LCLS-II TN-17-12

LCLS-II Beam Dumps∗

J. Welch, J. Blaha, P. Cutino, D. Hanquist, A. Ibrahimov, M. Kosovsky, Z. Li, M. Santana

October 18, 2017

Contents

1 Introduction 3

1.1 Beam parameters . . . . . . . . . . . . 3

1.2 Dump usage . . . . . . . . . . . . . . . 3

1.3 Power cycles . . . . . . . . . . . . . . 3

2 Mechanical 4

3 Temperature and Stress 4

3.1 Beam heating . . . . . . . . . . . . . 4

3.2 Peak temperature . . . . . . . . . . . . 4

3.3 Overall thermal expansion . . . . . . 6

3.4 Dump face heating . . . . . . . . . . . 6

3.5 Transients and bunch charge . . . . . 9

3.6 Peak Stress . . . . . . . . . . . . . . . 9

3.7 Cyclic stress . . . . . . . . . . . . . . 11

3.8 Cooling radius . . . . . . . . . . . . . 11

3.9 Heat transfer . . . . . . . . . . . . . . 13

3.10 Cooling sensitivities to mechanical tol-erances . . . . . . . . . . . . . . . . . . 13

3.11 Steel end cap . . . . . . . . . . . . . . 14

3.12 Steel sleeve . . . . . . . . . . . . . . . 15

3.13 Shielding heating . . . . . . . . . . . 15

4 Beam Position Control 16

4.1 Rastered beam . . . . . . . . . . . . . 16

4.2 Non-rastered beam . . . . . . . . . . 18

5 Cooling System 18

∗Work supported in part by the DOE Contract DE-AC02-76SF00515. This work was performed in support of the LCLS-II project at SLAC.

6 Operation and Maintenance 18

7 Installation and Removal 18

8 Safety 21

9 Radiation 21

10 Performance Margins and Rating 2410.1 Melting . . . . . . . . . . . . . . . . . 2410.2 Number of cycles . . . . . . . . . . . . 2410.3 Heat flux and burnout . . . . . . . . . 2510.4 Bunch charge . . . . . . . . . . . . . . 2510.5 Power ratings . . . . . . . . . . . . . . 25

11 Failure Modes and Effects 26

12 Time to Damage 27

List of Tables

1 Performance Requirements . . . . . . 32 Estimated average usage of LCLS-II

high power dumps. . . . . . . . . . . 33 Electron beam properties at the dump

faces at nominal and extreme condi-tions. . . . . . . . . . . . . . . . . . . 5

4 Calculated peak temperatures . . . . . 65 Maximum thermal expansion. . . . . 66 Peak von Mises stress in the Inner Core 107 Cooling fin parameters. . . . . . . . . 138 Selected heat transfer parameters. . . 139 Hydraulic sensitivities to inner core fit. 1410 Rastering parameters . . . . . . . . . . 17

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11 Diagnostics and various values. See [1]for details. . . . . . . . . . . . . . . . 19

12 Dump beam power ratings and perfor-mance margins . . . . . . . . . . . . . 24

13 Time to begin melting for variousworst case scenarios. . . . . . . . . . 27

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1 Introduction

We describe the complete beam dump systemsfor LCLS-II. Starting from requirements, we coverthe design (including design of the cooling system,planned operation, and installation and removal),the calculated performance (including failure modes),and state the “rated” performance in terms of maxi-mum beam power.

The starting point for the design is the overall per-formance required by LCLS-II and listed in Table 1for two undulator dumps, DUMPS and DUMPH,and the BSY dump DUMPBSY [2, 3]. Variousschemes were studied, including the use of the exist-ing D10 dump/collimator for the DUMPBSY [4, 5],an aluminum sphere/water- style water dump for alldumps, and a dump with a solid aluminum core withedge cooling for the undulator dumps. The most crit-ical design issues were to minimize the hazardous ef-fects of radiation, both prompt and long term, and toobtain a robust thermo-mechanical design. The endresult of this effort is a single dump design, related toan existing tune-up dump at CEBAF [6] which hasoperated for a number of years, that we believe canperform reliably at 250 kW with beam rastering, andat 120 kW without rastering.

Table 1: Performance Requirements

Dump Beam Power Beam Energy(Max.) (Nominal)

[kW] [GeV]

DUMPBSY 250 4DUMPS 120 4DUMPH 120 4

1.1 Beam parameters

Due to the extremely small emittance of the electronbeam, LCLS-II is capable of producing very smallspots on the face of the dumps. While the designoptics was chosen to avoid such small spots, tuningthe beam, even at the upstream end of the accelera-tor, can cause wide variation in the beta functions at

Table 2: Estimated average usage of LCLS-II highpower dumps.

MW h / y h / y kW average

DUMPS 135 5000 27DUMPH 239 5000 48BSYDUMP 450 5000 90

the dump and inadvertently cause a small spot size.Values for beam parameters at the dump faces undernominal and most extreme (minimum beam size) op-eration conditions are given Table 3. The minimumbeam sizes listed shown for the DUMPBSY were ob-tained by purposely distorting the optics to minimizethe overall beam spot [7]. The resulting BMAG [8]mis-match parameter was 5. A BMAG mis-matchparameter of less than 2 is expected in practice. Forthe undulator dumps, the minimum was obtained byscaling the beta functions down by a factor of 10.The beam is not expected to be round, but for sim-plicity the geometric mean beam size σr is listed inthe Table.

1.2 Dump usage

Dump usage refers to the average beam power on thedump, where the average is assumed to be taken overa long period time such as a year. The estimate ofthe dump usage is described in detail in [3], and thehighlights are shown here in Table 2. It is assumedthat the machine operates for 5000 hours per yearwith the remaining time used for shutdowns, mainte-nance, etc.

1.3 Power cycles

Beam power delivered to the undulators is expectedto vary considerably during normal operation de-pending on experimenter’s needs. A typical sessionmight involve a very low power tune-up beam, witha sequence of steps up in power, followed by a sud-den drop when access to the experiment is needed.To avoid coupling the beam for one undulator withthe beam from another as the experimenters needschange, the linac beam is held constant and the beam

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in excess of demand is sent to the DUMPBSY by thespreader/kicker. As a result, all three dumps are ex-pected to see a number of cycles, some of which havebeam power up to or near the rating of the dumps.Based on anticipated experimenter usage, the esti-mated number of cycles that come within 25% of therated power in thirty years is 4000 [9].

2 Mechanical

The basic components of the dump are shown in Fig-ure 1. An aluminum vacuum beam tube is weldedto the aluminum Inner Core and the assembly is in-serted into the Outer Core and welded at the up-beam end only. Cooling water is brought in throughDump Water Supply tubes to the down-beam end ofthe assembly and then flows up-beam concentricallyaround the Inner Core. Cooling fins (80) are ma-chined into the solid Inner Core to provide enhancedheat transfer from the aluminum to the water. Thegeometry of the fin cooling structure is shown in Fig-ure 2. The number and aspect ratio of the fins wasoptimized to increase the wetted surface area whilemaintaining adequate mechanical strength for han-dling and manufacture. Once installed in the outerbody, the fins will only be in contact with water andwill bear no external forces. Clearance between theouter diameter of the fins and the inner diameter ofthe shell is provided for a loose slip fit at maximumthermal expansion of the core.

Beam power is converted to radiation and is mostlyabsorbed in the Inner Core. A small fraction is ab-sorbed in outer core and is easily cooled by the DumpWater. A layer of steel shielding is put around theregion of most intense radiation to absorb most ofthe remaining radiation that would otherwise heatthe steel shielding in the dump. See Section 3.12. Itis cooled by contact with the Outer Core.

A steel end cap is bolted to the downstream end ofthe dump to absorb a small amount of beam powerthat would otherwise penetrate the aluminum coreand escape along the beam axis. The bolts are tackwelded to the steel end cap to assure they cannotloosen. The higher density of steel compared to alu-minum effectively shortens the overall length of the

dump.A Burn Through Monitor (BTM) is encapsulated

by the Steel End Cap. It consists of a empty sealedvolume that is pressurized to 5 to 10 psig. If beamwere to somehow burn through the entire Inner coreand the BTM the pressure would drop and a pressureswitch would cause a trip of the PPS system. Thistype of event is not expected to ever happen.

3 Temperature and Stress

Temperature and stress issues are described in thissection in some detail. They are critical to the per-formance of the dumps at high power.

3.1 Beam heating

Energy deposition by the beam was studied in [10, 11]for beam energy ranging from 2 to 8 GeV and forraster radius ranging from zero to 3 cm. FLUKA [12]was used to generate the dose maps which were thentransferred to the thermal analysis program FHeat3D[13, 14]. The same dose maps were also used as in-put to independent thermal and stress analyses usingANSYS and TEM3P. The results of these simulationsare used in the discussion that follows.

3.2 Peak temperature

Calculated peak temperatures at the rated beampowers are given in the Table 4. The peak tempera-ture generally occurs on the axis about 40 cm fromthe front face. The calculated peak temperatureswere obtained by adding the average temperature in-crease of the dump water to the peak temperaturein the aluminum inner core calculated in simulation.In simulations the bulk water temperature all alongthe dump is assumed to be 35 C and the continuousrise of water temperature as it flows along the innercore is neglected. Results from different simulationswere averaged. Peak temperatures for all cases arewell below the melting point of 660 C.

While beam parameters and the design of DUMPSand DUMPH are identical, they are connected in se-ries in the cooling water system with DUMPH being

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Table 3: Electron beam properties at the dump faces at nominal and extreme conditions.

DUMPBSY DUMPS DUMPH

εn µm 1.0 1.0 1.0Power kW 250 120 120Energy GeV 4 4 4

Nominalβx m 363 42 31βy m 354 4 6√βxβy m 367 13 14

σr µm 217 40 42

Extreme√βxβy m 30 1.3 1.4

σr µm 125 13 13

Beam Tube

DUMP Water Return

BTM pressure tubes

Steel Shielding

Steel End Cap Aluminum Outer CoreDump Water Supply

Inner CoreOuter Core

Steel Shielding

Electron Beam

Dump Water Supply BTM

Steel End Cap

Figure 1: Dump subassembly.

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Figure 2: Section showing fin geometry.

Table 4: Calculated peak temperatures

Dump Beam Beam PeakName Energy Power Temperature

[GeV] [kW] [C]

DUMPBSY 4 250 311DUMPS 4 120 409DUMPH 4 120 393

DUMPBSY 8 250 276

Table 5: Maximum thermal expansion.

DUMPBSY DUMPS/DUMPH250 kW 120 kW

Length mm 1.6 1.1Radius mm 0.14 0.09

cooled first. As a result DUMPH has slightly lowerpeak temperature than DUMPS. The values in thetable reflect this arrangement.

The temperature distribution and profile calcu-lated by simulation for the DUMPBSY is shown inFigure 3. This model includes 80 cooling fins thatextend over the central 85 cm of the core. The graphshows the temperature profile along the center of thecore where it reaches a peak of 556 K (283 C) for4 GeV. The peak temperature is slightly lower at8 GeV as the energy loss is spread out slightly andthere is a small increase in escaped energy.

Analogous calculations were done for DUMPH andDUMPS and results are shown in Figure 4. In thisnon-rastered case the temperature is more sharplypeaked toward the axis.

3.3 Overall thermal expansion

Two of the same analysis programs used to calculatetemperatures in the dump core were used to calculatethe thermally induced expansion. Using a coefficientof expansion of 2.4 × 105/C and Young’s modulusof 68.9 GPa, and a Poisson ratio of 0.33, assumingno yielding, the calculated maximum expansion isgiven in Table 5. Slight yielding may occur duringthe first thermal cycle to maximum power, but theanalysis programs assumed no yielding, so actual ex-pansion will be slightly more than calculated here.Clearances and tolerances take linear expansion intoaccount, and additional clearance was included to ac-count for the possibility of some yielding.

3.4 Dump face heating

When the beam first enters the aluminum of thedump core at the dump face, before there is any

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Figure 3: Calculated temperature radial and longitudinal temperature distribution for a beam of 250 kW,4 GeV, and a 2 cm raster radius

shower development, it can be very small, (see Ta-ble 3). Electrons start to lose energy by generatingbremsstrahlung radiation and through ionization ofthe aluminum atoms. Though the rate of energy lossfrom bremsstrahlung is far higher than from ioniza-tion, such energy is not absorbed near the dump facebecause the bremsstrahlung radiation is very pen-etrating and simply goes deep into the dump corebefore it is absorbed; leaving ionization as the onlysignificant source of absorbed energy near the dumpface [15]. Fortunately the ionization energy loss rateis so much lower than for bremsstrahlung that thenet effect of the very small beam size on the peaktemperature at the dump face is tolerable.

An estimate of the peak temperature at the dumpface can be made using an analytic expression forheat flow under the assumption of cylindrical symme-try, no longitudinal heat flux, and constant absorbedpower per unit length, dPabs/dz. The minimum ion-ization loss rate for aluminum is 4.37 MeV cm−1 [16].If we employ the simplifying assumption that all thebeam power is deposited uniformly within a radius

equal to the gaussian beam size σr, then

∆T ≈ dPabs/dz

2πK· lnRedge

σr,

where Redge = 4.0 cm is the radius of the water cool-ing and K = 167 Wm−1C−1 is the thermal conduc-tivity, and ∆T is the temperature difference betweenthe water cooled surface and the center of the beamas it enters the dump. For σr = 10 µm DUMPS andDUMPH beams, which are not rastered and limitedto 120 kW, dPabs/dz works out to 131 W/cm and∆T ≈ 103 C. There is virtually no film drop to thedump cooling water and with a 35 C water supply,the peak temperature at the face will only be about138 C. Even if the beam size were reduced to 1 µm,the peak temperature on the face would only reach167 C. For the rastered case the effective beam sizeis very large and the beam heating at the face can beneglected. See Section 4.1.

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Figure 4: TEMP3P calculations of radial and longitudinal temperature distributions for a 120 kW, 4 GeV,beam with no rastering.

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Tem

pera

ture

Incr

ease

Time [us] 80 10060402000

40

80

120

160

C

Figure 5: Transient temperature response in alu-minum for 30 µm, 1 MW, 1 MHz, 4 GeV beam

3.5 Transients and bunch charge

A single bunch causes an essentially instantaneoustransient temperature rise in the dump. If the chargeis large enough, the amplitude of the temperaturetransient can be significant [17, 18]. However, for therange of bunch charge for which LCLS-II is expectedto be operated the transient temperature rise is quitesmall.

Transient heating by the LCLS-II beam was stud-ied in [19], [13], and by A. Lutman (unpublished).There are two primary features of interest:

• Instantaneous temperature rise due to a singlebunch.

• Transient response to a sudden increase in beampower.

For the beam parameters of LCLS-II the transientresponse to a sudden application of high beam poweris the more important feature. The highest bunchcharge for operation, only 300 pC, produces a modestsingle bunch temperature rise in the dump as can beseen in Figure 5. The “teeth” on the temperaturecurve correspond to individual 300 pC bunch and areapproximately 7 C high. On the other hand, it isnot hard to come up with materials which, in the

event of a sudden increase in LCLS-II beam power,would reach melting points in less than 1 ms. For thedumps however, with the relatively low Z and highthermal conductivity, the transient response is muchmore favorable.

3.6 Peak Stress

The design of the dump is such that as the temper-ature changes the inner core may expand freely fromthe single welded upstream end connection. This fea-ture avoids build-up of differential thermal expansionbetween the core and the outer body. Nevertheless,within the core there can be large thermal stress asthe hotter central axis tries to expand more thanthe outer surface where the water cooling is applied.In addition to the ‘bulk’ internal stress caused bythe gross distribution of heating and cooling, thereare small areas of enhanced ‘local’ stress associatedwith the fin structure. Table 6 lists calculated peakstress for various conditions and the yield and ulti-mate strength of the core material.

Stress was simulated independently usingTEMP3D and ANSYS with good agreement.Particular attention was given to the von Misesstress. Zero pre-stress at room temperature wasassumed. Typically as a function radial coordinatethe von Mises stress goes through a minimum around20 mm and then increases with radius. The highestvon Mises bulk stress occur on axis where the stressis compressive. Material on-axis could yield but aslong as the integrity of the material around it isintact, it cannot go anywhere. Near the outer radiusof the inner core the material is in tension. For the120 kW un-rastered case the peak von Mises stressis seen to reach 223 MPa in the groove between fins.See Figures 7, 8 and 9. For the 250 kW rasteredcase, the peak stress in the fin core reaches 249 MPa,and 330 MPa in the fin groove. See Figure 6. Thelongitudinal coordinate for this plot was chosen tocoincide with the peak thermal flux and highesttemperature. Simulations without fins show no localenhancement of stress at the outer boundary andpeak von Mises stress considerably lower.

In the 250 kW rastered case, the calculations indi-cate the peak stress in the groove can reach slightly

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Figure 6: von Mises stress as a function of radialcoordinate for a beam of 250 kW, 4 GeV, and a 2 cmraster radius, for the hottest longitudinal location

above the yield point and the material is in tension.Over a region of a few longitudinal centimeters weexpect there will be yielding of the material on thefirst thermal cycle to maximum power. Nominallythe stress on the first cycle would also be above theultimate tensile strength. But the high stress pointof the fin groove is longitudinally confined by adja-cent cool material where stresses are well below theyield point, so breakage should not occur. The cyclicbehavior of this case is discussed detail in Section 3.7.

Aluminum 6061 alloy was chosen over the 1100 se-ries alloys because of the increased strength. Thermalstress scale inversely with thermal conductivity, indi-cating a higher stress for 6061 alloy compared with1100 by a factor of approximately 220/167 = 1.3.However the yield strength of 6061 is greater thanthat of 1100 (H14) by a factor of 310/105 = 3.0,more than making up for the higher stress level.

It is possible material properties may change foraluminum that is subject to the most intense radia-tion dose and the high strength advantage conceiv-ably could be lost. This is only of concern in the

Figure 7: von Mises stress as a function of radialcoordinate for a beam of 120 kW, 4 GeV, and a 2 cmraster radius, for the hottest longitudinal location.

Figure 8: von Mises stress distribution over a sliceof the inner core for a 120 kW, 4 GeV beam at forthe hottest longitudinal location.

Table 6: Peak von Mises stress in the Inner Core

Location 120 kW 250 kWno raster 2 cm raster

Fin Groove MPa 223 330Fin Core MPa 180 249Central Core MPa 322 132

Yield MPa 276 276Ultimate Tensile MPa 310 310

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Figure 9: High (colored red) von Mises stress distri-bution in groove between fins.

central portion of the core. At the outer boundary,where the cooling water is applied, the radiation doselevels are not unusually high and the material canbe expected to maintain its integrity. Stress relatedmodification of the material near the axis is not ex-pected to lead to any external or performance changesin the core since such material is confined.

Similarly the temper of the alloy will eventuallybe lost in places with the temperature above about150 C. But such places are completely confined bycooler, largely un-radiated material, whose strengthshould not degrade.

3.7 Cyclic stress

Cyclic stress generated by thermal expansion is aconcern in the inner core of the dump. Data for6061 Aluminum shown in Figure 10 indicate that forstress ratio1 of zero, the first failure was observed af-ter 20,000 cycles to 275 MPa (40 ksi). Simulations ofthe 120 kW case show (see Table 6) peak stress onthe central axis of 322 MPa, which is above the yieldpoint. Evidently the material in that portion of thecore will yield on first cycle to full power. Upon cooldown that same material will end up in a compres-sive state so subsequent cycles will not get to as highpeak tensile stress and the stress ratio will become

1Stress ratio is the ratio of minimum stress to maximumstress in one cycle of loading. Tensile stresses are consideredpositive and compressive stresses negative [20].

slightly negative. Material surrounding material onthe beam axis is not expected to yield because thestresses there are much lower, so the integrity of thecore should be maintained.

A similar situation occurs at the base of the gapbetween the fins. Locally the stress is enhanced toas high as 330 MPa (48 ksi) in the 250 kW case. Inthis small region the aluminum will yield and then,upon cool down, it will be in a compressive state.Unlike the core, however, the base of the groove isnot surrounded completely by material that is belowthe yield point so the integrity is not guaranteed. Inthe test data shown in Figure 10 a curve is fitted todata for zero stress ratio which intersects the peakfin stress between 10,000 and 20,000 cycles - uncom-fortably close to the estimate number of lifetime highpower cycles of 4000. However the test materials weresubject to controlled amounts of axial tension andcompression, the same for each cycle. As the mate-rials yielded and lengthened the applied stress wasartificially held constant. Subsequent cycles causemore and more elongation until failure. In the caseof thermal stress, if the material yields the stress isrelieved for future cycles. Consequently we expectfar more cycles will be tolerated before a local failureof the fins than predicted in Figure 10. Failure, if itshould occur, would probably take the form of thedevelopment of radial and longitudinal cracks origi-nating in gap between the fins. Such cracks wouldnot cause a performance problem unless they propa-gate enough to allow a fin to break off entirely. Thisis also unlikely since the upstream and downstreamportions of the fins are in low stress regions.

3.8 Cooling radius

Heat transfer from the aluminum core to the coolingwater was a critical factor in the optimization of thedump design. In establishing the cooling radius a bal-ance had to be made between lower heat flux,(whichlowers the film drop at the aluminum/water bound-ary) and higher temperature drop due to conductionin the aluminum core. In a cylinderically symmetricmodel of the dump which includes the film tempera-ture drop and the conduction temperature drop, theradius at which the peak core temperature is mini-

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Figure 10: Fatigue life in cycles for 6061 Aluminum for various stress ratios [21].

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mized is

Ropt =K

h

where K is the thermal conductivity of the core and his the heat transfer coefficient at the water boundary.For example if K = 167 Wm−1C−1 (6061 Al) andh = 7000 Wm−2C−1 the optimum cooling radius is24 mm. At such a small radius the radiation fromthe electron beam would highly activate the coolingwater and that activity would be spread throughoutthe cooling system. To minimize such radiologicalproblems we chose a larger cooling radius of 40 mmwhere irradiation of the cooling water is much less.

3.9 Heat transfer

High flow velocity reduces the film drop and low-ers the core temperature, but too high flow veloc-ity can cause excessive erosion. We based the de-sign around a flow velocity limited to approximately2.3 m/s. At a radius of 40 mm the peak heat fluxat the water boundary for the 250 kW dump is es-timated to be about 170 W/cm2 assuming a purelyannular geometry [22]. We employ cooling fins to in-crease the contact area of the water. The net effectis to reduce the heat flux by about a factor of fiveto around 34 W/cm2. High resolution simulationsshow the temperature drop in the aluminum of thefins is negligible compared with the film drop, so thefins could be made thinner and the film drop reducedfurther. However, as the film drop is only 29 C, atmost, there is little to be gained and it was felt thatthicker fins are more robust.

A feature was added to the design of the core toenhance the transition from laminar to turbulent flowand thereby increase the heat transfer coefficient.Following [23] the development of turbulent flow fol-low the Reynolds number Rex = (x/2.7) × Re∞,where 2.7 is the hydraulic diameter in millimeters ofthe cooling fin channel, Re∞ ≈ 6000 is the Reynoldsnumber for fully developed flow, and x is the distancealong the flow channel from the start. The transitionfrom laminar to turbulent flow has been measuredto be in the range of Rex = 8 × 104 to 5 × 106 de-pending on the roughness of the flow channel and the

Table 7: Cooling fin parameters.

Number of fins 80Fin height mm 6.33Fin width mm 1.212Fin fillet radius mm 0.72Bare core radius cm 3.367

Table 8: Selected heat transfer parameters.

CommonFlow speed m/s 2.3Reynolds number 6088Nusselt number 57Heat transfer coefficient W

m2C 12794

DUMPBSY 250 kWFilm drop C 29Wall peak temperature C 96

DUMPS/DUMPH 120 kWFilm drop C 14Wall peak temperature C 72/57

initial state of the flow. This leads to a range of dis-tance to develop turbulent flow in the fin channel of36 to 2250 mm. To ensure that the flow is developedearly on we added a special feature at the beginningof the fin channel that forces the water to developtransverse momentum to encourage turbulent eddydevelopment.

3.10 Cooling sensitivities to mechani-cal tolerances

The fit or clearance between the inner and outer corecan have an effect on the hydraulics and cooling. Ifthe fit is very loose then for a fixed water volume flowrate the flow velocity at the heated surfaces will de-crease and the film drop will increase. The pressuredrop along the fins, ∆Pfin , will also decrease. Thesensitivities of the some hydraulic parameters werecalculated for the nominal and maximum clearancesspecified. Nominal clearance is defined to be at max-imum material conditions. It represents the closest

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Table 9: Hydraulic sensitivities to inner core fit.

Nominal Loosest Loosest

Flow gpm 30.0 30.0 37.8Speed fps 7.0 6 7.6∆Pfin psi 6.7 4.6 6.7Film drop C 13 14 12

fit of the inner and outer cores (without taking intoaccount thermal expansion during operation) and isa diametrical clearance of about 0.6% of the diame-ter. The results are tabulated in Table 9. Nominalvalues are listed normal font, and bold font highlightsthe values that change as result of the fit. The mostobvious affect of the fit is on the fin pressure drop,which will be almost 2 psi lower than nominal forthe loosest fit if the flow rate is held at 30 gpm. Inthis constant-flow-rate case, the change in the filmdrop due to the reduction in flow velocity cause bythe loose fit is only about 1 C. If instead the pressuredrop is held constant, the flow rate for the loosest fitwill increase from 30 to 37.8 gpm.

If the fit is too tight there are two possible con-sequences; the parts won’t go together, or when thedump is operated at high power, the inner core willthermally expand until it reaches the outer core di-ameter and bind with it. In the former, the inner corecould be re-machined to a slightly smaller diameter.Of course it would be important not to jam the partstogether with such force that they cannot be pulledapart. In the latter case there is some risk if the innercore binds up with the outer core that a portion ofthe fins will be radially compressed and could yield orconceivably break. As mentioned in Section 3.3 theradial thermal expansion is up to 0.14 mm or less,depending z-location and power level. That expan-sion uses up about half of the clearance at maximummaterial condition. This expansion only affects theinner core as the outer core has little heat generationin it.

3.11 Steel end cap

For a raster radius of 2 cm and a beam energy of4 GeV the steel end cap absorbs about 0.35% of thebeam power, which is 875 W for a beam of 250 kW.For a beam energy of 8 GeV the steel end cap wouldabsorb about 0.602%, or 1.5 kW from a 250 kWbeam. These powers were actually calculated for acopper end cap of the same dimensions [11], but aresufficiently accurate for our purposes.

Heat absorbed in the 30 cm diameter steel endcap is transferred to the water-cooled aluminum coreby conduction with a nominal contact area of about700 cm2. For a 4 GeV beam running at 250 kW thetotal heat load from the cap is 875 W and the nom-inal heat flux is 1.25 W/cm2. The end cap is boltedto the aluminum core, so the issues of thermal re-sistance of the contact and differential expansion areworth exploring.

A safe upper limit of the effective contact resis-tance can be made by assuming heat conductionfrom the steel end cap to the aluminum core is onlythrough an air gap with a thickness roughly equalto the flatness tolerance of mating surfaces. In re-ality, while bolt forces provide metal to metal con-tact over an area much smaller than the nominalcontact area, they also should reduce the size of airgap by causing the parts to conform to each other’sshape. If we assume the flatness tolerance is suf-ficiently tight that the effective air gap is 0.1 mm(0.004 inch) and use the thermal conductivity of dryair Kair = 0.0263 Wm−1K−1 (no convection) for thethermal resistance, then the calculated upper limiton the temperature drop for across the gap wouldbe 47 C for a 250 kW, 4 GeV beam. The temper-ature of the dump body should be close to the wa-ter cooling inlet temperature which is held to be lessthan 35 C for DUMPBSY and DUMPH and 41 C forDUMPS. The internal temperature drop within thesteel end cap is estimated, based on the assumptionof uniform heat deposition, to be only 20 C. Thusthe hottest spot in the steel end cap is less than47+35+20 = 102 C. Similarly, for the corresponding8 GeV beam where the cap absorbs about twice asmuch power it should not be possible for any part ofit to get above 149 C. We judge it unlikely to suffer

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degradation due to oxidation that can occur at veryhigh temperature.

Differential expansion over the diameter of the steelend cap and the aluminum core would be less thanof order∼ 0.24 mm for 4 GeV, 250 kW beams andless than 0.42 mm for 8 GeV, 250 kW beams. Re-peated cycling, over a long period of time, could con-ceivably lead to bolts loosening and even falling out,thereby allowing separation of the steel end cap fromthe cooled aluminum core. For this reason the boltsare tack welded to the steel end cap after they aretightened.

3.12 Steel sleeve

The design includes a 75 cm long section of concentri-cally attached steel sleeve plates bolted to the cooledouter core of the dump to capture more beam energythan would have been captured with a purely alu-minum structure. This was done to reduce the heatload on the surrounding shielding. See Section 3.13.This sleeve is cooled by direct contact with the alu-minum outer core. As in the case of the steel end cap,issues of issues of thermal resistance of the contactand differential expansion are given careful consid-eration. In this case the maximum power the sleevewill absorb is about 5 kW and the contact area of thesleeve is about 7000 cm2 — roughly ten times morethan the contact area of steel end cap. The max-imum average heat flux is about 0.7 W/cm2 whichis about one-half that of the steel end cap. Scalingfrom the steel end cap result, all other things beingequal, at maximum power we should expect the tem-perature of the contacting surface of the sleeve to bean average of no more than about 25 C above thatof the aluminum outer core. The temperature dif-ference could lead to a differential thermal expansionover the length of the sleeve between the outer coreand the sleeve of between 0.1 and 0.2 mm. The sleeveis made of many small pieces each secured with a sin-gle bolt in the center to allow for this expansion. Inaddition, the bolts are tack welded to the sleeve toprevent loosening.

3.13 Shielding heating

Not quite all the beam power is absorbed in the dumpand carried off by cooling water. A fraction of thepower is converted to radiation which escapes thedump and is essentially entirely absorbed in the steeland concrete shielding surrounding the dump. Theescaped power will heat the adjacent shielding.

To reduce the escaped power to a level below arule-of-thumb value discussed below, a one inch thicksteel sleeve surrounding the outer core of the dump inthe region where the radiation is highest was includedin the design. For a 250 kW beam, without the sleevethe escaped power would be about 8 kW [10] whilewith the sleeve it is about 3 kW.

To set the scale for the heating of the escaped radi-ation we calculate the thermal gradient due to a heatflux of 1 W/cm2 in steel with a thermal conductivityof 50 Wm−1C−1. That is dT/dx = 1 × 104/50 =200 C/m. As the thickness of the steel shielding isof order 1 meter we might expect the shielding totemperature near the dump to eventually reach oforder 200 C for the heat flux of 1 W/cm2. The figure1 W/cm2 has in the past been used as a rule-of-thumbfor the limit on shielding heating (D. Walz, personalcommunication). We too adopt this rule.

The actual heat deposited will vary with positionalong the dump reaching a maximum about 45 cmfrom the front face and falling off rapidly before andafter the maximum (see Figure 3). The peak powerloss per meter for a 250 kW BSY dump is about4270 W/cm [24]. If 3 kW out of 250 kW escapes, thepeak power loss per meter that heats the shielding isapproximately 4270 × 3/250 = 51 W/cm, which fora radius of 15 cm is equivalent to a peak heat flux of51/(2π × 15) ≈ 0.5 W/cm2 — a factor of two belowthe rule-of-thumb value.

Questions that are not answered by instead usingthe rule-of-thumb are:

• How hot is too hot?

• What is the effective thermal conductivity of apile of steel plates.

• What is the three dimensional flow of heat.

15

• What is the appropriate usage factor given thevery low heating rate (of order 12 hours per de-gree C) and the resulting long time for the shield-ing temperature to get near the equilibrium tem-perature.

We assume the answers to these questions are benign.

4 Beam Position Control

Beam position control is vital to the safe operationof the dumps. For DUMPBSY to run above 120 kWthe beam needs to constantly move in a prescribedway, while for the undulator dumps the beam needsto be maintained at a constant position to within aprescribed radial tolerance.

4.1 Rastered beam

The DUMPBSY may see continuous beam power upto 250 kW and would reach damaging temperaturesand stress if the beam spot was not kept moving tospread out the heat. This motion of the beam on theface of the dump is called rastering. If the raster-ing were to fail, the dump could be damaged. Forthat reason the rastering magnet and the BPM sys-tems are part of the BCS and subject to special pro-tections. Rastering is not needed for the 120 kWDUMPH or DUMPS. Detailed requirements for therastering system are given in [25] and recapitulatedhere.

Rastering is accomplished using deflecting magnetsto move the beam, two BPMs to verify the motion,and a feedback system to maintain the central beamtrajectory. The rastering magnets are located ap-proximately 60 m upstream of the dump. There areno focussing magnets between the raster magnets andthe dump, so from the rastering magnets to the dumpthe beam should travel in a more or less straight line,only affected by static magnetic fields such as theearth’s field. Furthermore, with no focussing in be-tween, a circular kick is needed to produce circularmotion. A BPM located near the rastering magnetsand a second BPM is located near the dump are usedto monitor the beam position. The BCS will trip ifthe beam position near the dump deviates beyond

Figure 11: Dependence of stress in the Fin Core forrastered and non-rastered beams.

the allowed range given in Table 10. Orbit feedbackis used to help keep this from happening. A BCScurrent sensor on the raster magnet power supplydetermines if the rastering magnets are powered atthe proper strength, again tripping the BCS if thecurrent deviates from the allowed range. The BPMnear the dump should show the proper motion of thebeam going around in a circle in the right locationand the right diameter. The MPS will monitor thisBPM and issue a fault if it should exceed allowedlimits.

The limits on the rastering radius are determinedon the high side by the need to avoid hitting thebeam pipe wall with the beam halo and on the lowside by the increase in stress that results when theradius is decreased. If the raster radius is reducedthe stresses increase. Ultimately the stresses will in-crease by more than 50% for zero raster, in the linearapproximation. See Figure 11 With about 25% in-crease they will exceed the ultimate tensile strengthin the fin core. It seems reasonable to limit the in-crease in stress to about 5%, corresponding to 10%of the raster radius, i.e. 2 mm.

The choice of rastering frequency (see Table 10) isbounded on the low side by two factors:

• The need for the heat to evenly distribute aroundthe rastering circle before it diffuses to the cool-

16

ing channels.

• The needs for the beam spot to move a distancegreater than the diffused heat from the previousbunch so as to avoid thermal ‘pile-up’.

It turns out that the second factor is the determiningone.

The time scale ∆t for heat to diffuse the distanceand distance ∆L is ∆L2/D , where D is the diffu-sion constant which for 6061 aluminum alloy is about6.9×10−5 m2/s [15]. The circular raster path is about20 mm from the cooling channel, so the characteristicdiffusion time is ∆t = 0.022/6.9 × 10−5 ≈ 6 s. Theraster period must be much shorter than this char-acteristic time if the heat is to be spread out by thetime it reaches the cooling channel.

The distance heat diffuses in the time betweenbunches depends on the repetition rate. Followingcalculations in [15], for the highest repetition rateof 929 kHz and a rastering radius of 20 mm, theheat diffuses about 9 µm in the time interval betweenbunches, and the minimum rastering frequency thatinsures the spot has moved more than the heat hasdiffused is 311 Hz. This means that, roughly, for a1 Hz rastering frequency and a 929 kHz repetitionrate, the heat from about 300 pulses would pile upbefore it had a chance to diffuse away. Calculationshave shown that for LCLS-II parameters the instan-taneous temperature rise due to an individual bunchis around 7 C or less (see Figure 5 which used 4 GeV269 pC bunches). Clearly a 1 Hz rastering frequencywould lead to unacceptably high transient tempera-tures.

In reality the pile up effect is diluted because thereis substantial thermal diffusion occurring. Further-more there is a limit on beam power which decreasesthe bunch charge, and therefore the instantaneoustemperature rise, at high repetition rates where thediffusion time is shorter. For example, a 929 kHzbeam cannot have 269 pC bunches since that wouldfor 4 GeV amount to 1 MW and exceed the dumppower rating by a factor of 4. Beams limited to250 kW can have bunches up to 67 pC which willgenerate only about 1.7 C instantaneous temperaturerise.

Table 10: Rastering parameters

Min Nominal Max

Radius mm 18 20 22Center mm 0 0 2Frequency Hz 10 30 100

With this reasoning in mind, a 10 Hz rastering fre-quency would imply transient temperature rise up toroughly 30 C which is about 10% of the total maxi-mum equilibrium temperature rise and is acceptable.A nominal 30 Hz rastering frequency would implytransient temperature rise up to of roughly 10 C .These values are included in Table 10.

On the high side, the rastering frequency limit isdue to eddy current shielding by the vacuum cham-ber. If we require the skin depth to be 10 mm ormore, so that there would be relatively little shield-ing in the few millimeter thick vacuum chamber wall,then for an aluminum chamber the frequency wouldhave to be less than about 100 Hz (skin depth is about8 mm). For stainless steel chambers the frequencycan be higher by roughly a factor of 5.

Specifications for rastering magnets are not yet de-fined. One possible arrangement consists of threeconventional corrector magnets, arranged sequen-tially with a 60 degree rotation (roll) between ad-jacent magnets. A single conventional three-phasemotor controller can then be used to power the mag-nets with a different phase for each magnet. Thisarrangement takes advantage of the reliability andstability of the three phase power of the grid. Inprinciple two correctors with two power supplies 90degrees out of phase could produce the required cir-cular motion, but amplitude, phase, and frequencyof the supplies would have to be tightly controlled.With a three-phase arrangement, only the amplitudeneeds control. A third possibility is to make one sex-tupole magnet and wire the poles as three pairs ofdipoles. This is the arrangement used at CEBAF (C.Sinclair, personal communication).

17

4.2 Non-rastered beam

Deviation of a non-rastered beam from the dump cen-ter has the same types of hazards that are detailedfor deviation of a rastered beam if the raster radiusis too large. Tolerance on the maximum radial de-viation for a non-rastered beam is chosen based onthe one of the same criteria as is used for rasteredbeam, namely that irradiation of the cooling waterdoes increase more than a factor of two. This occurswhen the rastered beam is more than 22 mm fromthe center, so 22 mm is the tolerance on the relativeradial position of the non-rastered beam with respectto the dump center

5 Cooling System

Requirements for the Cooling System, given in [1],provide diagnostics to detect fault conditions as wellas a means of measuring beam power to calibrate theBCS ACMs. Schematics and tables of nominal andmaximum values of the parameters taken from [1]are repeated here for reference in Figures 12 and 13,and Table 11. Discussion of the power limits of thecooling system can be found in Section 10.

Briefly, a closed-loop, nominally 30 gpm water flowis recirculated through a dump, a heat exchanger andan expansion tank. A small amount of the flow isbypassed to a resin tank and a filter to remove corro-sion/erosion debris which can become radioactive. Inthe case of the undulator dumps, the dump water isconnected in series. Flow is monitored for each loop.Pressure and temperature monitoring are provided atthe input and output of each dump.

6 Operation and Maintenance

Operation of a dump consists of keeping the dumpin a state where it is safe to put beam on it. Exceptfor occasional maintenance the dump is continuouslyready to accept beam and the cooling system mon-itored and running within parameters in the rangegiven in [1].

Before high power beam can be put safely onthe dump, a low power beam is established with

feedbacks controlling the beam position so that itproperly strikes the dump face. For the rasteredDUMPBSY the beam parameters are given Table 10and the topic is covered in detail in [25].

Ordinary maintenance consists primarily of occa-sional replacement of the resin tank cores and filtersas well as flushing the system to remove some accu-mulated erosion corrosion products. The frequencyneeded is not well determined but is low, possibly lessthan once a year. It could be at intervals of months oryears depending on water chemistry, scheduled down-times, etc. Following an earthquake event, or fromtime to time when convenient, a check of the align-ment of the dumps should be made. Repairs and/orreplacement of pumps and diagnostics, and mainte-nance/repair to the ancillary Cooling Water systemare to be expected, but only infrequently.

7 Installation and Removal

Installation and removal procedures are the same forDUMPS and DUMPH and are briefly described here.For the first installation the process starts with theremoval of the existing LCLS shielding and dump.This exposes the concrete walls of the acceleratorhousing and the installation of the Core can proceed.Referring to Figure 14, the Girder Pedestals are lo-cated and installed, and bottom layers of shieldingare placed and grouted. Since bottom shielding platestack-up may not provide the proper profile due torough housing floor flatness and shielding plate thick-ness tolerance accumulation, we use shims in the gapbetween the top of the bottom shielding and the basefor the Core with iron.

The next step is to place the assembly of the DumpSubassembly, a wedge shaped steel piece that makesup for the 3.83 degree vertical angle of the beam,and shielding concentrically placed around the BeamTube. See Figure 15. The Outer Core is equippedwith tooling balls to facilitate proper position and ori-entation alignment. Three vertical adjustment screwsprovide three degrees of freedom (+/-Y, roll, andpitch). Three additional degrees of freedom (+/-X,+/-Z, and yaw) may be adjusted by temporary hy-draulic or screw jacks. When the Outer Core is in the

18

Table 11: Diagnostics and various values. See [1] for details.

Undulator Dumps Symbol Units Nominal Max Power27+48 kW 2 × 120 kW

Dump Water flow FU gpm 30 30DUMPH water inlet temperature TH in C 35 35DUMPH water outlet temperature TH out C 39 41DUMPS water inlet temperature TS in C 39 41DUMPS water outlet temperature TS out C 43 56

Pressure drop across combined dumps PH in − PS out psi 23 23Pressure at Pump inlet PP U in psig 30 30Pressure at Pump outlet PP U out psig 53 53Pressure at DUMPH inlet PH in psig 53 53Pressure at DUMPH outlet PH out psig 42 42Pressure at DUMPS inlet PSin psig 42 42Pressure at DUMPS outlet PS out psig 30 30

Cooling water flow FC U gpm - -Pressure at HX cooling water outlet PC U psig 100 100Pressure advantage of Cooling Water ∆PH U psig 70 70Cooling water HX inlet temperature TC U in C - -Cooling water HX outlet temperature TC U out C - -

Resin system flow FR U gpm 1 1Expansion tank level LU % 56 56

BSY Dump Symbol Units Nominal Max Power90 kW 250 kW

Dump water flow FB gpm 30 30DUMPBSY water inlet temperature TB in C 35 35DUMPBSY water outlet temperature TB out C 46 67Pressure drop across DUMPBSY psi 12 12

Pressure at Pump input PP B in psig 30 30Pressure at Pump output PP B out psig 42 42Pressure at DUMPBSY input PB in psig 42 42Pressure at DUMPBSY output PB out psig 30 30

Cooling water flow FC B gpm - -Pressure at HX cooling water output PC B psig 100 100Pressure advantage of Cooling Water ∆PH B psig 70 70Cooling water HX inlet temperature TC B in C - -Cooling water HX outlet temperature TC B out C - -

Resin system flow FR B gpm 1 1Expansion tank level LB % 56 56

19

DUMPH

DUMPS

TANK

HX

Level

Temperature Pressure TransducerFlow

Differential Pressure SwitchPressure Switch

+

-

TH in

TH out, (TS in)

TS out

RESIN FILTER

PUMP

PBTM

PBTM

Dump Water

Cooling Water

FU

LU

FC U

TC U inTC U out

TU HX out

PC U

PS out

PP U in

PP U out, (PH in)

PH out, (PS in)

�PH U

Figure 12: Schematic of undulator beam dump cooling system with diagnostics.

proper position, we weld the Wedge to the shim platesto make a solid monolith to protect the structure frommotion during seismic event. Once the Outer Core isin the proper position and the secured, the shims arewelded in place and the rest of the shielding is placedand grouted.

The procedure to remove the Core is somewhat dif-ferent because of the high radiation levels present.The procedure avoids direct exposure to radiationfrom the Core by setting up a shielded box calleda “coffin” into which the Core is pulled by a cable.See Figure 16. The Coffin is needed because steelshielding around the Core will become too activated.Fresh iron and lead is needed to shield the Core.

First there must be a wait period after the lastbeam for a time that is designated by RadiationPhysics. After that we cut off the vacuum, cool-ing water and nitrogen lines using a hydraulic cut-

ter, and then remove the Diagnostic Table. A CoreEnclosure (Coffin) is then put in place of the Diag-nostic Table on top of the Girder Pedestals. TheCoffin is opened by pulling on a heavy steel verti-cal sliding door and a cable is attached to the eyebolt on the Outer Core. The shielded dump assem-bly is supported by four track bearings that allowmotion parallel to the beamline. A winch then pullsthe shielded dump assembly into the Coffin. Oncethat is done the Coffin door is closed and the Coreplus Coffin assembly can be lifted and removed.

To install a replacement Dump, the winch system isset up on top of the Diagnostic Table and the gravityis used to roll the new Core into the position of theold Core.

20

DUMPBSYTANK

HX

Level

Temperature Pressure TransducerFlow

Differential Pressure SwitchPressure Switch

+

-

RESIN FILTER

PUMP

PBTM

TB in

TB out

FBLB

Dump Water

Cooling Water

PT B

TC B outTC B in

TB HX out

PC B

FC B

PP B in

PB out

PP B out, (PB in)

�PH B

Figure 13: Schematic of DUMPBSY cooling system with diagnostics.

8 Safety

The beam dumps are components of the Beam Con-tainment System (BCS), which is a automated beamsafety system that ensures the electron beams do notescape their intended channels nor exceed allowedbeam powers. This system monitors the beam powerand trips off the beam if the power exceeds the al-lowed beam power for that location. The allowedbeam power can never be more than the dump powerrating.

The dump design is deliberately conservative. Itis inspired by the design of dumps operated at CE-BAF for over two decades [6]. The peak tempera-tures are far from the melting point, and there aremultiple indicators of cooling faults. In addition, aburn-through monitor is located on the downstreamend of the dump. In the very unlikely event the beammelts its way through two meters of aluminum andsteel, when it hits the BTM it is expected to burnthrough and cause a PPS fault to turn off the beams.

The expected lifetime of the dumps is set by the

lifetime of the facility. That is, there are no rea-sons to expect the dumps to wear out or fail prior tothe end of use of the facility. Considerable thoughtand planning has gone into the design to minimizepersonnel exposure when removing the shielding andradioactive dumps when the facility needs to be de-commissioned, or in the very unlikely event of a dumpfailure.

9 Radiation

All three dumps are very well shielded to preventprompt radiation from reach occupied areas and forresidual radiation following a one hour cool-down pe-riod. The shielding is also designed to limit the pro-duction of tritium in the adjacent soil to below theEPA detection limit [24]. Prompt radiation insidethe accelerator housing (but outside the dump localshielding) which might affect electronic equipment isestimated to be moderate. Radiological issues con-nected with the dump cooling water are discussed in

21

Girder Pedestals

Shielding

Grout

Shims locations

Figure 14: Configuration just before a shielded dump assembly is placed.

22

Shielded dump assembly

Wedge (3.83 deg)

Top Plate

Figure 15: Configuration just after a shielded dump assembly is placed.

23

Coffin

Winch

CoffinDoor

Figure 16: Radioactive core inside the “coffin” offresh steel and lead.

[26].

If the dump is required to run up to 8 GeV somechanges to the shielding may be necessary to obtainthe same level of radiation in the soil as for 4 GeV.Possibilities are discussed in [27].

10 Performance Margins andRating

The dumps are designed to safely operate up to thebeam power and energy given in Table 1, and for us-age factors given in Table 2. Testing to find the actualfailure point is unfortunately not practical. In thissection we discuss the estimated performance mar-gins.

10.1 Melting

Estimated beam power required to reach the meltingtemperature in the inner core are listed in Table 12.These are obtained from the calculated temperaturesat the design power limit given in Table 4 using the

following:

T̂ = Tinlet +∆TH2O

2+ ∆Tcore + ∆Tfilm

where T̂ is the peak temperature in the aluminumcore, Tinlet is the temperature of the cooling wa-ter supply, ∆TH2O is the temperature rise across thecooling water, ∆Tcore is the temperature drop acrossthe aluminum core, and ∆Tfilm is the temperaturedrop from the aluminum to the cooling across theboundary layer. All of the quantities on the righthand side except for Tinlet, which is fixed, are pro-portional to the beam power, assuming the flow rateis held constant. Because the thermal conductivity isexpected to be almost independent of temperature, alinear extrapolation of the core temperature drop asa function of power is reasonably accurate. Failure isassumed to occur when T̂ reaches the melting tem-perature, 660 C. Such melting could eventually leadto beam penetration of the dump as was seen in thehigh power destructive beam tests in [28].

Because the two dumps are connected in series inthe cooling circuit DUMPS fails at a lower power thanDUMPH. In Table 12 the Failure Power for DUMPSwas calculated with the assumption that DUMPHwas simultaneously at its Failure Power. If DUMPHwas limited to 120 kW the Failure Power for DUMPSwould be about 265 kW.

Table 12: Dump beam power ratings and perfor-mance margins

Rated Failure FailurePower Power Mode[kW] [kW] [kW]

DUMPBSY 250 565 meltingDUMPS 120 200 meltingDUMPH 120 270 melting

10.2 Number of cycles

What beam power would it take to cause a stress-related failure assuming the aluminum does not first

24

melt and that heat is removed normally by the watercooling system? What would such a failure look like?

It is enlightening to note that thermal expansion ismuch smaller than elongation required for breakageof aluminum. Overall thermal expansion, given Ta-ble 5, converted to strain yields overall strain for the250 kW case of 0.35% radially and about 0.1% lon-gitudinally. This can be compared with elongationat break in the range of 12-17% [29]. It would takebeam power in the 10 MW range for single thermalcycle could rip the aluminum apart via thermal ex-pansion. Melting failure would certainly precede suchan event, and such high beam power is not possibleat LCLS-II. However, failure from a number excessivepower cycles is possible.

As noted in Section 3.7 thermal expansion does notproduce the same stress each cycle once the materialhas started to yield. For the mild yielding expected atthe rated maximum beam power, yielding in tensioncauses compressive stress for zero beam power that isjust sufficient to prevent yielding on the next cycle.In this case there should be no limit to the numberof cycles.

If the beam power is far above the rated beampower, it is possible the expansion is so large thatthe residual compressive stress is at the yield point,the stress ratio is −1.0 and material continues to flowradially each cycle. This might lead to failure in theform of the fin groove filling in with material. To getto this state the beam power would have to be at leasthigh enough to produce twice the theoretical yieldstress in the fin groove, which for the rastered 250 kWcase implies a beam power greater than or equal to2 × 250 × 276/330 = 418 kW . If the beam powerwere raised to 418 kW, after one cycle the stress ra-tio would be -1.0 and each cycle the material wouldreach the yield point in tension and in compression.According to data in Figure 10 we might expect thefin groove to lose integrity after around 1000 suchcycles of the rastered beam on DUMPBSY. For theun-rastered beam, a similar scaling calculation leadsthe beam power of 2 × 120 × 276/223 = 297 kWthat would be sufficient cause fin groove failure afteraround 1000 such cycles.

10.3 Heat flux and burnout

Though the peak wall temperature is near the sat-uration temperature the system is very far from thecritical heat flux needed for burnout. The peak heattransfer coefficient is at least two orders of mag-net less than the critical heat flux under the mostconservative assumptions. Using tables provided in[30], assuming no vapor phase in the cooling wa-ter (zero Quality), 30 gpm, and conservatively as-suming no sub-cooling and atmospheric pressure, thecritical heat flux is estimated to be about 18 timeshigher than the maximum heat flux when operatingat 250 kW. Under such conditions even a trickle flowthat is completely vaporized would provide enoughcooling to avoid burnout.

10.4 Bunch charge

The temperature difference between the meltingpoint, 660 C, and the highest equilibrium tempera-ture for maximum power operation, 409 C, is 251 C.We posit that, even under maximum average op-eration, we can safely allow a bunch charge up tothat which corresponds to 10% of that difference,25 C. Scaling from the 300 pC case, described in Sec-tion 3.5, the maximum allowable charge is 300 pC ×25/7 ≈ 1 nC. Much higher charges could be allowed,but not at the maximum average power.

10.5 Power ratings

The purpose of stating a dump power rating is to pro-vide a convenient, clearly understandable, value forthe beam power below which safe performance canbe assuredly maintained. It is understood that op-eration above the rating might be possible, but mayinvolve dump failure risks. The dividing line betweenassured performance and failure risk is not sharp.Because power related failures can involve consider-able facility down-time and risk radiation exposureto those involved in replacing dumps it is better tosomewhat bias the rated power toward assured per-formance and away from risk of failure.

The beam power ratings for high power operationare given Table 12. They take into account the ex-

25

tensive modeling and analysis on beams with en-ergy from 2 to 8 GeV, much of which is describedin this paper. The Cu linac will be able to gener-ate higher energy beams, perhaps as high as 17 GeV.The dumps can easily accommodate such high energybeams. They are very low power, usually less than1 kW and BCS limited to less than 5 kW. The innercore of the dumps is about 1.6 m long, which is about18 radiation lengths. We don’t expect much differ-ence in the stopping ability of the dumps for beamsup to 17 GeV compared with that of 4 GeV beams.A 17 GeV electron will be about 6 GeV after aboutone radiation length, leaving 17 radiation lengths forattenuation of the shower. Thermally the higher en-ergy spreads the heat out a bit more and increase thebeam power performance margin slightly.

11 Failure Modes and Effects

An Failure Mode Effects Analysis (FMEA) processwas undertaken to identify possible failure modes forthe dump system, what the hazardous consequencesmight be, how such failure would or would not bedetected, and what mitigation actions could be un-dertaken to reduce the likelihood of occurrence orseverity of the consequences. The results of completeanalysis is in the form of a spreadsheet. After filter-ing, 49 types of possible failure events are analysed.The types of failures are categorized into:

Cooling failures events which might cause thecooling system to not operate within it’s designparameters,

Diagnostic failures events caused by diagnostic er-rors that might lead to non-operation or actualfailures of dump systems components,

Core failures events which might lead to a burn-through of the core,

Alignment failures events which might lead tobeam hitting the dump outside the design range,

Radiation failures events which might lead tohigher than expected exposure,

Installation/Removal failures events that pre-vent planned installation or removal,

Other all other events that cause the dump to per-form in a way it was not intended.

The most numerous types of failure events listed arefor Cooling failures (18). They included a variety ofleaks, obstructions, corrosion related problems, resintank failures, mistakes in plumbing, cavitation, andmore. In all cases some reasonable mitigation ac-tion is identified including making test welds, pres-sure and vacuum testing of all joints, locating filtersdownstream of the resin tanks and upstream of thedumps, and a number of maintenance actions.

Diagnostic failures listed include software errorsand failure of diagnostic instruments, but mostly areconcerned with damage to the BTM which could leadto a dump replacement. Special features are includedin the design to protect the BTM and its lines.

Core failures, though felt to be highly unlikely,include melting through by the beam, the effect ofbolt breakage, transmutation gas build-up and dam-age to the core by a mis-steered high power beam.In one case, transmutation gas build-up, there is nodirect detection mechanism identified. But the con-sequence would be swelling and degradation of thecooling channels causing a failure of the cooling sys-tem which is well instrumented.

Alignment failures identified, which might becaused by an earthquake or even a loss of alignmentdata, could cause the beam to strike the dump too farfrom the center and generate excess radioactivity inthe dump water and additional thermal stresses. Spe-cial alignment features and procedures are includedas mitigation for these types of failures.

Only two Radiation failure modes are identified:one due to unexpected material composition and onedue to corrosion. The mitigations are to verify ma-terial composition for the dump core and to add ananti-corrosion black-oxide coating to some shieldingsleeve pieces.

Installation/removal failures generally might leadto additional exposure to personnel who have to fixthe problems. There are four such failures listed.

The final category, Other failures, includes fire,flood, rastering failure, excess beam power, over-

26

focussing of the beam, quality control problems, andloss of operational information about how to run thedump.

It is likely that one or more significant earthquakeswill occur during the service life of the dumps; smallerearthquakes being more frequent. The acceleratorhousing surrounding the Undulator Dumps is very ro-bust with reinforced concrete walls, ceiling and floorthat are many feet thick. The undulator dumps aremounted very securely to the steel shielding which istrapped by and grouted to the accelerator housing.Consequently the most probable scenario is that, fol-lowing an earthquake, the dumps will not be damagedand it is only necessary to check the dump is still inthe proper position. Additional verification of theintegrity of cooling and vacuum systems (which areexternal to the shielding) will, of course, be needed aswell. Position integrity is determined by observationof fiducials on the vacuum chamber that is welded toand extends from the outer dump core.

Corrosion at a modest level is expected, but not se-vere corrosion that would be associated with core de-compostion or serious shielding contamination. Cor-rosion, and in particular corrosion in a high radia-tion environment, could conceivably limit the use-ful lifetime of the dumps. There is an historical ac-count that warrants some optimism regarding corro-sion of the wetted aluminum surfaces. At SLAC 600to 800 kW beams were run on the SL10 collimator,which was installed in the mid-1960s and consists ofan aluminum chamber containing aluminum spheresand water. The wall thickness in the 6061 aluminummodules from the vacuum interface to the cooling wa-ter is 1.25 mm. To date no failure has been observedof this system (D. Waltz, private communication, Au-gust 2016)

Corrosion of surfaces exposed to air and high radia-tion will likely oxidize. The AP9 dump at FNAL hadto be replaced because of a water leak. Among otherthings, ‘there was rust all over everything’ whichmade containment issues more difficult (R. Lebeau,FNAL, private communication, April 2017). We arecoating some steel pieces that see the highest radia-tion to reduce corrosion. Overfocussing of the beamis expected to occur on occasion during tuning at lowbeam power. All other listed events are not expected

Table 13: Time to begin melting for various worstcase scenarios.

Event Time

Rastering is suddenly re-moved from the DUMPBSYrunning at 250 kW

2.0 - 4.1 s

A 1.2 MW beam is suddenlysent to DUMPBSY previ-ously running at 250 kW

1.8 - 3.4 s

The cooling system stopswhile DUMPBSY is runningat 250 kW

18 - 24 s

A 250 kW beam is suddenlysent to DUMPS or DUMPHpreviously running at 120kW.

8.5 - 12.5 s

DUMPS or DUMPH is run-ning at 120 kW and the cool-ing suddenly stops.

35 - 38 s

to occur during the lifetime of the facility.

12 Time to Damage

Estimates of the time it takes for dump damage tooccur in a few ‘worst case’ excess beam power fail-ure scenarios have been made using the tools devel-oped to study the transient heating of the dumps[10]. Note that ‘damage’ is assumed to occur whenthe aluminum core reaches the melting temperature.Burn-through times are much longer. The results arelisted in Table 13. All times are substantially longerthan a typical BCS response time.

References

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[2] T. Raubenheimer, “LCLS-II Parameters,”Physics Requirements Document LCLSII-1.1-PR-0133-R3, SLAC, May 2016.

[3] J. Welch, “LCLS-II Electron Beam Lossand Maximum Credible Beam,” Physics Re-quirements Document LCLSII-2.7-PR-0079-R1,SLAC, October 2015.

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[8] F.-J. Decker, C. Adolphsen, W. J. Corbett, andP. Emma, “Dispersion and betatron matchinginto the linac,” in Particle Accelerator Confer-ence, vol. 2, pp. 905–907, IEEE, May 1991.

[9] J. Welch, “Estimated number of high powercycles on LCLS-II dumps.” LCLS-II memoran-dum, November 2016.

[10] M. Santana-Leitner, “Energy deposition andheat transfer studies for LCLS-II CEBAF typedumps,” SLAC Radiation Physics Note RP-16-10, SLAC, June 2016.

[11] M. Santana-Leitner, “Energy deposition andheat transfer studies for Oct-2016 version ofLCLS-II CEBAF type dumps,” SLAC RadiationPhysics Note RP-16-25, SLAC, October 2016.

[12] A. Fasso, A. Ferrari, and P. Sala, “Electron-photon transport in FLUKA: Status,” in Proc.

MonteCarlo 2000 Conference, Lisbon (A. Kling,F. Barao, M. Nakagawa, L. Tavora, and P. Vaz,eds.), pp. 159–164, 2001.

[13] M. Santana-Leitner, “FHeat3D, a heat evolutionplug-in for FLUKA monte carlo code: Exam-ple of use for LCLS-II BCS response time defini-tion’,” Radiation Physics Note RP-14-07, SLAC,April 2014.

[14] M. Santana-Leitner, “Benchmarks of FHeat3Dheat evolution code,” Radiation Physics NoteRP-16-04, SLAC, February 2016.

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[17] M. C. Ross, R. Iverson, K. Jobe, D. McCormick,P. Tenenbaum, and P. Raimondi, “Single pulsedamage in copper,” SLAC Pub SLAC-PUB-8605, SLAC, September 2000.

[18] M. Schmitz, “Description of the beam dumpsystems for the European XFEL,” specification,DESY, December 2013. Revision 2.1.

[19] M. Santana-Leitner, “Minimum estimated timeto damage . . .,” RP Memo RPG-140506-MEM-02, SLAC, May 2014.

[20] http://www.instron.us/en-us/our-company/library/glossary/s/stress-ratio,August 2017.

[21] Anonymous, MILITARY HANDBOOK,METALLIC MATERIALS AND ELEMENTSFOR AEROSPACE VEHICLE STRUCTURES.No. MIL-HDBK-5H, Dept of Defense, 1998.

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