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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 a field 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 magnetic field 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 generator field, the 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. of 800 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 focussing field 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.
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