Kikutani, E

Suetake, M

Tobiyama, M

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BUCKET SELECTOR SYSTEM FOR KEKBE. Kikutani, M. Suetake and M. TobiyamaKEK, Oho 1-1 Tsukuba, Ibaraki 305-0801, JapanAbstractA bucket selector system has been developed for the KEK-B collider. Three VME computers for actual bucket-selecting, bunch-current reading and operator interface areworking. The system provides a tool for generating the de-sired filling patterns in the rings.1 INTRODUCTIONAt KEK in Japan, the commissioning of an asymmetricelectron-positron collider, KEKB, is now in progress. Thecollider consists of a 3.5 GeV positron storage ring (LER)and an 8.0 GeV electron storage ring (HER). In order toachieve the target luminosity the currents to be stored inthe rings are 2.6A and 1.1A for LER and HER, respective-ly. These beam currents are distributed to many rf bucketsand ultimately, the number of buckets filled will be 4607,which is 90% of the total number of rf buckets, namely5120. At the present stage[1], the maximum storable cur-rents are about 500mA for both rings and the shortest bunchspacing with which beams are stably stored, with the helpof feedback systems, is 4ns.The electron and positron beams are provided by an in-jector linac complex[2]. For the synchronized operation ofthe linac and the rings, a fast timing-manipulator systemwas constructed, and as a sub-part of this system, the buck-et selector system was developed. With this system, we caninject the beam into any desired rf bucket of the rings.In the previous paper[3], we described the basic designof the system. In this paper, we will explain mainly thesoftware structure of this system.2 FUNCTIONS OF THE BUCKETSELECTOR SYSTEM2.1 Generation of desired filling patternsIn general, it is not easy to store a large number of buncheswith a small bunch-spacing. The difficulty arises from var-ious causes: instabilities (mainly coupled-bunch); heatingof the beampipes; degradation of vacuum caused by highbeam-current; etc. The strengths of the instability, the heat-ing, etc. are functions not only of the total beam-current butalso of the bunch spacing, bunch-currents, etc. Thus, a fill-ing pattern of bunches, i.e., which rf buckets are filled withthe beam, is a very important factor in raising the luminos-ity as high as possible.In principle, the number of possible filling patterns isvery large and we can not explore all these patterns. Usual-ly the bunches are filled in trains, and it is convenient to cat-egorize the filling patterns by the following parameters: (1)the number of the bunch trains, (2) the number of bunchesper train and (3) the bunch spacing in a train. By surveyingthese parameters, we find the optimum filling pattern undera given operation condition. Figure 1 shows an examplefilling pattern: 32 bunch-trains, 40 bunches/train and 6nsbunch spacing.Figure 1: An example of the filling patterns in LER.The optimum filling pattern may change with differentvarious operation conditions, decrease of the vacuum pres-sure, change in the strength of the instability depending onthe operating point, etc. Thus, it is important for operatorsto be able to change the filling patterns very easily.2.2 Control of filling orderNext we explain the requirements on the filling order. If weinject the beam into the same rf bucket continuously at themaximum injection rate (50Hz), the bunches in the bucketssurrounding the injection-bucket will be kicked repeatedly.Since the transverse radiation damping time of the rings isabout 45 ms, the kicks will be given to the bunches with-in one-half the damping time. In order to avoid this, theinjection-bucket should be changed from pulse to pulse.2.3 Equalizing the bunch-currents in the ringWhen the beam is lost, either slowly due to vacuum orrapidly due to instabilities, we re-fill the ring. It is typicalthat the bunch currents are lost unevenly and the numberInternational Conference on Accelerator and Large Experimental Physics Control Systems278International Conference  Accel rator and L rge Experim ntal Physics Control Systems, 1999, Trieste, Italyof particles to refill is different, bucket to bucket. In orderto regulate the bunch-currents we have an optional bunchcurrent equalizer (BCE) in our bucket selector system.3 HARDWARE CONFIGURATIONThe rf bucket to be filled with the next linac-pulse is de-termined by the trigger timing for the electron gun. Thismechanism is essentially common in both electron andpositron modes. Five VME-packaged delay modules areused for controlling this timing, and the delay values ofthese modules are set by a VME computer which is in-stalled in the common VME sub-rack to the delay mod-ules. Hereafter, we call this computer the “linac computer”because the sub-rack is located near the gun.An accelerator operator chooses the filling pattern (ac-tually, chooses the parameters explained in Section 2.1).These parameters are accepted by a computer installed inthe Central Control Room (the “CCR computer”) . TheCCR computer and the linac computer are connected bydedicated optical fiber cables using the interface of theShared Memory System, which we described in a previouspaper[3].Additionally, we have one more VME computer to con-trol two bunch-current monitors for the LER and HER.They output the 5120 bytes of data per ring synchronizedwith a signal which triggers the injection kicker. This com-puter, the “bunch-current computer” is also connected tothe linac computer with the optical cables and the bunch-current information is transferred to the linac computer.Figure 2 shows the hardware configuration of the sys-tem. The distance between the bunch-current monitor andthe CCR is about 300m and that between the CCR and thelinac is about 1000m. By using dedicated optical cables,the transfer speed of the bunch-current and the filling pat-tern information is not dependent on the control networkunder which all the control components are supported.4 SOFTWARE4.1 GeneralThe control system of the KEKB accelerators, includingthe injector, has been constructed based on the EPICSenvironment[5]. In this environment, each piece of hard-ware is controlled by a VME computer, while the opera-tor interface is performed by UNIX computers. The VMEcomputer is Motorola Power PC 750 (266MHz clock) withmemory of 64Mbytes. The operating system is VxWorks5.3.1.The program running in the linac computer plays a mainrole in executing the functions explained in Section 2. Indesigning the software, the most important point is the reg-ulation of the execution time of the program. The maxi-mum injection rate of KEKB is 50Hz, that is, all the pro-cedure should complete within 20ms. In order to keep theexecution time constant, we adopted a scheme as follows:Fuji buildingControl buildingLinac buildingShared Memory(VME board)VME computeroptical cable forshared memorycontrol networkdelay modulesbunch currentmonitorsFigure 2: Hardware configuration of the bucket selectorsystem.1. The number of EPICS records is limited in the linaccomputer to make it approximately dedicated to thebucket selecting,2. communication with the operators, which is asyn-chronous with the beam-injection, is done by the CCRcomputer to keep the linac computer free from opera-tors’ interrupts.4.2 Software on the bunch-current computerThe program in the bunch-current computer acquires the10240 bytes (two rings) of bunch-current data from themonitors. Its execution speed is fast enough to synchro-nize to the injection kicker, whose rate is 50 Hz at the high-est. It is coded as a device-support of the EPICS systemand the data is available through an EPICS record. Besidesthis record, the bunch-current information is written intothe Shared Memory. After writing all of the bunch cur-rents, an interrupt signal is also sent to the linac computerthrough the Shared Memory System.4.3 Software on the CCR computerAs described above, the parameters specifying the fillingpattern are accepted by the CCR computer through a num-ber of EPICS records. They are written into the SharedMemory and copies are available on the Memory Boardunder control of the linac computer. The device support for279the Shared Memory loaded in the CCR computer functionsas the device support of the bucket selector system.The operator interface was developed with MEDM,which is the standard user interface in the EPICS system.Figure 3 shows the operator-interface panel. Through thispanel, an operator can specify the filling pattern and sever-al options, such as turning the bunch current equalizer onand off. In addition, the operator can read some useful in-formation: the last-filled bucket; total number of bunches;and so forth.Figure 3: Control panel of the bucket selector system.4.4 Software on the linac computerA flow-chart of the program in the linac computer is shownin Fig. 4. This is coded not as device support but as a sim-ple C-language program under VxWorks. The hardwareaccessed by the code are the Shared Memory Board andthe delay modules, both of which are VME modules.The code consists of some flow-control part and the cal-culation/setting of the delays including the 12 access timesof the Shared Memory and 5 access times of the delay mod-ules. Execution time for one trigger was measured to beabout 40 micro seconds with the bunch-current equalizeroff. A separate measurement showed us that one access ofa VME module takes approximately 1 micro seconds. Ifthe bunch-current equalizer is switched on, reading of thebunch-current information (4.6kbytes at most, 5ms) will beadded. Adding the processing time of the interrupt underVxWorks, 21 micro seconds, the total time should be some5ms, which is much shorter than 20ms.initializationsemTakeread information of the fill patternmultiple bunchtrain (auto)BCE?read bunch current and find thebucket of the minimum currentset delayset delayset delaycalculate thenext bucketcalculate thenext bucket yesnoyesnosemaphoreFigure 4: Flow chart of the code in the linac computer. Theexecution of the program in the linac computer is initiatedby an interrupt from the bunch-current computer. The in-terrupt management is done with code prepared under theVxWorks System.5 EXPERIENCEWith the bucket selector system, the beams are smooth-ly injected to the rings. When the operation scheme ischanged, changing of the optics, for example, the optimumfilling patterns are easily surveyed with this system.6 REFERENCES[1] K. Oide et al., in Proceedings of the 1999 Particle AcceleratorConference, p.288.[2] A. Enomoto, in Proceedings of the 1-st Asian Particle Accel-erator Conference (KEK Proceedings 98-10), p.292.[3] E. Kikutani et al., in Proceedings of the 1-st Asian ParticleAccelerator Conference (KEK Proceedings 98-10), p.525.[4] M. Tobiyama et al.,, in preparation.[5] A. Akiyama et al., in Proceedings of the 1999 Particle Accel-erator Conference, p.343.280

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