advanced light source's approach to ensure conditions for safe top-off operation

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Nuclear Instruments and Methods in Physics Research A 608 (2009) 2–18

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Corr

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journal homepage: www.elsevier.com/locate/nima

Advanced light source’s approach to ensure conditions for safe top-offoperation

Hiroshi Nishimura, Barry Bailey, Ken Baptiste, Walter Barry, Warren Byrne, Patrick Casey,Michael Chin, Richard Donahue, Robert Duarte, Alex Gavidia, Jin-Young Jung, Michael Kritcher,Patrick McKean, Robert Mueller, Greg Portmann, David Robin �, David Rodgers, Fernando Sannibale,Tom Scarvie, Alexis Smith-Baumann, Christoph Steier, Max Vinco, Tony Warwick, Weishi Wan,Jonah Weber, Russell P Wells

Lawrence Berkeley National Lab, 1 Cyclotron Road Mail Stop 80R0114, Berkeley, CA 94720-8229, USA

a r t i c l e i n f o

Article history:

Received 25 March 2009

Received in revised form

30 May 2009

Accepted 30 May 2009Available online 21 June 2009

Keywords:

Synchrotron radiation source

Third generation light source

Top-off/top-up injection

Radiation safety

02/$ - see front matter & 2009 Elsevier B.V. A

016/j.nima.2009.05.196

esponding author.

ail address: [email protected] (D. Robin).

a b s t r a c t

The purpose of this document is to outline the Advanced Light Source (ALS) approach for preventing a

radiation accident scenario on the ALS experimental floor due to top-off operation. The document will

describe the potential risks, the analysis, and the resulting specifications for the controls.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the storage ring of a synchrotron light source, the beamcurrent continually decays due to a number of beam lossmechanisms. In the traditional mode of operation, user beamtime is interrupted every few hours in order to inject electronsinto the storage ring thus restoring the beam current. Top-offinjection (sometimes called top-up injection) is a newer mode ofoperation where beam electrons are frequently (about everyminute) injected into the storage ring to keep the current in thestorage ring quasi-constant. By frequently replacing the electrons,the integrated brightness is higher and the thermal variations onthe synchrotron user’s beamline optics due to changes in beamcurrent remain lower. Both of these effects make top-off injectionvery attractive as compared with the more traditional ‘‘decaymode’’ of operating. The example shown in Fig. 1 is that of theAdvanced Light Source (ALS) where the older decay mode iscompared with the newer top-off mode of operation.

Top-off is rapidly becoming the standard injection mode formodern storage ring based synchrotron light sources. TheAdvanced Photon Source (APS) was the first light source tooperate with top-off injection [1], and several others such as the

ll rights reserved.

Swiss Light Source (SLS) [2], Spring-8 [3], Taiwan Light Source [4],and the Advanced Light Source are presently operating with top-off injection. In addition there are plans for Stanford SynchrotronRadiation Laboratory (SSRL), SOLEIL, DIAMOND, Shanghai Syn-chrotron Radiation Facility (SSRF), and others to transition to top-off operation in the near future.

From a radiation safety point of view there is a qualitativedifference between decay and top-off mode operation. In decaymode, the safety shutters for each beamline remain closed duringthe injection period. Doing this assures that the injected beamcannot exit the storage ring shield wall through the beamlines andcreate unacceptably large radiation doses on the experimentalfloor. The present top-off plans call for injecting a charge ofabout 1 nC approximately every 30 s. In contrast to decay mode,top-off mode requires that the safety shutters be left open duringinjection. Thus for top-off, alternative controls need to be inplace to provide protection equivalent to the safety shutters.Implementing such controls involves detailed safety analysissuch as electron beam tracking studies and radiation trans-port simulations, and additional mitigating design features suchas controlled beam apertures and additional safety interlocksystems.

In this paper we describe the approach taken at the ALS toensure adequate protection. We start with the radiation hazards,go through the analysis to determine the controls, and finallydefine the specifications for the controls. Such information and

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Fig. 2. Clock diagram of the advanced light source. The source of radiation from

each beamline is labeled as nominal bend (B), superbend (SB) or as insertion

device (ID).

Fig. 3. Two sectors of the ALS. A normal sector on top and a sector modified to

include superbends. There is also an additional family of quadrupoles, QDA, in the

modified sector.

Fig. 1. Comparison of ALS ‘‘decay mode’’ injection operation with top-off injection

operation.

Fig. 4. Typical arc sector sho

H. Nishimura et al. / Nuclear Instruments and Methods in Physics Research A 608 (2009) 2–18 3

experience, including lessons learned, can be valuable to otherfacilities that are considering operation in top-off mode.

The paper is organized in the following way. Section 2 presentsspecifics of the ALS that are relevant for top-off operation. Section 3quantifies the hazards of the injected beam defining theacceptable levels. Section 4 presents the general approach thathas been used to evaluate each beamline and to guide specifica-tion of in specifying the controls necessary to ensure safeoperation. Section 5 presents the specific field profiles and rangesthat are used in the tracking simulations. Section 6 presentsthe apertures and tolerances used in the tracking simulations.Section 7 presents the accuracy and time response for thetolerances are specified for the controls. Section 8 presentssummary remarks including general lessons learned and advicefor others. Finally in Appendix A a tracking example is presentedfor a subset of the beamlines.

2. Specifics of the ALS

Before describing the approach that has been taken at the ALSto ensure safe top-off operation we will give a brief description ofthe ALS facility. Particular attention will be paid to points that arerelevant for top-off.

The ALS is a 1.9 GeV third generation synchrotron light source.The clock diagram of the ALS showing the accelerators andbeamlines is shown in Fig. 2. As of December 2007 there are 43beamlines emanating from the storage ring. The source of theradiation for each beamline is either an insertion device (labeledID in Fig. 2), a normal bend (labeled B in Fig. 2), or a super-conducting bend (superbend, labeled SB in Fig. 2).

The storage ring consists of 12 sectors. Each sector has astraight section approximately 6 m long and an arc approximately11 m long. The numbering of the sectors starts with the injectionstraight and proceeds clockwise. Each arc consists of a triple bendachromat structure in which each of the three dipoles has abending angle of 103. In three sectors of the storage ring (arcs 4, 8,and 12), the central bend, whose field on the stored beam orbit is1.3 T, has been replaced with a higher field superbend, whose fieldreaches up to 6 T. Therefore, in total there are 36 dipoles—33normal-conducting and 3 super-conducting. Fig. 3 shows themagnet layout for two sectors—a sector without a superbend andone with a superbend drawn from the middle of the straightbefore the arc to middle of next straight.

Each sector contains up five ports where a beamline can beplaced. Fig. 4 shows the arc magnets and vacuum chamber alongwith all of the ports of a sector. In the figure, ‘‘X’’ represents thesector number. The X.0 port is fed by the light emitted frominsertion device(s) in the previous straight. The X.1 port emergesat 6:33 through the first bend, the X.2 port emerges at 2:63 throughthe second bend, the X.3 port emerges at 7:43 through the secondbend, and the X.4 port emerges at 2:663 through the third bend.

The ALS injection system, shown in Fig. 5 and consists of a50 MeV linac followed by a 1.9 GeV booster synchrotron. Beam isemitted from the thermionic gun in a pulse train. The number ofbunches in the train is 10 or less and the total charge is a few nC.

wing all possible ports.

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Fig. 6. Dose from a 1.9 GeV, 1 nC electron beam hitting an optimum thin target

radiator. Dose is calculated as a function of incident electron beam angle and at a

distance of 30 cm from the target. Results were calculated with the SLAC SHIELD11

[12] analytic model.

Fig. 5. Diagram of the ALS injection system.

H. Nishimura et al. / Nuclear Instruments and Methods in Physics Research A 608 (2009) 2–184

The 50 MeV linac has a maximum repetition rate of 10 Hz butusually is operated at 1 Hz or slower. The booster synchrotron canaccelerate a bunch train from the linac at up to 1 Hz from 50 MeVto 1.9 GeV.

3. Top-off injection and radiation safety implications

In decay mode, electrons are injected into the storage ringapproximately once per 8-h shift, bringing the stored beamcurrent from �250 to 500 mA (see Fig. 1). As discussed earlier, inthis mode the safety shutters inside the storage ring shield wallsare closed to prevent the possibility of the injected beam frombeing inadvertently sent down a beamline frontend and out ontothe experimental floor resulting in unacceptably high dose rates.

The worst case potential dose is illustrated in Fig. 6 plots theradiation from a single 1 nC electron bunch hitting a target oftheoretically optimal thickness. The dose is calculated as afunction of angle with respect to the incident electron beam andat a distance of 30 cm from the target. Such a model is highlyidealized but represents the theoretical worst case. The calculateddose is highly forward directed due to the angular dependence ofbremsstrahlung X-rays (dotted line) while the neutron component(dashed line), mainly due to giant dipole resonance reactions, isroughly isotropic. As can be seen from the vertical dose scale, thepossibility of a single injected electron bunch escaping the shieldwall must be prevented. Because of the large number ofexperimental complexities involved in booster and storage ringoperations, the impossibility of this scenario can only bedemonstrated through detailed tracking studies of injectedelectron beam. These studies are discussed later in Section 4.

Other processes directly or indirectly related to top-offupgrades may result in increased stray radiation levels on theALS experimental floor. To differentiate between these we definetwo categories of beam loss (and resulting radiation doses):

�
Type 1 losses—those beam losses resulting from the injectedbeam being lost beyond the point that is 1 m inside of thestorage ring shielding wall. � Type 2 losses—all other beam loss processes.
3.1. Type 1 losses and the ‘‘1 m criteria’’

Above we demonstrated that the injected beam exiting theshielding wall could create unacceptably high radiation levels.Therefore one must determine a point inside the storage ringshield wall where the injected beam can be allowed to impactunder certain conditions and still be considered safe to personneloutside the shield wall. This point may be at the inside surface ofthe shield wall or it may be some distance further upstream fromthe inner surface of the shield wall.

Simulations were performed with the MCNPX [5] radiationtransport code to determine the dominant forward directedbremsstrahlung dose rates from an electron beam hitting thefrontend beam pipe at various distances from the inner surface ofthe storage ring shield wall. A worst case, 6 in. diameter steelbeam pipe with 1

8 in: wall thickness is modeled as penetrating atypical storage ring transition wall normal to its surface. A typicaltransition wall consists of a 3 in. thick Pb belly band (approxi-mately 1 ft high centered at beam height) followed by 3 in. of Pbfloor-to-ceiling and then the 18 in. thick concrete plug (14 in. highby 14 in. wide) replaced with 6 in. of Pb and 12 in. of polyethylenewhere beamlines have been constructed. Since dose rates in theforward direction are dominated by bremsstrahlung X-rays (seeFig. 6), neutron production and transport are turned off. Thepolyethylene section of the storage ring transition wall is modeledas a void. A 1.9 GeV electron beam is allowed to impact the beampipe at various distances from the shield wall and at variousangles with respect to the pipe surface.

The results are summarized in Fig. 7. Dose rates are calculated atvarious distances outside the shield wall and at 30 cm from thebeam pipe surface. These results indicate that the highest dose ratesare on the order of 25–30 mrem from a single shot which is allowedto impact the beam pipe at a distance as close as 1 m from the innersurface of the storage ring shield wall. Based on these results astarting point of 1 m from the inner surface of the shield wall is usedas the distance of closest approach for the tracking studies.

3.2. Additional radiation monitors

Using the 1 m criteria, the levels created by the injected beamwill be at most 30 mrem/injected pulse. Given the low probability

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Fig. 7. Summary of calculated dose rates due to electron beam impacting beam

pipe inside the storage ring shield wall. Dose rates are plotted as a function of

distance from the outer surface of the shield wall. Symbols indicate various impact

angles of electron beam on the beam pipe surface as well as distance between

impact location and inner shield wall surface. All results calculated with the LANL

MCNPX transport code.


of the worst case event, this level may be acceptable for a one timeevent but is not a condition that can be allowed to persist.Therefore, as part of the top-off upgrade process the ALS is addingits existing beamline ion chambers to the Radiation Safety System(RSS) interlocks. These ion chambers are located above the firstoptic or beamline aperture on all insertion device and hard X-raybeamlines. As part of this upgrade, keep-alive radioactive sources(1 uCi 137Cs) were mounted outside the detector active volumes.Current and ongoing measurements of these systems indicate thata maximum window of 32 s is sufficient to safeguard against falsealarms while providing sufficient protection from injection beamlosses.

Current plans are to inject in top-off mode about once every15–30 s. The interlocked ion chambers can be set to trip for anintegrated dose of �5 mrad=h in as fast an integration timeinterval as 4 s (fast trip mode) and as long an integration timeinterval as 1 h (slow trip mode). Both the fast and slow trip modesoperate in parallel. An errant injected beam loss at 1 m inside theshield wall (a highly unlikely condition) producing a dose of25–30 mrem outside the shield wall would trip the safetyinterlocks before the next injected shot. Failure of the radiationmonitors would be detected within 30 s because of the keep-alivesource function. The probability of dumping an errant beam at the1 m point (off energy injection, optimized missetting of magnetcurrents, optimized displacement of important beam apertures,etc.) at the same instant the interlocked radiation monitors havefailed and will not be detected for 30 s is considered negligible.Additional interlocks added to the existing ALS RSS interlocksystem as a result of planned top-off operations are detailed laterin this document.

3.3. Type 2 losses

These are losses other than injected beam losses while theinjection safety shutters are open. Some examples of these lossesmight be from: shortened beam lifetime due to decreased(improved) beam emittance, losses from RF interlock trips,localized losses from new stored-beam scrapers, beam loss dueto increased gas pressure, etc. The possible increased radiationlevels from these conditions have been, and will continue to be,studied and mitigated by ALS staff. ALS policy does not require

experimenters to wear radiation dosimeters. Some experimenterschoose to wear them and about 60–70 of the ALS staff are asked towear them. Results over the life of the facility have identified onlya few personnel dosimeters with results above the dosimeterreporting threshold of about 15 mrem. Ambient radiation levels atthe ALS are measured by a combination of methods. The followingis an approximate summary:

1.
�12 active interlocked neutron and gamma monitors locatedon the external shield walls.
2.
�30 active (existing but interlocked with top-off upgrade)alarming ion chambers located adjacent to beamline firstoptics tanks or beam defining apertures.
3.
�60 OSL passive gamma dosimeters (TLDs prior to 2006,overlap 2006), and CR39 neutron dosimeters, located on theexperimental floor close to the shield wall (‘‘in-close moni-tors’’). Results are analyzed monthly.
4.
�12 passive OSL gamma and moderated LiB neutron monitorslocated on the periphery of the experimental floor. Results areanalyzed quarterly and annually.
These methods provide flexible spatial and temporal coverage.Passive dosimeters can be quickly and easily located or relocatedclose to potential sources to provide an early warning of increasedlevels.

4. The general approach

In Section 3 we discussed why it is necessary to prevent anyType 1 event from ever occurring. Therefore, it is essential todemonstrate that assuming reasonable possible failures andconditions, the injected beam cannot pass the 1 m point. This isthe goal of the ALS approach. We show that with sufficientcontrols in place and then by scanning over the parameters(energy, position, angle, field strengths, aperture misplacements,etc.), it is not possible to create a Type 1 event. By exhaustivelyattempting to generate an event and not being able to do so onehas demonstrated that a Type 1 event is virtually impossible.

4.1. Computer simulation versus experimental measurement

The scan is done via computer simulation. Computer simula-tions are used rather than experimentation because it is notpossible to exhaustively simulate all possible scenarios experi-mentally. There are three reasons. The first is that there are a largenumber of magnets between the injected line and mostbeamlines. It is highly impractical to experimentally evaluate allpossible combinations of settings. The second is that one does nothave a precise knowledge of the injected beam parameters. Third,unlikely events such as partial magnet shorts would be verydifficult to produce experimentally. Other laboratories such as theAPS, ESRF, and SSRL have also come to the same conclusion that itis impractical to sufficiently simulate Type 1 events throughexperimental means. Therefore computer simulation is essential.

4.2. Forward tracking

It is conceptually straightforward to imagine an injectionsimulation that begins with electrons launched at the storage ringinjection section with different initial energy, position, andangular offsets, and then track them to the beamlines to see ifany condition creates a Type 1 event.

We will refer to this type of tracking as ‘‘forward tracking’’ asthe tracking is done in the direction of the electron beam. Even

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though it is conceptually simple to understand, forward trackingfrom the injector is impractical. There are many beamlines that liefar from the injection point and requires an unreasonably largenumber of quantities to be varied in combination. Another issuewith forward tracking is that it provides no definitive demonstra-tion of safety. Many initial particles will survive for an indefinitenumber of turns. Even if a particle does not reach the beamline onthe first turn, it is not clear whether the particle could reach thebeamline on subsequent turns.

4.3. Backwards tracking

An alternative approach, termed ‘‘backwards tracking’’ [6], isdefinitive and more practical than forward tracking. With back-wards tracking one starts at the beamline and tracks towardsthe injection point. If no particle can get sufficiently close to theinjection point, then the beamline is declared safe. As thelegitimacy and practicality of backwards tracking may not beimmediately obvious it will now be explained.

In a system where there are only magnetic fields, the equationsof motion are the same if one reverses the direction (i.e. particlevelocity) and reverses the sign of the charge (i.e. electrons becomepositrons). In other words a trajectory of an electron going frompoint A to point B will have the same trajectory as a positron goingfrom point B to point A (see Fig. 8). Thus if one tracks particlesbackward with all possible initial conditions in energy, position,and angle, starting from the beamline pipe, and does not reach theinjection point because they have all struck an aperture in thestorage ring, then one has definitively demonstrated that noparticles from the injection point can exit the beamline. This isillustrated in Fig. 9.

We now explain why backwards tracking is more practical. Theinitial conditions in forward tracking are centered around theideal orbit; therefore, particles with many initial conditions will

Fig. 9. Backtracking simulations with the beam starting at the beamline (right in the fi

beam is lost.

Fig. 8. In a pure magnetic environment, the path of an electron going from point A

to B is the same as the path of a positron going from B to A.

survive for many turns. However, in backward tracking this is notnecessarily the case. The initial conditions originate in thebeamlines that are far from the ideal orbit. Thus one could easilyimagine that they may not propagate far. One can define aposition, C, that is just a few meters upstream of the beamlinelocation, B, and perhaps well downstream of the injection point, A,and declare that C will be the end of the backward trackingsimulation (see Fig. 10). If no particle can reach beyond C thenclearly the particle cannot reach the injection point. This approachis conservative in that it does not mean that if a particle passes Cthat it will reach A.

The choice of the location C is a practical one. Having too manymagnetic elements between B and C will make the simulationimpractically time-consuming. However, B and C should not betoo close or it will be impossible to arrive at practical controls toprevent all possible initial conditions from passing point C. In thecase of the ALS point C has been chosen within the same sector asthe one that the beamline is in. We will refer to C as the ‘‘trackingend-point’’. The backward tracking approach was pioneered at theAdvanced Photon Source (APS).

To summarize, for each beamline there are three required stepsto backward tracking:

1.

gure

Figequ

Determine the range of the initial coordinates of the beamlineto be backward tracked.

) and traveling toward the injector (which is to the left of the figure) until the

. 10. Illustration that if apertures are in place such that no rays go from B to A is

ivalent to showing that there are no rays that go from A to B.

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2.

Figfor

Define a backward tracking point, C, that should not be passed.
3.
Fig. 12. Four extreme rays projected to the initial tracking point.

Backward track all initial coordinates from the beamlinetowards the injector by systematically scanning all settings ofthe magnets between B and C (component failure scenariosshould be included as well). If none pass point, C, the beamlineis declared safe.

These steps will now be explained in more detail.

4.4. Defining initial backtracking coordinates

Tracking depends upon the initial particle coordinates (posi-tion and angle), the energy of the particle, the fields of themagnets, and upon apertures in the storage ring which if struckwill stop the particle. Starting with the backward trackingapproach the first step is to determine the range of initialconditions. To determine the range of angles and positions, onemust choose at least two pairs of apertures in the frontend plusthe initial tracking point. There is one constraint regarding thechoice of apertures:

�

Fig. 13. Four extreme rays plotted as points in phase space ðx; yÞ at the initial

tracking point. The phase space is then determined by connecting the points and

filling in the space enclosed.

The apertures need to be to the upstream of the storage ringside of the 1 m point to ensure that a single shot loss remainsacceptably low (see ‘‘1 m point criteria’’—Section 3.1 andFig. 7).

The range of angles and positions to be tracked from the initialtracking point then needs to be determined. Doing so requiresknowing the location and size of the apertures and the initialtracking point and the strength of any magnetic fields betweenthem, as is illustrated in Fig. 11. With this information, the initialtracking phase space can be determined. In the simplest possiblecase there are only two pairs of apertures with a field free regionbetween the pair of apertures and between the apertures and theinitial tracking point. To determine the initial phase space the firststep is to determine the four extreme rays and determine theircoordinates at the initial tracking point. This is illustrated inFig. 12. The full phase space is then determined by drawing astraight line between the four points in phase space. This isillustrated in Fig. 13.

The initial phase space can also be determined in the case of anon-field free region. In the special case of just having dipolefields, the initial phase space will be shifted from the field freeregion, but the shape will remain the same. If the fields are morecomplicated the shape of the phase space will also change.

For some beamline geometries using only two pairs ofapertures gives an unrealistically large phase space. One pair ofapertures might effectively limit the positional spread; not the

. 11. Example layout of beamline frontend including typical apertures credited

the safety tracking.

angular spread, another pair of apertures may effectively limitsthe angular spread but not the positional spread. To extend theforegoing analysis to more than two pairs of apertures one drawsthe phase space from each of the different pairs, plotting them ontop of each other, and using only the region where the regionsoverlap (the intersection of all phase spaces). For very complexbeamline geometries with multiple credited apertures or verycomplex aperture shapes, one can also employ simple trackingcodes (performing straight line tracking in field free regions) thatdetermine the complex shape of the overall beamline phase spaceacceptance.

4.4.1. Treatment of the vertical plane

This analysis for the initial phase space can be done for boththe horizontal and the vertical plane. However, for practicalcomputational reasons it is desirable to minimize the dimension-ality of the initial conditions. Therefore in the ALS studies theanalysis is only done in the horizontal plane. This limitation isallowable because the effect of being off axis vertically can beincluded as an increase in the vertical magnetic field range for thehorizontal tracking. One needs to assume only horizontalobstacles in the storage ring. (We will take no credit for thepossibility of a beam striking a vertical aperture.) Therefore, theonly loss mechanism is for a particle to strike an aperture inthe horizontal plane. This is a conservative assumption. Thesecond assumption is that the horizontal motion is determined byonly the interaction of the particle’s velocity in the horizontalplane with the vertical magnetic field. (We neglect the interactionof the vertical particle velocity with the longitudinal fieldsbecause it is negligibly small.) Now vertical offsets can beaccounted for by extending the range of the vertical fields, as itwill be shown in Section 4.4.1.

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4.4.2. Energy range

The remaining initial parameter is the range of energymismatch—the difference between the stored beam and thebeam coming from the injector. This range can be very large if notcontrolled. In the ALS case the tracking simulations will show thatthe range of allowable energy mismatch will need to be restricted.

4.5. General tracking assumptions

Once the initial tracking coordinates have been defined, it isnecessary to determine if any of the initial conditions will bepotentially unsafe by backtracking towards the injection point.Hence in addition to the initial tracking rays one must know thevalue and location of fields and apertures in the storage ring.

4.5.1. Magnetic fields

The field at a given location is a function of the magnetgeometry, power supply current, vertical offset of the particle, andmagnet shorts. For most of our magnets we use a 1-D field profilebut check the impact of longitudinal fringe fields for the magnetsgiving ByðxÞ. An example is the field of a quadrupole magnet (seeFig. 14). As seen in the figure the field is far from linear and turnsover as one gets beyond the poles of the magnet.

4.5.2. Assumption of small angles

As the injected particles travel at large amplitudes, the fieldsneed to be rather accurate at large distances from the center of themagnets. Because the angle of the trajectory can be largecompared to the trajectory of the nominal stored beam, onecannot assume a small angle approximation which is often donein tracking codes. This simplification was not made in oursimulations.

4.6. Accounting for abnormal and failure scenarios

For the tracking, human errors as well as system failures wereassumed to determine the range of the fields. To study howdifferent effects can be combined to determine the range ofparameter scans, we looked at the probability of the occurrence ofevents considering two categories. The first was exceedingly lowprobability events that might happen at most once in the lifetimeof a facility. The only fault that fell into this category is theprobability of certain combinations of magnet coil short faults.This was done by examining the various magnets (quadrupole,sextupole, and dipoles) looking at which pairs of coils were closeand determined what the field impact would be if they shorted.With the exception of one coil short that was discovered whencommissioning the ALS in 15 years there were no other shorts. We

Fig. 14. 1-D field ðByðxÞÞ for a quadrupole magnet.

also surveyed a great deal of light sources including (NSLS UV andX-ray rings, CAMD, SPEAR, SRS, NINA, APS, ELETTRA) and mostfacilities had no shorts.

The second category contained higher probability events suchas human missettings, feedback systems, power supply trips,shunt failures. These events would occur more often than once inthe lifetime of a facility. The way that the events were combinedwas in all possible combinations of higher probability event witheither none or just one low probability event.

4.6.1. Checking assumptions and code

A simulation code was developed for the backward trackingstudies. An example of the output of the code is shown in Fig. 9. Inthis simulation it is possible to set the size of the phase space andenergy range to be tracked, the range of the vertical fields of themagnets and location of the apertures. In addition it is alsopossible to specify all possible coil shorts in a magnet.

Due to the complexity of the calculation and the fact that thisis a safety relevant analysis there were significant checks of boththe assumptions as well as the code. The simulation code waschecked in a number of ways. The effect of ignoring thelongitudinal fringe field was checked and the discrepancy wassufficiently small [7–11]. The code was checked by an indepen-dent coder, using an independent code, with a different integrator,and the discrepancy was sufficiently small (again see Refs. [7–11]).

4.7. Determining potentially unsafe regions

The goal of the backward tracking studies is to identifypotentially unsafe regions. As previously specified an area of thephase space will be defined as safe if no initial condition in thatphase space can be backward tracked past point C. The choice of Cis made for practical reasons. In Fig. 9 one sees the backtrackingrays for a X.3 beamline port. The point C is located to the right ofthe first QFA magnet (QFA1).

The initial phase space representation is a very usefulrepresentation in illustrating potentially unsafe regions. Simplyput the phase space representation allows the sorting thebeamlines into a small number of groups. For instance all thebend magnet beamlines can be sorted into six groups—X.1 normalbend, X.2 normal bend, X.3 normal bend, X.4 normal bend, X.2superbend, and X.3 superbend. This means that for all beamlineswithin one of these groups, the fields and apertures that theparticles will be backward tracked through are identical. The onlydifference is the difference in the range of initial coordinateswhich is given by the beamlines. Therefore for each group one canplot the phase space of all beamlines on a single plot and

Fig. 15. Beamline phase spaces and potentially unsafe regions are plotted on one

graph. Potentially unsafe beamlines (such as beamline 3) are identified as the

intersection of the beamline and the phasespace.

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backward track a phase space that includes all beamline phasespaces. So on the same plot one can also plot the regions ofconcern (as determined by the tracking) and see if there is anoverlap for any given beamline. This is illustrated in Fig. 15.

The plan is to survey a relatively large phase space region thatwould include all present (and hopefully) all future beamlines inorder to identify the potentially unsafe regions (black region inFig. 15). By initially tracking a larger space allows one to judge afuture beamline by just drawing the beamline phasespace in thefigure seeing if it does not intersect a potentially unsafe region.This also provides information about how close a beamline is to apotentially unsafe region. Now lets look some of the specifics ofthe magnetic field and tracking assumptions that were used in thesimulations.

5. Field profiles and ranges

5.1. General considerations

In the backward tracking simulations we use the full verticalmagnetic field profile calculated by finite element analysis (FEA)codes. The example in Fig. 16 shows the field profile used for theALS quadrupoles. The nominal profile is shown as well as thedifferent profiles when the particle is vertically offset. It is evidentthat in the ALS quads (and in any real quadrupole design as well),the intensity of the field depends also on the vertical position ofthe particle, especially when a particle is strongly displaced

Fig. 16. ALS quadrupole field profiles for different vertical offset of a particle.

Fig. 17. ALS quadrupole

horizontally and goes in the proximity of a pole. A similar scenarioapplies to the case of sextupole magnets.

We have analyzed this effect for all the magnet types andextended the magnet scanning range in order to properly accountfor this vertical offset field variation without the need of trackingthe particle also in the vertical plane. The different cases for thedifferent magnet typologies will be given later in this section. Inthe coil short analysis, we consider only shorts in a single pole coiland no shorts of the whole magnet or between different poles. Thereason for such a choice is due to the geometry of the ALSmagnets. All of them have been designed and built in such a waythat the input and output magnet terminals as well as the inputand output terminals of each of the pole coils are all mutually verydistant, making it impossible to have shorts between any of theseterminals. The example in Fig. 17 shows the case for an ALSquadrupole. The field of a shorted quadrupole is shown in Fig. 18.

Additionally, protective lexan insulating covers are over thecable connections in all the magnets and prevent falling metalobjects over the terminals from shorting the magnets. All highpower cables between magnets are enclosed in cable trays and areconnected inside the chassis of the power supplies, making themfully protected from accidental shorts.

In defining the scanning range for the magnet fields to be usedin the tracking simulations, we assumed the full power supplyrange capability unless the power supply current is interlockedagainst faults and tuning/mis-setting. Apart from the correctorand skew quadrupole power supplies, fixed polarity has beenalways assumed. We also accounted for the effect of the ‘‘shunts’’present in our QFA quadrupoles. The maximum current that fuses

terminal position.

Fig. 18. Example of the field profiles used in simulations. Quadrupole with and

without shorts cases.

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Fig. 19. Calculated vertical field at the pole position in a quadrupole as a function

of the particle vertical offset.

Fig. 20. Cross-section of the ALS arc vacuum chamber.


and cables allow to flow through the shunts is about 5% of thenominal value.

Failures in the power supply control crates and water coolingflow-meters have also been considered. Such failures will eithertrip the current to zero or to a fixed value within the power supplyrange. And both these scenarios are included in our assumption toscan all the power supplies within their full range (unlessinterlocked). At the ALS the skew quadrupole fields are generatedby hybrid magnets that also include dipolar (corrector) andsextupolar field components. In our simulations, the main effectof the skew quadrupole field is to generate a horizontal kick formaximum vertical offset. Such a kick has been incorporated in thecollocated corrector range in order to avoid the need of scanningthe skew quadrupoles independently and with a significantbeneficial impact on the required computing time for thesimulations.

The effect of the ALS feed-forward (tune and orbit compensa-tion for insertion devices) and feedback (slow and fast orbit)systems has been accounted for as well. They affect a limitednumber (48) of quadrupoles and can use only a limited currentrange (10%) of the corrector magnets. Such systems effect isaccounted for by the fact that all the magnets (including the onesaffected by these systems) are independently and simultaneouslyscanned during the simulations. Last but not least, all the interlockthresholds and magnet ranges have been defined by finding theoptimal condition that ensures full safety and maintains at thesame time flexibility in setting the operation lattice for the ALS.The next paragraphs will give more details for the quadrupoles.Similar treatment was done for the normal bends, superbends,sextupoles, and correctors.

5.2. Quadrupole field

The field profiles used for the quadrupoles have been alreadyshown in Fig. 14. As for the normal conductive dipole case, 1-Dand 2-D models have been compared showing that the 1-D modelis appropriate [9].

5.2.1. Quadrupole shorts

As we said before, a total short of a quadrupole is not allowedby the mechanical layout of the ALS magnets, so only single poleshorts will be considered. The field profiles for different singlepole coil shorts are shown in Fig. 18.

For symmetry reasons, we can say that shorts in poles 2 or 3have the same vertical field profile along x for y ¼ 0. Shorts inpoles 1 or 4 have the same By profile along x for y ¼ 0 buthorizontally flipped respect to the poles 2 and 3 case.

Backtracking with the QFA field profiles with 50% and 100%shorts in one pole are performed for both the horizontally flippedcases. So the simulations have to include two different shortprofiles and two different short amplitudes for a total of fourdifferent cases.

5.2.2. Vertical range

Fig. 16 shows an example of the field horizontal profiles for thecase of a particle on the reference orbit and for a particle with a5 mm vertical offset. The vertical offset increases the vertical fieldin the proximity of the poles. The effect can be accounted in thesimulations by properly increasing the scanning range of thequadrupole strength.

Fig. 19 shows how the vertical field increases as a function ofthe vertical offset at the pole position increases. A cross-section ofthe ALS vacuum chamber is shown in Fig. 20, the maximumvertical offset that a particle can have with respect to the orbitplane is 0.5 cm if it is close to the quadrupole pole tips. This is

defined by the dimension of the antechamber slot in the vacuumchamber. For such an offset according to Fig. 19, the upper limit forthe quad scanning range must be increased by 6.2%. An analogousanalysis has been made for the sextupoles.

5.3. Field profiles and ranges summary

Table 1 shows the scanning range for the currents of all themagnets, calculated in order to account for all the constraintsdiscussed above. The tracking studies (see Appendix A) revealed anecessity to limit the range of certain power supplies, resulting inseveral interlocks. These interlocked power supplies are inevidence in the table (current interl. under PS). In addition tothose there is also a stored beam interlock that restricts the rangeof the normal bend field in case of a short.

6. Apertures

The tracking studies used to demonstrate the safety of top-offoperation, as described in Appendix A, credit multiple apertures,both in the beamline frontend upstream of the 1 m point, as wellas in the accelerator. Those apertures therefore become anessential part of top-off safety. At the ALS, they will be treatedas other shielding, put under tight configuration control and theirposition will be regularly measured. The following sectionoutlines general aspects of the credited apertures, as well as adetailed alignment tolerance budget and the means to ensure thatall credited apertures are within those error tolerances. In order toaccount for possible errors of all credited apertures, all apertureswere artificially enlarged in the simulations described in SectionAppendix A. The possible error sources (e.g. alignment errors) arediscussed in more detail below. The overall error budget turns outto be in the range of 2–5 mm, depending on which type ofaperture is concerned.

The only practical way to account for all possible apertureerrors in a conservative way in the simulations, without having tocarry out a detailed probabilistic analysis, is by simultaneously

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Table 1Summary table of magnet ranges.

Parameter PS tuning Shunt Girder water and controls crate Skew quad Vertical offset Total

QFA 71% �5% 71% NA þ6:2% 94–107.2% of

(current interl.) (current interl.) knom ¼ 2:954353 m�2a

QDA 0–174% NA 0–174% NA þ6:2% 0–180.2% of

(fixed polarity) (fixed polarity) knom ¼ �1:779475 m�2

QF 0–138% NA 0–138% NA þ6:2% 0–144.2% of

(fixed polarity) (fixed polarity) knom ¼ 2:23711 m�2a

QD 0–123% NA 0–123% NA þ6:2% 0–129.2% of

(fixed polarity) (fixed polarity) knom ¼ �2:2511045 m�2a

SF 50–106% NA 50–106% NA þ6% 50–112% of

(current interl.) (current interl.) Lknom ¼ 16:0 m�2

SD 50–139% NA 50–139% NA þ6% 50–145% of

(current interl.) (current interl.) Lknom ¼ 12:0 m�2

BEND 70:5% NA 70:5% NA 0% 70:5%

(current interl.) (current interl.)

S-BEND 70:5% NA 70:5% NA 0% 70:5%

(current interl.) (current interl.)

SF correctors 72:2 mrad NA 72:2 mrad 70:4% mrad þ1:68 mrad (Sext Offset) �2:6 to þ4:28 mrad

SD correctors 72:25 mrad NA 72:25 mrad 70:4% mrad �1:68 mrad (Sext Offset) �4:33 to þ2:65 mrad

Standard correctors 72:5 mrad NA 72:5 mrad NA 0% �2:5 to þ2:5 mrad

a Nominal values are slightly different for the superbend sectors.

Fig. 21. Vacuum chamber of one of the 12 ALS arcs. Clearly visible is the relatively

wide antechamber as well as the actuator and cooling mechanisms for the six

photon stops.


widening every aperture on both sides of the aperture by the fullerror amount. One should remark that this is a very conservativeassumption. Widening every aperture leads to a much wider patha beam could pass through in the simulations, whereas in reality,relative misalignments of multiple apertures that should be in astraight line usually leads to a significant narrowing of anyremaining opening.

All apertures credited in the tracking studies have beenanalyzed in terms of their error budget. In general, the creditedapertures fall into two categories: photon beamline apertures andstorage ring apertures, which will be described in more detail inthe following sections. For a typical beamline, at least twobeamline apertures are credited, plus the storage ring arc vacuumchamber of the sector the beamline is in, including all photonstops mounted to that chamber.

An important fact is that all tracking studies described inSection Appendix A are local and constrained to a relatively shortsection of accelerator upstream of the beamline in question.Therefore, all alignment tolerances are also local.

6.1. Storage ring apertures

In the ALS storage ring, all elements (magnets, vacuumchambers, etc.) within one arc are mounted to one commongirder (see Fig. 21). In addition to their mounting on the girder,every magnet, as well as the vacuum chambers can also be alignedindividually. This has been done in the past and is typicallychecked on an annual basis. The typical relative misalignment ofelements within one girder is around 50mm and the maximummisalignment smaller than 150mm. For the purpose of top-offsafety we decided to carry out an error tolerance analysis for theposition of all storage ring apertures and to periodically verify theaperture position using precision surveys.

Table 2 contains a detailed list of all contributions to theoverall error of storage ring apertures together with the tolerancetaken into account for all contributions. The tolerances arejustified in the following paragraphs. To come up with anoverall error, all individual contributions are then simplysummed up (conservative worst case assumption).

The reference system in which all aperture positions areestablished by survey of all magnets of a single arc, which reduces

the overall error in the determination of the reference system to avalue much smaller than the other errors tabulated above. Thesmall error in the alignment of individual magnets is not relevantfor the aperture positioning error and is instead taken intoaccount in the conservative assumptions for the scan ranges of themagnets in the tracking studies.

The sum of all error contributions if all the apertures are withinone arc is 71:6 mm for internal storage ring vacuum chamberapertures. In case the apertures extend beyond one arc (which isthe case for some beamlines, where single apertures in theadjacent straight are credited), the total is 72 mm. To use oneconsistent value for all beamline apertures (avoiding potentialmistakes) and be on the conservative side, the value of 72 mmwas used in the simulations.

One special type of storage ring apertures is the photon stops.Since they are movable during the process of activating thetitanium sublimation vacuum pumps, their alignment is not asprecise. Therefore one has to add 2:4 mm additional error to thetotal quoted above. The drive mechanism will be physically lockedfor top-off operation and the key to the locks will be controlled,however, the additional error in horizontal position due to thedrive mechanism remains. This results in a total error budget for

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Table 3List of contributions to the tolerance budget for beamline apertures.

Relative measurement Method in determining

differences from ideal

Tolerance

budgeted

Internal aperture relative to

external fiducial markers or outer

beam pipe circle

Manufacturing tolerance and

fiducialization

70:25 mm

Outer fiducial markers to SR

magnets

Survey measurement/alignment 71:2 mm

Survey accuracy Laser tracker calibration (short

distances)

70:05 mm

Expected beamline fiducial

motion until next survey

Worst case ground settlement

between SR girder and beamline

based on past surveys

70:5 mm

Table 2Contribution to the overall tolerance budget for the credited storage ring apertures.

Tolerance category Method in determining

differences from ideal

Tolerance budgeted

Aperture

manufacturing

precision

Construction tolerance, in

some cases CMM verified

70:25 mm

Alignment error of

vacuum chamber

relative to magnet

centers

Direct survey of magnets

and vacuum chamber

71:2 mm (can be

determined with much

higher precision than this,

this is the allowable value

before realignment is

necessary)

Survey and alignment

error

Calibration measurements

of laser tracker, precision of

alignment targets, etc.

70:05 mm (multiple tracker

setups, over short distances

of about 10 m)

Expected magnet and

chamber motion

Statistical analysis of 70:1mm (one arc);

until next survey

(ground settlement)

past survey measurements 70:5mm (arc to arc)

Additional: Transverse

precision of

Construction and 72.4 mm

vertical photon stop

drive system

guidance tolerance


photon stops of 74:0. Again, as a conservative choice thesimulations used a slightly larger value of 75 mm. The followingsubsections explain the individual contributions to the overalltolerance.

Fig. 22. Fault event and interlock response timing.

6.2. Beamline apertures

In addition to the storage apertures, typically two apertures arecredited in each beamline (see Section 4.4 and Fig. 11). These canbe aperture plates in beamline frontends, specific apertures addedfor top-off safety, or just plain vacuum chambers. Again, allcredited apertures are labeled, treated as shielding and put undertight configuration control (also see following section). Most ofthose elements have been fiducialized before installation, so theirposition relative to fiducial markers outside the vacuum chamberis known with very high accuracy. However, even for straightvacuum chamber pipes, the manufacturing tolerance is not worsethan 0.25 mm. All apertures can also be easily surveyed relative tothe storage ring (single arc sector) coordinate system that wasreferenced in the previous section about storage ring apertures,since they are still inside the storage ring tunnel, so a direct line ofsight exists for the laser tracker. Table 3 summarizes theindividual contributions to the error budget for beamlineapertures. Most of the tolerances are very similar to the ones forthe storage ring apertures and have already been explained there.

The overall error to be used in tracking simulations is againestablished by summing up all individual contributions (con-servative worst case assumption). The sum is 72:0 mm, which isthe value that has been used in simulations. As in the storage ringaperture case, 1.2 mm of the total 2.0 mm error budget have beenallocated for the actual misalignment of the fiducial markers ofthe beamline apertures relative to the storage ring magnets. Thisis not determined by the precision with which the position can bedetermined (the precision of the survey itself is and order ofmagnitude better than this), but rather by the desire to minimizethe number of times chambers have to be realigned. If at a givensurvey, the position of the credited beamline apertures is withinthese 1.2 mm then it does not have to be realigned, since even ifthe maximum observed ground settlement would happen overthe next year, it still would be within the total tolerance until thenext survey.

The tolerances quoted above are the horizontal tolerances,perpendicular to the direction of the photon beamlines. Thetolerance for the longitudinal direction (parallel to the direction ofthe photon beamlines) can be conservatively estimated by takingthe maximum angle between any of the extreme phase space raysand the directions of the beamlines. Taking the worst case of anyof the beamlines of the ALS and putting an additional safetymargin on top of it, the longitudinal error tolerance wasdetermined to be 10 mm or much less stringent than thetransverse tolerance.

7. Interlock requirements

The tracking studies revealed that three types of interlockswere necessary. The first is an interlock on the stored beamcurrent. The second is an interlock on the difference in energybetween the injected beam and the stored beam. The third is aninterlock on the power supply currents of some magnets. We willrefer to these three interlocks as the beam current, energy match,and lattice match interlocks. There are requirements for both thevalue and time response for these interlocks and will be describedin the next subsections.

7.1. Time response

The time response of the system needs to be fast enough thatonce the event occurs it is detected and the injection is haltedbefore it reaches a potentially dangerous condition. This illu-strated in Fig. 22, where the total response time of the interlocksystem (t2–t0) must be smaller than (t3–t0), the time it takes toreach a potentially dangerous situation. The shortest time that ittakes to reach a dangerous condition was analyzed and found tobe greater than 1 ms. This included effects such as screening

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Table 4Energy and lattice interlock requirements.

Power supply Full scale

output (A)

Nominal

current (A)

Intrlk. accuracy

7% of nominal

Maximum

response time

(ms)

BR bend 1050 1025 0.1 1

SR normal bends 1000 897 0.1 1

SR superbends

sectors (4, 8, 12)

350 298 0.1 1

Main SR QFA 550 517 0.1 1

QFAs sectors (4, 8,

12)

550 518–526 0.1 1

SR SF 400 374 10 1

SR SD 400 215 10 1

Table 5Stored beam interlock requirements.

System Beam current full

scale (A)

Intrlk. accuracy 7% of

full scale

Current detector accuracy %

of full scale

BPM 0.5 1.0% (5 mA) 0.5% (2.5 mA)


effects of vacuum chamber and time constants of power suppliesand magnets. This drives the time response for the interlocks.

7.2. Accuracy

The accuracy of the interlocks was also analyzed for and theresults are summarized in the following Tables 4 and 5. With thecombination of these accuracies and the submillisecond timeresponse the system is guaranteed to turn off the injector before apotentially dangerous condition is reached.

8. Concluding remarks and lessons learned

We presented our approach to ensuring that the injectedelectron beam cannot pass down a beamline, reaching far enoughto create an unacceptably large radiation dose. Many elements ofthe approach were adopted from work done at other facilitie-s—particularly the Advanced Photon Source. This is a verycomprehensive and conservative approach that logically followsfrom the hazards to the controls. The effort was much moreinvolved than any of us previously had imagined. During thecourse of the work several preconceived notions were found to befalse. In this final section we wish to share some of the lessons welearned.

Because the details of each storage ring are different, theexperience in doing the analysis at one facility may not translateto another facility. There were several rules of thumb that we hadentering into this process that turned out to be disappointinglyfalse for the case of the ALS:

�
An interlock on stored beam current is sufficient.Unfortunately just being off in energy as shown in Appendix Ashows this to be false for the ALS.
�
Beamlines that are emitted from the front of a bend (such as aninsertion device beamline) are naturally safe because theelectrons will be deflected by the rest of bend and should notmake it through the beamline. Therefore, the ALS insertiondevice beamlines that are emitted at the entrance of a101 bend should be okay without modification.
This is true if the bend is wide enough. However, in the case of the

ALS, there is a combination of very wide vacuum chambers and

relatively narrow gradient bends. Therefore the electrons can

travel to points that are largely horizontally displaced and where

the field does not significantly bend the beam.

One of the other things that added to the time to do the ALSanalysis is that almost all beamlines are different. Even though inour approach we are able to mitigate the analysis time somewhatby having the beamlines grouped into smaller number of sets, thedata gathering as well as the individual aperture solutions greatlyadded to the time.

In total, 19 of the 43 beamlines required the addition ofapertures to reduce the beamline phase space. In two casespermanent magnets were also installed in the beamline to shiftthe beamline phase space. This was all in addition to the fairlyrestricted active interlocks that were added.

So our advice or lessons that could be applied to other facilitiesis that from the view of top-off the following items are helpful:

�
Standardize the beamlines. � Avoid large horizontal chambers and beamline apertures. � Try to make beamlines and the storage ring top-off compatible
from the start. It is easier to design it in than to retrofit later.

The status of the top-off project is that at present all beamlinesare running in top-off and the users of the ALS are enjoying thebenefits of higher flux and improved stability.

Acknowledgments

During the course of this study we received a lot of help andencouragement from many. We wish to particularly acknowledgethe help and support of Louis Emery, Laurent Favraque, SamKrinsky, Sayed Rockne, Bob Hettel, James Safranek, Jeff Corbett,Laurent Nadolski, Richardo Bartolini, Ben Feinberg. The work herewas supported under DOE BES Contract DE-AC03-76SF00098.

Appendix A. Tracking study example—X.3, normal bend

In this section we present an example of our top-off tracking.As mentioned in Section 4.7 one of the attractions of ourtechnique is that beamlines can be grouped into a small numberof beamline sets and the tracking then needs to be done only oncefor each set of beamlines. Any future beamline that is added to abeamline set does not require any further tracking. The exampleshown in this section is the set of all X.3 normal bend beamlines.There are 13 X.3 normal bend beamlines at the ALS that are inoperation. In our approach the same plot is used to plot the phasespace of all X.3 normal bend beamlines together with thepotentially unsafe regions. Each beamline is declared okay fortop-off if there is no overlap of the beamline phase space and thepotentially unsafe region. If there is overlap, then additionalcontrols are needed.

The approach taken for these X.3 normal bend beamlines is thesame as for the other six types of beamlines—normal bend,superbend, and insertion device (see Section 4.7). The steps in theapproach are:

�
define the tracking start-point and initial coordinate system(see Section 4.3), � determine the tracking end-point (see Section 4.3), � determine the phase space of the beamlines at that the start-
point (see Section 4.4),

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Table 6


�

Figape

Shorted cases for bends, quadrupoles, and sextupoles.

Magnet type Description

backtrack towards the tracking end-point using a set of initialphase space rays that encompass the phase space of all thebeamlines to determine the potentially unsafe conditions (seeSection 4.7), and

Bend short 2.5% short, top
� 5% short, top
Quadrupole short 50% short, out

100% short, out

50% short, in

100% short, in

Sextupole short 50% short, out

100% short, out

50% short, top

100% short, top

50% short, in

100% short, in

Table 7Coarse grid phase space (38� 87� 7 ¼ 23;142 points).

Init. phase Nominal Min Max Step size

x0 0.32 0.23 0.415 0.005

y0 0.195 0.16 0.289 0.0015

DP=P0 0.03 �0.03 0.03 0.01

if there are overlaps of the beamline phase space with thepotentially unsafe conditions, then more stringent interlocks orapertures need to be included.

Now we will go through these steps for the set of X.3 normal bendbeamlines.

A.1. Initial coordinate system and beamlines phase space

Fig. 4 shows one sector of the ALS. The initial coordinatesystem (x; s) for the X.3 normal bend beamlines has its origin inthe center of the second QD magnet. The s coordinate is defined bya ray starting in the center of the magnet (at the origin) andtraveling axially through the center of the QD2 magnet in thedownstream (electron beam direction) direction. The x coordinateis defined by a ray that starts at the origin and is perpendicular tothe s ray and pointing outward. This coordinate system is shownin Fig. 11.

It is in this coordinate system that the extreme rays in position(x) and angle (y ¼ arctanDx=Ds) for the phase space are deter-mined. The phase space plot is plotted in the initial coordinatesystem (x; y). All beamline phase spaces include the 72 mmaperture widening tolerances (see Section 6.2). We will start withthe phase space for the 13 X.3 beamlines as they were in 2007 (seeFig. 23). As seen in the figure several of the beamlines have largephase spaces, particularly the beamlines 9.3.x and the 7.3.xbeamlines. These were two of the oldest beamlines.

A.2. Tracking end-point

Now that the tracking start-point and the beamline phasespaces are determined, the next step is to define a tracking end-point. As mentioned in Section 4.3 the tracking end-point isdetermined from practical considerations—allowing the analysisto complete in a reasonable time while not overly restricting thecontrols. In some cases the actual location of the tracking end-point is determined after gaining some experience with thetracking.

. 23. X.3 Beamline phase spaces for beamlines in 2007 before any top-off

rtures were added.

In the case of the X.3 normal bend beamlines, the tracking end-point was chosen to be just downstream of the first QFA magnet,QFA1, in the sector. So the backtracked ray, originating in abeamline phase space, should not reach into the QFA1 magnet(see Fig. 4). Between the tracking start-point and the tracking end-point there are six magnets (SF1, B2, SF2, QFA2, SD2, and B3). Thefield of QD2 is negligibly small for all rays that pass far from thecenter.

For each beamline, magnet shorts need to be considered. Forthe quadrupoles and sextupoles, each short corresponds to a newmagnet field profile. The number of shorted cases for quadrupolesand sextupoles is shown in Table 6. Bend magnet shorts are alsoconsidered. A bend short can be modeled as a parameter variationof �2:5% ð�4:36 mradÞ and �5% (�8:73 mrad). Larger shorts than�5% need not be considered for they cause a loss in stored beamwhich will be interlocked. In five of the six magnets (SF1, B2, SF2,QFA2, and SD2) one needs to consider the cases of shorted poles. Itis not necessary to consider shorting B3 because the field isalready so small for the location of the beam that a �5% short is avery small perturbation (o2 mrad) in absolute scale.

The time to do the tracking studies is determined by:

�
the granularity of the initial phase space, � the number of parameters that need to be varied for each case
tracked, and
� number of cases that need to be tracked.
In the next subsections we discuss the granularity of the initialphase space and the parameter variation. In terms of the numberof cases, they are shown in Table 6. There are 24 shorted cases inaddition to one non-shorted case, or 25 cases in total that need tobe tracked. This turns out to be feasible in terms of tracking time.But further increasing the number of magnets or cases rapidlyresults in the calculation becoming prohibitively long.

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Fig. 24. Magnetic field profiles at the maximum power supply current.

Table 8Maximum step size for each magnet family.

Magnet

family

Maximum kick at KLnom

(rad)

Delta for a 0.002 radian

kick

Simulation

delta

Simulation

minimum

Simulation

maximum

Simulation

nominal

Number of

points

HCM NA 0.002 0.00166666 �0.0025 0.0025 0 4

HCSF NA 0.002 0.001693 �0.00260 0.004172 0 5

HCSD NA 0.002 0.001718 �0.00422 0.002652 0 5

QF 0.0401 0.1116 0.1112384 0 3.225914 2.237111 30

QD 0.0188 0.2674 0.270355 �3.24427 0 �2.511045 13

QFA 0.0544 0.1453 0.09749 2.777093 3.167053 2.954353 5

QFASB 0.0544 0.1453 0.08239 2.933565 3.345515 3.120815 5

QDA 0.0133 0.2676 0.267217833 �3.206614 0 �1.779475 13

SF 0.0181 1.77 1.653333 8 17.92 16 7

SD 0.0136 1.77 1.74 �17.4 0 �12 11

Fig. 25. Full phase space back tracking of the nominal lattice (no errors) with a 720% scan of injection energy about the stored beam energy.


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Fig. 26. Nominal lattice (no errors) with no lattice errors and with a single back tracked ray that has a 20% increase of the energy compared with the stored beam energy.

The ray easily back tracks past the tracking end-point into the next arc showing the sensitivity to high injection energy.

Fig. 27. Beamline phase space and potentially dangerous regions (in white) with

no lattice errors and when just the energy is varied by 720% about the nominal

stored beam energy. (Same data that were plotted in Fig. 25.)


no lattice errors and when just the energy is varied by 73% about the nominal

stored beam energy.


A.3. Phase space range and granularity

The phase space range needs to cover all the beamlines in theset. The phase space ðx; yÞ grid size and granularity must belimited or the simulation time will be too long. A good rule ofthumb that was adopted here was the phase space grid should be

yx ðstep sizeÞ ¼ x ðstep sizeÞ=bx at the launch point:

The concern with increasing the granularity of the phase spacescans is that one could potentially miss an important area. Ofcourse one always has the option of reducing the granularity forany area that requires more scrutiny. A reasonable approach is todo coarse scans on the large phase space area and fine scans onany region of concern. The range and step size for energy variationis fixed at, DP=P ¼73% energy change, 1% steps. The phase spacegrid is shown in Table 7.

A.4. Parameter range and granularity

The max/min range of a parameter (knob) is based on powersupply limits and interlocks (see Section 5.3). The step size for theparameter variation will be fixed for all simulations. The numberof steps is based on limiting the parameter steps to less than a2 mrad change. Fig. 24 shows the kick strength and field profile forthe storage ring magnets. Table 8 shows the parameter ranges.

A.5. X.3 tracking results

For the final steps of the analysis we need to backtrack fromthe start-point towards the tracking end-point using a set of initialphase space rays that encompass the phase space of all thebeamlines to determine the potentially unsafe conditions. If thereare overlaps of the beamline phase space with the potentiallyunsafe conditions, then more stringent interlocks or aperturesneed to be included.

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Fig. 29. Beamline phase space and potentially dangerous regions (in white) when just the second QFA magnet is decreased by 40% from its nominal value. The ray easily

back tracks past the tracking end-point into the next arc showing the sensitivity to low QFA2 values.

Fig. 30. X.3 beamline phase spaces for beamlines in 2009 after top-off apertures

were added.


all lattice errors and when just the energy is varied by 73% about the nominal

stored beam energy.


In Table 1 a list of magnet ranges is given for the tracking. Asseen in the table, some of the magnet power supplies areinterlocked. Also there is a restriction in the 73% in the energymismatch of the stored beam and the injected beam. Theserestrictions were determined by the tracking. We now show howthe tracking is used and guides us towards the appropriaterestrictions.

In Fig. 25 a plot is made for backtracking the ideal lattice with a720% energy variation about the stored beam energy. One seesthat many rays terminate on apertures downstream (safe side) ofthe first (left most) QFA, however, many rays pass through. InFig. 26 one specific case is given for a ray that has an energydeviation of þ20% showing the electron path successfullynavigating past the tracking end-point. In Fig. 27 we plot all theconditions shown in Fig. 25 in initial phase space. The darkness ofthe color corresponds to how far back the rays got. The whiteregions correspond to areas where rays have passed by thetracking end-point and thus is declared potentially unsafe. As onecan see from the plot, the potentially dangerous region overlaps

many of the beamlines. Now let us restrict the energy range andsee what happens.

We restrict the energy range of the injected beam to 73%variation about the energy of the stored beam. The trackingresults are plotted in Fig. 28. In comparing Fig. 28 with Fig. 27 onesees that the potentially unsafe region shrinks considerably. Inthis case the potentially unsafe rays only overlap with beamline9.3’s phase space. From this one sees that energy differences(particularly larger injection energy) is a parameter that needs tobe controlled. In fact when the energy reaches þ5% thepotentially unsafe regions begin to get uncomfortably close tosome of the X.X.2 ports without scanning any other parameters.To control the relative energy match requires that the powersupplies of the bends, superbends, and the injection booster bendsbe interlocked.

One sees how the tracking helps to define the interlocks. InTable 1 one also sees that the QFA is greatly restricted as well. Asimilar tracking analysis showed the potentially unsafe regionsare also very sensitive to low QFA2 fields. QFA2 focuses electrons

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that are at large horizontal outward displacement radially inward.If QFA is large, the rays that are greatly displaced will be stronglybent inward colliding on the inside vacuum chamber. However, forlow QFA2 fields the bending is not as strong, allowing theray to recover and get past the tracking point. This is shown inFig. 29.

Power supply interlocks by themselves were not sufficient toshrink the potentially unsafe regions so that they remainedoutside of all beamline phase spaces. In some cases the beamlinephase space needed to be reduced by adding apertures to thebeamlines. This was the case for 9.3 and 7.3 as well as some of theX.X.2 beamlines. As a result, the beamlines were modified. In mostcased by putting in a simple aperture and in other cases (such as9.3) it was necessary to make larger modifications. In Fig. 30 a plotis made of the beamline phase spaces as they are in 2009 withnew apertures installed. The result of all tracking with the newbeamline phase space is shown in Fig. 31. As one sees with theadditional apertures and the interlocks there is no overlap withthe beamline phase space and a potentially unsafe region.

This example showed the case for the X.3 normal bendbeamlines. Similar analysis was done for all the other beamlines.The requirements of the interlocks and apertures are determinedby the tracking analysis.

References

[1] L. Emery, M. Borland, Top-up operation experience at the advanced photonsource, 1999, pp. 201–203.

[2] A. Ludeke, M. Munoz, Top-up operation experience at the Swiss light source,in: EPAC’02, Paris, France, June 2002.

[3] H. Tanaka, et al., Top-up operation at spring-8—towards maximizing thepotential of a 3rd generation light source, in: Proceedings of the 9th EuropeanParticle Accelerator Conference, Lucerne, 2004, p. 222.

[4] G.H. Luo, et al., Operation experience of top-up injection at Taiwan lightsource, in: Proceedings of the 2007 Asian Particle Accelerator Conference,2007, pp. 151–153.

[5] Reference for LANL.[6] M. Borland, L. Emery, Tracking studies of top-up safety for the advanced

photon source (1999), in: Proceedings of the 1999 Particle AcceleratorConference, 1999, pp. 2320–2322.

[7] W. Wan, A note on the difference between the 1D and 2D field maps and themeasured and computed horizontal position of the gradient magnet (revised),ALS Light Source Number LSAP-303, 2007.

[8] W. Wan, A note on the difference between the 1D and 2D field maps of thehorizontal corrector magnet on the sextupole, ALS Light Source NumberLSAP-304, 2007.

[9] W. Wan, A note on the difference between the 1D and 2D field maps of thequadrupole magnet, ALS Light Source Number LSAP-305, 2007.

[10] W. Wan, A note on the difference between the 1D and 2D field maps of thesextupole, ALS Light Source Number LSAP-306, 2007.

[11] W. Wan, A note on the difference between the 1D and 2D field maps of thehorizontal corrector, ALS Light Source Number LSAP-307, 2007.

[12] T.M. Jenkins, Nucl. Instr. and Meth. 159 (1979) 265.

advanced light source's approach to ensure conditions for safe top-off operation

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