Nagaitsev, S S

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EXPERIENCE WITH MAGNETIC SHIELDING OF A LARGE SCALEACCELERATORS. Nagaitsev†, C. Gattuso, S. Pruss, and J. Volk,FNAL*, Batavia, IL 60510, USAAbstract It is not unusual to place multiple accelerators in acommon enclosure to save on civil construction costs.This often complicates operations, especially ifaccelerators are affecting each other. At Fermilab, theinfluence of a rapidly cycling Main Injector (MI)synchrotron on an antiproton storage ring (Recycler),placed in a common tunnel, was initially found to beunacceptable for a reliable operation of the Recycler.Initial closed orbit excursions in the Recycler ring duringthe MI ramp were in excess of 5 mm (rms). This paperdescribes a shielding technique, used to reduce these orbitexcursions by a factor of five.1 INTRODUCTIONFigure 1: Tunnel cross-section in a standard arc cellshowing a Main Injector dipole near the floor and aRecycler gradient magnet above it and near ceiling.Figure 1 shows a cross-section of the Main Injector(MI)/Recycler tunnel.  Stray magnetic fields originatefrom both the quadrupole and the dipole bus (supply andreturn) excitations during the MI ramp (8 to 150 GeV).At the top of the ramp the current in the quad and dipolebuses are 3.5 kA and 9.2 kA correspondingly.  Both busescontribute about equally to the transverse magnetic fieldat the Recycler beam location.Figure 2 shows the measured transverse magnetic fieldin the Recycler beam pipe without a magnetic shieldduring the MI ramp.Figure 2: Dipole bus current (2.5 kA/div) and transversemagnetic field (1.5 G/div) as a function of time (4.5 s fullscale) during the 150-GeV MI ramp.  The quad bus wasalso energized.The observed transverse fields were on average about 5G but in some places as high as 10 G. These fields aredistributed along the entire 3.3-km long Recyclercircumference. It was estimated that if unshielded, thesefields can move the closed orbit of an 8.9 GeV/c Recyclerbeam by as much as 30 mm.  The problem of straymagnetic fields was recognized early on during the designstage of the Recycler ring [1].  The implemented shieldingtechnique was to wrap the elliptic Recycler vacuum tube(48 mm x 100 mm) with two layers of high-permeabilityshielding material.  For the inside layer Carpenter HighPerm "49" alloy 12" wide, 0.004" thick foil was chosen.This foil material came as a coil, annealed (permanufacturer spec) and certified. According to themanufacturer’s spec this material has an initial µ of about8,000, a maximum µ of 150,000 and a saturationinductance of 13 kG.  This foil was wrapped along thebeam tube in pieces as long as one can handle andattached to the tube by Kapton tape.  The circumferenceof the Recycler tube is about 9" so there was about 3" orless of overlap between two edges.The outer layer was a silicone steel alloy AS-0 fromEagle Magnetic Co., Inc.  It came as a coil 14.5" wide and0.006" thick, certified per MIL-N-14411C specification. Itwas also wrapped along the beam tube in long pieces.The direction of wrapping was across the coil rolldirection and it created a one turn with a small (about 1")overlap.  The gap between the two layers (the "49" alloyand the "silicone alloy") was created by a fiberglass clothof about 1/8" - 1/4" thick.______________________________________________†nsergei@fnal.gov*Operated by Universities Research Association, Inc., under ContractNo. DE-AC02-76CH03000 with the U.S. Department of EnergyMagnetic fieldDipole buscurrent0-7803-7191-7/01/$10.00 ©2001 IEEE. 3371Proceedings of the 2001 Particle Accelerator Conference, ChicagoFigure 3 shows the measured transverse magnetic fieldinside the shielded (as specified above) Recycler beamtube at the same tunnel location as in Fig. 2.  Themeasurements of the magnetic field were made with aHall probe (zeroed prior to the MI ramp).Figure 3: Dipole bus current (2.5 kA/div) and transversemagnetic field (1.25 G/div) as a function of time (5 s fullscale) during the MI ramp.  The quad bus was alsoenergized.The installed magnetic shielding reduced the transversemagnetic field on average by a factor of 6.  The measuredrms Recycler closed orbit excursion during the MI rampwith this shielding was 5 mm in both planes.  This wasdetermined to be inadequate and tests were initiated todesign a shielding arrangement that would reduce thestray magnetic field by another factor of 10 or more.  Inaddition, the field of 10 G should not saturate the shield.2 SHIELDING COEFFICIENTFor an infinitely long cylinder with a wall thickness tand diameter D of a constant permeability µ in a uniformmagnetic field the attenuation coefficient k is wellapproximated by the following expression:Dtk µ+≈1 . (1)This formula is also valid for a finite length cylinder if itslength is greater than 4D.   For an ellipse one shouldprobably use the larger of two sizes instead of thediameter for estimates.  For multiple isolated layers thetotal coefficient is somewhere between the sum and theproduct of individual coefficients (depending ongeometry).  Since in most cases the permeability of theshield depends strongly on the flux density, the initialvalue of µ should be used for conservative estimates inlow fields.3 TEST RESULTSTo test various shielding arrangements and to measurethe attenuation coefficient we have employed 20"diameter Helmholtz coils capable of producing fields ofup to 10 G (dc or ac).  The coils were large enough tosimulate field at "infinity" for tube samples of up to 4" indiameter.  The measurements were done by a 3-axismagnetic sensor [2] with a resolution of about 1 mG.Prior to measurements the probe was zeroed in a zero-field chamber.  Measurements were performed with a 1-Hz ac magnetic field.First, it was found that the silicone iron performsdifferently if wound in the roll direction (opposite to whatwas done initially).  Figure 4 shows a comparison of twomeasurements performed with silicone iron only, woundon a 4" OD tube.  The probe was placed in the center ofthe tube.  The external field, generated by Helmholz coils,was calibrated without the tube.-1.5-1-0.500.511.5-3 -2 -1 0 1 2 3External field, GInside field, GAlong the roll wrapAcross the roll wrapFigure 4: Comparison of two wrapping techniques for thesilicone iron (outer shield layer).  There is a factor ofthree difference between two wrapping styles.The shielding coefficient is enhanced by a factor ofthree if the wrapping is done along the direction of thespool roll (as supplied from the factory).  Along the beamtube the neighboring wraps were overlapped by 1".For the inside layer a different (from the originalCarpenter Alloy "49") was chosen.  We compared severalsamples of a "µ-metal" alloy, supplied by variousmanufacturers.  The tested materials had very similarmanufacturer specifications and their shieldingperformance was identical with the uncertainty on themeasurements.  It was also found that all tested materialshad a very isotropic nature: the direction of the roll didnot affect the shielding performance.  The material wasthen chosen on the cost basis.  It was AD-MU-80 highnickel steel supplied by AD-VANCE Magnetics, Inc [3].The material was delivered in 15" wide, 0.004" thickspools, annealed per manufacturer’s specs and certified.Three different wrapping techniques were tested.  In allthree the outer layer was the silicone iron, separated fromthe inside layer by a 1/8" gap and wound as describedabove.  The inside layers were: (1) Alloy "49" and AD-MU-80 on top of it, (2) AD-MU-80, 1/8" gap, Alloy "49"and (3) AD-MU-80, 1/8" gap, AD-MU-80.  Figure 5shows a comparison of these three styles (three layerseach) wound on a 4" OD tube.  The external field waschanged from -1 G to 9 G and back at 1 Hz.  Theperformance was compared by comparing the total insidefield swing when the external field changes by 10 G.Magnetic fieldDipole buscurrent3372Proceedings of the 2001 Particle Accelerator Conference, Chicago-0.2-0.15-0.1-0.0500.050.10.150.2-2 0 2 4 6 8 10External fie ld, GInside field, G123Figure 5: Comparison of three-layer wraps (see text fordetails).Styles (1) and (2) had very similar results 0.34 G and0.28 G field swings or the shielding coefficient of about30.  Moving the Alloy "49" to the outside of the AD-MU-80 resulted only in a small enhancement.  One can gainanother factor of two by adding one more layer of highnickel steel (3): 0.15 G field swing.4 IMPLEMENTATIONFor the final implementation we have chosen technique(1).  It required several steps: (a) removing the outersilicone iron shield, (b) cutting it in shorter pieces, (c)removing the cloth spacer, (d) wrapping the tube with onelayer of AD-MU-80 and the spacer, and (e) winding thesilicone iron in its original roll direction.  This was donefor the entire Recycler ring.  It resulted in a factor of 30reduction of stray magnetic fields as compared with Fig.2.  The rms closed orbit excursions for both planes weremeasured to be about 1 mm.5 DISCUSSIONShielding the Recycler beam tube only eliminates thetransverse fields at the orbit.  It might not reduce the fluxthrough the Recycler orbit.  A 10 V per turn energy gainis needed to change the Recycler beam energy by 1 MeVduring the MI ramp.  This energy gain can exist if there isa 100 kG-m2/s flux change through the orbit.  If onedivides this number by the Recycler circumference of 3.3km it gives 30 G-m/s per meter of orbit length.  This isnot a very large number if one thinks of how much steelthere is in the tunnel walls, magnets and shielding.  Sincethe closed orbit variations were reduced to an acceptablelimit we will be able separate the effect of energy changeand the deflection.  This betatron-style energy variation, iffound, would require intercepting the flux before itreaches the Recycler orbit plane.  Active devices are alsonot out of consideration.  However, the beam energydeviation is equivalent to the change in the vertical fieldintegral Bydl.  We have observed that the horizontal closedorbit moves to the inside of the ring in the areas of highdispersion.  The measured move is equivalent to a changein the vertical field integral by 6×10-4 or to a presence of0.34 G vertical magnetic field everywhere on the Recyclerorbit, which is consistent with results in Fig. 5.6 CONCLUSIONS1. The effective µ calculated from Eq. (1) for themeasured attenuation coefficents is an order ofmagnitude (or more) smaller than the lowest oneprovided in the material certificate.2. Alloy "49" material had a very low attenuationcoefficient.  It did not perform as expected.3. The attenuation coefficients for multiple layers ofshielding material (if separated by a small gap) addrather than multiply.4. It is important to keep in mind that some materialscan be anisotropic and the roll direction can affect theshielding performance.7 ACKNOWLEDGEMENTSWe are grateful to our colleagues B. Brown, A.C.Crawford, G.W. Foster, and J. Marriner for discussions.We would like to thank the entire staff of the mechanicalsupport department of the Fermilab’s Beams division forhelping us to install the shielding in the tunnel.8 REFERENCES[1] G. Jackson, "The Fermilab Recycler Ring TechnicalDesign Report", Fermilab preprint FERMILAB-TM-1991, Nov. 1996.[2] Magnetoresistive sensor hybrid HMC2003,Honeywell sensor products.[3] http://www.advancemag.com3373Proceedings of the 2001 Particle Accelerator Conference, Chicago

Experience with Magnetic Shielding of a Large Scale Accelerator

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