Superconductive composites

Superconductive composites

Superconductive composites B. J. M A D D O C K * Superconductors have no electrical resistance at sufficiently low temperatures and they are therefor...

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Superconductive composites B. J. M A D D O C K *

Superconductors have no electrical resistance at sufficiently low temperatures and they are therefore now commonly used in the construction of powerful electromagnets. However, it has proved necessary to combine them with conventional conductors to obtain satisfactory electrical performance of the magnet windings. The reasons for this are discussed. The essential properties of superconductors and the manufacture of the composite conductors are described. Examples of the use of superconductive electromagnets are given and some comments made on future conductors

INTRODUCTION

SUPERCONDUCTOttS

By no means all composites are made just to obtain improved mechanical properties. Expanded polystyrene is a good thermal insulator because the plastic prevents convection of the gas, while resin cored solder provides the correct amount of flux for each joint in a very convenient way. Superconductive composites are made principally to obtain improved electrical properties, for although superconductors have no electrical resistance at very low temperatures they have some undesirable features as we shall see. The recent rapid development and application of superconductors' stems from the discovery, late in 1960, that some materials remained superconducting in much higher magnetic fields, than previously observed and that thesg materials could carry high direct currents. It is with superconductors which have these properties that we shall be mainly concerned. They make possible the operation of large and high field electromagnets without prodigious and often prohibitive power consumption. Compactness and hitherto unobtainable field gradients are other virtues. Some comments on superconductive composites for power transmission are made in the last section.

High field high current superconductors are k n o w n as bulk hard type 2 superconductors because they can carry current without resistance throughout their bulk. This is in contrast to the earlier known superconductors which can only carry such current in very thin layers at their surfaces. = The bulk superconductors of technological importance at present are either alloys of niobium with titanium and zirconium or the compound of niobium and tin, Nb3Sn. The way the electrical resistance of a typical alloy of niobium and titanium changesas its temperature is lowered is shown in Fig l. Notice the slight fall followed by a sudden drop to zero at a critical temperature. Also shown is the variation for a high conductivity copper. For this there is a marked fall by a factor of about two hundred to a residual value, but no sign of superconduction even at temperatures down to the lowest attainable; a few millidegrees above absolute zero. The residual resistance is lower the higher the purity, and the better the copper is annealed. The critical or transition temperature, the ambient magnetic field and the current density in the superconductor are related as shown in Fig 2. This is drawn for the popular alloy Nb-44wt%Ti but similar pictures apply to the other bulk superconductors. If the point representing the

* Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, England

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COMPOSITESDecember 1969

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operating conditions lies within the surface shown, the material is superconducting; otherwise it is resistive. Salient figures for several superconductors are given in Table 1 : the transition temperature is that for zero field and current and the critical field is that at 4.2°K (-269°C, the boiling point of liquid helium) with no current. Only the first two materials listed are in widespread use at present. Niobiumtitanium has largely replaced niobium-zirconium because it is now easier to produce and can be used in magnets generating higher fields. The current carrying capacity of a superconductor is crucially dependent on the way the material is prepared whereas the critical fields and temperatures are more fundamental parameters. For high critical current densities the material needs to be very inhomogeneous on a microscopic scale. Fine precipitates and complex dense networks of dislocation appear to be beneficial in the alloys, which nevertheless are reasonably ductile, and these networks are produced by heavily cold working the metal such as by wire drawing. Reductions of the cross sectional area by factors of at least 104 are often necessary. The mechanisms controlling the critical current densities are not well understood, particularly in the compounds. These have a complex crystal structure and are very brittle. Consequently, niobium-tin is manufactured as thin layers on a substrate tape by processes quite different from those used for alloys. Despite the heavy currents which can be carried without resistance, these bulk superconductors have to be combined with conventional metals to form composite conductors so as to obtain satisfactory performance.

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WH Y C O M P O S I TE C O N D U C T O R S

Many of the' early attempts to make magnets with bulk superconductors met with disappointment. The expected critical currents and fields were not reached, the coils becoming resistive prematurely. The problem and a solution are really contained in Fig I. If for any reason during operation the temperature of the superconductor rises momentarily above its transition temperature, its resistance

Table 1 Critical temperature and critical magnetic field of the technically important superconductors (1 T(tesla) = I0 k il ogauss)

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Critical field (at 4.2°K), T

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COMPOSITES December 1969

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shoots up to a high value. Much heat is then generated by the current. This heat spreads, driving more of the coil resistive and sometimes the wire melts. By adding a good ordinary conductor such as copper or aluminium, the rise of resistance and hence the heating can be limited. Tile first composite wires and tapes were electroplated and this produced a considerable improvement. Still better performance is now obtained by cladding the alloy wires with copper at an early stage in their manufacture and then drawing the composite to size. This gives a thicker coating of higher purity, and hence of lower resistance, and one which is better bonded mechanically and electrically to the superconductor. Many strands may be incorporated in a copper matrix to give a larger total current. For the compound superconductor niobium-tin, the extra copper is soldered on as foil.

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Cryostatic stabilisation For small coils the optimum amount of copper was found by experience; too much being unsatisfactory simply because it seriously reduced the amount of space available for the superconductor. However, for large systems which must work first time the conductor has to be carefully designed. A quantitative basis for doing this was first provided by Stekly and Zar 2 in 1965. They argued that although the transient thermal disturbances in the coils were not understood one could cope with their consequences. (These disturbances are caused by the irregular motion of the magnetic flux within the superconductor, but what governs this motion is not precisely clear). Cooling has to be provided throughout the coil and sufficient copper added in parallel with the superconductor such that, whenever the latter become resistive so forcing the current to flow in the copper, the rate of heat generation is less that that which can be carried away by the coolant. The conductor will then recool and become superconducting again. The condition is

i p/,4 < Qe where 1 is the current (amps), p the resistivity (~2cm) of the copper or other added metal and A its cross sectional area (cm z). P is the surface area per unit length (cm) of the conductor exposed to the coolant and Q the heat per unit surface area (W/cm 2) which can be transferred to the coolant. (For a full discussion of Q, see reference 3. A typical value for liquid helium is 0.3W/cm2). This expression enables the minimum quantity of copper to be calcuhted. One finds that tile required cross sectional area of copper usually lies between two and twenty times that of tile superconductor. This technique, which ensures that the full performance of the superconductor is obtained, is known as steady-state or cryostatic stabilisation. Clearly, the lower the resistivity of the added metal the better. However, the resistivity of pure metals at low temperatures is quite strongly increased by high magnetic fields and the enhanced value must be used in the condition above. Some typical data are given in Fig 3, the lower curve

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COMPOSI TES December 1969

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for each metal being for higher purity. Aluminium has not been used as much as this picture would suggest, because although the necessary purity is available, it is very soft, is more difficult to co-process with the superconductor and its resistivity is more easily increased by mechanical stress than that of copper.

Other forms of stabilisation An alternative approach is to understand and control the disturbances within the superconductor in such a way that it never becomes resistive prematurely. Two schemes are possible,4 both of which are still at the experimental stage. In one, the magnitude of the disturbances is reduced sufficiently so that the transient temperature fluctuations do not rise above the critical value. It turns out that for this technique, known as adiabatic or enthalpy stabilisation, to succeed, the superconductive wires or layers must be thin, typically less than 50tam. It is also helpful if the heat capacity is as high as possible and some composite conductors have incorporated a sheath of lead. There is no need to add copper or to allow the coolant to permeate the winding. To obtain the required total current, many filaments are used in parallel. It is convenient to produce these simultaneously in a suitable matrix, and if this is copper, the Filaments must be twisted round each other or even transposed. If this is not done, the high electrical conductivity of the matrix couples the filaments together and makes them behave as one thick unstable filament. The second method is to slow down the motion of magnetic flux. Thus there is time for the heat generated to

be conducted away to the coolant without a sharp rise in temperature. This is known as dynamic stabilisation. A material of high electrical conductivity will damp the flux motion. For this reason copper or aluminium is chosen. These metals also have a high thermal conductivity. Again it is necessary for the superconductor to be thin, typically 50/am, since bulk superconductors have poor thermal conductivities (about one thousand times lower than that of copper). There is no special need t o twist the f'daments with this technique. Both these methods of stabilisation require less matrix material and less space for helium within the winding than ~'or cryostatic stabilisation. This allows higher overall current densities and more compact winding to be achieved.

Protection To cope with unforeseen disturbances or failures which may drive the superconductor resistive despite stabilisation, protection systems for superconducting magnets are needed. The simplest scheme is to allow the current to decay by passing it through a resistor in which the magnetic stored energy is dissipated. The inductance of the winding limits the rate at which the current can be switched-off in this way and the conductor must not overheat during the discharge. Again, the best solution is to combine copper with the superconductor to limit the resistance (Fig 1) and hence the heating in the windings s

Forces The electromagnetic forces, (arising from the interaction of the current and the magnetic field and which act on the conductors) are large in superconducting magnets because of the high fields generated. For a solenoid the forces are radially outward and axially inward and so the important stresses in the conductor are a hoop tension and a sideways compression. Care must be taken to avoid the stress in the copper exceeding significantly the yield value (about 8kg/mm: at 4.2°K for annealed material) otherwise the resistivity would be increased and hence the stability impaired. There would also be a small change in the dimensions of the winding. Filaments of niobium-titanium in the copper help only a little because the elastic moduh,s of this superconductor is not markedly greater than that of copper. If the forces are severe they may be partly taken by extra metalwork within the coil or by incorporating strands of a stiff and strong metal into the composite conductor. An alternative, applicable to composites with a copper matrix and particularly helpful if the sideways compression is high, is to slightly work harden the copper. 6 This can be done without too serious an increase in its low temperature resistivity. For example, an area reduction of 5% increases the yield strength to about 20kg/mm" while increasing the resistivity in a field of 5T by only 10%. For ihe brittle niobium-tin, it is important to avoid any tensile stress in the superconductor itself. We shall see in the next section how this is ensured.

MANUFA CTURE OF SUPERCONDUCTIVE COMPOSITES Conductors using,niobium-titanium The alloy is made by arc melting the elements in vauum to form ingots, taking care to avoid excessive concentrations of oxygen, carbon and nitrogen which lower the ductility. The ingots are clad with the required quantity of copper and then processed by the standard metal forming techniques such as forging, swaging, extrusion, rolling and drawing. In order that the superconductor suffers sufficient cold work a typical single core wire is drawn down to an overall diameter of 0.4ram with a core diameter of 0.25mm. Such a wire is used for currents of the order of 50A. Higher current conductors are produced by combining a number of rods at an early stage in the process to give a multilqlament conductor. Finally, the composite is heat treated at about 400°C to improve the critical current of the superconductor and simultaneously to anneal the copper. This reduces its low temperature resistivity to the residual value determined by the impurity content. If a strengthened conductor is required, the heat treatment is carried out before the final cold drawing stage. Three examples of multifilament composites are shown in Fig 4. The centre conductor has a volume ratio of copper to superconductor of 8 : 1 and carries a current of 250hA in a field of 6T. "Fhe transverse grooves improve the access for the liquid helium coolant. Also shown is a hollow conductor for use in an electromagnet cooled by circulating very cold high pressure helium gas through the conductor. There are fourteen filaments which have become crescentshaped in cross section. The Niomax-M conductors were one of the first co-processed composites produced and were

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FIG 4 Multifilament niobium-titanium-tr)pper composites. The width of the centre conductor is lOtnm. Left: hollow 14 lilament Niomax-M. Centre: 60 filament (each 0.24ram diameter) grooved Niomax-M. Rig/at: 648 filament (each 441am diameter) Niomax-Fbl (the copper has been etched away at the end to show the filaments of the superconductor) (Photograph by courtesy of Imperial Metal htdustries Ltd)

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developed jointly by the Central Electricity Research Laboratories and Imperial Metal Industries Ltd. The smaller filament diameters required for adiabatic and dynamic stabilisation are produced simply by drawing the larger conductors down to a smaller size. High current conductors needing many fine filaments may be made by a two stage process in which rods already containing a number of filaments are combined and further processed. An example is shown at tile right in Fig 4: it has a copper to superconductor ratio of 4 : 1. If copper is used as the matrix for a conductor designed to be adiabatically stable, t.he required spiraling of the filaments round each other is produced by twisting the conductor at a'late stage in the processing with a pitch of perhaps one turn every few centimetres. Alternatively, an alloy of copper and nickel may be used for the matrix. This has a much higher resistivity and so less twisting is required. Fine filament composites, particularly twisted versions, and tile use o f a copper-nickel matrix have been developed by Imperial Metal Industries Ltd in collaboration with the Rutherford Laboratory. Enamel coatings for electrical insulation may easily be applied to all these conductors if separate insulation is not provided during winding. For cryostatic stabilisation the coating thickness should not exceed about 20#m otherwise heat transfer to the liquid helium is impaired. However, a thickness of about 10pro improves tile permissible heat flux.

Conductors using niobium-tin Being brittle, NbaSn has to be produced as thin layers on a substrate: there are two principal processes. Tile first pioneered by the Radio Corporation Of America and known as vapour deposition, relies on the simultaneous decomposition with hydrogen of vapourised niobium and tin chlorides. The overall reaction is 3NbCI4 + SnCI2 + 7H2 = NbaSn + 14 HCI and the deposition occurs on a metal tape maintained at about 900°C by passiqg a current through it as it is fed through the reaction chamber. Hastelloy (a nickel molybdequm based alloy) is often used for the tape which has a width from 2 to 13ram and a thickness o f a b o u t 45pm. The material chosen for the substrate must obviously withstand the temperature of the process and not react with the chloride vapours. It should also contract on cooling somew.hat more than the niobium-tin so that the latter is under modest compression. This allows tile composite tape to withstand tensile forces without the deposited layer fracturing. Concrete is often prestressed with steel for just the same reason. The higher the elastic modulus of the substrate, the higher these forces can be. The layer thickness is usually about 6/am, but can be from 3 to 20/am, on each side of the tape. A typical 2.3mm wide conductor can carry a current of 100A in a field of 10T. For small coils some stabilisation is provided by a thin coating of electroplated silver but for larger coils the tape is soldered between strips of high conductivity copper foil.

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COMPOSITES December 1969



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The foil thickness can be from 25 to 100/am depending on the degree of stabilisation required. Reliance is usually placed on a combination of adiabatic and dynamic stabilisation. Tile other method of producing a niobium-tin conductor, used by General Electric of America, the Plessey Co and others, is based on a diffusion process. A niobium substrate ribbon is coated with a thin layer of tin and then passed through a furnace at about 950°C. The tin diffuses into the niobium and reacts to form a layer of Nba Sn about 3/am thick on each side. widths up to 13mm are available. It is difficult to make the reacted layer much thicker. So two or more tapes may be soldered together to obtain a higher current for a given width. Copper foil is added for stabilisation as for the vapour deposited niobium-tin. The diffused niobium ribbon is usually quite tldn, about 10/am, and so has little strenth, it can, however, be bent round radii as small as 3mm without fracturing the reacted layers because these lie close to tile neutral plane. Stainless steel tape (eg 25pm thick) is soldered on to one or both sides to provide additional strength and to help prestress the niobium-tin as the composite is cooled down from the soldering temperature. 7 An example of a diffused niobiumtin composite is shown in Fig 5.

APPL ICA TIONS Superconductive composites have so far been used almost exclusively for making electromagnets. Many hundreds of small solenoids with bores up to a few centimetres in diameter are in use in research laboratories, particularly for solid state physics research. Many medium sized magnets have also been constructed and some typical examples of their use are given at tile top of Table 2. Two particularly large windings have been completed within the last year:

Some present and future application of superconductive magnets

Table 2 Size

Mediu m (aperture less than about l m )

Large (aperture greater than about l m )

Present day

Testing composite superconductors Experimental motors and generators Plasma containment Pulse energy storage

Bubble chambers

Next two years

Bending and focusing beams of nuclear particles Prototype MH D generators Electron microscope lenses

Homopolar motor field winding Spark chambers for nuclear physics Purification of kaolin

Future

In medicine and surgery, eg intravascular navigation Suspension units for high speed trains

Thermonuclear fusion power stations DC and AC generators Magnetic separation of ores High energy particle accelerators Energy storage Spacecraft propulsion

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FIG 6 One half o f the winding o f the Argonne bubble chamber magnet with part o f the helium cryostat can being Iowered htto place. The conductor has a cross-section o f 2.5ram x 51ram and was manuJbctured bj, Supercon (Photograph by courtesy o f Argonne National Laboratory, J llhtois )

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these provide the magnetic field for two liquici hydrogen bubble chambers which are used for studying nuclear particle reactions. The largest is shown under construction at the Argonne National Laboratory in Fig 6. The winding, which generates a field o f 1.8T, has an inner diameter o f 4.2m and uses a copper/niobin-titanium composite. Some forthcoming applications are also listed on the Table: some are certain, others less so, and no doubt there will be others not yet considered. Fig 7 shows the superconductive field winding o f a homopolar m o t o r ~ being built by International Research and Development Co Ltd.

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PTG 7 Field whlding o[ the IRD 3250 JlP homopolar motor encased itt its annular cryostaL The lacge central aperture, in which part o f the motor stator can be seen, is 2.2m in diameter. The winding uses a l.Smm x lOmm Nionuo:-M conductor [Photograph by courtesy o f hltentational Research & Dcrelopment Co Lid)

COMPOSITES December 1969

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There is of course another potential large scale appllcation of superconductors and that is for electrical power transmission. Much research is in progress on the technical and economic aspects of this but it will be some years bel\~rc a iwototype cable is in operation. It remains to be seen what the actual costs will be in comparison with conventkmal cables. Low temperature transmission systems pose challenging refrigeration and high voltage insulation problems as well as those of the conductor. Many of the ideas and composites developed for magnets are adaptable to DC transmission. Magnetic fields are smaller being just those generated by the current flowing in a straight conductor. For AC transmission, which is of greater importance, new designs of composite conductor are needed which avoid the heating or loss which occurs in bulk superconductors when the field or current changes rapidly. In effect, bulk superconductors do not have zero resistance to alternating currents. Some possible designs are mentioned in the next section.

FUTURE COMPOSITES What will superconductive composites be like in a few years' time? There will be further development of those already described: improvements in performance, improvements in processing and some standardisation of sizes leading to reduced costs. Some of the uses given in Table 2, for example generator field windings and particle accelerators, require the magnetic field to be changed quite rapidly. This aggravates the problem of stabilising the superconductor and makes the total heating arising from the movement of magnetic flux within the superconductor more important. Proper transposition of the filaments may prove necessary and this will require a braiding or weaving process. The fine filament materials described earlier and which have just become available commercially will be developed further to meet these requirements. Three and even four component conductors may be needed. For example, a composite matrix of copper and a high resistance alloy could be made to meet the requirements of protection, adiabatic stabilisation and low heating. The superconductor itself may be a composite, either to raise its critical current or to make this current increase with temperature, at least around the working temperature. (Compare with Fig 2 in which it decreases everywhere). Such a positive temperature coefficient would make the superconductor intrinsically stable and immune to small temperature fluctuations. At 30Hz the power losses are serious, especially in high fields. (Each watt dissipated at 4.2°K requires about one kilowatt of" refrigeration power). It does not at present seem likely therefore that the output windings of large alternators can be made superconductive, nor do the prospects for superconductive power transformers look good. For low fields, such as in power cables, the losses can be reduced to an acceptable level by using a surface superco,ductor, or just possibly a bulk superconductor with a very high critical current density which confines the current to a thin surface layer. Large surface areas may be

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COMPOSITES December 1969

obtained by using many fine wires or, more conveniently but less compact, a large diameter tube. Niobium is the most suitable surface superconductor. It can carry an almost lossless current of about 50A rms/mm of tube circumference, though this is crucially dependent on the surface finish as the current flows in a skin less than l/am deep. Just as for magnets, a composite conductor is necessairy. Transmission systents have to withstand heavy current overloads and these would drive the niobium resistive. A substrate of bulk superconductor or very pure high conductivity metal such as aluminium is required to limit the heating during overload. Protection from overheating following a failure, such as in the cooling system, and adequate strength to withstand the pressure of the coolant are also needed: the conductor may therefore have to incorporate copper and sieel as well. Electrodeposition, sputtering and conventional metal processing are the principal techniques for producing thin layers of niobium. Each has its particular manufacturing problems. Of course, as far as the properties of superconductors themselves are concerned, the most dramatic development would be the discovery of a material which superconducts at an appreciably higher temperature. At present this does not seem very likely. The highest transition temperature known is 20°K for a compound of niobium aluminium and germanium. It is an even more intractable material than niobium-tin..There has been speculation that a material consisting of alternate layers of superconductor and dielectric, each a few nanometres thick, would have an enchanced transition temperature and also that certain complex organic substances taught superconduct at high temperatures.

CONCL USIONS Superconductive composites are made to obtain the right combination of electrical, thermal and mechanical properties. Their development has been successful and very rapid: it is only nine years since the high field high current capabilities of superconductors were discovered. Niobiumtitanium/copper composite conductors are available which are strong and reliable and which can be bent and handled by standard engineering methods. Large magnet systems using these conductors are already in operation. Niobiumtin composites are available which, although slightly worse mechanically, enable fields well over 10T to be generated. Rapid development is taking place of the fine filament conductors following the increased understanding of the behaviour of superconductors, tile demand for more compact magnets and for those whose fields can be changed quickly. Many new composites are likely, including ones suitable for power transmission.

A CKNOWL EDGEMENTS I am grateful to several of my colleagues for helpful comments. This paper is published by permission of the Central Electricity Board.

REFERENCES 1 ROSE-INNES, A. C. and RHODERICK, E. H., Introduction to superconductivity, Pergamon Press, 1969

TAYLOR, M. T., WOOLCOCK, A. and BARBER, A. C., Strengthening superconducting composite conductors for large magnet construction, Cryogenics, 8, 3 1 7 - 9, 1968

2 STEKLY, Z. J. J. and ZAR, J. L., Stable superconducting coils, IEEE Trans N S - 12, pp 367 - 72, 1965

BENZ, M. G., Mechanical and electrical properties of diffusion-processed Nb3Sn - c o p p e r -stainless steel composite conductors, J A p p l Phys, 39. pp 2533 - 7,

3 MADDOCK, B. J., JAMES, G. B. and NORRIS, W. T., Superconductive composites: heat transfer and steady state stabilization, Cryogenics, 9, pp 261 - 73, 1969

1968

8 APPLETON, A. D., Motors, generators and flux pumps, Cryogenics, 9, pp 147 - 57, 1969

4 CHESTER, P. F., Superconducting magnets, Rep Prog Phys, 30 (pt 2), pp 561 - 614, 1967

5 MADDOCK, B. J. and JAMES, G. B., Protection and stabilization of large superconducting coils, Proc lEE, 115, pp 543 - 7, 1968

9 HAMPSHIRE, R. G., SUTTON, J. and TAYLOR, M. T.,

Effect of temperature on the critical current density of Nb - 44wt% Ti alloy, paper given at the conference on low temperatures and electric power, London, March 1969

COMPOSITES December 1969

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