In view of increased processing bandwidth at demanding  experiments and the need for rapid compensation of noise  spikes and new, yet unknown excitations, a fast orbit feedback  aiming at noise suppression in the 1Hz 50Hz range  has become mandatory for the 3rd generation light source  BESSY II. The fast set point transmission plus the replacement  of all corrector power supplies is scheduled as a first  step. Later   in combination with top up operation   orbit  stability can be further improved by replacing today s multiplexed  analog beam position monitors by state of the art  fast digital units. This paper describes how the pilot installation  of a small subset of fast corrector power supplies allows  to tune performance and study the benefits for today s  most sensitive experiment

Franksen, B.

Görgen, R.

Kuske, P.

Lange, R.

Müller, I.

Müller, R.

Rahn, J.

Schneegans, T.

HZB Repository

INSTALLING A FAST ORBIT FEEDBACK AT BESSYRoland Mu¨ller∗, Benjamin Franksen, Rainer Go¨rgen, Ralph Lange,Peter Kuske, Ingo Mu¨ller, Joachim Rahn, Tobias SchneegansHelmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, BESSYII, Berlin, GermanyAbstractIn view of increased processing bandwidth at demandingexperiments and the need for rapid compensation of noisespikes and new, yet unknown excitations, a fast orbit feed-back aiming at noise suppression in the 1Hz-50Hz rangehas become mandatory for the 3rd generation light sourceBESSY II. The fast set point transmission plus the replace-ment of all corrector power supplies is scheduled as a firststep. Later – in combination with top-up operation – orbitstability can be further improved by replacing today’s mul-tiplexed analog beam position monitors by state-of-the-artfast digital units. This paper describes how the pilot instal-lation of a small subset of fast corrector power supplies al-lows to tune performance and study the benefits for today’smost sensitive experiments.INTRODUCTIONThe level of photon beam stability needed by experimen-tal signal conditions or resolution goals varies from exper-iment to experiment. At 3rd generation light sources, sen-sitivity of measurements to electron beam motion typicallyrequires micron and sub-microradian electron orbit stabil-ity, time scales range from hours to microseconds.Many experiments today typically average data for asshort as 100ms. Perturbations in the frequency range 1Hzto 50Hz have to be kept as low as possible to ensure thatlimits for high resolution experiments are not set by thebroad-band noise floor. Any irregular transients and noisespikes generated by experimental activities at multi-userfacilities have to be suppressed as fast as possible. Con-sequently, modern light sources put much effort into thedesign of all components crucial for the orbit stability andprovide a sophisticated fast orbit feedback (FOFB) in addi-tion.THE BESSY SETUP: FEATURES,LIMITATIONSHunting and eliminating the sources of smallest beammotions has a long tradition at BESSY: the smaller the per-turbations to correct, the less noise is introduced by imper-fections of a feedback loop running at high gain.To achieve these goals, power supplies have been ad-ditionally stabilized by ripple suppression circuits. Ultra-high precision, thermally stabilized I/O boards have beendirectly plugged into the power supplies, enabling shortanalog cable length and preventing signal cross talk. Set∗Roland.Mueller@helmholtz-berlin.depoints and commands are transmitted to the I/O boards indigital form via ethernet and CAN bus network links. Thissetup results in a competitive high intrinsic stability of thelight source.Initial focus on the beam position monitor (BPM)data acquisition was precision and high dynamic range,achieved by integration and averaging of the BPM signalsfor 1s. Data collection from the 16 front end computers(IOCs) is done unhurried by a master node via softwarehandshake over the standard network link every 2s.This slow orbit feedback (SOFB) setup allows for a pre-cise global orbit drift control with an excellent basic elec-tron beam position and energy stability [1, 2]. The in-stalled orbit correction scheme, running at a cycle of 0.2Hz,corrects orbit drifts as well as imperfect compensations ofslowly moving insertion devices and guarantees good ex-perimental conditions with respect to run-to-run and fill-to-fill reproducibility and uniformity.Figure 1: Recent example of successful noise eliminationat the source. Analyzing the fast BPM data, the cen-tral bending magnet of the femtosecond slicing chicanecould be identified as the origin of horizontal beam mo-tions at 1.25Hz and 2.5Hz. Appropriate countermeasureshave been taken with obvious success.The major bottlenecks that have to be overcome for aFOFB at BESSY are corrector power supplies and set pointtransmission. Possible solutions differ from those avail-able for new installations (Diamond, Soleil, ALBA) [3, 4],where the complete chain from BPM data acquisition tocorrector actions has been tailored to the requirements of aFOFB at an early design stage.Proceedings of IPAC’10, Kyoto, Japan WEPEB03006 Beam Instrumentation and FeedbackT05 Beam Feedback Systems 2749UPGRADE OF COMPONENTSA system typical for a first generation FOFB setup [3, 4]can be obtained by pushing the existing installation to thelimits achievable with moderate investments.Beam Position Data AcquisitionParallel to the initial slow reference BPM data acquisi-tion system, the fast BPM data collection with a samplingrate of maximal 2.4kHz and a 300Hz low pass filter hasbeen in place since several years for diagnostic use in the0.1Hz to the several 100Hz region.The initial BPM data end station has now been aug-mented by dedicated compute and control workstations. Ahigh throughput reflective memory network connects work-stations and all corrector IOCs. During one cycle, all BPMreadouts are available at the central computing node(s),where operation modes are controlled, set point changescalculated and distributed to the power supplies IOCs. Thisarchitecture allows for feedback correction loops tailored tospecific mode space characteristics [5].After these modifications, BPM data access is simpli-fied and flexible. Fig. 1 shows the result of a beam motionanalysis and the success of the attempted countermeasures:Doubling the switching frequency of the readback ADC onthe I/O board, combined with an experimentally optimizedpairing of a selected I/O-board and a specific power supplyunit removed this perturbation (compare lower curve withupper trace in Fig. 1) and pushed this setup one order ofmagnitude beyond its specifications.Fast Power SuppliesThe new system places highly demanding requirementson new fast power converters:An output current of up to +/-8A at 40V is needed to al-low for 3mrad kicks1. The new power supplies have to havethe same characteristics with respect to stability, precision,ripple, and thermal drift (typically +/- 25ppm) as the exist-ing units, since they have to be replaced sequentially groupby group without any noticeable effect on the storage ringperformance.Multiple options for set point inputs have to allow for fu-ture developments: if the CAN bus input to the present I/Oboard turns out to be insufficient, the slow control part viaCAN bus could be disentangled from the FOFB system viathe analog modulation input (Fig. 2). Even if componentobsolescence or technical improvements require or advisethe replacement of the whole power supply controller [8],no major modification should be needed.The transfer functions measuring the effect of fast cor-rector magnet changes on the beam show the limits of thechosen WME PA4008\11 converters appear at 200Hz.1The diagnostic value of this capability has recently been used to lo-cate a beam destructive obstacle in the beam pipe with meter precisionthus allowing for repair and resume of user operation withing about oneweek.Power Supplies WME  PA4008\11ADA 16 IO8 InterfaceboardAnalog SetpointResolution up to 19 BitTwo analog readbacks 16 bit resolution.One Slow High precisionADCOne 100 kHz ADCDigital StatusDigital controlsTrigger Setpoint+DCCT2-MonitorDCCT1-RegulationLinear Power Stage+Second Analog Setpoint+/-10V for +/-ImaxStability: ±100 ppmripple +drift3db bandwidth: 1 kHzINom=    ± 8 AUNom= ± 40 VCAN-BusEmbeddedControllerAnalog Modulation InputMagnetMainsTerminalExtInterlockTerminalDCCT1-RegulationDCCT2-MonitorFigure 2: Power supply control flexibility is given by twosymmetrical input interfaces: digital and analog set pointsare internally added (or optionally multiplied).Set Point TransmissionSince the power supply I/O boards allow for set pointchanges at rates up to 200Hz, only the link between the cen-tral computing node and the power supply I/O boards hadto be significantly modified and upgraded. This has beenachieved by linking the 8 power supply IOCs and the FOFBcomputing nodes via reflective memory and by modifyingthe CAN based distribution from the IOC to the power sup-plies (see [6] for details).Figure 3: Measured data transmission time and jitter: 1msafter the clock trigger (upper, yellow line) all set points aredelivered via reflective memory to the IOCs (middle, cyanline, 1ms/div). Without a sync event, 3ms later, with a jitterof 1ms, the last CAN object provides the set point to thepower converter (bottom, green line).The measurement depicted in Fig. 3 shows that set pointsare available at all power converters within 4ms. On thesync trigger, all power supplies reach their new values witha jitter of 0.1ms. Orbit data are delivered to the centralVME crate with a scan rate of 600Hz (up to 2.4kHz). Thecentral computing nodes can be chosen powerful enoughWEPEB030 Proceedings of IPAC’10, Kyoto, Japan275006 Beam Instrumentation and FeedbackT05 Beam Feedback Systemsto complete the computation task for one cycle well within2ms. Thus the known time budget of the present installa-tion seems to allow for a closed FOFB loop running at afrequency around 100Hz.Feedback AlgorithmThe orbit correction capability of the Matlab MiddleLayer (MML) toolkit [7] turned out to be a flexible andinstructive supplement to the operational SOFB. It is alsoused on the RT-linux computing node of the FOFB as arapid prototyping development environment and will be re-placed by a custom program as soon as experiments havedetermined the optimal algorithm.FOFB PHASE IPilot InstallationA small subset of 8 power supplies has been installedto be able to gain experience with the proposed system.Their position and orientation have been chosen to addressthe dominant components of known residual perturbationsoriginating from the superconducting wavelength shifters,the femtosecond slicing chicane, fast polarization switch-ing elliptical undulators, etc.Global FOFBOnce extended to all corrector power supplies, we expectthe full scale FOFB to eliminate most noise spikes or infre-quent jumps harmful for difference measurement methods.We also expect a significant reduction of broad-band noise,which will improve intensity and contrast conditions forcertain sophisticated experiments.Maintenance/Upgrade OptionsDependent on the progress of recent developments [8] ormaintenance requirements of components at BESSY II, apossible replacement of the power supply controllers mightgive room for further improvements. The deterministic andreliable data distribution investigated at NSLS-II seems tobe a very promising approach [9].FOFB PHASE IIIn a second phase we intend to replace the BESSY BPMsby state-of-the-art, parallel processing, fast digital units tofurther increase correction precision: In combination withtop-up operation, the large dynamic range (especially forhigh single bunch currents in decaying beam mode) is nolonger necessary, and the precision of the beam orbit mea-surement could become better by nearly an order of mag-nitude.By the time a final decision has to be made on whichsystem to use, there will be sufficient experience availablefrom other FOFB installations to be able to assess the ad-vantages of the different solutions developed for the mostrecent facilities (e.g. NSLS-II). In addition, novel ingredi-ents like the jitter minimized data distribution system [9]will be better understood.SUMMARYA fast acquisition of BPM data and a correspondinglyfast distribution of these data via reflective memory to thenodes involved has been achieved, all in a reliable and eas-ily configurable way. 8 out of 112 corrector power supplieshave been seamlessly replaced by fast units, without anyadverse effect on routine operation. The fast data distri-bution needed to close the feedback loop for any subset ofFOFB phase I is installed and performs to specifications.The pilot installation is expected to already provideworthwhile suppression of certain frequency componentsand will provide valuable information on possible short-comings of the envisaged setup. The running-in processfeatures identical data representation on all relevant inter-mediate levels of the SOFB and FOFB systems (BPM data,power supply set points, correction algorithm, and controlparameters). Thus, slowing down the new fast system tothe Hz level, the FOFB system has to show behavior thatis identical to the well known SOFB system. This consis-tency check has been successfully completed. Now, ma-chine development time is needed to tune the fast feedbackperformance for the subset already available.REFERENCES[1] Orbit Stability at BESSY, J. Feikes, K. Holldack, P. Kuske,R. Mu¨ller, RPAE035, PAC’2005, Knoxville, USA (2005)[2] Orbit Stability in the ’Low Alpha’ Optics of the BESSYLight Source, J. Feikes, P. Kuske, R. Mu¨ller, G. Wu¨stefeld,THPLS017, EPAC’2006, Edinburgh, UK (2006)[3] Global Fast Orbit Feedback System Down to DC using Fastand Slow Correctors, N. Hubert, L. Cassinari, J. Denard, A.Nadji, L.S. Nadolski, MOOC01, DIPAC’2009, Basel, Switzer-land (2009)[4] Overview of Fast Beam Position Feedback Systems, D. Bul-fone, TUZM02, EPAC’2008, Genoa, Italy (2008)[5] Introducing Fast Orbit Feedback at BESSY, R. Mu¨ller,R. Go¨rgen, R. Lange, I. Mu¨ller, J. Rahn, THP059,ICALEPCS’2009, Kobe, Japan (2009)[6] Preparing Slow Controls at BESSY for Fast Orbit Feedback,R. Lange, R. Go¨rgen, I. Mu¨ller, R. Mu¨ller, J. Rahn, THP063,ICALEPCS’2009, Kobe, Japan (2009)[7] Optic calibration at the MLS and BESSY II, P. Schmid,D. Engel, J. Feikes, P. Kuske, R. Mu¨ller, G. Wu¨stefeld,WEPEA014, IPAC’2010, Kyoto, Japan (2010)[8] Control Architecture of a New Power Supply Controller forDiamond Light Source, M. Heron, M. Abbott, J. Dobbing, A.Rose, J. Rowland, I. Uzun, TUP062, ICALEPCS’2009, Kobe,Japan (2009)[9] Power Supply Control at NSLS-II, Y. Tian, EPICSCollaboration Meeting, Vancouver, Canada (2009)http://isacwserv.triumf.ca/......epics09html/FR1/PS control EPICS09.pptProceedings of IPAC’10, Kyoto, Japan WEPEB03006 Beam Instrumentation and FeedbackT05 Beam Feedback Systems 2751

Installing a Fast Orbit Feedback at BESSY

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Installing a Fast Orbit Feedback at BESSY

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