Theorbitalmagnetandpo Wersupplyofthe

THE ORBITAL MAGNET AND POWER SUPPLY OF THE 10 GEV PROTON SYNCHROTRON AT THE AUSTRALIAN NATIONAL UNIVERSITY

J. W. BLAMEY Australian National University, Canberra

Summary

The proton synchrotron under construction in Canberra, Australia, has an air-cored orbital magnet of peak field 80,000 gauss, energised by a homopolar generator of energy storage 5 × 108 joules. The problems associated with this departure from convention are discussed, and the main features of construction described.

Introduction When we first embarked on this project (1953), it appeared that application of the strong-focusing principle might The proton-synchrotron under construction in the reduce the volume of strong-field region and hence the Australian National University is an experimental approach power demand. Detailed analysis showed that any gain to the problem of accelerating particles to very high energies which could be made in this way was more than offset under conditions where resources are small, both in by the complications and uncertainties involved. Accordingly, manpower and money. It was felt that a bold effort a weak-focusing orbital magnet has been designed should be made to demonstrate the practicability of a and is described below. machine relatively small in dimensions and cost. This Energy is most easily and compactly stored in a could well point the way to future development in high rotating mass. The weakest factor in the design of rotating energy accelerators. The dimensions of an accelerator electrical machinery, for pulsed operation, is the fastening for 10 Gev or more can be reduced appreciably only by of conductors into position so as to withstand the large abandoning the use of iron in the magnetic circuit and by mechanical and electromagnetic forces involved. It was providing much greater pulsed powers to energise the natural, therefore, that we should think in terms of a homopolar "air-cored" magnet. The overall cost of an accelerator, generator which has no windings, especially as we including buildings and fondations, is determined almost possessed a magnet with 148" pole diameter and a large wholly by its physical size. The capital cost of the power-plantgap, together with four forged discs of steel for homopolar and the running costs can be kept at a reasonable rotors, capable of storing a total energy of about 5 × 108 figure by reducing the rate at which pulses are repeated. joules when rotating at 15 revolutions per second.

Fig. 1. J. W. Blamey 345

The low pulse repetition rate is a disadvantage which in any case must be faced in the future; it makes high demands on control techniques and limits the number of nuclear experiments that may be performed. This limitation is regarded as tolerable in a developmental project, especially as it could be overcome by the development of suitable techniques in the future. Our financial resources barely cover the cost of this project and we are also severely limited in personnel. Consequently although original technical development of some magnitude is required, we are unable to make an Fig. 2. Current and speed variation during pulse for β= 0.25. adequate design study. Much reliance has therefore been placed on simplicity of design and ultimate solution of problems using the homopolar generator itself as an experimental tool, rather than the creation of a theoretically to break the circuit at this point and accelerate the rotors sound design. to full speed in the reverse direction for the next pulse 10 minutes later. General description The rotors are accelerated by passing through them Fig. 1 shows diagrammatically the basic elements of the 3300 amp from a grid controlled rectifier. Brush contacts synchrotron with the aircored magnet, injection cyclotron for this and for the pulse current are made with liquid and homopolar generator approximately to scale, and in metal (sodium-potassium alloy) jets. their true relative positions. An appendix at the end of the The proton beam is injected at afield of 850 gauss from paper lists the more important dimensions and data. an 8 Mev cyclotron and accelerated in one of the straights The synchrotron magnet which is essentially ironless with radio frequency applied to a broadly tuned ferrite provides a peak magneticfield of 80,000 gauss in a space toroid and cavity. 25 cm. wide and 40 cm. high, in four quadrants with a The vacum box is a stainless steel tube 8.5" internal mean orbital radius of 480 cm., separated by straight diameter and 0.08" wall thickness, opening out to a larger sections 2½ metres long. The circular section vacuum section in the straights. box within has a clear internal diameter of 22 cm. The power supply is a large homopolar generator which has 4 steel discs each about 139" in diameter and 19 tons Choice of parameters in mass. These, rotating at 900 r.p.m. in a magnetic field of 16000 gauss, have an energy of 5 × 108 joules and The target values of proton energy, power supply energy develop an e.m.f. of 800 volts when in series. and voltage, and the parameters e.g. orbital radius, mass of copper, etc. which determine these, were dictated largely When suddenly connected to the air cored magnet of by our resources—that is, size of buildings, the 136" inductance L and resistance R it acts as a condenser of cyclotron magnet as homopolar generatorfield, th e four capacitance C 1700 farad and thus results in a current pole tips as rotors, a rectifier set ordered earlier for a similar 1 purpose, a 30" cyclotron magnet, crane loads, machine -Rt R2 Vo capacity and Australian manufacturing resources, and i = exp( ) sin ωt where ω = √ - 2 ωL 2L Lc 4L of course, money and staff. Parameters could be optimized only within narrow limits. which may also be written as The discovery of the strong focusing system in USA Rt Vo 2 Q 2 2 led to the serious consideration of an air cored magnet ½Li = exp (- ) sin ( • √1 - β ) t 1) 1 - β3 L √2QL to replace the cyclo-synchrotron . This in turn led to the conception of the use of the large cyclotron magnet solely √2QL L as a homopolar generator, with kinetic energy up to where β = 8 2Vo / R 6 × 10 joules, and later, an e.m.f. of800 volts. and Q joules is the initial energy of the rotors. With such a power supply as a starting point, various With the parameters chosen this represents a fairly air cored magnets were examined and the present system heavily damped oscillation in which the peak currentchosen of , mainly on the grounds of efficiency and relatively 6 1.6 × 10 amp is reached in 0.8 sec. (see fig. 2.). At thissimple theoretical analysis due to the basic circular section. point the magnetic energy is1 Li2 1 2 2 Q. It was at first proposed to incorporate strong When the current again passes through zero the rotors focusing; but this was abandoned, as the reduction in are rotating at just under half speed in the reverse direction aperture was offset by the space occupied by the focusing and have lost about 80% of their energy. It is proposed conductors, with a net gain not sufficient to justify the 346 Magnet problems

N is the number of turns. Each turn carries current ip around a full circuit on both sides. β is as noted on page 345.

-2β arcos β α = exp ( 2 is a measure of the useful energy √1-β )

Fig. 3. of the generator

1 Li 2 = αQ. i.e. 2 p complication. Also great doubt about tolerances on the The energy restored after one pulse is precision of the focussingfield existed at that time. 2 The copper conductor system has a section based on Q = exp{ - 2πβ/√(1-β )} two overlapping circles with the central common space free of copper forming the aperture. The current flows in Thus for β = 0, and hence α = 1 the energy loss is zero opposite directions through the two sides and the field is uniform throughout the aperture—in the case of circular β=1 corresponds to critical damping. sections and straight conductors, which is a sufficiently Values of β/√α and β√α are indicated in fig. 4. good approximation for general consideration of parameters. The inductance per unit length varies only slightly with a and d/a (about 20%) and much less than this in the range The five approximate equations below show the relationships 0 < d/a < 0.5. This variation has been ignored in the between important parameters. Numerical values above equation8 s as has also the error in M for small have been substituted for the three fixed quantities Q = 5 × 10 , 7 values of d/a. Further analysis must include other considerations V = 800 and Hρ = 4 × 10 ; where Q is o e.g. resistance, inductance and cost of busbar the rotational energy of the rotors in joules, V is the o leads and connections, size and cost of supporting structure, initial voltage and Hρ gauss cm. is the product of field forces, power costs etc. and radius i.e. a measure of the proton energy (11 Gev). Some sort of practical restriction applies to each variable a = 1.6 √r α (1) apart from those set by the equations above. The equations were set out in this form to show why a large value of β 0.25 was accepted. For instance, appreciable i = 9 × 109 √α r/dM • r/M (2) p reduction in β increases i and Z too much, unless M is

3 greatly increased. M cannot be increased without also Z = (6.25 × 10 )/β√α • r/M (3) increasing r (1st equation) if the aperture is not to be sacrificed. The value of βi s most easily varied by changing M = 16 r a (a-d) (4) N, but reduction of N leads to trouble in the conductor system as explained later. It was proposed to reduce N 8 N = (1.27 × 10 a)/ipr (5) to increase efficiency and peak energy if the machine were

-2 3/2 very successful, but it will almost certainly be impracticable or N = 2.2 × 10 β • M/r to do so.

where a, d and r are as shown infig. 3, in cm.

ip amp. is the peak current from the generator

Z sec-1 is the quantity ∂H (∂ t )t = 0 which corresponds Hρ max.

to the initial rate of rise of proton energy.

∂i This also equals ( ∂t)t = 0

ip

M c.c. is the volume of copper (approx.) Fig. 4. J. W. Blamey 347

Thefinally chose n values effect a compromise between cost of copper, large currents, rate of rise of field for R.F. injection, eddy currents and controls; and too low a value of N with many parallel conductors. The aperture must also be kept suitably large (the uncertain relation between aperture, Z and r is not included above). The aperture depends very much on the H ρ and Q if r is limited. The design proton energy was in fact reduced from 14 Gev to 11 Gev to allow a reasonably large aperture as well as to greatly lessen other difficulties such as forces, conductor sizes and eddy currents. The values chosen were approximately Fig. 6. Basic conductor section (eccentricity exaggerated).

a = 25 d = 12.5 (half aperture) Orbital Magnet r = 480

N = 4 An extensive magnetic field of 80,000 gauss is inevitably 6 ip = 1.6 × 10 accompanied by large forces and energy storage. The Z = 2.5 design adopted for the copper section minimizes these by virtue of its symmetrical nature, its dimensions and its P = 0.25 shape. About 27% of the magnetic energy is developed in the volume enclosed by the conductors, 28% within These values are not quite self consistent in this particular the copper and the rest outside. The magnetic field is set of equations. Insulation space within the copper maximum and almost uniform in the interior space and was allowed for by using a higher value than the actual falls through zero to relatively low negative values within one for resistivity of copper. Since preliminary design, the copper. It is possible to improve on this considerably, β has increased, as the resistance has increased for several for instance: by using a thinner section for the conductor, other reasons, without change of dimensions. by departing from uniform current densities, by the use of very eccentric ellipses instead of circles to give an aperture of better shape—a slight improvement only—or by more intense cooling to increase the conductivity and thus reduce the dimensions. For this magnet such complications are not warranted.

Important requirements are that the magnet be able to resist the forces without serious mechanical or field distortion, that it provide a field of the correct shape and that it be either insensitive to eddy currents induced by the high rate of rise offield, or capable of correction during a pulse. While the choice of parameters in the whole machine and adoption of a particular shape for the copper section achieves a great deal in these directions, it is still necessary to give the most careful attention to structural rigidity andfield shape in the further details of design, methods of construction, and accuracy of dimention for the many components. Flexibility infield correction is also necessary.

The magnet is shown in plan and section infig. 5. The four straight sections are used for the single radiofrequency induction accelerator, for injection, for targets and experimental ancillaries, and for pumping.

The copper conductor section is based on a property of ellipses. If the two full ellipses shown in Fig. 6. are sections of two straight conductors carrying opposite currents of the same uniform density, then in the region Fig. 5. common to both ellipses the magnetic field on the x axis is 348 Magnet problems

Hy = 2πj(s + x)(1 - δ) + 2πj(s - x)(1 + δ) = 4πjs(1-xδ/s) where j is the current density, 2s the distance between centres of ellipses, a(1 + δ) and a(1 - δ) the major and minor half axes, and x is measured from the mid-point. The resultant current density in the common region is zero, so that the copper there may be omitted, leaving a free aperture with a field of uniform gradient throughout. For circular section, δ= 0, and thefield is uniform. If instead of being straight the conductors are bent to form a ring of radius r = AC, the field due to circular sections is no longer uniform but has a gradient given by n = 0.25 where n = r/H • ∂H/∂r excluding second and Fig. 8. n value on median plane in a quadrant. higher orders of a/2r and x/2r. Similarly for ellipses, the gradient is changed so that n = 0.25 - r/s δ 8. Thus any desired small gradient can be obtained by appropriate The adopted arrangement of bars now gives afield of choice of δ. In this magnet r = 480, s = 12.5, δ= 0.0225 n = -0.55 in the median plane (fig. 8). Furthsr correction giving n = - 0.62. The value of n finally adopted was of this plane and also of field errors from other sources -0.55. (assembly, eddy currents etc.) is to be made by a large A true elliptical section was not suitable for fabrication number of small conductors disposed on a circle outside from the smaller elements necessary for construction and the vacuum box. The current in each, or in groups of reduction of eddy currents. Square section elements rather these, is to be separately controlled. than shaped sections were chosen for convenience and to ease other problems such as bending, production and The conductors are made from high conductivity phosphorus-de-oxidized assembly. The irregular contour resulted in field distortionelectrolytic copper (98.5% IACS) of which was partly corrected by re-arrangement of bar 1.150" square section hollow bar, extruded and drawn positions, following extensive numerical computation. half hard to lengths from 27 ft. to 32 ft. All these have been made well within the tolerance ± .001". The hole through the centre is for water cooling (0.25" ±0.05"). The bars are assembled as shown infig. 7 and the ellipses have been obtained by varying the insulation space between bars. Thus the outer ellipse has insulation between horizontal layers of 0.090" and vertical insulation between bars 0.030", with this arrangement reversed in the other ellipse. This form wastes conducting space and reduces the efficiency of the magnet considerably but was adopted rather than the employment of rectangular bars oriented accordingly on either side, because of the increased flexibility in design, and because the smaller cross-section just fitted the size of the extrusion ingots permitting the desired bar lengths to be obtained. The whole magnet has effectively four turns on each side and there are 128 bars on each side. The number of turns isfixed by the matching to the generator and the desired rate of rise of current, etc. as indicated in the section on choice of parameters. Each turn takes the full generator current of 1.6 × 106 amp. so that each bar takes 50,000 amp. The dimensions of the bars are a compromise between the complication of a great number of bars and the eddy currents induced in them. The reduction of eddy currents and their time constants to desirably small values requires insulated copper bars of less than one cm. square. But this means a very large number of connections, e.g. 1,250, at the end of each quadrant. In the accepted Fig. 7. Section through air-cored magnet. arrangement there are 128. The four quadrants are Fig. 9. Magnet Model, cross connection at closed end.

348a Fig. 10. Model. Input end connections.

Fig. 11. Foundation for Orbital Magnet.

348b J. W. Blamey 349

connected in parallel to the generator, and the top and permanent set or distortion of the copper, to certain particular bottom halves of each quadrant are also paralleled. This forms, occurs in sufficient magnitude to affect leaves in each half quadrant (upper or lower), 16 turns, conditions at injection. i.e. four circuits in parallel. As each turn has a different inductive coupling with the whole, each of these four The insulation around the copper is leatheroid or elephantide circuits of 16 turns has to be made up from selected bars in full length angle strips fitted around the corners, to give the same induced back-e.m.f. This has been done with over-lappingflat strip s and sheets. It is proposed theoretically to within ½% by computation of the coupling to vacuum impregnate the whole of the copper and insulation of each bar (neglecting end effects), and choice of a convenientwith a cold setting casting resin such as araldite to arrangement. But the effect on current distributionprovid e sounder insulation and greater rigidity. The is magnified in practice and difficult to calculate,dura i plates are also to be embedded in a sand-araldite so it will be necessary to ensure equal currents in the mix for rigidity; the outer silicon steel plates (see below parallel circuits by certain external arrangements, i.e. by andfig. 7 ) form a vacuum tight envelope for this purpose. running separate bus-bars suitably proportioned to give As this mixture has great compressive strength, it can be the best balance of resistance and back-e.m.f. It is also used tofill in the gaps between the copper and the dural. practicable in the bus-bar run to pass the parallel currents Thus only those plates necessary for location of the copper in pairs in opposition through a transformer of sufficient during assembly need be accurately machined—about flux capacity to prevent a substantial difference in currents. one in ten. Three such transformers are required to equalize the current in four parallel circuits. The number 64 was, An overall radial bursting force of about 4 tons/inch therefore, regarded as an upper limit in complication of of circumference also exists. It tends to increase the diameter end connection and coupling errors. The eddy currents of the orbit. In the case of four separate quadrants within each bar produce to afirst orde r afield of 68 gauss the radial force on each quadrant is only 11 tons but the at the centre and 131 gauss at the edge of the aperture, in force just mentioned still appears in one form or another opposition to the mainfield. A t the injectionfield of according to the restraints within a quadrant. It can 850 gauss this alters the gradient appreciably. The resultantappear, for instance, as a tension along the quadrant, and field shape also varies with time. One function of theas large forces (750 tons) on the end connecting bars. auxiliary conductors is to correct this error. These forces are taken by 1½% silicon steel plates (see fig. 7) which surround the copper and durai. Thick The end connections and the straights give an unknown austenitic stainless steel plates would have been more variation offield over a wide region. A full scale model suitable, but were rejected on the grounds of expense, of a half quadrant, i.e. 45° length of full section with end scarcity and production troubles. However, because of 3 connections as proposed, has been made using /8" square the symmetrical disposition of the silicon steel plates, their aluminium in place of the centre of each 1.15" square magnetic properties and lower resistivity are not as harmful copper bar, and measurements are being made of the field asfirst appears. The effect of the eddy currents in the distribution at the ends of the quadrant using 50 c.p.s. copper is, in fact, greater. It should perhaps have been alternating current. This does not give eddy current mentioned earlier that eddy current effects in the dural values. Figs. 9 and 10 are illustrations of the model showingtend to nullif y those in the copper. By suitable choice the complex end connections. of thickness, e.g. 2", a very favourable first order balance The interaction of the magneticfield an d the current appears. The exact nature of the balance was, however, gives rise to large forces. The copper sections on either too problematical to risk the use of the thick plates. The side of the aperture repel each other with a force of 40 tons/silico n steel enclosure is to be prestressed i.e. the end inch of circumference. The copper is, therefore supported connections are to be pressed inwards by tension in these by ½" thick duralumin plates, of the shape shown in plates acting on end clamping plates. fig. 7. These plates are in radial vertical planes, and adjacent around the circumference, there being 2000 in The four quadrants are seated on a rigid annular concrete the upper half and 2000 in the lower half. The stresses foundation, and are held by bolts and grouting to in the copper and in the duralumin depend on the accuracy this. The whole is within a wooden rotunda-like structure and rigidity of construction. The computed maximum 60 ft. in diameter with a wooden floor. Fig. 11 shows stresses in the copper for full support are 2 tons/sq. in. this in the process of construction. vertical compression and 4 tons/sq. in. horizontal compression. This ignores stress concentrations and unpredictableTh e vacuum box is a stainless steel tube of 0.080" local bearing loads, which the methods of support (14 s.w.g.) wall thickness and 8.5" internal diameter opening and assembly tend to minimise. The copper has a yield out into larger straight sections. The eddy currents produce point of over 7 tons/sq. in. and ultimate strength of a field of 40 gauss with changes in gradient of δn 0.03 22 tons/sq. in. at injection with a negligibly small time constant so that errors infield due to this are easily dealt with and are small Deflection of the durai plates under the load is appreciablecompare d with the copper errors. It appears that the but not important in its effect on thefield a s long as no tube will have to be fabricated from short sections. The 350 Magnet problems welds if properly disposed have a negligibly small effect. The tube has to be made to fine tolerances. The copper is cooled with water circulating through a heat exchanger, cooled by an evaporative cooling tower used for other purposes as well. The temperature rise per pulse is 25 °C assuming no recovery of energy and the R.M.S. power is about 800 kW. Connection to the generator about 120 ft. away is by aluminium bus-bar, with part of the run in copper if necessary, where space must be conserved, e.g. near the generator and in equalizing transformers if these are employed. The resistance of the bus-bars represent 5% of the total resistance and the end connections in the aircored magnet 15%.

Homopolar generator Fig. 12. Generator field magnet. General

The type and much of the design and details of constructionField magnet of the homopolar generator were set by the decision to use our large cyclotron magnet and pole tips The magnetic field of 16,000 gauss is supplied by the for this purpose. With a free choice possible, itmagne is probablet shown in fig. 12. It is a conventional cyclotron that many of the problems that have arisen couldmagne t except that it has no bolts apart from 12 used be avoided. Pursuit of such a line of thought is without during construction and left in place. The yokes are profit to us at the moment, but it is undoubtedly of greabuilt t from laminations of thickness dictated by ingot importance for future development. In this paper, size in the Australian rolling mills (7 tons). Each pole therefore, we shall deal with the problems associated with piece is also a fabricated welded structure 148" diameter, our generator without critical assessment. Many of the and 40" deep with a large recess in it for the bearings of the problems are too complex for analysis and the policy rotor. The horizontal yokes have central 8" square holes accepted is that the magnitude and solution of many of for the shafts. The magnet was completed in 1954. the difficulties would be best realized during and after actual construction and operation. It has been found that An interesting feature of the magnet is that the pole attempts at full theoretical and experimental investigation gap does not vary in length by more than 0.002" when the such as construction of models are not sufficiently realisticfield is switched on (to 16,000 gauss). Normal construction or are in themselves major projects. On the whole it would show a gap change of 0.050" due to bending seems almost as efficient and far less costly to treat the of the horizontal yokes. This has been achieved by full size machine itself as experimental, with speed the balancing the force in the gap against the force between essence of the contract. It is, however, difficult to refraithn e pole piece and a central portion of the yoke which from spending much time trying to avoid possible but moves .030" relative to the pole piece, and to the rest of the uncertain defects and changing the design as emphasis yoke, without touching the pole piece. on certain points necessitates re-assessment of the relative The number of coils in the magnet has been reduced importance of the various difficulties. by 25% to allow more room for the homopolar generator attachments. It was originally designed for 15,000 gauss The main features of design were fixed for us by considerations with air gaps of 4" and 14" at 330 kW with 1000 kW already mentioned. Dimensions and safe available, but now gives 16,000 gauss in gaps 8", 6", 8" speed of rotation set upper limits to the energy and voltage at 16,000 gauss, at about 600 kW with 750 kW available. per rotor disc. Efficiency and reduction in number of With the 62" gap the field is 7,500 gauss. Further parallel circuits on the orbital magnet made it advantageous information is tabulated in the appendix. to run the rotors in series to obtain maximum voltage. This also influenced the arrangement of rotors within the gap and the location of the bearings. The maximum Rotors current is limited by forces and thermal effects. The rotors rotate in opposite directions for balanceFou ofr momentum mild steel discs, 139" in diameter and 10" thick, and suppresion of some forces. This, and problemsfor m the two rotors. These were originally intended to of assembly, space and rigidity limited the choice in bearingbe pole tips for the cyclotron and were forged from two design. The general arrangement is shown in fig. 12. 120 ton ingots for homogeneity. J. W. Blamey 351

at radii 24" and 69.5". The voltages and interconnections are as shown. With counter rotation, the resultant angular momentum is zero; also direct brush connection between the two rotors near the centre is possible. However as in this particular arrangement, current flows through conductors attached to the pole pieces, the main magnetic field acting on these currents exerts opposite torques of 1,250 foot-tons on the pole pieces. This is taken by shear in the magnet frame and the resultant on the whole magnet is zero. With 400 volt output, these torques can be eliminated. However they present little difficulty. If all four discs rotated in the same direction, the resultant unbalanced torque on the magnet would be 5000 foottons. With some strengthening the magnet could accept this, but the foundations would need to be closely examined. For an air cored magnet of lower voltage and current requirements (but same energy) such an arrangement is the most attractive as a symmetrical bearing is possible and most insulation difficulties are avoided.

The rotors experience a variety of forces. Being magnetic they are attracted to the pole pieces and each other. The gaps are chosen so that these vertical forces are balanced to within 40 tons. A displacement of 1" increases the force by 80 tons, this information being estimated from a 1/28 scale model. This balance can be destroyed by the pulse current due to saturation of the steel of the current flows through the steel itself. The rotors are also unstable against tilt. If the rotors are tilted by 0.001 radians, i.e. 0.070" from centre to edge a torque tending to increase the tilt, of 3 × 106 inchlb. is developed. It is proportional to the tilt. This value corresponds to 70 tons at the guide bearings. The theoretical value of the torque Fig. 13. NaK brushes, rotors and bearings. B2 R4 Φ dyne cm where B gauss is the magnetic field in 8d air gaps of length d cm. on each side of a rotor, R the effective radius of the rotor in cm (allowing 0.4 d for The rotors are arranged as infig. 13. The upper pair fringing field), and Φ radian the angle of tilt, was recently of discs are attached to but insulated from each other, and confirmed in a model test, for values of B up to 7000 gauss, also secured to a large shaft coupling which is part of the to within the limits of experimental error (less than 10%). double guide bearing in the upper pole piece. This unit The possible tilt is set by clearances in the guide bearings is supported by a shaft and thrust bearing above the magnet. and errors in construction. Bending of the coupling and The other unit is similarly supported from below. With the presence of the second rotor must also be taken into counter rotation and limited space such an arrangement account. The former adds 30% to the tilt and the latter is virtually necessary. effectively halves the value of d in the intermediate gap. At 900 r.p.m. each disc contributes 200 volts between The quotedfigures include the second correction. brushes. Rotational stresses are 15,000 lb/sq. in. with Under the pulse current of 1.6 × 106 amps the rotors concentration to 30,000 lb/sq.in . around the central bolt repel each other with a force of 27 tons and are decelerated hole, or more around non-central holes. Additional and accelerated by the peak torque of 2,500 foot stresses are produced by thermal gradients, bolt pressures tons about the axis of rotation. Thefirst forc e is balanced and several other forces as mentioned later. Methods of automatically by the rotor and current arrangement, and securing the rotors to each other and the coupling are at the second results in only small stresses of the order of present being reconsidered. Earlier, twelve bolts were 200 lb/sq. in. The rotors brake from 900 r.p.m. to zero to perform this function. Consideration is being given in 0.8 sec. (and then accelerate in opposite direction) to the use of one large central bolt and the bonding of the so the inertia of unbraked portions, e.g. the shaft and rotors with araldite. bearings, result in shear stresses which are not great but The brushes are located as shown by the arrows in fig. 13,requir e transmission through keys etc. If the current 5 being continuous jets of liquid metal around circumferences goes accidentally to 5 × 10 amp. which is unlikely with- 352 Magnet problems

out arcing damage the repulsion force would be 250 tons accelerating current, mercury is the most efficient. It is plus other forces. The system is designed to take about possibly better for the main pulse also, but has been excluded 400 tons repulsion and torque corresponding to this mainly on the grounds of expense and health hazards. current, as an abnormal load. Although mercury has ten times the specific resistivity of sodium, the heating and power loss is not as important The resistance of the four rotors in series is negligibly -7 a factor as deflecting forces and eddy current losses. The small (6 × 10 ohm); but as the material is magnetic the greater density of mercury means a reduction in volume current tends to flow along the surface—penetrating flow to one quarter that for Na and simpler drainage, and almost completely by peak current. Thus local heating the high resistance still further reduces impedance to occurs, especially near the inner jet. The current produces a flow in a magnetic field. Its cost would be over 100 times field of 5000 gauss in air so that the steel becomes saturatedthat. of a sodium. It was used in a model homopolar generator This has some effect on the mainfield an d affects the which gave 150,000 amps. balance of forces in the rotor, although time constants are large enough to reduce the effect considerably. For It should be realized that the suitability of any one of this and other reasons the rotors will have copper sheaths these materials cannot be fully assessed until the limiting if one can feel confident about the method of attachment. factors in operation are more accurately known. Efficiency At present it is not proposed tofit them. At the line of in particular aspects is meaningless if break-down occurs entry of current into the rotors current densities are very for some other reason, e.g. excessive local heating, formation high in a very small region and dangerously large tem­ of emulsion, unreliable operation, insulation failure, peratures (over 100 °C) leading to fatigue failure can occur.excessiv e and difficult maintenance etc. We could not Copper inserts at these places may be advisible. They justify the expense and health risk of mercury while would also be replaceable after arc damage. sodium or NaK has a chance of success. Sodium although very cheap requires temperatures of over 100 °C. The Windage losses at full speed are thought to be about extra cost and difficulties associated with operation at this 200 H.P. temperature are great and justify the use of the more The rotors are to be balanced statically and dynamically expensive NaK alloy—at least for a first machine such as on an oil pressure bearing of the zero friction type. this. Apart from the reduction in operating temperature it is problematical which is better, Na or NaK. It is When rotating, the rotors if tilted can produce eddy believed that Na is the better because it is less reactive and currents which tend to dampen out the tilt and hold the has higher density, specific heat and conductivity. The rotors stable at the expense of acceleration. The orders last is not necessarily an advantage, and could in fact of magnitude indicate that this, if it occurs, will take placeb e a serious disadvantage. Therefore, after further at low speed (i.e. < 1 r.p.m.). Model tests to show this investigation and a few simple tests which showed the have failed constructionally but only weak attempts have alloy to be not as dangerous as we had previously believed, been made. The bearings are designed to take the load we recently decided to change over to the alloy (Na 50%, due to tilt. K 50%, melting point 11°C). It is still necessary, of course, to operate in a sealed atmosphere of an inert gas—in Brush system our case, nitrogen.

The four discs, being in series for an output pulse, The four inner jetsflow vertically, i.e. parallel to the must be connected to each other and to the output terminalsfield of 16,000 gauss in a continuous cylindrical sheet at eight places as shown in fig. 13. Each connection 1 mm. thick and 2 ft. in radius, at a peak velocity of is between a moving and a stationary surface and is required 2000 cm/sec across a gap of 2.5 mm. The four outer jets to take the peak current of 1.6 × 106 amps. At 900 r.p.m. flow horizontally in the fringing field which is essentially the rotor is moving at 12,000 ft/min. at the inner contact horizontally and up to 4000 gauss according to position. and 35,000 ft/min. at the outer contact. The high current They thus form annular sheets ½ mm. thick across a densities preclude the use of solid brushes, so a liquid metal4 mm. gap at a velocity 1200 cm/sec. The corresponding is employed. The brush contacts take the form of high pressures are 25 lbs/sq. in. and 9 lbs/sq. in. with a supply speed jets because, at least with this form of homopolar pressure of up to 100 lbs/sq. in. the radius of the inner jets generator, we were unable to conceive a design employing is a compromise between loss of voltage, and current stationary pools or slow feeds to a sealed or partially density, and magnetic fields. The jets are in the form of sealed system, which was anywhere near satisfactory from sheets rather than many small solid cylinders for simplicity, the point of view of forces, heating, eddy currents, voltage lateral stability, smoothflow an d reduction of forces and gradients on the surface of the rotor, and assembly. These deflections, and better throw off (less spray). problems are serious even with jets, and their solution The jets are acted upon by many forces. These are too requires high velocities thus introducing great difficulties complex to analyse and a full account of them is too involved with supplies, drainage and insulation. to present here. They arise from the interaction Sodium, sodium-potassium alloy and mercury have been of three types of currents—main pulse current, eddy considered for this purpose. For the jets used to pass the currents, and currents due to the voltage gradient on the J. W. Blamey 353

rotor surface—with three types of magnetic field—the on the inside surface. Earlier experiments with such mainfield, th e field of the pulse current and the fields of thepipe s showed contrary to theoretical expectation that eddy currents. with mercury the induced currents and back pressure even at 25,000 gauss were negligible. However, the Any movement of the sodium transverse to a magnetic flows now are so great and the pipes of such a large diameter field produces eddy currents. The sodium must move that the experiment must be repeated with sodium transversely to all of thesefields at some stage of entry, more carefully. This is now under way. For some parts transit and throw off. It then experiences forces and of theflow insulation is of no value and here the velocity accelerations and new high velocities which result in a new must be kept low enough to reduce back pressure to system of current,fields etc . The result is a redistribution allowable values. For example, where the NaK flows of velocities and current densities within the jet. across the distribution ring behind the jets, a back pressure 2 -4 2 In general the secondary and induced effects may be 0.25 B u × 10 dynes/cm /cm is developed. If u the made small by decreasing the thickness of the jet. This radial velocity is 100 cm/sec (corresponding to 2 cm. depth of channel) and B = 1.6 × 104, the back pressure increases resistance to eddy currents and the jet behaves 2 more predictably against the primary forces. The main is 0.6 kgm/cm for each cm. of the radial travel (23 lb/sq. force is the reaction of the main current (1.6 × 106 amp.) in/inch). to its own magneticfield (5000 gauss at the centre jets). It is desirable to seal the jets with valves near the orifice The jet is deflected inwards (towards the centre of the and to use these to initiate theflow for reproducible starting machine). In order for the jet to cross the gap it should conditions, smooth flow and good sealing when the jet have a velocity v cm/sec of the order is off. No suitable simple valve could be designed. As i a compromise it was decided to have valves at the supply V = √l,t tanks and low pressure jet valves which would open under 10 r√2ΠP low sodium pressure when the tank valves were opened. Thus the jets could be sealed and the pipe lines filled before where i amp is the current,l cm . the length of the jet, t cma. pulse . With sodium at 100°Cflexible rubber diaphragm its thickness, r cm. the radius and the density of the liquid valves were not considered strong enough and more solid metal (0.9 gm/cm3). spring-loaded valves (with rubber facings) were designed. With the NaK alloy rubber diaphragms may again be It is seen that as the jet is reduced in thickness thefeasible velocity. At present, however, jet valves have been omitted must be increased but the flow 2 π rtv is reduced. It altogether. is desirable for the flow to be as low as possible. A lower limit to the thickness of the jet is set by production tole-tances,On leaving the jet the liquid metal isflung off the rotors the risk of blockage, heating effects, pressure supplydirec t into large trays for the outside jet. For the inner and presumably wear. ½ mm. is considered the smallest jet the sodium is deflected partly into trays between the practicable jet. It is possible that in practice some of the rotors, while much must travel across the face of the rotor secondary effects are favourable and that a thicker jet may which is coated with an insulating film to ease drainage. be used without having to increase theflow whic h is already Effective disposal of the NaK after it leaves the jet is undesirably great. It is unlikely that at ½ mm. thickness difficult without introducing complexity of structure and the secondary effects are deleterious in this respect. They assembly. The present arrangements are rather optimistic may, however, have other serious effects such as limiting and must await test under operating conditions. theflow into the jet and interfering with throw off and drainage. Resources for good experimental investigation The sodiumflows fro m each tray into a sump through of these effects are not available and the generator itself a multidrop fall at a controlled rate. It has been found by has to be regarded as an experimental tool for this. experiments with sodium that such a device provides insulation between sump and drain pipes, atflows o f The temperature rise in the jet is not great (about 20 gallons per minute, which is more than required. 20°C). With NaK the rate of rise is 60,000°C per sec. Similarly, sodium is pumped by a centrifugal pump into the but the high velocity counteracts this. Experiments have pressure tanks which have pressurized insulating drops shown that contact resistance is small compared with jet above them. resistance for cylindrical jets on steel. The current density in the steel of the rotor is, however, very high and temperatureThe curren t passes from the sodium to the copper jet rises of 100°C just below the surface of the steel structure at the orifice and thence through copper rods are possible. The surface of the steel is cooled by the jet. and plates to the outside of the structure where external For this reason a copper insert may be used, or the contact connections are made. surface raised to reduce stresses and fatigue effects. The framework supporting the jets and bus bars and The liquid metal is supplied to the jets through pipes supply lines was to be of stainless steel but it is now of from 16 external pressure tanks. As these pass through mild steel, with non-magnetic stainless steel between rotors the magnetic field to the inner jet they must be insulated and near the rotors. Seals are by welding and rubber 354 Magnet problems

gaskets. Synthetic rubber is moderately satisfactory when carry a current i amp where air is excluded, and so far silicone rubber has shown no signs of deterioration other than some permanent set in, 2 for instance, Saunders valves, where other rubbers had i 60 va /l failed. General experience has been gained with sodium in a system of two 18c.ft .tank s with a nitrogen compressor If v is replaced by √2p/ρ where p dyne/cm2 is pressure as the driving force. An Australian manufacturer is at the jet and ρ gm/cm3 is the density, the same result supplying a sodium pump to deliver 120 gall/min at 100 appears to hold for mercury. This current is the one at lb/sq in. A pump made up in the laboratory has not yet which breaks in current occur. Breaks at the rate of been tested with sodium. 1/sec. appear to do no damage. The mechanism by which a break occurs is uncertain. Equating the pinch pressure The framework consists of a number offlat units about of the magnetic field i2/200 πa2, (or rather the resultant 16 ft. in diameter and 5 inches high—seefig. 14. Four hydrostatic pressure i2/100 πa2, at the centre of the jet) units carry the outer jets and trays, four units carry another to ½ pv2 where v is the velocity for zero current gives the set of trays for the outer jets, and three other units similarly correct current for a jet of length two to three times the serve the three sets of inner jets. Some of these must diameter. A long thin jet was observed to pinch in the be in place before the rotors are bolted to the coupling, middle with very little change in resistance, but as always, but nine of the units can be extracted through the space the break or incandescence occurred at the electrode between the rotors (after lowering or raising the units). opposite the orifice. It is probable that a severe pinch Each unit is continuous around the circumference, i.e. arising from instability and wave motion occurs suddenly without gasketed joints in vertical planes. Each unit is at this electrode with the final break by sudden boiling. sealed from the outside atmosphere. Circumferential The resistance of the jet varies only slightly with the current gaskets are necessary to seal the small space between units. before break, and the breaking current with mercury is The whole is bolted together and to the pole pieces, preserving about the same as that for Na (for the same pressure). insulation between units which are at five different potentials from - 400 to 400 volts. It is necessary to cool That is, mechanical movements are more significant the structure. Arrangements for this are as yet uncertain. than electrical resistance. The experiments were not sufficiently precise or extended in range to establish Acceleration of rotors relationships or laws, for detailed accurate analysis. Ranges were ½ mm. to 1½ mm. in jet diameter, 100 to The rotors are accelerated by passing through them a 1000 amp. in current, 2 mm. to 10 mm. in length and current of 3,300 amp from a grid-controlled mercury velocities 500 to 3000 cm/sec. The results agreed fortuitously arc rectifier, which consists of 4 steel tanks in parallel, andwell with design values accepted on a cruder provides up to 480 volts (1650 kW). The output voltage theoretical basis. of the rectifier is automatically adjusted with the back voltage of the rotors, i.e. the speed, to give an approximatelyThe experiment s showed that the flow required for a constant accelerating current. given current is independent of the diameter of the jet. There is, therefore, no loss or gain in having more than one Eight jets are required if the rectifier (designed for anotherje t at the same radial position on a rotor. It is proposed purpose) is to be used with maximum efficiency. Each to use several jets for symmetry in torque and reduction of these jets will consist of at least two, probably four or in arc voltage in the event of one jet failing. In no experiment six, in parallel. Experiments have shown that a sodium jet so far has arc damage at a jet orifice been observed, of velocity v cm/sec radius a cm. and length l cm. will even with 105 amp. The surface struck by the jet has been grooved out in some cases by deliberately induced arcing. It was also established that contact resistance on surfaces either stationary or moving at high speed (30,000 ft/min) was small—less than the resistance of the jet itself.

So far little work has been done on the accelerating jet system, and the number and disposition of these jets has yet to be decided. They may, for instance, be located at or away from the main jets. The latter is more desirable but leads to more insulation and potential difference troubles. The inner jets nearest the pole pieces may be omitted, and the current passed into the rotor through the shafts, via mercury cup brushes or solid brushes at the outer ends of the shafts. The time required to accelerate Fig. 14. Diagrammatic section of jet structure around rotor. the rotors from zero to full speed is about 12 minutes. J. W. Blamey 355

External sodium system shallow pockets in each, from which it can escape only through the bearing clearance. Thus the pressure depends The use of sodium or sodium potassium alloy requires on the inverse cube of the clearance which adjusts itself special techniques which have now been developed in many to take the load. The pumps allow pressures of up to countries. It is, of course, necessary to have a first-hand500 0 lb/sq. in., and the normal operating pressure is acquaintance with these and work on a small scale was 1500 lb/sq. in, for a 150 ton load and corresponding begun several years ago. A larger sodium system circulatingclearance of 0.004". Sudden loads may compress the oil about one ton of sodium has now been built in a to higher pressures than above as a non-return valve is room kept at 105°C and is used for testing techniques, fitted to each pocket. The flow to each pocket is also items of equipment, and materials, and for experiments. restricted somewhat for stability. The frictional horsepower Our problems are much simpler in many respects than those at full speed is 30 for 150 ton load (effective co-efficiency normally encountered as we need to operate only at low friction 0.005) and the pump horsepower is temperature. On the other hand we have much greater 20 for each bearing. complication with pulse operation and pipe network and five different electrical potentials. We have only Although this type of bearing is excellent, it is not used just ordered a supply of sodium-potassium alloy. The much in industry because of the danger of oil supply failure. tests mentioned earlier were on samples prepared in the Many precautions have been taken to guard against laboratory. this, including the use of two motors each operating two pumps. One motor is operated from a diesel generator The liquid metal for the jet system is pumped from a set and the other from the ordinary mains. Either one common sump, and through the insultating drops, into of these can carry the whole load in the event of failure of 16 pressure tanks each about 12 cu. ft. in volume. Two the other. The bearings are mounted on a large ball joint ; of these, half filled, and at a nitrogen pressure of up to also two other lightly loaded bearing surfaces on the shaft 100 lbs/sq. in. supply each jet, through a system of large exert some control on stability. The bearings are at a pipes and distribution rings. The jets are operated by potential of about 200 volts to earth. opening all 16 tank valves simultaneously or nearly so. The air cored magnet external switch is then closed, if Other types of bearing were considered, but this type allowed by interlocks. It is not necessary for the jets to be is both the simplest to make in the laboratory and the most running at full speed before this happens, as the speed suitable in view of the loads and variable speeds from zero required is proportional to the current and both rise to full speed in either direction. linearly with time. The velocity of the sodium is allowed to fall off after peak current, to conserve sodium. Guide bearings After a pulse, the sodium drains slowly to a sump through insulating drops, and is pumped in the next 5 to 8 minutes The guide bearings in the pole piece presented severe back to the pressure tanks, which do not completely empty difficulties. These bearings control the position and tilt during a pulse. The quantity of nitrogen in a tank may of the rotor. Both bearings on one si de of a rotor instead be kept constant, or some may be released during a pulse of one on each side is, of course, most unattractive, but to lower the velocity of the sodium, according to our was accepted for reasons mentioned earlier. The distance findings on operation characteristics. A nitrogen compressorbetween the tw o bearings is also limited and the room for and 500 cu. ft. low pressure reservoir maintain them is very small and access is poor. They must be pressure and supplies as necessary. The reservoir which insulated (200 volts) from the pole piece. The load to be is a variable volume gas holder, establishes a constant low taken by each is 10 tons for each 0.001" error in alignment base-pressure for the whole system—slightly above atmosphericand each 0.001 " of radial clearance, as well as large loads pressure. Other high pressure nitrogen and air due to static and dynamic unbalance. Thermal expansion supplies are to be used for pneumatically operated valves of the coupling must also be taken into account—with and possibly the air cored magnet switch. sodium the temperature of the coupling would be over 100°C while the pole piece would be at 30°C. It would be difficult to predict or control temperatures and tolerances Thrust bearings well enough to prevent seizure and in any case it is desirable to run the system cold for test purposes. Rotating The two thrust bearings carry a normal load of up to loads of 150 tons could well occur. The use of sodium-potassium 80 tons but are designed to carry 150 tons continuously alloy reduces the severity of this problem considerably, and to deal with short duration accidental loads of up to but does not eliminate all thermal expansion 400 tons. The load can be taken in either direction—up troubles and tolerances. or down. Each shaft has a collar, 21" in diameter and 7" thick, the upper and lower annular faces of which are As mentioned earlier induced currents tends to make the the rotating bearing surfaces. The fixed bearing faces rotor run true and to damp out oscillations. It is expected, above and below this allow a total vertical clearance of therefore, that if free to do so, the rotor when moving 0.020". Oil is supplied from pumps at a constant flow will assume some stable position or perhaps precess slowly. of 15 gall/min through eachfixed bearin g block to four If this is so, it would be desirable for the bearings 356 Magnet problems

to beflexibly mounted so that they may align themselves sweep is 1 mc/sec, to 9 mc/sec and the required peak voltage suitably. This would also allow relaxation in tolerances 4 kV. The system is capable of developing 8 kV. and leave room for thermal expansion and centrifugal Accelerating voltage is produced in one tuned ferrite loaded unbalance. At the same time, if this natural self-alignmentcavity. Th e ferrite (Stemag type 03186) is in the form of does not occur a positive restoring force proportionalsquar e frames, fabricated from bricks of the one size to displacement is necessary with a stiffness sufficient to (36 cm. × 10 cm. × 2 cm.). During a pulse the cavity overcome the tilt forces. It appears, however, that space will probably be turned coarsely by a variable condenser is too limited to allow flexible supports which are sufficientlyand more accuratel y by saturation of the ferrite. The stiff without distortion and possible seizure. A radiofrequency power is 8 kW, but 20 kW is to be bearing which seems to overcome all these objections provided for safety. has been designed but on account of its somewhat complicated nature and the possibility of failure in some componentsProgres s to March 1956 or something overlooked in its analysis it is at first to be tried in a modified semi-rigid form. Essentially The design of the air cored magnet has been completed it is similar in principle to the thrust bearings, but it has except for ancillary equipment such as current control, six pads for each bearing. The .003" (nominal) gap busbars, heat exchanger, vacuum system etc. Techniques between each pad and the coupling is supplied with oil of assembly are being further studied. The copper is on at constantflow fro m a pump, i.e. 24 pumps in all. hand as finished straight bars, the duralumin as rectangular Each pad is either a piston or a diaphragm which may be plates. Satisfactory quotations for fine machining (after pushed towards the coupling by oil pressure from behind. sawing to shape here) have been obtained. The model is It is possible to automatically control the volume of oil in ready for testing. Foundations have been installed and this space with the pressures in the oil between pad and the frame of the wooden building erected. coupling as an input signal, through what is in effect a One might say, without disparagement, that the design of simple external servo-mechanism. The whole system the homopolar generator has been completed several locates the shaft coupling in the position which gives times—one might also say that completion of construction minimum pressures or variation in pressure around the and tests must precede completion of design. The most pads, i.e. it gradually locates the rotors in the position of intricate part—framework and jet structure—has been zero or near zero tilt. It can still deal with large forces revised several times. The design of the thrust bearings and impulsive forces. It is proposed at first to do without and associated equipment has been completed ; construction the automatic control and to locate the pads manually and assembly are well advanced. The full design of the during tests. It may be found for instance that they can be bearings is almost finished. The field magnet and all its permanently set in some optimum position, and that further ancillaries were completed some time ago. Rotors and control is unnecessary. Such a system allows relaxation couplings are partly machined. The rectifier and some of tolerances, and adjustment of bearing parameters—clearances,controls have bee n tested. The full layout of the sodium pressures and flows, and location of shaft to system has yet to be drawn up. give minimum waste of power and bearing loads. Operating characteristics are given in the appendix. The injection cyclotron is under test. The main features of the R.F. system have been worked out. Injection system Personnel An account of this is not required in this paper. Injection from a cyclotron though not recognized as ideal, is The whole project is confined to part of the Research by far the cheapest method of obtaining a proton beam of School of Physical Sciences which has many other interests sufficient energy. We already had a small cyclotron as well. We are unable to contract out any work of an magnet—built in 1951, and used as a model homopolar investigation or experimental nature. Much of the heavy generator, and intended subsequently as a neutron source. machining on the field magnet was done by Naval Dockyards, It is now in operation (under test) as an 8 Mev proton and the duralumin plates are to be machined elsewhere. cyclotron. Special attention has been paid in the design Otherwise practically all work is done in the laboratory, to the production of a high beam intensity with a minimum except of course for purchases of units such as of radial oscillation. It is as yet too soon to say whether rectifiers and motor-generator sets. We have had valuable this has been achieved. The extracted beam is to be co-operation from firms in Australia and the U.K. in focused and electrostatically inflected into the orbital supplying in special form steel, copper, aluminium and magnet. duralumin, sometimes well outside normal production.

Radio frequency system The project was conceived and has been closely directed by Professor M. L. Oliphant. The other members of the An account of this is also not required, but a few notes team in order of date of joining, with their chief concern are given below for interest. The required frequency in the project are :— J. W. Blamey 357

Mr. J. W. Blamey — Homopolar generator and initial Cross section of each bar 1.150 in square design studies on air-cored magnet. Peak current per bar 50,000 amp. Mr. D. B. Shenton, Engineer — Homopolar generator Number of separate parallel circuits especially bearings and framework, but also mostin magnet 32 electrical and mechanical engineering, and draughtingPeak magneti c bursting force between office. two sides 40 tons per in. length Peak tension in quadrant constraints 750 tons Dr. W. I. B. Smith — Injection and cyclotron. Equiv. radial bursting force 4 tons per in. length Mr. H. Morton — Ph. D. student assisting Smith. Dr. L. U. Hibbard (1954) — Aircored magnet. Supports:— Dr. D. Robertson (1955) R. F. system, succeeding the late Number of duralumin plates 4,000 Dr. R. S. Wilson — R. F. and design studies. Dimensions — length (radial) 42" Mr. P. Carden (1955) assisting Shenton, guide bearings and height 24" rectifier control system. thickness ½" Dr. E. K. Inall (1955) — Sodium System. Weight of duralumin 90 tons In addition there arefive technical officers assisting with experiments and carrying out construction, assembly Envelope:— etc., two design draughtsmen, and a computer; also a well Width of horizontal silicon steel equipped machine shop shared with the rest of the laboratory.plates 44 inches Nearly everything, including negotiation with Height of vertical silicon steel plates 55 inches manufacturers and suppliers, inspection, etc., has been 5 Thickness of plates /8 inches done by the people named above. Vacum box (circular section) 22 cm. wall thickness 0.080" Conclusion material Stainless Steel

An account of the main considerations in the design of the air cored magnet and homopolar generator has been Homopolar- Generator given. Whilst difficult problems have yet to be solved and some solutions accepted are doubtful, no obviously insuperableEnergy storage 5 × 108 joules problem has yet arisen. E.m.f. 800 volts Peak current 1.6 × 106 amp. Equivalent capacitance 1700 farad A.N.U. 10 Gev PROTON SYNCHROTRON - DATA Number of rotors 2 Number of discs per rotor 2 Orbital magnet Rotor speed (max) 900 r.p.m. Beam energy 10.6 Gev Disc dimensions — diameter 139" Peak magnet field 80 kilogauss thickness 10" Time of rise to peak field 0.8 sec. weight 19 tons Initial rate of rise of field 2 × 10 gauss/sec material mild steel Pulse repetition rate, once in 10 min. Radius of orbit 480 cm. Field Magnet Useful aperture (circular) 22 cm. Length of each straight section 250 cm. Length 372" Number of turns 4 Width 169" Peak current (per turn) 1.6 × 106 amp. Height 252" 3 3 Inductance 2 × 10-4 henry Yoke laminations 55" × 2 /0", 2 /4 and 53" × 4" Resistance 2.2 × 10-4 ohm Pole diameter 148" Pole depth 40" Weight 1400 tons Conductors:— Gap length 62" Cross section (overlapping ellipses) 25 cm. mean radius Total air gap with rotors in 22" Distance between centres of ellipses 25 cm. Magneticfield (operatin g value) 16 kilogauss Separation in median plane 25 cm. Coils — Number of turns 540 Ratio of major to minor axis 1.045 Max. current 2000 amp. Weight of copper 77 tons Resistance 0.16 ohm. Number of square bars in cross Inductance 9 henries section 256 Weight of aluminium 34 tons 358 Magnet problems

Section of aluminium 1.68" square Total number of pumps 24 Length of alum. in double pancake 2300 ft. Total oilflow (nominal) 60 gall/min Bore 0.9" Nominal oilfilm thickness 0.003" Water flow 90 gall/min Radial adjustment of pads 0.010" Heat exchanger capacity 1000 kW Diam. of large bearing 18" Evaporation forced draught cooler Diam. small bearing 13" capacity 2000 kW Centre separation 20" Overall coil diameter 20 ft. Power supply (M.G. set) 1000 kW, 480 V Pulse jets Diam. of bearing recess 17" and 31" Number 8 Shaft hole in yoke 8" square Liquid metal Na-K 50-50 Height above floor of pit 7' Inner jets — radius 24" Magnet seat steel bars each 4" × 2" × 15 ft. thickness 1 mm. Magnet seat on another bar each 18" × 5" × 15 ft. length (vertical) 2.5 mm. Magnet seat on concrete base each 11 ft. × 18 ft. peak velocity 2000 cm/sec Air gap between rotors 6" peak flow 75 litre/sec Air gaps between rotors and poles 7.6" Outer jets — radius 69.5" thickness 0.5 mm. Thrust bearings length (horizontal) 4 mm. peak velocity 1200 cm/sec Allowed normal load (eitherdirection) 150 tons peak flow 66 litre/sec Allowed max. load 400 tons Supply pressure (max.) 100 lb/sq in. Expected steady load (field on or Number of tanks per jet 2 off) 40 40 tons Volume of tanks (18" diam., 5 ft. high) each (includes large reserve Type—constant oil flow, hydrostatic for tests) 8 c. ft. pressure self adjusting Bearing face—outer diameter of annulus 21" Accelerating jets (details not fixed) Total oil flow (4 bearing faces) 60 gall/min Current 3,300 amp. Oil inlet pressure (150 ton load) 1850 lb/sq in. Rectifier up to 480 volts Oilfilm thickness 0.004" Typical diameter 2 mm. Bearing friction total 60 h.p. Typical velocity 1500 cm/sec Total number of pumps 4 Typical number for one bridge 4 Max. pump pressure 5000 lb/sq in. Typical flow through 4 jets (one Pump horse power (total) (150 bridge) 0.2 litres/sec ton load) 40 h.p. Typical nominal current capacity for any one of these 4 1800 amp. Guide bearings (vertical journal) Max. power available for acceleration Load per 0.001 radians tilt of of rotors 1600 KW rotor coupling 90 tons Allowed max. load. impulsive 400 tons Allowed max.load . "steady" 100 tons Injection Type—constant flow, hydrostatic, adjustable for alignment Type Cyclotron Number (2 per rotor) 4 Energy 8 Mev Number of pads per bearing 6 Value of magnetic field in synchrotron 850 gauss

LIST OF REFERENCES

1, Oliphant, M.L. The Cyclosynchrotron. Acceleration of heavy particles to energies above 1000 Mev, and the homopolar generator as a source of very large current pulses. Nature, 165, p, 466-8, 1950.