Kikutani, E

Tobiyama, M

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COMMISSIONING OF KEKB BUNCH FEEDBACK SYSTEMSM. Tobiyama and E. Kikutani, KEK, Tsukuba, JapanAbstractCommissioning of KEKB started at the end of the lastyear. In the early stages of the commissioning, we observedtransverse instabilities in both rings and started tuning of thebunch feedback systems. Even though the tuning is now atthe initial stage, the feedback systems are contributing toan increase of the maximum storable currents in the KEKBrings. Feedback-related systems, tune meters and bunch-current monitors, are also working well.1 INTRODUCTIONKEKB, an asymmetric electron-positron collider facilityat KEK, is now in the first stage of commissioning. Itconsists of an 8 GeV electron ring (HER) and a 3.5 GeVpositron ring (LER), both fed by a common injector-linacfeeds beams. About 5000 bunches/ring will be stored andthe total current will amount to 1.1 A and 2.6 A, respec-tively. As a part of this complex facility, bunch-by-bunchfeedback systems have begun operation. Each of the feed-back systems consists of (1) wide-band position detectionsystems, (2) high-speed digital signal processing systemswith large-scale memories, and (3) wide-band kickers fedby high-power amplifiers. At the same time, betatron tunemeters and bunch current monitors, which share some com-ponents with the feedback systems, have also begun opera-tion. With the requirement that the systems be able to han-dle 5120 bunches/ring at a bunch frequency of 509MHz, thedesign of these systems is very challenging. For example,all parts of the systems have a bandwidth of half the bunchfrequency, 255 MHz. The final design of the feedback sys-tems is described in reference[1]. In this paper, we describethe initial experiences with these systems in the commis-sioning of the KEKB rings.2 OUTLINE OF THE KEKB FEEDBACKSYSTEMSFor both the transverse and longitudinal planes, the feed-back monitors use specially designed button electrodes andthe 2GHz (4£ the rf frequency) component of the signal isused to detect the bunch position. Figure 1 is a photo of themonitor chamber installed in the LER.For the transverse feedback, two sets of button elec-trodes are installed in two points in each ring. The signalfrom these electrodes needs to be combined appropriatelyto make the feedback signal.A common digital signal processing board is used inthe transverse- and longitudinal-plane feedback systems. Itcan work up to 500MHz bunch-frequency owning to spe-cially fabricated high-speed de-multiplexers (FDMUX) andFigure 1: Feedback monitor chamber in the LER. Speciallydesigned button pickup are used in this monitor system.multiplexers (FMUX)[2]. The board works as a two tapfilter[3], which is powerful for the longitudinal feedback,and a simple digital delay.For the transverse kicker we use two types of striplinekicker: one is wide-band with a length of 40cm and theother is narrow-band with a length of 1.2m. Each electrodeof the kicker is fed by a 250-Watts amplifier.The longitudinal kicker is a low-Q cavity, which has orig-inally been developed at FRASCATI for DA'NE[4], andwe modified it to fit our parameters. As the longitudinal in-stability is expected to be the least serious in the HER, only2 sets of the longitudinal kickers are installed in the LER.The photo in Fig. 2 shows these kickers.The technology employed in the bunch feedback sys-tems has wide a variety of applications. The most straight-forward application is transverse tune measurement. Thetransverse kicker with the amplifiers kicks the beam and, wemeasure beam’s proper oscillation frequency, i.e., the tune,from the response to this kick.Another system which can be understood as a byproductof our feedback systems is the memory board. The digi-tal processing board of the feedback systems is modified torealize a kind of a transient digitizer. It utilizes the samemother board and the same input part (A-to-D conveter) asthe signal processing board. But the output part is removedand the storage size, which was 2MBytes for the processingboard, is increased to 20MBytes.0-7803-5573-3/99/$10.00@1999 IEEE. 1138Proceedings of the 1999 Particle Accelerator Conference, New York, 1999Figure 2: Two longitudinal kickers installed in the LER.Each kicker has four input ports and four output ports. TheQ -value of the kicker lowered to about 5.3 PRESENT STATUS OF THE MACHINECOMMISSIONINGWe started the commissioning[5] of the HER at the begin-ning of December 1998 and the LER at the beginning ofthis year. Since then, the storable currents of the rings in-creased rapidly. At present (middle of March 1999), thehighest stored currents are 240 mA and 370 mA for the HERand the LER, respectively. The minimum bunch spacing is10 ns (HER) / 4 ns (LER).In both rings we observe transverse instabilities, thoughwe find some difference between them. In order to suppressthem we started to operate the transverse feedback systems.On the other hand, up to present time, we have not observedlongitudinal instabilitiy and we have not started to operatethe longitudinal feedback system.4 THE FIRST EXPERIENCE WITH THETRANSVERSE FEEDBACK SYSTEMS4.1 Hardware configurationAt present, we are operating the transverse feedback sys-tems not with the final configuration but with some simpli-fied one.The signal process system is working as a simple digi-tal delay, not as the two-tap filter, because the two-tap filterapproach is complicated when the optics parameter is notcompletely known to us. When the optics become well un-der the control, we might be able to operate it with the two-tap filter mode.Out of two sets of the kickers, we started to operate thefeedback system only with the wide-band systems, whichcover the frequency range of 100 kHz to 250 MHz. Bythis simplification, we are free from the complexity com-ing from unwanted effects in the overlapped band. How-ever, this system is less powerful for lower-mode dominantinstabilities such as the resistive wall instability.4.2 Performance with the present hardwareIn order to check the performance of the feedback systems,we compared the frequency spectra of the signal from apickup, with the feedback on and off. When the feedbackis off, a large peak (> 40dB higher than the noise level) wasobserved at the tune-frequency, as shown in the left photoin Fig. 3, while with the feedback on, this peak has disap-peared (comparable with the noise level), as showin in theright photo. This phenomenon is more or less common tothe both rings and the both planes (horizontal/vertical).Figure 3: Frequency spectra of the signal from a pickup ofthe HER (vertical). The center frequency was adjusted tothe frequency correnponding to the tune. In the left photo apeak is found, which showed us the instability occured. Onthe other hand, in the right photo, we found no peak at thisfrequency.In addition, when the feedback system was turned offwith stored current higher than 250mA in the HER, non-negligible fraction of the stored beam was lost and the beamcurrent was decreased to about 60% of the original one. Inthis case the fill pattern, i.e., bunch-current distribution, gotirregular. Particularly, particles in buckets close to the tailof a train have more chance to be lost, in a bunch-train op-eration.We can estimated the damping time of the feedback sys-tems from the several parameters, sensitivity of the monitor,gain of the power amplifiers (gain 53dB, maximum power250 Watts), shunt impedance of the kicker (order of 10k› ),and losses in cables etc. and betatron function at the mon-itor and the kicker (10m» 25m). Estimated damping times(at present stage) are summarized below:horizontal verticalHER 1.2 ms 2 msLER 1.3 ms 1.8 msMore precise esitmation of the damping time with actualmeasurements will be done in near future as explained later.Shorter damping times will be realized by tuning the powerbalances in the systems.4.3 A problem to be solvedAt present, the KEKB rings are suffering from unwanteddrift of closed orbit, particularly in the vertical plane. The1139Proceedings of the 1999 Particle Accelerator Conference, New York, 1999stationary change of the beam position is seriously harmfulfor the feedback systems, since it wastes the dynamic rangeof the A-to-D converter whose resolution is 8-bit. In orderto use this dynamic range effectively, we must subtract theDC component from the detected signal. A simple high-pass filter approach does not work well, because output ofthe filter depends not only on the DC-component but also onthe bunch-filling pattern, which is essentially the duty fac-tor of the pulses corresponding to the beam positions.We are planning to introduce a DC-canceling circuitwhich detect “real” DC-component with a relatively high-speed sample/hold circuit. This circuit is now under tuningand will be included in our systems, in near future.5 RELATED SYSTEMSWe have started to operate two feedback-related systems,the tune meters and the bunch current monitors. At present,the tune-measurement systems are operated with the narrow(lower band) amplifier/kicker systems. The signal sourcefor the kicker is a spectrum analyzer equipped with a track-ing generator. The kicking frequency is swept around 2-GHz, and this signal is down-converted to the base-band ofthe tune, roughly from 5kHz to 500 kHz. Figure 4 schemat-ically explains the tune measurement system.tracking analyzerin tg outRFLO4 x fRFbutton pickup stripline kickerIFGPIB control(center frequency = 4 x fRF + n x frev)base bandbunchFigure 4: Blockdiagram of the tune measurement system.Relatively higher frequency components, about 4 times ofthe RF frequency, is used for this system. This choice en-able us to increase the S/N ratio in the measurement.We are operating the bunch current monitor consisting ofa front-end detector circuit, which is essentially the sameas the longitudinal position detector, and the memory boardwith a fast-read-out interface. Blockdiagram of the systemis shown in Fig. 5.The bunch-current is measured every 20ms (i.e. 50 Hz),and obtained bunch-current distribution is transferred to theinjector linac to be used by the bucket selection system ofthe KEKB rings[6]. Simultaneously, the bunch-current dis-tribution is graphed out in a display in the main controlroom.detectorcircuitFADC memory509MHz clock(as the A-D trigger)FDMUXssynchronizing signal (fr)MEMORY BOARDcontrolnetworkVME interfaceVME computerfrompickupstopFigure 5: Blockdiagram of the bunch-current monitor. Out-put of the bunch-current detector circuit is A-to-D con-verted (509MHz sampling rate) and stored in the memoryboard. The contents of the memory is read-out via the VMEbus.6 PLANS IN NEAR FUTUREThe following items are our targets which should be real-ized in near future.† As we mentioned, we are using only wide-band kick-ers (the short kickers) fed by the wide-band amplifiers.In the next step, we must employ the lowest-band longkickers, which should be effective to suppress the in-stability caused by the resistive wall impedance.† The next theme is an operation of the large-size mem-ory board which can store 20MBytes of data, that cor-responds to 4096 turns of 5120 bunches. This is avery powerful beam-diagnositic device for the inves-tigation of the instabilities observed in the rings. Itis also useful for measuring the damping time of thefeedback systems, by catching the beam behavior justbefore/after the feedback system ON and OFF.† The third theme is the operation of the longitudinalfeedback system, particularly the study of the longitu-dinal kicker. As described first, we have not observedthe longitudinal instability. However, the surveyingthe performance of the longitudinal system is a veryimportant study-issue for us.7 ACKNOWLEDGMENTSThe authors are grateful to the commissioning group ofKEKB for their operation of the machines during our study.They thank also Dr. Y. Ohnishi for his efforts to develop theoperator interface for the feedback and the related systems.8 REFERENCES[1] M. Tobiyama et al., Proceedings of APAC98 p. 540 .[2] E. Kikutani et al., Proceedings of EPAC96, p. 1893.[3] Y. Minagawa et al., Nucle. Instr. Meth. A416, 193 (1998).[4] R. Boni et al., Part. Accel. 52, 95 (1996).[5] K. Oide, these proceedings.[6] E. Kikutani et al., Proceedings of APAC98, p 525.1140Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

Commissioning Of Kekb Bunch Feedback Systems

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