At the 3rd generation light source BESSY II the first phase of a fast orbit feedback system    has been completed and put into operation in 2012. In this first phase the aim was to achieve noise suppression in the 1Hz to several 10Hz range, mostly avoiding expensive upgrades to existing hardware, such as beam position monitors and the CAN based setpoint transmission to the power supplies. Only the power supplies were replaced with newer, faster versions. This paper describes the capability of the phase I FOFB with respect to beam motion transient suppression, low frequency damping and high frequency noise generation as well as aspects of operational integration and stabilit

Birke, T.

Diehn, M.

Engel, D.

Franksen, B.

Görgen, R.

Kuske, P.

Lange, R.

Müller, I.

Müller, R.

Schindhelm, G.

Schälicke, A.

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FAST ORBIT FEEDBACK AT BESSY-II: PERFORMANCE ANDOPERATIONAL EXPERIENCESR. Mu¨ller∗, T. Birke, M. Diehn, D. Engel, B. Franksen, R. Go¨rgenP. Kuske, R. Lange, I. Mu¨ller, A. Scha¨licke, G. SchindhelmHelmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, BESSYII, Berlin, GermanyAbstractAt the 3rd generation light source BESSY II the ﬁrstphase of a fast orbit feedback system (FOFB) has been putinto operation in September 2012. In this ﬁrst phase theaim was to achieve noise suppression in the 1 Hz to a few10 Hz range, mostly avoiding expensive upgrades to ex-isting hardware, such as beam position monitors and theCAN based set-point transmission to the power supplies.Only the power supplies were replaced with newer, fasterversions. This paper describes the capability of the phase IFOFB with respect to beam motion transient suppression,low frequency damping, high frequency noise generationas well as aspects of operational integration and stability.INTRODUCTIONThe photon beam stability requested by experiments at3rd generation light sources typically requires micron andsub-microradian electron orbit stability, where time scalesrange from hours to microseconds. BESSY has a long tra-dition in diagnosing and eliminating sources causing small-est beam motions. Thus in standard user mode the orbit,drift corrected by the slow orbit feedback (SOFB), has al-ready reached a competitive level of stability [1].But at a multi-user facility experimental activities gen-erate irregular transients and noise spikes, that have to besuppressed as fast as possible. And for experiments averag-ing data as short as 100ms, beam motion in the frequencyrange 1 Hz to 50 Hz has to be kept as small as possible.Further more, in low-α mode, routinely offered at BESSY,any perturbation results in a typically exaggerated horizon-tal beam motion [2]. Consequently, in addition to the ex-cellent base stability of the orbit, a “not-so-slow”, or evenbetter, a sophisticated fast orbit feedback (FOFB) providesoperational head-room and improved experimental capabil-ities.IMPLEMENTATION STATUSTwo Phase ApproachThe introduction of a FOFB system at BESSY has beensplit into two phases [3]. In a ﬁrst functional set-up theworn out slow corrector power supplies have been replacedby fast modern devices. Express data lanes have been setup, collecting the 600 Hz parallel read-out of the old multi-plexed beam position monitors (BPM) and distributing thecorrector set-point on a dedicated reﬂective memory. The∗Roland.Mueller@helmholtz-berlin.deFigure 1: Simpliﬁed operators views with and withoutFOFB. In the current state the FOFB is most effective inthe low frequency part (below 10 Hz booster line) of thebeam motion spectrum.existing CAN bus attached power supply controllers couldbe utilized by a custom set point transmission, bypassingEPICS slow controls on dedicated CAN segments conﬁg-ured for data rates up to 200 Hz.From this set-up a possible follow-up phase II foresees afull replacement of the BPMs by fast, precise digital BPMunits, an appropriate data and trigger distribution networkas well as suited fast power supply controllers.Phase I - Current StandingThe phase I implementation has been completed in mid2012. System tests showed, that running the existing hard-ware near speciﬁcation limits, a robust set-point through-put at 150 Hz with acceptable jitter can be achieved, allow-ing for a 4x averaging of the BPM readings. Thus, withproper control loop adjustments and additional ﬁne-tuning,a system bandwidth of 30-40 Hz seems feasible.Due to a lack of commissioning opportunities the sys-tem had to go into operation “raw”, i.e. as set up andassembled. Further developments targeted robustness andreliability, but the reﬁnements of the control loop are stillmissing. In this state the clear improvements <10 Hz (seeFig. 1) are already a success. Additional improvements ofthe system bandwidth beyond 10 Hz are subject of currentresearch.OPERATIONAL ASPECTSCaveatsThe exclusive corrector access in “FOFB enabled” or“FOFB active” mode creates delicate operational con-straints: feed-forward compensations (slicing bump clo-WEPME002 Proceedings of IPAC2013, Shanghai, ChinaISBN 978-3-95450-122-92920Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)06 Instrumentation, Controls, Feedback and Operational AspectsT05 Beam Feedback SystemsFigure 2: Sketch of the vertical orbit stability for the different combinations of SOFB/FOFB and top-up/decaying beammode. Top: Beam current (red) and regions of all FB off, SOFB/FOFB on. Middle: BPM data of one sector showing theremaining beam motion in top-up mode for SOFB-only, FB off, as well as FOFB on. In addition the well known beamwander for decaying beam is clearly visible even with FOFB on. Bottom: Increased regulation activities of the correctorsin the same sector in FOFB mode, compared to the SOFB case.sure, wave length shifter ﬁeld changes), formerly easilyhandled by offset values to the EPICS slow control set-points, as well as save/restore or application program ac-tivities now require new operational procedures.Top Up OperationFOFB has been put into operation in parallel to the in-troduction of top-up as the new standard operation mode.The latter additionally stabilizes the beam on time scalesconnected to thermal equilibrium, that is at least helpful inmitigating any possible DC or beam wander problems ofthe FOFB (see Fig. 2).General BehaviorDespite the differences in the slow and fast BPM dataacquisition the “seamless” switching between SOFB andFOFB is feasible without speciﬁc provisions: resulting or-bit jumps are on the few or even sub- μm level with negligi-ble effect on the usable beam. Integration into the operatorsuser interface features the same look and feel for both theSOFB and the FOFB.For the path length correction it was originally plannedto integrate the RF frequency into the horizontal re-sponse matrix and minimize the average horizontal correc-tor strength in the SVD calculation at each correction step,a method successfully applied in the SOFB.Fast delivery of set-points to the master oscillator hasto use the analog input. Here noisy frequency requestscould cause phase jitter jeopardizing the required injec-tion efﬁciency. As a measure of precaution a much sim-pler approach has been installed ﬁrst: the appropriate RFfrequency change is calculated from the excess averagestrength of the horizontal corrector families, according topreviously observed relations [4], and “cautiously” applied(special care is taken to avoid frequency correction right atthe injection shot).Proceedings of IPAC2013, Shanghai, China WEPME00206 Instrumentation, Controls, Feedback and Operational AspectsT05 Beam Feedback SystemsISBN 978-3-95450-122-92921 Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)Table 1: Integrated rms beam motion (0.02-300 Hz) withand without FOFB in its present status, comparison with10% beam stability target in brackets (bandwidth∼ 10 Hz).See R. Bartolini [5] for a facilities overview.Mode Horizontal VerticalFB OFF 4μm(25μm) 1.5μm(2.5μm)300 mA Hybrid 5.5μm(25μm) 3μm(2.5μm)13.5 mA SB 9μm(25μm) 5μm(2.5μm)100 mA low-α 9μm 4μm15 mA low-α 14μm 7μmException HandlingCurrently software interlocks on major deviations guar-antee stable operation of the FOFB. A fast, hardware beamloss trigger attached to an extra BPM unit has been pre-pared, but was not needed so far. The FOFB runs well con-strained with literally no built-up of DC effects - both shortterm and in multi-week operation.PERFORMANCEWith respect to beam stability, operational robustnessand additional capabilities in speciﬁc operational modesthe FOFB behaves as expected. Problems of transientsdisappeared, basically no FOFB induced problems havebeen encountered, beside teething problems most of thefew FOFB outages have been caused by non-FOFB com-ponent failures or mishandling.Beam MotionThe multi-year, long-term operational reference of the100ms averaged slow orbit data is maintained by the FOFB,now additionally kept constant to the bit resolution level,see Fig. 2 for details. Up to the 10 Hz booster line orbitperturbations are practically extinguished. On fast beammotion analyzers it can be seen, that even the tail of thehorizontal injection shot perturbation is damped.But beyond 10 Hz additional noise is visible, that needsto be understood and is currently under investigation. Thisis also evident in the resulting integrated rms beam motion,see Table 1, which is far from satisfying. Probably it has tobe accounted to deﬁciencies in the set point data synchro-nization that need to be eliminated – even if some 2 μmoriginate from the 10 Hz booster synchrotron.Operational ModesThe most obvious improvements in standard user mode(300 mA hybrid) are related to transient imperfections: dueto the asynchronous set-point forwarding the femto secondslicing beam separation bump could only be advanced atvery low speed to enable the SOFB to keep the perturbationwithin 10 μm. Now with FOFB active it can be set at aspeed of 20 μrad/s without measurable beam disturbance.In single bunch (SB) decaying beam mode the huge dy-namic range is the major challenge for the BPM measure-Figure 3: Orbit stability, two complete weeks of low-αoperation. Light colors, noisy lines: Oct. 2012, SOFBonly. Dark colors, smooth lines: May 2013, FOFB. Green:average position[-0.02 mm,0.02 mm], blue: rms devia-tion [0.01 mm,0.05 mm], red: alternating beam currents100 mA high intensity/15 mA non-bursting mode.ment units. Now with top-up operation this issue lost rel-evance, the BPM signals are still more noisy compared tomulti-bunch mode, but the FOFB is fully functional.In low-α mode the FOFB opens a new regime of hori-zontal beam stability, see Fig. 3. Both with respect to beamcentering (average position) and orbit deviations (RMS)even at currents as high as 100 mA and unlimited changesof insertion device gaps the FOFB controlled orbit behavesas benignant as in standard user mode.SUMMARYEven the “raw”, unreﬁned status of the FOFB phase Isystem provides signiﬁcant improvements: transients areefﬁciently suppressed, the over-all operability and reliabil-ity is convincing. Beam motion in the frequency range<10 Hz is reduced substantially.Objective evidence as well as clear valuation of theachievements of the present FOFB set-up on user experi-ments is not easy to get and still pending. Accordingly thepossible beneﬁt of a FOFB phase II for the experimentsperformed at BESSY is not assessable yet.REFERENCES[1] J. Feikes, K. Holldack, P. Kuske, R. Mu¨ller, “Orbit Stabilityat BESSY,” PAC 2005, Knoxville, USA, RPAE035 (2005).[2] J. Feikes, P. Kuske, R. Mu¨ller, G. Wu¨stefeld, “Orbit Stabil-ity in the ’Low Alpha’ Optics of the BESSY Light Source,”EPAC 2006, Edinburgh, UK, THPLS017 (2006).[3] R. Mu¨ller, B. Franksen, R. Go¨rgen, R. Lange, P. Kuske,I. Mu¨ller, J. Rahn, T. Schneegans, “Installing a Fast OrbitFeedback at BESSY,” IPAC 10, Kyoto, Japan, p. 2749-2751(2010).[4] R. Bakker, K. Holldack, P. Kuske, R. Mu¨ller, “Orbit Controlat BESSY II: Present Performance and Plans,” EPAC 2000,Vienna, Austria, p. 666 (2000).[5] R. Bartolini, “Performance and Trends of Storage Ring LightSources,” EPAC 2008, Genoa, Italy, TUXM02, Slide 20 inaccelconf.web.cern.ch/AccelConf/e08/talks/tuxm02 talk.pdfWEPME002 Proceedings of IPAC2013, Shanghai, ChinaISBN 978-3-95450-122-92922Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)06 Instrumentation, Controls, Feedback and Operational AspectsT05 Beam Feedback Systems

Fast Orbit Feedback at BESSY II  Performance and Operational Experiences

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Fast Orbit Feedback at BESSY II Performance and Operational Experiences

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