Gresham, A T

Sheldon, R

Stapleton, G B

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Particle Accelerators, 1972, Vol. 4, pp. 43-51 © GORDON AND BREACH, SCIENCE PUBLISHERS LTD. Printed in Glasgow, ScotlandMAGNETS FOR RADIATION-RESISTANT ACCELERATORSA. T. GRESHAM, R. SHELDON AND G. B. STAPLETONRutherford High Energy Laboratory, Chilton, Didcot, Berkshire, EnglandThe ?aper discusses the prob~em of radiation damage likely to be met with in particle accelerator installations and~onsId:rsmethods of p~otectIonbased on the total exclusion of radiation sensitive materials. The use of mineralInsulatIon for magnets IS explored and a detailed description of a number of actual prototype magnets is included.1. INTRODUCTIONNumerous studies have been made of the effect ofhighly ionizing radiation on materials and com-pon:nts required in the construction of high energypartIcle accelerators. 1 In general, radiation damagehas been countered by determining the expectedradiation dose in various regions of the installation,and selecting suitably resistant components andmaterials for the proposed application. Thistechnique must be backed up by tests to failure ofradiation sensitive components in similar environ-ments; this enables predictions of service life to be~ade, and indicates where there is a need to developsImple replacement techniques. In order to specifythe service life of a component with confidence itis necessary to consider many and often synergi~ticenvironmental variables such as radiation quality,dose rate, oxidation, ozone attack, temperature andmechanical and electrical stress. This is a lengthyp~oce~ure and could be unacceptable in a projectWIth tImescale and cost limitations. An alternativeis to provide adequate shielding for radiation-sensitive components.A major problem, however, arises with thedesign of the magnets for the accelerator, for thebeam lines and the target stations. Magnets areespecially sensitive to damage since the coil, usuallylocated and insulated with epoxy resin glass fibrecomposite, is normally close to the particle beam orexperimental position. The purpose of this paperis. to describe in detail the construction of prototypedIpole and quadrupole magnets with mineralinsulation, resulting in a fully radiation-hardcomponent.Mineral insulation performs much better follow-ing exposure to high levels of radiation dose thanorganic materia~s where the formation of gaseousbyproducts results in loss of material, and ultimatepolymer chain fragmentation. 2 The main problemwith magnets made from mineral materials lies inthe need to reconcile the intractable nature of suchmaterial with the many requirements expected froma magnet insulator, namely, provision of mechani-cal and electrical support for the copper windingsunder stress and, constraint of the windings to closedimensional tolerances under operating conditions.These requirements have dictated the need for thecoil and coil support material to achieve a degreeof integrity that is usually associated with the useof glass tape wrapping and vacuum impregnationwith epoxy resin.2. INORGANIC INSULATING MATERIALSIn order to discuss these materials it is con-venient to characterize them under headings,namely, ceramics, glasses and refractory cements. 32.1. CeramicsThe electrical properties of these materials, forexample, porcelain, steatite, alumina, magnesia,quartz, have been well characterized, with theemphasis on high-temperature or high-frequencydielectric loss. Their electrical behaviour is morethan adequate for the low-duty requirement ofmagnets. To make a conventional magnet fromceramics two techniques can be considered:(i) The refractory powder is slipcast or press-formed around coils accurately prelocatedinto position by a suitable support jig andthen fired to produce the required integrity.44 A. T. GRESHAM, R. SHELDON AND G. B. STAPLETON(ii) Strips or blocks of ceramic are used to 'egg-crate' the copper coils during winding andthe coil clamped to retain configuration.The difficulty with the first technique is that thehigh firing temperatures needed to sinter thematerial exceed the melting point of copper.Further, the sintering process would result in grossdimensional changes probably leading to brittlefracture.The second technique could be used to constructelectromagnets, the main requirement being thehighly sophisticated clamping necessary to preventcoil movement and provide insul~tion throughoutthe whole coil configuration. A further problemarises from using a hard and abrasive material withthe soft copper of the coil, any slight movement ofthe ceramic in contact with the copper results incopper smearing which can lead to breakdown.2.2. GlassesGlasses are available which melt at much lowertemperatures than the predominantly crystallineceramic materials and which exhibit much betterdimensional stability on cooling, but the problemsdescribed under ceramics apply also to the vitreoussystem at present. Some low temperature meltingglasses and in particular glass-bonded mica can bemoulded into complex shapes, using plastics fabri-cation techniques, to good dimensional tolerances.Though the pressures required for moulding arelarge and temperatures in excess of 350°C must beused, these materials might afford a possiblealternative to conventional plastic insulationmaterials. The mechanical and electrical propertiesof glass-bonded mica are good, although theperformance of this material when subjected toionizing radiation is not clear at present, but it isthought to be much superior to the organicplastics.2.3. Re.fractory CementsRefractory cements are used extensively inelectrical applications such as the bedding offurnace windings, thermocouple wires and packagedheating elen1ents. In general for these applications,mechanical stress requirements and electrical dutyto be supported are low, but since these materials,and the ceramics and glasses also, give an electricalresistivity temperature coefficient which accordswith the Hinrichson-Rash law InR = aJT+b, theresistivities obtained at high temperatures can besufficiently poor to warrant concern. Therefore,in the published papers, resistivities are usuallyquoted for high temperatures that are not relevantto the construction of magnets. In the main, theserefractory cements have embodied the High Alu-minous Cement (HAC) material which is madefrom a calcined and ground mixture of limestoneand bauxite or alumina. This material with orwithout various types of filler or aggregate is mixedwith water and then slowly baked out to give arefractory insulator.It is apparent that the most successful techniquefor the construction of heating elements, thermo-couple sheaths and mineral-insulated cables makesuse of preformed ceramic beads or compressedceramic powder such as magnesium oxide. Magnetshave already been designed and constructed usingmineral-insulated cables now obtainable in squarecross section. 4 The obvious problem with thesematerials, however, is the low conductor packingfactor that follows from the use of the coppersheath round the insulator and conductor bar.3. HYDRAULIC CEMENTS ANDCONCRETEIt is considered that, of the materials discussed inthe last section, the castable cements offer the mostpromising solution to magnet construction. Astudy of available literature shows that hydrauliccenlents and concretes can be prepared with goodresistivity and mechanical integrity, althoughpresent data on their resistivity and other propertiesis extremely variable. 5 The reason for this in-consistency must lie in the ill-defined nature of thehydraulic cements themselves together with thevariable composition and bulk properties of theaggregate. Hammond and Robson give a goodaccount of this work with volume resistivities ofgreater than 1013 ohm cm for HAC and 1011 to1012 ohm cm for Portland cements, both beingdried at 105°C and cooled in desiccated con-ditions. 6 They also demonstrate that surfaceresistivity, whilst of a similar high order to thevolume resistivity when desiccated, falls rapidlywhen exposed to atmospheric moisture, the bestRADIATION-RESISTANT MAGNETS 45results being obtained with the high-aluminacements.The study of breakdown voltage indicates thatvoltages of 10 to 20 kV cm -1 could be obtained, thehighest values again being obtained with HAC.These results indicate that after the first breakdown,subsequent ones occur at a voltage slightly lowerthan the initial breakdown but thereafter remainfairly constant. Other authorities give resultswhich although not comparable through formu-lation variables and test procedures bear out thegeneral conclusion outlined above. 74. EXPERIMENTAL WORK AT RHELWork on the development of concrete insulationhas been carried out at the Rutherford Laboratoryunder three general headings:(i) Studies of the electrical and mechanicalproperties of cements and cement/aggregatesystems.(ii) Experiments to study the behaviour of bulkquantities of cement and cement/aggregatesystems.(iii) The design and construction of sITlall modelprototype electro-magnets with concreteinsulation.Specimens used for electrical tests were preparedin polystyrene boxes 10 cm x 15 cm x 7.5 cm andvibrated to compact the material. These areillustrated in Fig. 1. The electrodes incorporatedwere guard ring systems of 10 cm2 area of copperFIG. 1. Compacted specimens for resistivity tests.46 A. T. GRESHAM, R. SHELDON AND G. B. STAPLETONon epoxy/glass laminate substrate, the electrodespacing being 1 em. Specimens removed from thedrying process must be isolated from atmosphericmoisture either by sealing or storing in a dryenclosure such as a desiccator. Table I presentshinder impregnation of the closely packed coilturns. Most mixes so far investigated use aggregatesmaller than 7 mesh sieve size. In the work dis-cussed in this paper, choice of aggregate wasrestricted to silica or alulnina n1aterials.TABLE IResistivity values (typical samples)Cement/ Water/ ResistivityAggregate Cement ohmcmCement type ratio ratio at 500V.DC RemarksHigh Alumina 1/4 0.39 2 x 1013 Desiccated(SECAR) 5 x 109 Exposed toatmosphereHigh Alumina 1/2 0.32 3 x 1013 Desiccated(SECAR) 1 x 109 Exposed toatmosphereHigh Alumina 1/4 0.50 1.3 x 1011 Desiccated(CIMENT FONDU)White Portland 1/4 0.39 1.5 x 1013 Desiccated(SNOWCRETE) 3 x 109 Exposed toatmosphereWhite Portland 1/4 0.52 3 x 1013 Desiccated(SNOWCRETE) (Different 5 x 108 Exposed toaggregate) atmosphereWhite Portland 1/2 0.33 5 x 1013 Desiccated(SNOWCRETE) 1 x 1014 DesiccatedOrdinary Portland No 0.26 3 x 1013 DesiccatedCement aggregatevalues of resistivity obtained from typical speci-mens. These data show that both White HighAIl:1minous Cement (SECAR) and White Portlandcement perform well electrically under dry con-ditions.The aggregate systems were found to be generallysimilar in performance but some very good resultshave been obtained from systems not containingaggregate at all.Concrete is characterized by high compressivestrength but poor strength in tension. Thischaracteristic is accommodated in designs formagnets by arranging for reinforcement to beapplied in tension. One feature of concrete thathas given concern is the shrinkage on cure.Aggregate additions will reduce the cure shrinkagebut adversely affect the mechanical properties, forthis reason it is very important to optimize theaggregate gradings for minimum shrinkage withbest strength characteristics, at the same timeavoiding the use of large particles which wouldIn addition to the laboratory studies of theproperties of cement/aggregate systems it wasconsidered necessary to investigate constructiontechniques with these materials. A programme ofdevelopment work was, therefore, carried out todevise the best technique for making both dipoleand quadrupole magnets. In the first instance asmall model H magnet was made (Fig. 2). Thisconsisted of twelve turns of0.8 em square conductoron a small split yoke approximately 22.0 em x12 em x 8 em, the whole being encased in aluminacement with external post-stressed bolts, stressbeing applied in one direction only. Interturn andother spacing requirements were achieved in thisand all other magnets by the insertion of a numberof strips of mineral material during the coil windingand location operations. During power tests,cracks were produced in a direction parallel to theexternally-applied stress.The resistance of the insulation on this modelwas similar to that previously obtained fromRADIATION-RESISTANT MAGNETS 47FIG. 2. Model H magnet.laboratory test samples exposed to normal atmo-spheric conditions.Next it was decided to build a small H magnetsuitable for use in an accelerator beam line (Fig. 3),based on the existing Daresbury Nuclear PhysicsLaboratory H5 magnet which has a moderate fieldof 11.5 kG at a power rating of 15 kW.The nlagnet has 80 turns of 6 mm x 6 mm con-ductor with a water connection to every 8 turns.Its design did not involve any alteration to the coilgeometry, except that a minimum spacing of 1.0 mmis desirable between adjacent conductors. This isnecessary to ensure penetration of the mortar ofcement and fine aggregate mix. At each end of theyoke where the coils project, a temporary castingprovided a mould for the mortar and a means forpre-stressing both the steel-work holding the coilends and also the stainless steel flight tube throughthe magnet gap.The mortar was introduced into the top of thecasing at each end of the magnet while on avibrating table and small holes were provided inthe centre of the yoke top to allow trapped air toescape. The magnet was completed and performedsatisfactorily on test. Table II gives the perform-ance data.A second H5 magnet, shown in Fig. 4, wasdesigned to overcome some of the shortcoming ofthe first prototype. This magnet was designed tobe filled with concrete pumped in under pressurein order to ensure that the concrete would bepermanently under compressive stress in alldirections even when raised to a temperature of100°C, thus preventing hair-line cracks forming in48 A. T. GRESHAM, R. SHELDON AND G. B. STAPLETONFIG. 3. H5-type magnet (first prototype).Field intensityCooling water flowTotal weightTABLE IIMagnet performance data H5 (first prototype)Area of fieldHeight of gapCross section of flight tubeLength of flight tubeNormal energizing currentsteel and copper sections to represent the cross-sectional areas to be filled. Filling was carried outon a vibrating table.Concrete can achieve resistivities adequate forinsulation of magnet coils. This is conditional oncomplete dryout and the maintenance of sealed ordesiccated conditions and it is questionable whetherthese conditions can be always guaranteed.,Accordingly, the technique of lining the yoke withthin sheets of high-resistivity ceramic material wasadopted.The second H5 prototype on test gave thefollowing results:Insulation test between conductors 400 MQConductors to earth 400 MQIt was also flash tested satisfactorily at 2 kV for= 11,400 G(14,800 G maximum)= 32x20cm= 5cm=20x5cm= 50cm= 375A(600 A maximum)Power dissipation of windings = 15 kW(48 kW maximum)= 1300 Ih- 1= 600 kgthe concrete and initiating a breakdown whensubjected to a high-voltage test.The procedure for filling this magnet wasrehearsed on a number of perspex tubes filled withRADIATION-RESISTANT MAGNETS 49FIG. 4. H5 magnet filled with concrete (second prototype).1 minute. This magnet has been installed and isoperating in a beam line at the Daresbury NuclearPhysics Laboratory.Figure 5 shows a quadrupole magnet preparedusing similar techniques and installed in an electronaccelerator beam line.The prototypes so far considered have beendesigned specifically for use in beam lines. Aconceptual design for an accelerator magnet for amultihundred GeV installation based on the use ofconcrete as insulator has been prepared by theauthors. 8 Figure 6 shows the layout of this designin which the concrete provides support for both coilpack and lanlinated yoke.5. RADIATION RESISTANCE OFCONCRETEThe radiation resistance of concrete is con-siderably higher than that of organic materials asis shown in a recent review ofstudies ofits behaviourin reactor environments.9 Whilst there are nodetailed studies on the effect of long term acceleratorradiation on the mechanical properties of concrete,there are no records of failure of concrete in thebiological shield of atomic reactors that can beascribed to radiation. Evidence from the exposureof samples, and tests on concrete shields exposedto high levels of neutron and y-radiation, indicatesthat standard quality concrete preserves its mechani-cal strength under nuclear radiation. lo6. CONCLUSIONIt has been demonstrated by the production ofprototypes that magnets can be successfully con-structed using radiation hard materials. II ,12 Itfollows that if these techniques can be applied in the50 A. T. GRESHAM, R. SHELDON AND G. B. STAPLETONFIG. 5. Quadrupole magnet (photograph courtesy of Lintott Engineering).FillingPlugBeamTubeElectricalConnectionsConcreteFillingHydraulicAlignmentTubeCoilsFIG. 6. Concept of a magnet for the multihundred GeV accelerator.RADIATION-RESISTANT MAGNETS 51construction of synchrotron magnets and otherbeam handling magnets, such installations can bemade completely radiation 'hard' thus obviatingthe need for expensive replacement contingencies.It becomes less important from damage con-siderations, to achieve high particle beam extractionefficiencies and further, one can also consider thepossibility of internal targetting methods, hithertonot possible in such high energy accelerators. Thesefactors together constitute a valuable contributionto the operational flexibility of such an installation.ACKNOWLEDGEMENTSThe authors wish to thank Mr. P. Bowles, Mr.D. A. Gray, Dr. L. C. W. Robbis, and Mr. G. E.Simmonds of the Rutherford Laboratory andProfessor J. F. Raffle of Loughborough Universityof Technology for advice and encouragement andMr. J. H. Aram, Mr. R. Forbes and Mr. R. Tolcherfor the experimental work carried out in supportof the project.REFERENCES1. See for example, H. Brechna, in Proc. 2nd Int. Con! onMagnet Technology, Oxford 1967, p. 305; G.Pluym and M. H. Van de Voorde, ibid., p. 341;R. Sheldon and G. Stapleton, ibid., p. 352.2. B. Zimmerman and M. M. Fromm, Proc. ElectricalInsulation Conference, IEEE, Chicago, 1967, p. 285.3. See for example, Digest of Literature on Dielectrics.Publication 1959 National Academy of Sciences,Washington, D.C., Vol. 31, 1967.4. A. Harvey and S. A. Walker, Los Alamos ScientificLaboratory Report LA-DC-10380 (1969).5. M. E. Lambert, Bull. Soc. Franc. Elect., 10, 257 (1940).6. E. Hammond and T. D. Robson, The Engineer, 199, 78(1955).7. Y. N. Vershinin, Electrical Engineering Concretes, TrudySibirshogo Nanchno-Issledovatel skogo InstitutaEnergetiki No.2 (21), Novosibirsk (1964).8. A. T. Gresham; R. Sheldon and G. B. Stapleton, 'ADesign Proposal for a Concrete Insulated Magnetfor the European 300 GeV Accelerator,' ReportRHEL/R 184.9. B. T. Kelly, J. E. Brocklehurst, D. Mottershead, Mrs. S.McNearney and I. Davidson, Proc. 2nd InformationMeeting ofPre Stress Concrete and Reactor PressureVessels and their Thermal Isolation, Brussels, 1969,EUR-4531, pp. 179-83.10. A. N. Komorovskii, Design of Nuclear Plants (Atomiz-dat, Moskva, 1965) (AEC-TR-6722).11. A. T. Gresham, R. Sheldon and G. B. Stapleton, 'TheConstruction of Magnets for Particle AcceleratorPhysics Using Cementatious Aggregates,' ReportRHEL/R 185.12. R. Sheldon and G. Stapleton, 'Electrical Insulation withCementatious Material,' British Patent No. 1269052.Received 22 June 1972

Magnets For Radiation-resistant Accelerators

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