Birke, T.

Falkenstern, F.

Müller, R.

Schälicke, A.

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COMPREHENSIVE FILL PATTERN CONTROL ENGINE:KEY TO TOP-UP OPERATION QUALITY∗T. Birke†, F. Falkenstern, R. Müller, A. SchälickeHelmholtz-Zentrum Berlin für Materialien und Energie, Berlin, GermanyAbstractAt the light source BESSY II numerous experiments aswell as machine development studies benefit from a veryflexible and stable fill pattern. The fill pattern control en-gine is based on a state machine that controls the full chainfrom gun timing, linac pulse trains, injection and extractionelements as well as next shot predictions allowing triggeringthe next user experiment DAQ cycle. Architecture and in-terplay of the software components as well as implementedfunctionality with respect to hardware control, performancesurveillance, reasoning of next actions and radiation safetyrequirements are described.INTRODUCTIONAfter starting user operation in 1998, the 3rd generationlight source BESSY II provided users with synchrotron radi-ation in decaying beam mode with three injections per dayfor 14 years. During this time BESSY II offered specific sup-port for time-resolved experiments, pioneered low-α modewith coherent THz-radiation and ps-pulses and establishedthe most advanced femtosecond slicing [1] endstation with100 fs pulses. A purity controlled high current camshaftbunch in the ion clearing gap enables pump/probe exper-iments as well as pseudo singlebunch experiments at fullintensity with a mechanical chopper.Switching to top-up mode [2] extended these possibili-ties with a thermally stabilized machine and higher averageintensity. Today, more pseudo single bunch experimentswith reduced intensity are possible by using pulse picking byresonant excitation (PPRE) [3] with a selected bunch closeto the end of the gap.All these specific bunches have to be topped up with min-imal variation to programmable intensities.CONSTRAINTSRadiation SafetyBased on analysis of facility properties and various mal-function scenarios, several restrictions and modificationshave been set up to guarantee minimized losses and to makeall sources of possible damages measurable and hence allowbeamshutters to be kept open during injections:• Injection efficiency must be >60% for every shot.• Average injection efficiency must be >90% over 4 h-blocks.• Maximum allowed injection frequency is 0.1Hz.∗ Work funded by BMBF and Land Berlin† thomas.birke@helmholtz-berlin.de• Maximum increase in stored current per injection shot:2mA.• Minimum current in booster. For reliable and accurateefficiency measurement, a minimum current of 0.3mAduring an injection is mandatory.• Minimum and maximum current of stored beam to-gether with a corresponding minimum lifetime. Tocontrol and limit the normal average loss, a limit ofmax. 60mA/h has been defined. The nominal lim-its are 200mA-300mA stored beam with a minimumlifetime of 5 h.Top-Up InterlocksTwo separate systems are responsible for efficiency mea-surement and interlock. During top-up operation, both haveto permanently check all relevant parameters for compliancewith the top-up radiation protection restrictions and approvetop-up operation to continue. The efficiency interlock sys-tems are part of the radiation protection system.Any violation of any of the restrictions will activate thetop-up interlock system and pause or terminate top-up oper-ation while beamshutters are free to open. Top-up injectionsare continued as soon as all top-up conditions are restored.This may happen automatically (e.g lifetime temporarily be-low limit) or may have to be accomplished by operationsstaff after closing beamshutters (e.g. last injection efficiencybelow 60%).The fill pattern control engine operates in both top-up user-runs (beamshutters unlocked and free to open) and commis-sioning or machine development runs (beamshutters closedand locked). The latter is possible even without switchinginterlock systems to top-up mode. So the strict limitationsof top-up mode are not enforced in this case and the top-upinterlocks are inactive.Injection Frequency and TimingA proper compromise between minimizing variation inbunch currents and minimizing the number of disruptionsdue to injection kicker pulses had to be settled. The cur-rently consolidated injection-scheme is, to fill a maximumof 5 bunches (reduced to e.g. 1 or 3 when filling dedicatedsingle- or slicing-bunches) at every multibunch injectionshot with the highest possible bunch currents. The decayphase between two injections currently ranges from 10 s to200 s with an average of 120 s and every injection shot addsbetween 0.3mA to 1.8mA to the stored beam (ring currentvariation during top-up operation: ~0.5%).TOP-UP SERVICE AND FILLPATTERNCONTROL ENGINEThese core automation center for top-up operation atBESSY II is the "Top-Up Service and Fillpattern ControlEngine" that has been developed during tests, setup andcommissioning of top-up operation mode.It is implemented as a standard EPICS state-machine to-gether with a corresponding realtime database hosted on adedicated Software I/O-Controller (SoftIOC).During runtime, this system communicates via EPICSChannelAccess (CA) with several other I/O-controllers(IOCs).Input SignalsState and processed data of top-up interlock systemsDuring top-up operation, the status of all top-up interlocksystems is monitored to decide whether to continue top-up injections or to provide information about the reason ofa top-up interruption. If efficiency decreases, a possibleinjection-free time at the beginning of the next 4h-block isindicated.If top-upmode is not active and therefore the beamshuttersare locked, it is not necessary to obey the top-up restrictionsstringently. All this information has to be processed anddisplayed and may initiate state transitions.Fillpattern Monitor To decide which positions andbunch combinations to fill on the next injection shot, the fillpattern monitor is the central source of information and corediagnostic components for proper top-up operation.It is implemented using a stripline based high-performance ADC that delivers per bunch current measure-ment with high accuracy (up to 100 nA resolution) [4]. Deliv-ering an accurate fill pattern measurement of bunchcurrentsof all 400 possible bunches every second, it builds the foun-dation to decide where and how to place the next injectionshot.The fill pattern monitor needed to be reclassified whenpreparing of BESSY II for top-up operation. In the transitionfrom an optional diagnostic element to an essential coresystem of top-up operation, it has been duplicated with a hotspare system and can be complemented with high-resolutionphoton counter based on an avalanche diode that producesan accurate measurement of the fill pattern every 15 s – 30 s.Global beam current and lifetime measurementUsed to determine whether and when an injection shot isexpected to be initiated.Other inputs like the state of extraction- and injection-pulsers as well as the overall injector status from linac tobooster synchrotron are monitored and evaluated to decidewhether top-up injections can be carried on or not.Controlled ElementsAfter an injection, the injector is switched into suspendmode by switching off the extraction trigger pulse for thelinac-gun as well as the trigger for the booster-injection-pulsers. This measure is taken to avoid activation of theseptum as well as to protect the electron gun and the pulserklystrons from premature aging, because the complete injec-tor is operated with a 1Hz repetition rate.When preparing for an injection, the injector is resumed,and the linac is set up to produce the determined numberof pulses with the corresponding time interval in between.Currently, the number of pulses varies between 1 and 5 whilethe interval is typically 12 ns. This satisfies the requirementsof the currently used fill pattern (induced by the timing gran-ularity of slicing laser systems), but may change anytime.When all preconditions are met and an injection shotis imminent, the timing system is configured to place theinjected bunches at the selected position in the storage ringand the global injection trigger is switched on.Program StructureThe main program consists of three independent parts:The finite state machine implements the core enginethat controls and reflects the state of the top-up system andthe storage ring. It consists of five main states and some fewtransitional states (see Figure 1).Combining all incoming status information from inter-locks, measurements and configuration setup and performingthe appropriate state transitions, the state machine providesfull automation of the injection process for filling the ma-chine from scratch as well as running BESSY II in top-upoperation mode.Figure 1: Simplified state diagram showing only main statesand companion modules.The fill pattern analysis asynchronously processes themeasured fill pattern and decides which bunch group wouldbe topped up with the next injection shot regardless of whenit would actually happen.This is a process that is triggered every second on incom-ing fill pattern measurement data. According to the currentplanned fill pattern, all possible positions and bunch groupsto aim an injection shot at are checked, and the combinationthat lacks the most total current relative to the desired fillpattern is chosen.The EPICS realtime database is the sole interface tocontrol the state machine as well as the fill pattern analysis.It also reflects the internal status of the fill pattern control en-gine and holds many informational parameters for operationsstaff, analysis and statistics.The advantage of using an EPICS realtime database asthe sole control- and monitoring-interface to a backgroundserver process is, that all parameters, whether they are con-figuration or information, are provided as standard EPICSprocess variables (PVs) and hence integrate perfectly intothe underlying control system. Other applications may ac-cess these PVs just like any other PV in the control system.They are archived and alarm-monitored and can be moni-tored and controlled from any control system console as wellas displayed on user-information monitors and in web-basedstatus displays.In terms of control system infrastructure, the Top-Up Ser-vice and Fillpattern Control Engine behaves just like anyother device in the BESSY II control system.Human Machine InterfaceThe verbose user interface panel (see Figure 2) providesaccess to all configurable parameters of the control engineas well as all relevant diagnostic and interlock informationand several parameters internal to the control software.Target current and positions of all currently supportedbunch-groups are configured through this interface and de-fine the desired planned fill pattern.A simple event/message log provides a quick overviewof overall top-up operation events, and a reduced set ofinformational PVs like the result of the fill pattern analysisis displayed in this panel. The full set of internal statusinformation is accessible in a separate more verbose panel.Links to a few auxiliary software modules like injectorstandby/resume, linac controls and RF-knockout gap controlare also provided for easy access.The fill pattern diagram shows the current live fill patternas well as the underlying desired fill pattern calculated fromthe configuring parameters. It also displays the position andscaled up shape of the latest increase in the measured fillpattern, which is the last seen injection shot. Optionally therange cleared by RF-knockout during every shot for 50ms,to maintain a high purity of the camshaft bunch, is alsodisplayed.The top-up service determines some further diagnostic in-formation that are not of immediate relevance for the top-upinterlock systems, but might point to subsequent problems ifnot taken care of. An example is the per bunch booster cur-rent, that prevents injections on camshaft- or PPRE-bunchif it does not exceed the minimum limit.Fillpattern Control Engine — ConfigurationDevelopment over the last three years lead to the currentscheme of defining the desired fill pattern. The overall totalcurrent and four groups of bunches can be configured tomatch the current needs:Multibunch fill The part of the ring that will be popu-lated with equally filled bunches is defined by exclusion. Thelength and position of the ion-clearing gap are configuredand hence define the multibunch fill, which can in turn besetup to have every single or every n-th bunch filled.Camshaft bunch A single bunch at a fixed position inthe purity-controlled gap. Only the current stored in thisbunch can be configured. This bunch is used for standardpump/probe experiments, but also for single-bunch experi-ments at full rate and intensity using the phase locked MHzpulse selector [5], a chopper wheel synchronized to the cir-culating beam opening a 70 ns window for the light pulse at1.25MHz.PPRE bunch For a different scheme of pseudo singlebunch experiments, a dedicated single bunch in configurabledistance, but close to the end of the gap, is filled.Slicing bunches For femtosecond experiments, a groupof three bunches, usually opposite of the camshaft bunch,are filled with higher currents than the standard multibunchbunches. These are to be sliced one at a time with a fs-laserat a 6 kHz rate.All these four groups or any subset may be configured,defining the desired fill pattern for the next user-run.Error HandlingProblems that may occur during top-up mode are dividedinto five classes:Top-up interlock Causes pausing or termination of top-up operation (includes full or partial beamloss).Positioning mismatch A shot that is not appearing atthe position it was targeted at may be caused by a timingfailure, or a diagnostic problem. No further reliable exactpositioning is possible until resolved. Immediate fallback toround-robin positioning of injection shots.Injector failure Resulting in insufficient booster cur-rent will suspend injections for some or all bunch groups.Depending on the actual available booster current, the fillpattern control engine may leave out any bunch-groups thatcould not be filled without violating the minimum boostercurrent limit for a proper injection efficiency measurement.I2-limit exceeded Impedance related power deposit incomponents scales with the sum of the squared bunch cur-rents. To protect sensitive components, an appropriate limitis defined.Persistent efficiency problems Since top-up interlocksare inactive during beamscrubbing, the fill pattern controlengine itself will suspend injections to avoid activation ofcomponents and damages.Figure 2: Top-Up Service and Fillpattern Control panel in standard multibunch hybrid mode. It contains a multibunch train,a purity controlled high current camshaft bunch in an ion clearing gap for pump/probe experiments and a mechanical pulsepicking chopper, three high current bunches for femtosecond slicing and a specific bunch at the end of the gap for PPRE.All these error conditions are handled properly by eithersuspending injections until the error-condition disappearsor falling back into decay mode until problems are fixedmanually by operations staff.Reliable countdownSensitive experiments require a reliable prediction of startand end of the decay-phases between two injections.In top-up operation mode, after each shot an estimationis made on when the next injection shot will be necessarydepending on actual current and lifetime of the stored beam.Users can rely on this countdown, so even if beam-lifetimeis decreasing e.g. due to insertion devices moving at lowgaps, no injection is triggered before the reliable countdownhas elapsed.Fillpattern Control Engine – Next StepsAn update to the fill pattern control engine is currentlyin development, that has a more general approach. It willenable configuration of an arbitrary number of bunch-groupswith a few general parameters:• Affected bunches – range definition in MATLAB nota-tion: startpos:endpos:stepwidth.• Current to be stored in every bunch of this group.• Additional flags e.g. whether the given current may beadjusted to match the desired overall target current.• Prioritization of bunch groups.Besides the currently supported fill patterns, this schemewill enable special fill patterns like alternating bunches withhigh and low current which is used as a test setup for BESSY-VSR [6] studies, or additional bunches with varying currents(down to 5 µA) for even shorter pulses in low-α mode.CONCLUSIONA fully automated injection control software is absolutelyessential for top-up operation with a fill pattern tailored tothe users needs at BESSY II.It has also completely replaced the manual injectionprocedure during low-α runs, machine- and beamline-commissioning and machine studies as well as for beam-scrubbing at high stored currents.REFERENCES[1] S. Khan, et al., “Femtosecond Undulator Radiation from SlicedElectron Bunches”, Phys. Rev. Lett. Vol. 97, Iss. 7 (2006).[2] P. Kuske, et al., “Preparations of BESSY for top-up operation”,EPAC’08, Genoa, Italy (2008).[3] K. Holldack, et al., “Single bunchX-ray pulses on demand froma multi-bunch synchrotron radiation source”, Nature Commu-nications 5, Article number 4010 (2014).[4] F. Falkenstern, et al., “BunchView / a fast and accurate bunch-by-bunch current monitor”, DIPAC’09, Basel, Switzerland(2009).[5] D. F. Förster, et al., “Phase-lockedMHz pulse selector for x-raysources”, Optics Letters 40, (10), 2265-2268 (2015).[6] “BESSY VSR, the variable pulse-length storage ring proposedby Helmholtz-Zentrum Berlin”, [Online],http://www.helmholtz-berlin.de/zentrum/zukunft/vsr/summary_en.html

Comprehensive Fill Pattern Control Engine  Key to Top Up Operation Quality

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Comprehensive Fill Pattern Control Engine Key to Top Up Operation Quality

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