SLAC-265 (1985) 9. Kumagai, N. Et al., private communications (2003) 10. Asano, Y., Takagi, T., Overviews of induced activity at SPring-8 Proc.of the 2 nd international conf. on Radia. Safety at synchrotron radiation sources P.33 Grenoble France (2002) 11 Sasamoto,N. Asano,Y., Shielding design study for SPring-8 OECD document Proc. On SATIF P.235 Arlinton, U.S.A. (1994) 12. Takagi, T., Asano, Y., Radiation safety systems at SPring-8 Proc.of the 2 nd international conf. on Radia. Safety at synchrotron radiation sources P.123 Grenoble France (2002) 7