Schauer, W.

Specking, W.

Turowski, P.

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KFK. 12~~9!nstitut für Experimentelle Kernphy.s~kDevelopmsrrt of a Crvoqenic Synchrotron Magnet with HighPuritv Aluminium Ccilsw. Schauer, Wo Specklnq, Po TurowskiMay 1970KERNFORSCHUNGSZENTRUM KARLSRUHEInstitut für Experimentelle KernphysikKFK 1219Development of a Cryogenic Synchrotron Magnet with highPurity Aluminium Coils+w. Schauer, W. Specking, P. TurowskiGesellschaft für Kernforschung m.b.H., Karlsruhe·Paper presented at the "Thirä International Conference on MagnetTechnology", Hamburg, 19-22 May 1970.AbstractSmall iron magnets with aluminium exciting coila varying in purityhave been tested at ~~2 K and 20.~ K in magnetic fields upto 30 kG.Resistivity ratio and magnetoresistance results are reported forpancakes wound from aluminium ribbon. At 4.2 K the relative resisti-vity increase Ap!p due 1:0 the magnetoresistance tends to a satu-ration value of 1.8 at 30 kG, independent of the sample purity whichis given by the pancake resistivity ratio in zero field and wasmeasured to be 8000 for the best and 1500 for the worst materialinvestigated. At 20.4 K Ap!p becomes purity dependent in contra-diction to Kohler's rule and was found to be 3.6 for the best and1.5 for the worst material at 30 kG.Questions concerning the appropriate conductor material and confi-guration, temperature of operation and cooling performance are dis-cussed.ZusammenfassungKleine Eisenmagnetemit Erregerspulen aus Aluminium unterschiedI:-ichenReinheitsgrades wurden bei 4.2 Kund 20.4 K in Magnetfeldern bis30 kG getestet.Ergebnisse zum Widerstandsverhältnis und Magnetowiderstand werdenfür Flachspulen aus Aluminiumband berichtet. Bei 4.2 K zeigt dierelative Widerstands zunahme Ap!p aufgrund des MagnetowiderstandesSättigungstendenz bei dem Wert 1.5 in einem Feld von 30 kG, un-abhängig von der Probenreinheit. Der Reinheitsgrad ist durch dasWiderstandsverhältnis der Flachspule im Nullfeld bestimmt; 8000 wur-de für das beste und 1500 für das schlechteste der untersuchtenMaterialien gemessen. Bei 20.4 wird Ap!p im Widerspruch zur Kohler'-sehen Regel abhängig vom Reinheitsgrad: 3.6 für das beste und 1.5für das schlechteste Material bei 30 kG.Fragen, die sich auf das geeignete Leitermaterial und die Leiter-konfiguration, die Be1:riebstemperatur und die Kühlung beziehen,wurden diskutiert.- 2 -IntroductionThe generation of high magnetic fields by conventional techniquesleads to ~igh ohmic losses in the coils, which are in the megawattregion for a big magnet system like a synchrotron of several 10 GeV.To reduce these losses, that is to reduce the conductor resisti-vity, two ways can be seen to date promising an economic improve-ment. These are to use either a superconductor or anormal con-ductor of high purity at low temperatures to achieve a remarkabledecrease in resistance. For DC-magnets one would of course prefera superconductor, but in a pulsed synchrotron magnet this is stillan open question on account of the AC-Iosses in a superconductor,therefore a cryogenic normal conductor may be competitive in thiscase. The convenient conductor material for normal conducting cryo-magnets is allliüinium, which was proposed and used already oy Danbyet ale [1] for a cryomagnet. It can be purified by zone melting toa high degree thus giving a resistivity decrease of about foul"orders of magnitude between room temperature and ~.2 K. A secondimportant reason for its applicability is the fact that the magne-toresistance approximately tends to a saturation value in highmagnetic fields.The weIL known data of high purity aluminium are not sufficientto construct a synchrotron bending magnet, because these data re-sult from short sampie measurements. There is little informationabout the behaviour of the resistivity ratio when the material iswound into a coil and powered in a magnet at low temperatures.Therefore we have built a small iron magnet of the window frametype with exciting coils of high purity aluminium to investigatequestions which are of importance in the design of cryomagnets,such as problems concerning the resistivity ratio and cooling per-formance. We have chosen the window frame type magnet to attainhigh magnetic fields by low power consumption.- 3 -The experimental apparatusFig. 1 shows a cross section through the whole arrangement. The ironyoke of the small magnet is laminated for pulse experiments. Fourpancakes of aluminium tape are mounted in the window of the ironyoke. Inthe middle and on both sides of the coil arrangement ver-tical cooling channels provide for a removal of the generated heatout of the interior of the coil by convection cooling in liquidhelium 01" hydrogen. The magnet is suspended at a flange by meansof thin stainless steel sticks and mounted in a metallic cryostat.Two radiation shields reduce the heat input. A styropor insulationfills the space between the shields and the wall of the cryostat,thus the evaporated helium flows through the electrical currentleads cooling them at the same time,The electrical current leads consist of a copper pipe with copperfins at the inner and outer side to lead the gas flow alongtheenlarged surface. A safety valve avoids a dangerous pressure risein the vapor above the liquid. The central magnetic field is measu-"'"red by a low temperature Hall generator ').A flattened carbon resistor**> was embedded between two pancakesto observe the temperature riseinthecoilsbythe decreaseofitsresistance. This resistor and a secondone placedin the centralmagneticfield were parts of a symmetrical Wheatstone bridge inorder to compensate partially the magnetoresistance of the carbonresistor.ExperimentalresultsTh". ~'"m;",;"m +"''''0 ~..t';+'h 1? 'iU n ~ "".u_••2 Ln r-T".......vQ.~ ~~~+;-vt"'i ·w·ac ·w·v'-'Ut"iu..l_ •• ._.... _ .........._10 "" .... j:' ..... . .... ~A_6 ä60 ~ v.v. _. ... _.. _ _ ••into pancakes by a simple winding machine. The pancakes have aninner and an outer diameter of 4 cm and of about 12 cm, respectively.The windings were insulated by a 25 ~ thick Hostaphan foil.*) Siemens RHY 18**)Allen Bradley resistor 100 Q, 0.1 Watt- 4 -Four pancakes were put together to a magnet coil, electricallyconnected in series. Potential connections are installed at bothends of the exciting coil and at the connections between theindividual pancakes to evaluate the resistance of the entire coiland of the four pancakes separately. The current was measured bythe voltage drop of anormal resistor of 10-4n• At our investi-gations we observed at low temperatures a remarkable contactresistance between the connections of the pancakes. Thus to getreliable potential measurements one had to prepare the poten-tial connections carefully and had to place them away from thecurrent contacts.Fig. 2 shows the resistance of one pancake versus the centralcoil magnetic field at the temperatures of li~uiä helium anäliquid hydrogen. The aluminum* of this pancake was purified byzone melting and annealed at 5000C for one hour and cooled downfor about 10 hours after having been cold rolled to the ribbonof the desired geometry. Tbe resulting short sampie resistivityratio was about 9000. In the pancake we found a resistivity ratioof 8000. Due to the magnetoresistance the resistance of the pan-cakeincreaseS with1hcreasihg inagnetic field. At 4.2- K themagnetoresistance tends to a saturation value. At 20.4 K theresistance shows no tendency towards saturation up to fields ofabout30 kG and the magnetoresistance effect is obviously greaterthan at 4.2 K. The average increase of the resistance at a centralfield of about 20 kG amounts to a factor of 4.3 at 20.4 K. Thislatter result is not in agreement with Kohlerts rule of themagnetoresistance given for aluminum by several authors [2 J.Our results for aluminum indicate not only a temperature depen-dence of the magnetoresistance but also a dependence on purity.Fig. 3 shows a Kohler plot at 4.2 K and 20.4 K for 3 aluminum tapewound pancakes differing in purity. The relative increase of theresistance 6p/p in the self-field is plotted versus the argument•( P295 /PT) x H (H = central coil field ) as it is usuallydone. These ribbons are heat treated in the same manner as* delivered by VAW, Bonn, Germany- s -mentioned above. In the upper curve it can be seen that at 4.2 Kthe experimental values of the different materials are groupedalong a common mean curve. At 20.4 K, on the other hand, thereare signiEicant differences in the behaviour of the differentmaterials, as shown in the lower curve. The magnetoresistance in-creases with increasing purity. At 20.4 K and for a field of 30 kGin the coil center we found a relative increase in resistance of3.6 for the best material and 1.5 for the worst material investi-gated. The corresponding curve at 4.2 K is repeated for comparisonand shows a noticeable lower magnetoresistance.As our pancake measurements are performed in the self generatedmagnetic field we have to regard a temperature rise by the dissi-pated power which had an upper level of about 60 watts at ~.2 Kand of 180 watts at 20.4 K. The temperature in the exciting coilsapproaches a stationary value within aminute. The observed in-crease of temperature was low, at 4.2 K in the order of S810- 2K-"and at 20.4 K in the order of 5·10 4K (the absolute values of thetemperature rise are not quite reliable as the carbon resistorswere influenced by the magnetic field). At 4.2 K in the regionoftheresidual resista.ficethere Ts öhlya smarl irifluence Oftemperature rise on the resistance. From measurements of Dorey (3]the temperature increase in the heated coils khould not exceed0.3 K in the case of nucleate boiling being the common coolingcondition in channels. At 20.4 K on the other hand the resistancehas a strong dependence on temperature, thus a small temperaturerise leads to an appreciable increase in resistance. SupposingKohler's rule to be valid the observed additional resistanceat 20.4 K and 30 kG for the material of highest purity corres-pnds to a temperature rise of 6 K according to a T5-law temperaturevariation of the resistivity at 20.4 K. This value is toohigh in comparison with our experimental value. There is stillanother argument, which shows that our observation will benoheating effect. The material with the higher resistivityshows a smaller increase in resistance in spite of its higherJoule heating. In agreement with measuremnts of Borovik et ale [4]= 6 -there exists a considerahle deviation from Kohler's rule forhigh purity aluminum at 20.~ K.The short sample magnetoresistance effect will still be higherthan our results show, as in our experimental arrangement wemeasured a magnetoresistance averaged over the decreasing mag-netic field of the pancake.Up to now we have tested the aluminum under DC-conditions, hutin the next stage we will apply eyclic powering. To date we ob-served no change in the zero field resistance of the excitingcoils after several magnetic cycles up to fields of about 30 kG.The magnetic pressure acting on the windings was ealculated tobe about 36 atmeDiscussi'OnWe have used the high purity aluminum conductor in an arrangementcomparable to that in a future technical application. Thegeometri-cal dimensiofisof the alumifiWn. ribbofi, espeeiallythe thicKliessof 0.3 mm, were chosen to keep the eddy current losses and thesize effect small at 20.~ K, this means in a limit of lot of theohmic losses*. At 4.2 K these additional losses would be remark-able greater for the same ribbon because of its higher conduc-tivity.In our experimental apparatus we tried to generated magnetic..fields as high as possibel. The power input in liquid helium waslimited by the cooling performance of the cooling channels. Atan electri~al power of 59 watts in liquid helium we get a heatflow of 0.29 watts cm- 2 through the walls of the cooling channelswhich corresponds to the critical heat flow for our channel lengthof about 9.5 cm measured by Wilson [5]. In liquid hydrogen we didnot attempt to approach the upper limit of the cooling performance*The eddy current losses ware calculated from the usual relationP :: (1/2 4)"0 wl d 2 B2 [watts/ern 3] with the assumptions: centralpeak field ~O kG, 2Tr/w = 3 sec period and a resistivity ratio020/0295 of 1000.- 7 -for safety reasons. From ordinary theories of cooling in channels[5] one can expect a cooling capacity in liquid hydrogen ten timesgreater than in liquid ,helium because of the higher latent heat.In the next stage we will build a window frame magnet for 40 kG witha length of 40 cm and an iron yoke saturated up to a magnetic fluxdensity of 18 kG. Theexciting coils will be cooled down togetherwith the iron return yoke. The magnet will be placed vertically forapplying convection cooling. From our present results we expect apower dissipation of about 350 watts at 20.4 K at a current densityof about 4.5'10+3 amps/cm2• The winding section will be 4 x 10 cm2 andthe magnet gap will have a cross section of 4 x 7 cm2•~In correlation with thermodynamical considerations our measurementsindicate that liquid hydrogen is an economic cooling agent. The over-all gain in power calculated from ordinary formulas* will be 12 at20.4 K and 6.5 at 4.2 K. The temperature dependent magnetoresistancemay lead to another optimal temperature. Therefore we will spentmore attention ee this pr-öhLem in the next time by studjdng thetemperature and purity behaviour of the magnetoresistance.It is a difficult technical problem to bend the coil ends of a tapewound race track coil out of the coil plane. This is necessary forthe applicability of a window frame magnet to get access to themagnet gap. We are studying technical !ssibilities of bending tapewound coils. Another way is to use a twisted and transposed cable ofmany insulated thin aluminum wires, also under study.-1* 1 To-TThermodynamical gain G = (P295 / p{!' 'H» (1 .. t -y-) . To isroom temperature and t is the overall efficiency of the refrige-rator relative to the ideal thermodynamic efficiency. It may betaken to be about 0.3 at 20.4 K and 0.15 at 4.2 K.- 8 -Literatur[1] J. Allinger, G. Danby , J. Jackson, I_ Polk, A. ProdellIEEE NS l!, 728 (1969)(2) B. LUthi, Helv. Phys. Acta !!, 161 (1960);V. Arp, Brookhaven 1968 Summer Study, p. 1095[3J A. P. Dorey, Cryogenics i, 146 (1965)[4] E.S. Borovik, V. G. Volotskaya, N. Va. FogelJ. Exp. Theor. Phys. (USSR) ~t 46 (1963)~] M.N. Wilson, Bull, I.I. R. Annex S, 109 (1966)GASCOOLEDCURRENT LEADSSAFETYVALVERADIATIONSHIELDSHALL PROBEALUMINUMCOILSLIQUID N2SHIELDSTYROPORINSULATIONLIQUID He OR H2BATHIRaN YOKETEMPERA TURERESISTORS"~;" COOLING>~,CHANNELSFig.1: Cross section through the experimental apparatus200100o r--R~7950 I.o 10I20tRR582tRR3240IH [kG} 30Fig.2: The measured resistance of one pancake in variousmagnetic fields at the temperature of liquidhelium and liquid hydrogen;RR = resistivityratio ~295/9 T,H2I~g. R{H)-R{o) ° ... + 4,2°KA ~O-~+ ++ +1 R {ol A~o +5+2short samR/e RR1Öt + 900006130A 18105..RR-H [Oe]I I i,~__RR.H [Oe]RRI I I52Fig.3: Kohler diagram fordifferent Aluminumpancakes at 4.2 Kand 20.4 K.i i i I I. 107 2i52I5522255-151

Development of a Cryogenic Synchrotron Magnet with High Purity Aluminum Coils

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Development of a Cryogenic Synchrotron Magnet with High Purity Aluminum Coils

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