The six straight sections of the PEP II High Energy Ring (HER) serve various functions: lattice tuning, beam injection and abort, providing space for rf cavities, longitudinal and transverse feedback, beam diagnostics and the interaction point. A stainless steel vacuum system has been designed; prototypes are currently being built. Cooling is required due to radiation coming from the last arc dipole and resistive losses in the vacuum chamber. Although the nominal beam current of the HER is 1 A the vacuum system is designed for 3 A to provide margin and an upgrade path. 5 refs., 7 figs

Daly, E.

Kulikov, A.

Kurita, N.

Nordby, M.

Perkins, C.

Reuter, E.

Wienands, U.

UNT Digital Library

European Particle Accelerator Conference Stiges, SpainMay 1-5, 1995 LawrenceLivermoreNationalLaboratoryUCRL-JC-126717The Vacuum System for the PEP II High Energy Ring Straight SectionsU. Wienands, E. Daly, S. A. Heifets, A. Kulikov, N. Kurita, M. Nordby,C. Perkins, E. Reuter, J. T. Seeman, F. C. Belser, J. Berg,F. R. Holdener, J.A. Kerns, M. R. McDaniel, W. StoefflMay 1, 1995This 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.THE VACUUM SYSTEM FOR THE PEP II HIGH ENERGY RINGSTRAIGHT SECTIONS.U. Wienands, E. Daly, S.A. Heifets, A. Kulikov, N. Kurita, M. Nordby, C. Perkins E. Reuter,J.T. Seeman, SLAC, Stanford, CA; andF.C. Belser, J. Berg, F.R. Holdener, J.A. Kerns, M.R. McDaniel, W. Stoe, LLNL, Livermore, CAAbstractThe six straight insertions of the PEP II High Energy Ring(HER)[1] serve various functions: lattice tuning, beam in-jection and abort, providing space for rf cavities, longitudi-nal and transverse feedback, beam diagnostics and the in-teraction point. A stainless steel vacuum system has beendesigned; prototypes are currently being built. Cooling isrequired due to radiation coming from the last arc dipoleand resistive losses in the vacuum chamber. Although thenominal beam current of the HER is 1 A the vacuum sys-tem is designed for 3 A to provide margin and an upgradepath.I. DESIGN REQUIREMENTSThe PEP II Conceptual Design Report (CDR, Ref.[2]) callsfor an average pressure in the straights of 3 nTorr, mainlydetermined by beam lifetime considerations. In order toachieve this pressure, about 3 m pump spacing would berequired in the rst 6{7 cells of each straight section dueto the still signicant synchrotron radiation at 3 A current.Initially, lower intensities are expected and, therefore, forrst commissioning 220 l/s ion getter pumps will be pro-vided at every quadrupole only ( 7 m pump spacing).Fig. 1 shows the pressure prole and gas load at 1 A beam30100600Q (nTorr l/s/m) 51015P (nTorr)0 25 50 75 100Distance (m)Figure. 1. Pressure and gas load along an HER straightsectioncurrent. For this calculation, a photon-induced desorptionrate of  = 2 10 6 is assumed. To achieve this value, thevacuum components will be glow-discharge cleaned afterchemical cleaning. Initially,  will be higher but the clean-ing eect of synchrotron radiation will reduce , especiallyso at the upstream end of the straight. Over time,  is ex-pected to go down and eventually be less than 10 6. If it isWork supported by US Department of Energy under contract DE-AC03-76SF00515, DE-AC03-76SF00098, DE-AC03-81ER40050, DE-AS03-76ER70285 and W-7405-Eng-48Stainless SteelQuad & Drift ChambersCopper Quad &Drift Chambers"Standard" Regions (4)"RF" Regions (2)~4.5 m(End ofARC)e-Bel.1st "730"Quad &CorrectorQ. Chamb.1st RFRaftBel.RFDrf. ...2nd "730"Quad &CorrectorQ. Chamb.2nd RFRaftBel.Bel.RFDrf. Drf.~4.5 m(End ofARC)e-Bel1st "730"Quad &CorrectorQ. Chamb. Drift ChamberTeeIPTeeIPDrf.2nd "730"Quad &CorrectorQ. Chamb.Bel    Repeated Section(Typically 6 ~ 8 meters)TeeIP...(Ion Pump on each RF Cavity Waveguide)Drift Chamb.3rd RFRaftBel.RFDrf.Figure. 2. Layout of arc-straight transitionsfound that the pressure does not reach the required level,additional pumping tees and pumps will be installed in therst cells. The vacuum system of each straight section canbe separated from the adjacent arcs by gate valves locatedright after the transition in shape from the octogonal pro-le used in the arcs to circular pipe. The transition regionsare shown in Fig. 2.The expected distribution of the power deposition alongthe beam pipe for 3 A beam current is shown in Fig. 3At the upstream end of each straight section it is domi-nated by the synchrotron radiation; this causes the pres-sure rise in Fig. 1. Additional power is being deposited bydirect I2R losses due to image currents and by losses aris-ing from higher-order modes (HOM) traveling along thevacuum system, which will dissipate predominantly in thestainless steel walls. Cu extrusions will be used rather than3 A beam current0.0020.0040.0060.0080.00100.00120.000 5 10 15 20 25TotalSynchrotron radiationHOM (beam)s (m)dP/ds (W/cm)Figure. 3. Power deposition along beam pipeFigure. 4. Drift chamberstainless steel in the rst 4.2 m using the same shape as inthe dipoles of the arcs.[3], [4] The pump chamber providesa convenient, shielded area for four pump ports to holdNEG cartridges.The heating of an uncooled beam pipe as a function ofpower can be calculated approximately. We have per-formed a test heating a 1 m long sample of ss pipe bydirect current and measuring its temperature. Experimentand calculation are consistent and show that a linear powerdensity of 1 W/cm along the pipe will lead to temperaturesof about 70-80C if not cooled; this would lead to undesir-able outgassing.Due to the high currents and short bunches, a strictimpedance budget is enforced for the whole HER vacuumsystem. The total inductive impedance is limited by thebunch lengthening threshold to less than 200 nH for thewhole ring. Narrow band impedance limits are frequencydependent; but the contribution of the vacuum systemshould be insignicant compared to that of the rf cavi-ties. The existence of any gaps along the inner wall (e.g.at anges) is being avoided. Care is being taken to avoidany possibility of mode trapping between adjacent shapetransitions in the beam pipe. Almost every discontinuity isbeing tapered to less than 10 angle. An impedance com-mittee has been set up to scrutinize every signicant dis-continuity and transition in the vacuum system.[5]II. COMPONENT DESIGNThe straight section vacuum system is modular, with fourbasic components as shown in Fig. 2: Drift chamber,Quadrupole chamber, Pumping Tee and Bellows module.AISI 304L stainless steel is used throughout. The beampipe is required to be seamless to avoid the possibility ofseam-related problems. In addition, no welds at all arepermitted to be within any of the magnetic elements asimproperly carried out welds could change the  of thematerial away from unity. The  of the material has beenspecied to be less than r = 1:05 and will be inspectedupon receiving of the pipe stock.A. Drift ChamberThe drift chamber is designed using two concentric roundss pipes with a 1/16 in. gap for the cooling water. Asketch of the chamber is shown in Fig. 4. The concentricchamber design allows for fairly uniform cooling aroundthe azimuth of and along the whole chamber; a wire coilinserted between chamber and cooling jacket together withsome extra spacers at the ends assists to this end. Conatanges are welded on at each end.B. Quadrupole ChamberThe cooling-jacket concept used for the drift chamber cannot be used for the quadrupole chamber. The 50 mmpole radius of the quadrupole magnets and the inner ra-dius requirement of 44.5 mm or larger proved to be mutu-ally exclusive unless expensive custom-made pipe stock bepurchased. Therefore a dierent concept is being pursuedusing regular 44.5 mm inner radius \Schedule 10" piping(Fig. 5). The pipe is coated with a 1 mm copper layerand four 6.35 mm Cu cooling pipes will be soldered to theoutside. The copper layer increases the heat conductanceboth from areas of the pipe not directly under the coolingchannels. At each ange the cooling pipe will be wrappedaround the beam pipe for increased cooling. In this way,margin is provided against vertical displacement of the syn-chrotron radiation fan and areas under the anges receiveadequate cooling as well.C. Pumping TeeThe pumping tee module is shown in Fig. 6. The pumpis attached to the 8 in ange welded to a plenum. A slot-ted Cu insert provides the rf shield necessary to reduce theimpedance of the pumping tee. 8 in anges are used ratherthan 6 in to avoid the need for blind-tapped holes. The con-ductance of the whole assembly is calculated to be about470 l/s; this results in an eective pumping speed of 150 l/sfor a 220 l/s pump. Those tees located near the upstreamend of the straight sections have to be able to handle sig-nicant synchrotron radiation. In order to be able to doso, 1 cm wide strips at the side of the Tee are not slottedand a stainless cooling line is brazed to the insert.Figure. 5. Quadrupole chamberThe long slots are transparent to TE eld modes emanatingfrom the rf cavities and also generated by mode conversionof the TM elds associated with the beam. These can causeheating in the pumps and will therefore be screened by athin Cu sheet with 5 mmwide slots orthogonal to the slotson the insert. The long slots also increase the eectivediameter of the beam pipe and, thus, potentially can leadto trapped modes. To avoid this possibility the aperturewill be slightly restricted in the region of the slots, by about0.5 mm. MAFIA calculations conrm the eectiveness ofthis approach.D. Bellows ModuleThe bellows module is the most challenging part of the vac-uum system. The bellows themselves have to be protectedfrom too much power dissipation and would present an un-acceptable impedance to the beam; therefore, they will beshielded. The shield has to present as smooth a surface aspossible to the beam to reduce the inductive broad-bandimpedance and has to do this over the full operational ex-tension/contraction range of about 5 mm. In addition, ithas to sustain a contraction of up to 19 mm without dam-age if in situ bake-out were to take place.The shield uses a sliding-nder design with the ngers ofthe rf shield riding outside on the rim of a slightly conicalstainless stub. Figure 7 shows a sketch of the bellows as-sembly. The ngers are straight where exposed to the beamto avoid creating a bulge in the crossectional area thatcould act as a cavity. The total broad band impedance ofall  100 straight-section bellows modules is about 4.5 nHwhich is acceptable. The ngers are made of dispersion-strengthened copper (GlidCop) rather than BeCu, for im-proved strength and thermal handling and will be silvercoated. The stub, made of stainless, will be copper coatedon the inside to reduce I2R and HOM losses and will havea Rhodium coating at the contact area. A taper at the up-Figure. 6. Pumping TeeFigure. 7. Bellows assemblystream end of the bellows protects the ngers and the stubfrom synchrotron radiation.A test xture has been built to test the stability of thengers over at least 200000 compression-expansion cyclesat operating temperatures. Contact resistance and dustproduction can be assessed in this way. A current of 2 Aper nger can be injected into the xture, but the frequencyof 2 MHz is too low to fully test the current carrying abilityof the ngers.References[1] U. Wienands et al., \Status of the High Energy Ringof the PEP II B-Factory", these proceedings.[2] M. Zisman, Ed., \PEP-II An Asymmetric B Factory",Conceptual Design Report, SLAC-416.[3] C. Perkins et al., \Vacuum System of the PEP-II HighEnergy Ring", Proc. EPAC, London, UK, 1994.[4] E.F. Daly et al., \Stretch Forming Vacuum Chambersfor the PEP-II B-Factory High Energy Ring", theseproceedings.[5] S.A. Heifets et al., \Impedance Budget of the PEP IIB-Factory", these proceedings.Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

The vacuum system for the PEP II high energy ring straight sections

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The vacuum system for the PEP II high energy ring straight sections

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