The efficient exploitation of the CERN accelerator complex in the future, with new cycles to fill the LHC and possibly a dedicated neutrino cycle in addition to the actual fixed-target program, will require a rapid and coordinated response to adapt to the changing user requests. This paper reviews the general sequencing problem and describes some preliminary concepts and algorithms suitable for managing a network of accelerators. The benefits derived from the architecture that has already been implemented in the PS complex, since its start up in March, are presented. The last accelerator in the injector chain, the SPS, is currently running fixed super-cycles. Its event-based timing system will be integrated into the central control by the year 2001 in a way that is transparent to the SPS equipment

Bau, J C

Jonker, M

Lewis, J

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The central beam and cycle management of the CERN accelerator complex

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONCERN/PS 99-054 (CO)CERN/SL 99-064 (CO)The Central Beam and Cycle Management of the CERN Accelerator ComplexJ.-C. Bau, M. Jonker, J. LewisAbstractThe efficient exploitation of the CERN accelerator complex in the future, with new cycles to fill the LHC andpossibly a dedicated neutrino cycle in addition to the actual fixed-target program, will require a rapid and coordinatedresponse to adapt to the changing user requests. This paper reviews the general sequencing problem and describes somepreliminary concepts and algorithms suitable for managing a network of accelerators. The benefits derived from thearchitecture that has already been implemented in the PS complex, since its start up in March, are presented. The lastaccelerator in the injector chain, the SPS, is currently running fixed super-cycles. Its event-based timing system will beintegrated into the central control by the year 2001 in a way that is transparent to the SPS equipment.Presented at International Conference on Accelerator and Large Experimental PhysicsControl Systems (ICALEPCS'99), October 4-8, 1999, Trieste, ItalyGeneva, Switzerland18 October 1999THE CENTRAL BEAM AND CYCLE MANAGEMENT OF THE CERNACCELERATOR COMPLEXJ.-C. Bau, J. Lewis, M. Jonker, CERN 1211 – Geneva SwitzerlandAbstractThe efficient exploitation of the CERN acceleratorcomplex in the future, with new cycles to fill the LHC andpossibly a dedicated neutrino cycle in addition to theactual fixed-target program, will require a rapid andcoordinated response to adapt to the changing userrequests. This paper reviews the general sequencingproblem and describes some preliminary concepts andalgorithms suitable for managing a network ofaccelerators. The benefits derived from the architecturethat has already been implemented in the PS complex,since its start up in March, are presented. The lastaccelerator in the injector chain, the SPS, is currentlyrunning fixed super-cycles. Its event-based timing systemwill be integrated into the central control by the year 2001in a way that is transparent to the SPS equipment.1  HISTORYCERN is organized into divisions, among which twoare responsible for the accelerators of the PS and SLcomplexes. The PS complex consists of low-energy shortcycle machines such as the proton, ion and lepton Linacs,the Electron-Positron Accumulator (EPA), the ProtonSynchrotron Booster (PSB), and the CERN ProtonSynchrotron (CPS). The SL complex comprises the largerhigh-energy slow cycling machines such as the SuperProton Synchrotron (SPS) and the Large ElectronPositron collider (LEP). The evolution of the timingsystems controlling these two groups of machinesoccurred separately, and has in each case responded todifferent requirements. In particular, the SPS super-cycle(cycle-sequence) has not changed very often compared tothe PS machines. This is not only because it was notrequired, the SPS was not an injector for a higher energymachine (the filling of LEP was performed by twoparasitic cycles), but also because of machine constraintssuch as remnant magnetic fields and energy consumption.In the CPS, it is possible to ignore to a limited extentcycle-sequence effects and hence to consider cycles asindependent building blocks. The SPS timing system didnot need to make any major cycle change decisions. Forevery fixed SPS super-cycle with its predefinedrendezvous points at which the PS injector chain injectsparticle beams, a simple table driven event timing systemwas adequate to do the job. This simple timing executionsystem was, however, augmented by a real-time decisionprocess that could alter the usage of a cycle (to put theSPS in economy mode or into coast). On the other hand,the message-based PS timing system has been designed torespond to rapidly changing operational requirements,and can change super-cycle components on the fly basedon beam requests and interlocks. When the Large HadronCollider (LHC) comes on line, the SPS and the PSmachines will need to work together as a single entityexecuting many super-cycle changes per day in order tosatisfy LHC filling interleaved with SPS Neutrino usersand other fixed target physics. These new requirementshave motivated among others the SPS 2001 controlsproject [3], and specifically the proposal for centralizedmanagement of the timing systems via the Central Beamand Cycle Manager (CBCM).2  THE CENTRAL TIMING SYSTEMThe machines of the PS complex execute cyclesarranged to be small multiples of a Basic time Periods(BP) of 1.2 seconds in sequences. Each BP is describedby a telegram message which is distributed from a centralMaster Timing Generator (MTG) to each machine in thecomplex, telling them which cycle to execute next. TheMTG also interprets these messages itself and derivesfrom them a set of key timing events. [1] The messagesand key timing events are distributed on a timing drop netto timing reception modules which are able to producelocal output pulses and interrupts for real-time tasks. TheMTG thus has two functions: a) the cycle decision level,where decisions are made for all the acceleratorsdetermining which cycles they should be executing, andb) the timing execution level, which outputs the timingevents according to the chosen cycles.Figure 1 Central Beam and Cycle ManagerIn the SL complex, only a timing execution system wasimplemented and there was no high level function thatchooses cycles. With the new LHC requirements, itbecame necessary to include the SPS conditions andrequests into the central decision system, and to generatethe SPS timing from the resulting messages. The SPStiming execution system will be adapted such that it canbe driven by the timing messages from the central MTG.This approach will protect the investment in SPS timingreception systems a little longer. In addition, this LocalBeam and Cycle Manager (LBCM) system can continueto provide the fast real-time response required for energysaving, when the beam has not been properly injected, orto handle coast requests. The CBCM Fig 1 is thus anextension of the existing PS MTG to include the SPSrelated decision logic, and the interfaces needed to accessit form the new SPS control system.3  CONSTRAINTSIt is possible to control the accelerator complex on acycle basis, but there are many restrictions on how thesecycles can be sequenced. The cycles and sequences thatcan be executed depend on data that must be maderesident in the local equipment, on machine physics, andon other factors. For these reasons, the central timingsystem was constrained to producing predictablesequences that can be scrutinized in advance by theoperation teams of the machines. A general algorithm toperform this task reliably has not yet been devised. Amajor problem arises because the SPS and PS operationsteams work in separate control rooms kilometers apart,and each team has specialized knowledge about themachines under their control; so while a sequence may beexecutable by the SPS, it may not be executable by thePS. The PS may not be able to execute the requestedsequence due to short lived bad hardware interlocks,higher priority requests from other machines, or a host ofother physics and control system constraints.4  SEQUENCE BUILDINGA Compound Cycle is defined as a sequence ofCycles executed in one machine after another, across thechain of accelerators to produce a final product for an enduser, and a Cycle is a unit of work executed by a singleaccelerator [2]. As an example, the Compound Cycle tomake a beam for SPS fixed target physics begins withprotons in the linac, they are then accelerated in the PSB,a further acceleration in the CPS, then in the SPS, andfinally, the beam is ejected to the fixed target user.  Theoperator interface to the CBCM provides capabilities todefine cycles, to arrange these cycles into CompoundCycles, and to build sequences of Compound Cycle tospecify the operational requirements for CERN machines.This is an interactive process guided by the operationteams who implement the decisions taken at schedulingmeetings which are in turn guided by the physicscommunity needs, and priorities. The result of thisprocess determines a set of Compound Sequences, each ascenario for a particular set of operations, like an LHCpilot cycle followed by two neutrino cycles. Any of theseconstructed Compound Sequences can be loaded by theCBCM into the central MTG for execution. The CBCMselects the best possible sequence for execution, based onrequest priorities.When a sequence has become active in central MTGmemory, it is checked in real time against a rule baseconcerning hardware and software interlocks andrequests. At this stage a Compound Cycle may besubstituted or even canceled if need be. While editing thecompound sequences, the operator also specifies thealternative Compound Cycles that may be used forsubstitution.The SPS Compound Cycle sequence determineswhich cycles must be executed, where and when in theaccelerator complex, but this leaves many unused injectorcycles which can then be used by the PS local usersFig 2. Thus the final compound sequence is the result ofmerging the SPS and the PS requirements; and this isusually an interactive process requiring operator guidanceThe operator interface is able to check candidatesequences against a rule base for the more permanentconstraints to inform the operator of potential problems inthe sequence he is building.Figure 2 Cycles and Compound-CyclesThe CBCM must provide an interface that can betailored to provide each control room with a clear pictureof what is happening in their own terms and even hideirrelevant information on each side.  To set up a CBCMsequence for the SPS the Compound Cycles must first bedefined; this must be done in collaboration. A namedtemplate must be built to define the cycle time structurein each accelerator. Once this structure has been defined,the details of the cycles of the PS and SPS accelerators init are filled in separately by the machine specialistsaccording to the beam requirements, intensity, energy,particle type etc.  The SPS operators can then build asequence of defined Compound Cycles, they do not needto see what happens in the injectors, a sequence isnothing more than a list of cycle names which result in anSPS super-cycle. When the new sequence is sent to theCBCM, the PS operators must then manually define theirown Compound Cycles in the unoccupied space. Thesequence will then usually be made available forexecution if no rule violations have occurred. The processof constructing new sequences for the SPS will not bevery frequent, it will be done on a machine run bases,once a month.  The PS however will change itsCompound Cycles on average once per hour and mayhave several versions of the same SPS sequence residentin the CBCM but with different PS Compound Cycles forrapid PS response to its user requests.5  RUNTIME SEQUENCEEXPLOITATIONA Compound Cycle is a set of cycles to be executed insequence by a network of connected accelerators. Theprecise duration of each cycle, and the phase relationshipsbetween the cycles are all that is needed to address theproblem of determining what cycle each accelerator in thenetwork should execute at any given moment in time.Each basic period BP, the central MTG must send to eachaccelerator a message, instructing its control system whichcycle it should be executing. The timing event layer, theLBCM, also receives the message and sends out theindicated cycles timing events accordingly.Notice that the structures of the Compound Cyclesthemselves define the network topology, and the cycletime structure. With this information the sequencingalgorithm simply routes the beams through the networkone by one, in the order given, by arranging the cyclestart times under the constraint that no accelerator canexecute more than one cycle at a time. This implies astandard network routing algorithm where the nodecapacity for all nodes is one. Once the decision has beentaken it can not be revoked; a Compound Cycle is atransaction which once injected into the network must runto completion, even if during execution the beam is sentto the emergency dump, or killed in the linac. It should benoted that execution of a Compound Cycle begins at thetime of the earliest forewarning event. The central MTGexamines all the cycles in each Compound Cycle andtests for their “executability” against the interlocks,requests, and inhibits. If all these preconditions arefavorable, then it decides to commit the CompoundCycle, if not it looks for an alternative. Hence theresponse time of the central MTG to a changing conditioncan be several tens of seconds. Because of this, theemergency fast real time system behavior (<100 ms) mustbe implemented in the LBCM layer, and will occur laterduring cycle execution.6  RUNTIME SEQUENCE SELECTIONOnce the SPS sequences have been accepted by theCBCM and merged with the PS beams, they may beexecuted by the central MTG. A sequence is activated bythe CBCM based on a number of prioritized cyclerequests coming from the SPS control system. TheCBCM assigns a score to each sequence based on therequest priorities, and produces a list of candidatesequences ordered by score. The behavior of the systemsubsequently depends on what options have been set bythe control room operators, namely automatic or manual.In the manual case, the operators are informed that thereis a sequence ready for execution with a higher score thanthe current active sequence. There could be reasons whythe PS operators may whish to override the SPS Beamrequests, such as hardware problems, or high priority PSusers. In the automatic case, the new sequence will beactivated based on the highest score. Each residentsequence in the CBCM can have both the entry and exitconditions set to either automatic or manual. Notice thatthe SPS control system normally never requests directlythat a particular sequence is executed; it is the prioritizedCompound Cycle requests, which determines the order ofcandidate sequence priorities via an algorithm. Thisalgorithm takes into account machine usage: non-demanded and hence unused Compound Cycles in asequence will reduce the score, while each instance of arequested Compound Cycle will increase the score by anamount proportional to the priority of the request. Thealgorithm will have a weighting function that can betuned by the operators so that the CBCM sequence-picking behavior can be adjusted.7  CONCLUDING REMARKSMany of the features needed by the CBCM havealready been included in the PS MTG in a major redesignneeded to incorporate “loose-coupling” for the slowcycling Antiproton Decelerator [1]. Notably in March1999 the ability to switch between resident sequences wasadded. This has proved to be more flexible, and lessdemanding on the operation team, who no longer need tomanually program it so often.  However, the MTGbehavior depends on, the resident sequences, on requestsand interlocks, inhibits, the compound cycle and cyclestructure, and it is very often difficult for the operationteams to interpret; this has led to some confusion.Diagnostic tools, which explain the behavior of thesystem, are thus very important, and this will certainlyaffect the full implementation of the CBCM that we hopewill become operational for the start up by March 2001.At this time, we are still very much in the design stage,however we are planning to implement the CBCMentirely in Java using a client-server model makingextensive use of RMI, JDBC, and Swing.8  REFERENCES[1] J.-C. Bau, G. Daems, J. Lewis, J. Philippe, Managingthe behavior of a particle beam factory: the CERNProton Synchrotron Complex and its timing systemprinciples, Proceedings Real Time 97, Beaune,France, September 22-25, 1997, p.553.[2] J. Lewis, V. Sikolenko, Providing the new CERN PStiming system, Proceedings ICALEPCS'93, Berlin,Germany, October 18-23, 1999, p. 91.[3] P. Charrue, B. Denis, M. Jonker, M. Vanden Eynden,WC1P08 (259), Upgrading the SPS operationalsoftware for the LHC era - The SPS-2001 softwareproject; submitted to ICALEPCS'99, Trieste, Italy.

The Central Beam and Cycle Management of the CERN Accelerator Complex

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