The 9 GeV High Energy Ring (HER) of the PEP-II B Factory is an electron storage ring under construction at SLAC. Significant progress has been made in the last year on all systems. As of mid 1996, all 192 dipoles have been installed, with installation of the quadrupoles underway. The vacuum system, for design currents up to 3 A average, is in production using a recently commissioned e-beam welder. Beam instrumentation systems are being fabricated. The interaction region will bring the HER beam into collision with the 3 GeV beam of the Low Energy Ring; design of this section of the HER is in an advanced stage. 8 refs., 3 figs., 1 tab

Bellomo, P.

Daly, E.

Fisher, A.

Gracia, J.

Kulikov, A.

Kurita, N.

Pietryka, M.

Reuter, E.

Wienands, U.

UNT Digital Library

European Particle Accelerator Conference Stiges, SpainJune 9-14, 1996 LawrenceLivermoreNationalLaboratoryUCRL-JC-126719Status of the PEP-II B-Factory High Energy RingU. Wienands, E. Reuter, P. Bellomo, E. Daly, A. Fisher, J. Gracia, A. Kulikov, N. Kurita, M. Pietryka, J. T. Seeman, T. Taylor,C. Belser, L. Bertolini, M. Mugge, J. SwanJune 1, 1996This is a preprint of a paper intended for publication in a journal or proceedings.  Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.PREPRINTThis paper was prepared for submittal to theDISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.Figure 1. Quadrupole raft installed in the PEP TunnelSTATUS OF THE PEP–II B-FACTORY HIGH ENERGY RING*U. Wienands, E. Reuter, P. Bellomo, E. Daly, A. Fisher, J. Gracia, A. Kulikov, N. Kurita,M. Pietryka, J.T. Seeman, T. Taylor; SLAC, Stanford, CA 94309; andC. Belser, L. Bertolini, M. Mugge, J. Swan; LLNL, Livermore, CA 94550AbstractThe 9 GeV High Energy Ring (HER) of the PEP-II BFactory is an electron storage ring under construction atSLAC. Significant progress has been made in the last yearon all systems. As of mid 1996, all 192 dipoles have beeninstalled, with installation of the quadrupoles underway.The vacuum system, for design currents up to 3 Aaverage, is in production using a recently commissionede-beam welder. Beam instrumentation systems are beingfabricated. The interaction region will bring the HERbeam into collision with the 3 GeV beam of the LowEnergy Ring; design of this section of the HER is in anadvanced stage.1  INTRODUCTIONThe SLAC-LBL-LLNL PEP–II project is anasymmetric 3.1 GeV on 9 GeV e+–e– collider designedfor a peak luminosity of L=3×1033 cm-2s-1. In order toreach the design luminosity at moderate tune shifts (about0.03) the circulating beam currents are large—1 A in theHER and 2.1 A in the LER. A list of the important HERmachine parameters is given in Table 1. The facility isunder construction in the PEP tunnel at SLAC[1,2].The magnet lattice of the ring is six-fold symmetric tofit the existing tunnel and has been described before [3].The nominal beta function at the insertion point, βy*, is2 cm; the lattice and the insertion quads support βy* aslow as 1 cm. The dynamic aperture of the lattice atβy*=1.5 cm is more than 12σ.Table 1. Top-level HER machine parameters.Energy (GeV) 9Circumference (m) 2199.3Average beam current (A) 1Emittance, (horiz.,vert.) (πnm-r) (48,1.9)Number of bunches 1658Rf frequency (MHz) 476Beta functions at IP (x,y) (m) 0.5, 0.02Energy loss per turn (MeV) 3.692  ACCELERATOR SYSTEMSInstallationAs of Oct. 1995 all 192 refurbished PEP dipoles havebeen reinstalled in the tunnel. Quadrupoles are beingassembled and prealigned on rafts together with their* Work supported by US Dept. of Energy under contractsDE-AC03-76SF00515, DE-AC03-76SF00098, DE-AC03-81ER40050, DE-AC03-76ER70285 and W-7405-Eng-48.associated orbit correcting dipoles and sextupoles whereapplicable. The completed rafts are then transported to thetunnel where they are positioned and surveyed (Fig. 1).The quadrupoles are sorted by their integral gradient tomatch members of a family powered by one supply asclosely as possible. Surveying data gathered so farindicates that the rafts are being installed to within 1 mmrms of their final position before alignment. A slightsetback was encountered when it became evident that thefixtures used as quadrupole fiducials to locate the centerof each magnet did not give reproducible results. Noweach quadrupole is fiducialized individually.Quadrupole vacuum chambers (which are captive tothe magnets) are installed in the quadrupoles at raftassembly, before installation in the tunnel. Dipolevacuum chambers are installed in the tunnel as they slideeasily into the opening of the C-magnets. It had been decided early on in the project to reuse theexisting cable plant as much as reasonable, therefore, asignificant amount of cabling already exists in place butsignificant effort is required to reconfigure the cabling forthe different configuration of the HER magnets andinstallation of new cables for the LER. This activity isscheduled for completion by the end of 1996.MagnetsA fraction of the HER quadrupole magnets are newlybuilt, using the same lamination dimensions as theexisting quadrupoles. The laminations for these magnetshave been obtained from a local vendor, while the coilshave been built at BINP, Novosibirsk. Fabrication ofthese magnets is about halfway complete, withcompletion scheduled for August 1996. Magneticmeasurements of the first assembled magnets haveconfirmed that the magnetic properties differ onlyinsignificantly from those of the existing quadrupoles.Vacuum SystemThe HER arc vacuum system is a copper systemdesigned to absorb the 10 MW synchrotron radiationpower generated by a 9 GeV, 3 A beam. An electron-beam welder has been procured to allow welding of theup to 5.8 m long Cu chambers under clean-roomconditions. Production of the dipole chambers involvesseveral steps: Welding of the cooling bars, stretch-forming across a mandrel to follow the curvature of thebeam orbit in the dipoles, welding of the screenseparating the distributed ion pump (DIP) from the beamchannel, inserting the DIP and finally welding of theflanges and end-pieces of the DIP channel. The dipolechambers are glow-discharge cleaned before insertion ofthe DIP and the complete chamber is baked at 200 °Cbefore delivery to the tunnel. Measurements of thepumping speed of the first assembled dipole chamberhave shown a net pumping speed of approx. 180 l/s/m at10×10−9 torr. Fabrication of the quadrupole chambersfollows a somewhat similar procedure. All chambers arein full production and are being installed in the tunnel.The straight-section vacuum system does not have toabsorb much synchrotron radiation power and is thereforemade of stainless steel tubing with water cooling. Allcomponents are in production in the SLAC workshops,while cleaning is done in a facility built at LLNL.Prototypes of arc and straight-section bellows havebeen tested for rf heating and trapped modes. The testshave been successful: no evidence for excessive heatinghas been found in the straights bellows under rf power (at480 MHz) calculated to exceed the level expected at 3 Abeam current; also, no evidence of trapped modes belowcut-off of the vacuum system has been found. Thestraights bellows is described in detail in [4].Beam DiagnosticsThe suite of beam diagnostics under construction forPEP-II is described in more detail elsewhere.[5]Construction of the beam position monitor (BPM)system is progressing on schedule. The 15 mm pickupbuttons are welded into the quadrupole chambers duringfabrication; each chamber is subsequently calibrated atthe BPM frequency of 952 MHz. There are a number ofadditional BPM for special-purpose diagnostics.An extensive system of beam-loss monitors (BLM) isin design in order to allow localization of beam losses andto provide a large dynamic range—about 106—in loss-detection. The detector element is a Cherenkov counterequipped with a subminiature PMT; the whole detectorunit can easily be mounted close to the beam line. Leadshielding (10 mm) is provided against synchrotronradiation. All electronics are contained in CAMACmodules located underneath selected dipoles for shieldingagainst radiation.[6] This system also provides a signal tothe beam-abort trigger system (BATS).The dc current transformer (DCCT) uses acommercial unit by Bergoz. The housing is based on adesign used successfully at the ALS and has been built. Acurrent resolution of 5 µA allows for the fast, on-linebeam-lifetime calculations essential for tuning themachine.The bunch-current monitor (BCM) uses the signalfrom a dedicated set of BPM buttons, which are summedfor this purpose. High-speed conversion using an 8-bit,500 MHz DAC, parallel processing and accumulation ofthe data during one injection cycle yield an effectiveresolution of about 10 bit; essential for achieving aresolution of better than 0.5% for the charge measurementof a single bunch during one injection cycle (17 ms). TheBCM will be calibrated against the DCCT.The synchrotron light monitor is challenging becauseof the very high synchrotron radiation flux from the ringdipoles. The first mirror has to be able to withstand thisflux without significant distortion on a 10 nm scale. Thischallenge is being met by adopting a very shallow angleof incidence—4°—at the first mirror. A 4 mm high slot inthe mirror lets the X-ray fan, with its much narrowervertical spread and much higher intensity, pass through toa separate dump thermally decoupled from the mirror.[7]3  THE INTERACTION REGIONPEP-II is designed for head-on collisions in order toavoid issues arising from synchro-betatron coupling. Dueto the need to separate the beams quickly to avoidparasitic crossings, permanent-magnet dipoles are locatedwithin the detector. The innermost quadrupole, again apermanent magnet, acts on both the HER and the LERbeams. Outside of this quadrupole the magnetic elementsare separate for each ring. One of the HER insertionquadrupoles (Q4) will be offset both vertically andhorizontally to compensate for the bending of theasymmetric and tilted detector solenoid.[8]During initial HER commissioning, the sharedmagnets will not be present. The HER insertionquadrupoles Q4 and Q5 (Fig. 2) will run at highercurrents to provide the required focusing. By coincidence,the increased strength in Q4 will also just compensate forthe missing dipoles. The commissioning lattice is thentuned such that the beam sizes outside the insertionquadrupoles are identical to those in the finalconfiguration; in this way, background measurements thatwill make up an important part of the commissioningactivities in 1997 can be performed with a configurationvery close to the final one after installation of the LERand the detector.Both HER insertion quadrupoles are laminated iron-copper magnets. Design challenges arise from the largeQ4Q5Q1Q6Q7e-Fig. 2. Layout of the upstream HER Interaction Region(m)beam size in these magnets and the small separation ofthe beams at these quads, restricting the room for themagnet coils. Field-uniformity requirements are tight:non-linear field harmonics are not to exceed 10-4 of thequadrupole field at a radius of 59 mm.For QF5, the outer one of the insertion doublet, thecores of existing PEP insertion magnets are reused. Achannel is cut into the side of the yoke for the LER; aninsert restores the mechanical integrity of the magnet.New coils are being built that occupy significantly lessspace than the original coils. POISSON calculations donefor both the original and the new configuration confirmthat the effect of these modifications on the magneticfield harmonics is small. The shape of the end chamferswill be iterated in order to keep the integral fieldharmonics at or below the specifications.At QD4 the two beams are even closer together (15 cmseparation). There are additional restrictions on theoutside dimensions of the magnet due to its beingpartially within the detector envelope. A satisfactory coil-window was found by letting the coil protrude against thevacuum chamber. Field distortions in the vicinity of thecoil are avoided by letting the innermost layer of the coilfollow the field lines; this is essential in keeping thecurrent density reasonable.For both magnets, stray fields on the LER axis havebeen calculated to be a few Gauss. Iron or µ-metal shieldsaround the LER vacuum pipe will reduce these fields byabout one order of magnitude to acceptable levels.The vacuum in the IR upstream of the detector iscrucial for low-background operation of the facility. NEGpumping will be provided throughout the critical regionbetween the low-field bends and the detector to achievean average pressure of ≤0.5 nTorr (Fig. 3). Rather thanusing NEG strips, the pumps will consist of NEG discsstacked on a 6 mm tube, which in turn houses the heaterto regenerate and activate the NEG. Thus, pumping speedand capacity of the pump are enhanced by an order ofmagnitude compared to simple strips. Care is being takento ensure the NEG material is as free of magneticcontaminants as possible. The vacuum chambers arebasedon the same Cu extrusions as used in the HER arcs.4  BEAM INJECTION AND ABORTThe Linac beam is injected vertically into the PEP-IIrings. A horizontally bending Lambertson septum and avertically bending current-sheet septum act on theDistance (m)Pressure (nTorr)00.511.522.530 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0Pressure (nTorr)Paverage (nTorr)Fig. 3. Pressure profile, upstream interaction regioninjecting beam to steer it horizontally onto the ring axisand vertically inject at nominally 8.5σy. A local dc bumpdisplaces the ring orbit so that the 10σ beam envelopetouches the ring side of the septum; two fast kickers pulsethe orbit by another 4σ to receive the next linac pulse.The kickers are at 180° phase advance relative to eachother and thus produce a localised bump as well. The dcbump with four vertical dipole magnets allows adjustmentof position and injection angle to optimize the injection.At 3 A beam current, the stored energy in the HER isabout 200 kJ. Such large energies can cause damage tothe vacuum system if dumped in the ring. The beam abortsystem resides just upstream of the injection area. Akicker magnet will deflect the beam onto a beam dumplocated underneath the injection Lambertson septum. Inorder to protect the exit window and the dump itself, thekicker sweeps the beam vertically across an about  15 mmlong stripe. The beam dump itself uses layers of 6 X0 C,5 X0 Al and 12 X0  Cu in a water-cooled aluminumhousing.REFERENCES[1] J.T. Seeman et al., “PEP-II Status and Plans,” Proc.IEEE Part. Accel. Conf., Dallas, TX, May 1995,p. 486.[2] U. Wienands et al. , “Status of the High Energy Ringof the PEP-II B-Factory”, ibid., p. 530.[3] M.H. Donald et al. , “Lattice Design for the HighEnergy Ring of the SLAC B-Factory,” ibid., p. 591.[4] C. Belser, J. Berg et al. , “High Current Rf Shield forPEP-II Vacuum Expansion Joint,” these proceedings.[5] A. Fisher et al., “Diagnostics Development for thePEP-II B -Factory,” Proc. Beam Instr. Workshop,Argonne, Il, May 6-9, 1996, to be published.[6] J. Liu et al., “Calculations of the Radiation Doses tothe Electronic Equipment in the PEP-II Tunnel,”NIM A 359, 463 (1995).[7] A. Fisher et al., “Design of the Synchrotron-LightMonitor for PEP-II,” these proceedings.[8] L. Bertolini, M. Sullivan et al., “Interaction RegionDesign at the PEP-II B-Factory,” these proceedings.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Status of the PEP-II B-factory high energy ring

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Status of the PEP-II B-factory high energy ring

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