Bane, Karl Leopold Freitag

Irwin, J

Jobe, R Keith

McCormick, D

Raubenheimer, T O

Ross, M C

Stupakov, G

Tenenbaum, P G

Walz, D

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AN APPARATUS FOR THE DIRECT MEASUREMENT OF COLLIMATORTRANSVERSE WAKEFIELDS P. Tenenbaumy , K. Bane, J. Irwin, R.K. Jobe, D. McCormick, T.O. Raubenheimer, M.C. Ross,G. Stupakov, D. WalzStanford Linear Accelerator Center, Stanford, CAAbstractThe design of the NLC Beam Delivery System requires afirmer understanding of the effects of collimators on short,intense bunches than is presently available. We describe anexperiment to directly measure these effects through use ofa dedicated apparatus located at the 1.19 GeV point in theSLAC Linac. The apparatus consists of an outer vacuumvessel and an interchangeable insertion containing up to 5distinct collimator apertures. The insertion is capable ofremote-controlled translation, allowing the collimator aper-tures to be misaligned relative to the electron beam withoutchanging the incoming beam orbit; the wakefield deflectionis then measured by observing the change in the outgoingorbit on 32 beam position monitors. The parameters of theapertures have been selected to allow confirmation of thescaling laws for collimator wakefields, and to strongly en-hance either the geometric or resistive wall contribution ofeach aperture. Details of the apparatus design, the apertureparameters, and the experimental program are discussed.1 INTRODUCTIONThe Next Linear Collider (NLC) post-linac collimationsection is a 2.5 km long section of beamline the purpose ofwhich is to prevent particles at large amplitudes from en-tering the final focus, where they would cause unacceptabledetector backgrounds. The design of the NLC collimationsection is substantially based upon experience at the Stan-ford Linear Collider (SLC) with a similar collimation sys-tem interleaved with accelerator structures in the last 300meters of the linac.One of the central design specifications of the NLC colli-mation system is the contribution to luminosity degradationfrom collimator short-range dipole wakefields. A beamwhich passes off-center through a collimator experiences adeflection due to wakefields; therefore a collimation systemwill amplify incoming jitter. The NLC collimation systemhas been limited to a 25% increase in beam jitter due tocollimator wakefields.The present understanding of collimator wakefields isnot adequate to confidently predict the jitter amplificationof a given collimator design. The NLC collimators are ex-pected to utilize a smooth taper to match between a rect-angular aperture and a cylindrical beampipe. The analyticexpression for the deflection of an electron bunch due toWork supported by the Department of Energy, contract DE-AC03-76SF00515.y Email:quarkpt@slac.stanford.eduthe geometric wakefield of such a collimator is [1]:y0= 2Nreh2p(r0  r1)(r20  r21)zLTr20r21y0; (1)where y0is the beam offset in the collimator, N is thebunch population, reis the classical electron radius,  isthe relativistic factor, zis the RMS bunch length, LTisthe taper length, r0and r1are the radius of the beampipeand half-aperture of the gap, respectively, and h is thetransverse width of the gap (e.g., the horizontal width ofa vertical collimator). As expected, the deflection in Equa-tion 1 vanishes when r1! r0; however, the deflection isalso proportional to h, implying that the deflection from aninfinitely-wide collimator diverges.The resistive contribution to the wakefield deflection isexpected to be [2]:y0= 2R3=2e2NcLTEpzrZ0r0+ r1r20r21hfiy0; (2)where Z0is the impedance of free space,  is the con-ductivity of the collimator, e is the electron charge, E isthe beam energy, hfi is a constant arising from integrationof the kick over the (presumed) Gaussian distribution inz (hfi  0:724), and Ris a form factor equal to 1 fora cylindrical collimator and equal to 2=8 for a rectangu-lar collimator. Note that Equation 2 does not contain non-physical divergences and predicts a deflection even from aperfectly regular vacuum pipe (r0= r1). However, mea-surements of the deflection due to the wakefields from theSLC collimators have exceeded those predicted by Equa-tions 1 and 2 by up to a factor of 4 [3].Collimator wakefield measurements [3, 4, 5] have his-torically been difficult to interpret because they have beenmade on existing collimator installations, which are de-signed to minimize wakefield contributions rather than toamplify certain features of the wakefields over others. Forexample, the SLC collimator study discussed in [3] at-tempted to separate the resistive and geometric wakes bymeasuring the deflection as a function of bunch length:since Equation 1 predicts that y0 / 1=z, while Equa-tion 2 predicts that y0 / 1=pz, the contributions fromthe two components could be inferred by such a measure-ment. However interpretation of such a measurement is farfrom straightforward due to uncertainty in the actual bunchlength, response of beam monitors in the SLC under con-ditions of changing bunch length, and the impact of bunchlength on the wakefields from the RF structures which fillthe linac and complicate any trajectory measurement. A0-7803-5573-3/99/$10.00@1999 IEEE. 3453Proceedings of the 1999 Particle Accelerator Conference, New York, 1999more optimal measurement could be made at a dedicatedfacility, in which collimators of any design could be tested.As part of the NLC research project at SLAC, we have de-signed such a facility.2 OVERALL DESIGNThe collimator wakefield apparatus is a vacuum vessel withapproximate dimensions 1.7 meters 0.6 meters 0.3 me-ters, as shown in Figure 1. The vessel is designed with alid which can easily be removed and replaced to allow ac-cess to the interior. The vessel contains an insertion, whichis a long block of aluminum containing 5 apertures; eachaperture can be fitted with a collimator of any geometrydesired.67”60”ImpedanceMatcher (eitherend)FFTB Magnet MoverBeamInsertionBeamHoleVacuumBellowsStepper MotorFigure 1: Diagram of collimator wakefield test apparatus,side view (top) and end view (bottom). The long, coarsetranslation stage and stepper motor move the insertion inx to engage a test aperture; the FFTB magnet mover thenmoves the entire vacuum vessel in y to produce a beam-aperture displacement, which results in a measurable de-flection.The cartridge rides on a platform which is connected toan external horizontal translation stage; the stage engageswhichever of the apertures is to be used for experimenta-tion. In this way a series of related collimators can betested in sequence without entering the accelerator hous-ing or disturbing the vacuum vessel. When the translationstage moves to its full-out position, the insertion is removedcompletely from the path of the beam.The vacuum vessel is mounted on a vertical translationstage which has a range of 1:5 millimeters and a step sizeof 1 m. Once one of the test apertures is engaged, the ver-tical stage is used to make a precise translation of the appa-ratus. The wakefields due to the collimator in the aperturewill produce a deflection, which can be observed on beamposition monitors downstream of the apparatus.Wakefield deflections are inversely proportional to beamenergy, and therefore the apparatus should be installed ata location where the beam energy is relatively low. Thelocation selected is the SLAC linac spin rotator locationimmediately downstream of the point where the damped1.19 GeV beams re-enter the linac. This location is justupstream of the ASSET location, which allows a similarexperiment on RF structures using a drive beam from thepositron damping ring and a witness beam from the elec-tron damping ring[6]. The collimator wakefield experimentwill require only one beam. The deflection will be mea-sured by recording the beam position on each of 32 beamposition monitors downstream of the apparatus location, aswell as several monitors upstream of the apparatus. Thedeflection will be reconstructed after pulse-to-pulse jitter issubtracted using the upstream BPMs. Because the down-stream BPMs are largely interleaved with accelerator struc-tures, it is desirable to use a low bunch charge so that thewakefields from the structures can be neglected. Our testapertures are designed to produce a measurable deflectionfor a bunch charge of 1  1010. This is sufficiently belowthe threshold of instability in the linac that, if additionalresolution is called for, the bunch charge can be increasedmoderately for some measurements.3 INSERTION DESIGNThe bulk of the insertion should be constructed from amaterial with a high conductivity in order to avoid pro-ducing unwanted resistive-wall wakefields; it should be aslight as possible to facilitate installation and removal in thecramped experimental area; and it should be made from amaterial which is easy to machine. On the basis of thesecriteria, aluminum was chosen for the material.The insertion contains 1 circular beam channel and 4square ones, each with a full bore of 4 cm. The total lengthof the insertion is 1.5 meters, in order to accomodate thelongest collimators foreseen for measurement. In order tosimplify installing and removing collimator apertures fromthe channels, it was constructed in a split configuration witha top half and a bottom half which can be disassembled andreassembled easily.The clearance between the insertion and the impedance-matching flanges at either end of the vacuum vessel is 1mm. This produces a small discontinuity wakefield, but theclearance was deemed necessary to ensure that the insertioncould move easily on its horizontal stage. The deflection re-sulting from the discontinuity is believed too small to mea-sure, but this will be checked by engaging the cylindricalchannel without any collimator aperture and measuring thedeflection as a function of the vertical stage position. Thiswill allow us to subtract the deflection due to the vacuumvessel, discontinuities, and other construction features.3454Proceedings of the 1999 Particle Accelerator Conference, New York, 19994 APERTURE DESIGNIn order to separately measure the contributions of geomet-ric, resistive, surface finish, etc. wakefields, it is necessaryto carefully construct each test aperture to enhance one par-ticular source of deflections and reduce the rest. The firstset of apertures are intended to maximize geometric wake-fields. Therefore they are all constructed from copper, witha good finish, and all the apertures are short in z.Table 1 shows the parameters of the first set of test aper-tures to be measured. Note that the predicted deflectionsare calculated for a round pipe tapering down to either around or a rectangular aperture; we approximate these witha square pipe tapering down to a square aperture in the for-mer case, and a square pipe tapering down to a rectangularaperture in the latter. Computation of the wakefields forour precise geometry is difficult and uncertain, but the ex-pected deviation from the cylindrical approximation is onthe order of 20%. The expected resolution of the technique,based on ASSET measurements, is 1 microradian, and allof the expected deflections are large relative to this.Table 1: Parameters and predicted deflections for geo-metric wakefield test apertures, given a beam charge of1  1010, an energy of 1.19 GeV, and a 1 mm apparatusvertical offset.Channel Aperture Taper KickNumber (cm) Length (cm) (rad)1 4.0, round None  02 0.4, square 5.0 1.93 0.4, rect. 5.0 51.34 0.4, rect. 10.0 33.95 0.8, rect. 5.0 25.6The second set of test apertures will measure resistive-wall wakefields. This requires as axisymmetric a geometryas possible, in order to minimize geometric-wake contri-butions. The resistive wakefield can be further enhancedby using a material with a poor conductivity and includinga long untapered section with a small gap in between thetwo tapers, which should enhance resistive deflections butleave geometric deflections unaffected. Ideally these mea-surements would use a second insertion in which all chan-nels are cylindrical; machining considerations may man-date use of a square aperture once again. Table 2 shows thedesired parameters for a resistive-wall wakefield test inser-tion. While we have used stainless steel as our resistivematerial in Table 2, we are presently leaning towards useof a pure element for this purpose, in order to achieve auniform conductivity which does not depend upon relativedensities of various alloyed materials in the surface.Future insertions will test the effects of a subset of geo-metric features, namely surface finish and surface features,and also probe the behavior when geometric collimators ofvarying width are used. Ultimately an NLC-design colli-Table 2: Parameters and predicted deflections for resistivewakefield test apertures, given a beam charge of 1  1010,an energy of 1.19 GeV, and a 1 mm apparatus vertical off-set. Test apertures 1 and 2 are copper, while 3 through 5are steel.Coll. Aperture Taper Untapered Kick# (cm) Length (cm) Length (cm) (rad)1 0.4 5.0 0 1.92 0.4 5.0 100 2.63 0.4 5.0 100 7.14 0.4 5.0 50 4.55 0.8 20.0 100 3.2mator will be directly tested in this apparatus.5 EXPERIMENTAL PROGRAMThe vacuum vessel and insertion with the first set of testapertures is presently under construction, with installationin the SLAC linac scheduled for late April. Once the trans-lation stages are fully commissioned, experimental mea-surements will be made parasitically during PEP-II colli-sions, as it is expected that the PEP-II program can be in-terleaved with collimator studies with minimal disruptionof either program. Measurements are expected to continueon a periodic basis for the next several years.6 ACKNOWLEDGEMENTSThe authors wish to thank G. Bowden and D. Burke fortheir ideas and encouragement, and B. Brugnoletti for de-signing the test apparatus.7 REFERENCES[1] G. Stupakov, “Geometrical Wake of a Smooth Flat Collima-tor,” SLAC-PUB-7167 (1996).[2] NLC ZDR Design Group, “Zeroth-Order Design Report forthe Next Linear Collider,” SLAC Report 474, 589 (1996).[3] F. Zimmermann et al, “Measurements of Wakefields Gener-ated by the Newly Coated Collimators in the SLAC Linac,”SLAC-CN-415 (1997).[4] F.J. Decker et al, “Design and Wake Field Performance of theNew SLC Collimators,” Proc. Linac 96, 137 (1996).[5] K.L.F. Bane et al, “Measurement of the Effect of CollimatorGenerated Wake Fields on the Beams in the SLC,” Proc. PAC95, 3031 (1995).[6] C. Adolphsen et al, “Measurement of Wake Field Suppres-sion in a Detuned X Band Accelerator Structure,” Phys RevLett 74:2475 (1995).3455Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

An Apparatus for the direct measurement of collimator transverse wakefields

An Apparatus for the direct measurement of collimator transverse wakefields

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