SLAC RADIATION PHYSICS NOTE RP-07-04 DRAFTJanuary 8, 2007

Studies on Bremsstrahlung sources in the BTH and LCLS undulatorIrradiation of the FEE by Bremsstrahlung beams from these sources

M. Santana, A. Fasso

Radiation Physics Department, SLAC, MS 48,2575 Sand Hill Road, Menlo Park, CA 94025

Abstract

The bremsstrahlung power generated in the BTH and undulator and reaching the FEE has beensimulated with the FLUKA [1] intra-nuclear cascade code. Calculations contain a detailed descrip-tion of all 33 segments, quadrupoles, collimators, dump systems and surrounding walls and halls.

Three bremsstrahlung sources are surveyed. The first one describes the photons arising from theinterception of the beam by a wire scanner. The second analyzes the interaction of the beam halowith the C{x/y}31-38 collimators. In the third case a tune up beam dump is placed in the beam path.A separate paper [2] will cover the prompt dose rate implications of each of these scenarios.

Results for the first case are greater than previous assumptions (800 mW) for carbon wires andrender the use of tungsten wires inadvisable. The photon powers originated in the second and thirdcases are much lower.

Studies on Bremsstrahlung sources in the LCLS undulatorIrradiation of the FEE by Bremsstrahlung beams from the undulator

M. Santana, A. Fasso

1 IntroductionA part of the shielding design of LCLS consists in computing the bremsstrahlung losses from theundulator through the FEE to ultimately analyze the dose levels in the areas of personnel occu-pancy (NEH,. . . ). This task has been divided into two parts. The first one, covered in the presentnote, computes the bremsstrahlung power exiting the undulator sector and penetrating the FEE. Thesecond part, described in a separate note [2] tracks the showers caused by the photon beams in dif-ferent areas downstream of the undulator. As the photon losses are extremely collimated, the secondpart could essentially be computed separately, requiring only the intensity obtained in this note tonormalize the final results.

1.1 FLUKA, geometry and materialsThe tool used in the calculations is a complex Monte Carlo intra nuclear cascade code, FLUKA [1].The geometry of each segment has been carefully implemented as well as that of the quadrupoles,collimators and dumps, walls, etc. The latest version of FLUKA has been used, and the latticefunctionalities have been automatized with the bash script “makelat.sh” in order to build up theundulator with its 33 modules directly with the newest available [3] coordinates. In this study themagnetic fields have not been turned on for the sake of simplicity and for faster tracking, but futurestudies may include those if needed.

The prototype for a sector (see figure 1), based on an existing model [4], was translated intoa virtual parking gallery, following a similar computation technique to [9]. The vacuum chamberwithin the sectors was defined as a copper (density 8.96 g/cm3) box with inner cross section 12.5 x5.0 mm2. The 226 poles are made of Vanadium Permendur, defined as 49% iron, 49% cobalt and 2%vanadium, with a density of 8.2 g/cm3. As for the 226 magnets in between the poles, the compositionN2Fe14B has been used, with a density 7.40 g/cm3. Since Nd low-energy neutron cross sections arenot available in the FLUKA library, the iron cross section has been assigned to neodymium (for lowenergy neutrons only). The empty spaces around poles and magnets and behind the side racks havebeen filled with air. The base plate and side racks are made of aluminum (density 2.6989 g/cm3).The separation (4.5 mm) and dimensions (12.5 x 12.5 x 0.5 cm3) of the magnetic shield iron end-pieces (density 7.87 g/cm3) was obtained by visual inspection of a test segment hosted in the MagnetField Test Facility at SLAC. The undulator housing is made of titanium (density 4.54 g/cm3).

A cross section of the prototype for the quadrupoles, inspired in [5], is shown in figure 1), left.The collimators, of tungsten jaws, are described in section 3.1. The tune up dump, made of

copper and tungsten, is introduced in section 4.

1.2 Simulation settingsThe production and transport cutoffs used in the simulation have been set to 300 keV for electronsand positrons and 100 keV for photons, except at the position of the main dipole, where the transportcutoff has been fixed to top energy to represent the magnetic kick-out of the electrons.

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For faster computing, photo-nuclear reactions have been switched off1. Leading particle biasingwas applied to all electromagnetic interactions over the whole geometry.

1.3 Beam settingsIn all cases electrons have 13.64 GeV, but the beam loss rates differ from one case to another. For thebfw calculations, the beam power amouts 5000 W, for the CY38 collimator results are normalized to1 W of lost electrons and for tdund the beam is lost at 10 Hz (417 W).

2 Insertion of a beam wire scanner2.1 IntroductionThis section refers to a temporary and invading beam measurement consisting in inserting a wireperpendicular to the beam trajectory (bfw), at its central position. A huge flux of photons is thencreated as bremsstrahlung interactions between the electron beam and the field of the atoms of thewire. The worst possible scenario will occur the closest this wire is to the FEE, which happens forbfw33, placed right before the last segment, at z = 64548 cm. Simulations have been carried out forseveral wire materials (C, Ti, W) and diameters (30 and 40 µm). As for the beam size, a first setof computations dealt with a pencil beam (pessimistic approach) and refined calculations sampledelectrons from a Gaussian of 37 µm RMS in each of the transverse planes.

The simulations score the energy flux of photons through the pipe that crosses the FEE wall atposition z = 72180 cm. The scoring area is a disc of 1 cm diameter.

2.2 ResultsResults show that when the carbon wire is replaced by a titanium one, the photon flux (γ · E)increases by over a factor 5 and by another factor 10 if passing from Ti to W. Moreover, simulationswith a pencil beam indeed give results a factor two higher than those obtained with the “real beam”.For a 40 µm wire these are the results2:

C 1345 mW

Ti 7345 mW

W 72350 mW

Looking closer at the case of a 40 µm C wire, we see that a total power of 2450 mW (90.05 %γ, 9.90 % e−, 0.05 % µ, n ) hits the iron shield block3, and that only about 2 mW make it throughthe iron wall and the FEE shield. The rest of the transmitted power, 1350 mW, goes through thepipe, 99.999 % as photons and the rest as electrons.

For such fine wires the thin target approximation is valid, and Bremsstrahlung power scalesalmost linearly to the effective thickness. In the case of the 37 µ m RMS cicular beam, the averagetraverssed thickness is about 0.8 times the wire diamter.

1This feature is activated for other calculations.2These numbers are obtained for an electron beam of 5000 kW at 13.64 GeV.3A block of 434.3 x 365.8 x 122.9 cm3 of iron placed just before the FEE wall.

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3 Beam halo intercepted by the collimators3.1 IntroductionUnlike in the previous case, the photon flux created due to beam halo cleaning in the collimatorshappens continuously, though the consequences are less serious because the halo intensity repre-sents a small fraction of the beam power. All collimators (CX31, CY32, CX35, CY36, CX37 andCY38) have been included in LCLS FLUKA model as two-jawed block filters with the right length,aperture and position, all automatically retrieved from the LCLS twiss files before each calculation.A FLUKA source file has been written in order to represent a beam scraping with a small anglethe jaws of the last collimator in the sequence, CY38. Two cases have been considered in order tounderstand the importance of the grazing angle, 1 mrad and 0.1 mrad. The beam hits both jaws,each 8 cm long, 1.9 cm thick, separated by 0.64 cm, with uniform probability in the z axis andwith a Gaussian distribution of 37 µm RMS in the vertical plane. A set of detectors measuring theenergy flux of photons behind the last segment are activated to measure the throughput of photonsat different outgoing directions.

3.2 ResultsSimulations show a steep increase of the photon flux with decreasing grazing angles. From 1 mradto 0.1 mrad the flux increases more than two orders of magnitude. This situation is compatible withthe knowledge of Bremsstrahlung radiation, where the photons are expelled forming a sharp conearound the impinging electrons. In the second case the effective viewing angle is larger than inthe first. However, for nearly tangential angles the halo only sees a fraction of the jaw, which willdepend on the roughness of its surface, so the Bremsstrahlung emission falls down to almost zero.

At a 0.1 mrad grazing angle the integrated measured energy flux of photons was below 0.5 mWper Watt of electron power hitting the CY38 collimator. Most photons (0.487 mW) went all theway through the pipe, while only 0.007 mW fall out of that region. The high collimation indicatesthat losses at different locations (i.e. in different collimators) will directly add up at the FEE.Nevertheless, given that the nominal halo power losses are of the order of 0.1 mW [6]4, this casehas only limited impact in the FEE, although unlike the case of the bfw, here the flux is continuous.

4 Beam stopped at a tune-up dumpIn this case the beam is fully intercepted by the tune-up dump (tdund, z = 51285 cm), which is a15 cm x 15 cm block presenting three radiation lengths of tungsten (3 X0) at the extremes and 18X0 of copper in the central part [7]. As a consequence of the Bremsstrahlung reactions and of theself-shielding effect in the dump, a broadly scattered flux photons, 15 W in power, exits the dump,but only a tiny fraction has the right direction to reach the FEE. In the worst possible scenario,where the undulator segments are not yet mounted and therefore provide no additional shielding tothe scattered photon flux, it has been estimated that the power reaching the iron wall is about 15mW, of which only a few micro-watt go all the way through into the FEE.

Dose maps around the tdund in different conditions are closely presented in [8].4and that the maximum accepted loss stays below 20 W.

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5 ConclusionsThese results have been obtained after considerable efforts to match the input geometry to the avail-able design data. Further calculations may be carried out to account for the action of magnetic fieldsor with a more refined geometry. However, it has become clear that bremsstrahlung shower simula-tions through the FEE can reasonably count with a starting current of 1.5 W. This is true if carbonwire scanners are used. Otherwise this power ramps up quickly; a factor 60 in the case of tungsten.

With the present halo loss fraction, collimators are discarded as a worrisome bremsstrahlungsource. In the case of insertion of the tune up dump, photons are created and scattered in the dump,and only a negligible fraction (about 5 µW for a 420 W, 13.64 GeV beam) reaches the FEE.

References[1] A. Fasso, A. Ferrari and P.R. Sala, Electron-Photon Transport in FLUKA: Status, Proc. Mon-

teCarlo 2000 Conference, Lisbon, October 23–26 2000, A. Kling, F. Barao, M. Nakagawa,L. Tavora and P. Vaz eds., Springer-Verlag Berlin, p. 159–164 (2001)A. Fasso, A. Ferrari, J. Ranft and P.R. Sala, FLUKA: Status and Prospective for HadronicApplications, same proceedings, p. 955–960 (2001).

[2] A. Fasso, Shielding Design for the LCLS Front End Enclosure and Near Experimental Hall,SLAC Radiation Physics Note RP-07- (2007)

[3] P. Emma, LCLS Linac Current Beamline Design Optics Files,http://www-ssrl.slac.stanford.edu/lcls/linac/optics/

[4] A. Fasso, Dose Absorbed in LCLS Undulator Magnets, SLAC Radiation Physics Note RP-05-05, (2005)

[5] M. Jaski, Undulator Quadrupole Focusing Magnet Specifications, LCLS Engineering Speci-fications Document 1.4-102, LCLS Document L14305-00001, SLAC (2006)

[6] D. Dowell, P. Emma, J. Welch, Electron Beam Loss in the LCLS, LCLS Physics RequirementDocument 1.1-011, SLAC (2006)

[7] D.R. Walz, A. McFarlane, E. Lewandowski, Beam Dumps, Stoppers and Faraday Cups atthe SLC, page 2, SLAC-Pub-49, April 1989 (A)

[8] M. Santana-Leitner, Prompt dose study in the LCLS undulator, SLAC Radiation Physics NoteRP-07-05, (2007)

[9] M. Santana-Leitner, V. Vlachoudis, A. Ferrari, M. Magistris, K. Tsoulou, Energy DepositionStudies for the Betatron Cleaning Insertion, Particle Accelerator Conference 2005 proceed-ings, Knoxville TE, (2005)

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Figure 1: Transverse section of a segment and a quadrupole as implemented in FLUKA

Figure 2: Vertical-z cut of the undulator set-up at different locations

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