by 1. R. Smith, B. M. Novac and H. R. Stewardson

This article describes theflux-compression process-for the production 0 f a high-energy puke, and identijes two classes ofgenerator-one which produces high energy (MJ and high current (h54) in an external load and the other which concentrates a magneticfield into a small volume at a highflux density (MG; 1 MG = I 0 2 T) and a high enevgy density ( 1 OT Ml/m 3). For each class, a repyesentative generator is described and various possible applications are explained.

Introduction

t is on occasions necessary to undertake high-risk, high-energy experiments at remote sitec, or even in outer space, or to test new devices for which a high- I power, high-energy source is needed. In either

situation a capacitor bank may be too bulky to be transported, or too costly to be developed, especially at the niultimegajoule energy level of many proof-of- principle experiments. In these circumstances, the flux-compression generator can provide a compact and relatively inexpensive alternative energy source (lW3 WJ). Although commonly used, the term ‘flux compression’ can, however, be quite misleading, and care should be taken with its use, as in a typical simple generator design the final magnetic flux is only about 15% of the initial value. A more appropriate term is energy density concentrator, since the action of these devices is to generate a high energy density in the final (or load) volume remaining after the compression action is complete.

The principle of flu compression is readily illustra- ted by considerations of the flux and energy changes in a system in which the inductance is reduced h m Lu to LI with an accompanying increase in the current from l o to Ii. It follows that, for an ideal system,

bb = roro and that the initial stored energy is

EIJ =z&l: 1

The final stored energy is

However, for the typical simple and practical generator design mentioned above,

Lib = 0~15Lolo

50 that, for an inductance reduction of 1000, the current gain is 150 and the energy gain 22.5. These figures will, however, be considerably greater for special purpose generators designed for spechc applications.

A general statement of the action of a flux- compression generator could be ‘any closed conducting cage, surroundmg a magnetic field produced either by a current flowing through it or by external means, and which can be made to reduce its volume, can be considered to be a flux-compression device’. The movement should be su5ciently rapid 0.) to prevent massive magnetic diffusion through the cage and, wherever possible, the current density should be maintained below the level whch would vaporise the metal as a consequence of the nodnear Joule heating phenomenon. Although explosives-solid, liquid and gaseous-can be used to accelerate and deform the cage, and designs have even been proposed that use the energy h m a nuclear explosion, the process is sometimes more efficiently achieved by using external electromagnetic forces. The total energy ofthe system is increased by the work done as the cage moves against

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I 1

Table 1: The Megagauss Club maximum magnetic field, energy and current achieved with flux-compression generators

Magnetic field density, Energy, Current, Programme

MG (10'n MJ MA start

Russia (USSR) 17/25* 100 >300 1952 USA 10'/14* 50 320 1950 France 11.7 8.5 24 1961 EURATOM (Frascati) 54" 2 16 1961 UK 5 10 20 1956 Romania 5r7.5* 0.5 12 1982

Poland 3.5 - 0.8 1973 Japan 5.4 n 0 1970

PR China 0 U 2(?) 1967

*obtained only once data not available ? much hiaher fiaure Drobablv obtained

Germany - 1.2(?) 1975

- - . + inferred from X-ray pictures and a numerical simulation code

the internal magnetic field forces, and very high voltages and currents are induced with very rapid rates- of-change. These, and the cxplosive environment, have provided the impetus for the development ofa range of novel electro-optic devices for voltage and electric field measurement and magneto-optic devices for current and magnetic field measurement.

During the last 40 years or so, many different geometries for the cage have been investigated and tested, and types which are now f d a r include cylindrical, spherical, helical-cylindrical, plane and bellows, coaxial cylindrical, Archimedes spiral and disc- shaped. A summary of the levels of magnetic field (flux density) energy and current which have been reported as acheved by the mfferent countries involved in this work is given in Table 1. Included in this list are two quite merent classes of device:

e those in which high energies (MJ) and currents (MA) are produced in an external load, and whch is the conventional usage of a flux-conipression generator

e those which concentrate the magnetic field energy towards the centre ofthe device. in an

contained in an explosive charge into electm- magnetic enerm, via an intermediatr stagc of hnctic energy. In a helical generator, the detonation process accelerates part of the conduct- ing contour (the armature) to a very high velocity, and as this happens there is a correspondmg decrease in the generator inductance.

A general arrangcnient ofthe basic assembly ofan end-initiated helical generator is shown in Fig. la, where a priming current 10 from the capacitor bank Cis used to create an initial magnetic flux between the armature and the helical stator winding. When the explosivc contained within the cylindrical armature is detonated, this expands into the conical form of Fig. 1 b, with the point of contact made by the armature with the stator winding moving progressively to the right as the cxplosive fmnt travels in this direction. Contact between the cone

and the crowbar short-circuits the stator winding and compresses the magnetic field into an increasingly smaller volume and, with work subsequently being done against the magnetic field, the current in thc load increases towards a final value Ij as a substantial portion of the kinetic energy of the expandmg armature is converted into electromagnetic energy in the load coil. Since a typical explosive stores chemically about 5 MJ of energy pcr kg, and a significant proportion of this can be obtained at the output, it is clearly possible to produce a very lightweight and compact power source with a higl- energy, high-power output. In practice, of course, conditions at the site at whch the generator is fired wdl limit the amount of explosive that can be used. In addition, any generator design will have to meet requirements imposed by both the priming source and the current required by the load.

A simple flux-compression generator of the type shown unassemblcd in Fig. 2 can typically take more than 100 p to complete its compression process, with the energy gain from the capacitor bank to the MJ, MA

attempt to obtain ultrahigh mabetic fields (MG; 1 MG = 102 T).

In what follows, attention is concen- trated on one example from cach of these classifications, the helical- cylindrical generator and the cylindrical implosion device. It will be noted from Table 1 that in h s area of technology very large units are involved: MJ, MA and MG, and, with MV being generated for microsecond time scales, power is often measured in GW and sometimes even TW.

Helical generators

An explosive-driven flux-compression generator converts the chemical energy

a

load

detonation - products __ b

armature cane

Fig. 1 initiation

End-initiated helical generator: (a) prior to initiation; (b) following

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output in the load being of I 1 the order of tens. The corresponding time variation of the load current is as shown in Fig. 3. By using a more sophisticated design, involving coils on the stator winding with tilted turns and/or variable geometry, ths gain can be raised to a few hundred. For very high energy multiplication, of the order of lo6 to lo8, several generators can be connected through transformers in series or in parallel, with even a Marx type connection having been considered for some applications. Special tech- Flg. 2 Wire-wound multiple-section I MJ helical niques, such as the s h u l - generator: (i) helical coil; (ii) aluminium armature;

(iii) end rings; (iv) measurement probes; taneouS initiation Of the (v) assembled generator with concrete reinforcement explosive charge at different (length 2 m, weight 160 kg) [photograph used by points along the generator courtesy of DRA (Fort Halstead)] axis, can be used to ensure a

opening and closing switch techniques involving explod- ing metalhc fuses and plasma devices.

Cylindrical implosions

Even though magnetic fields exceedmg 20 MG (2000 T) were chmed to be produced near shaped rods carrying very fat-rising currents, and fields of the order of 100 MG (1 O4 T) are believed to exist in both fast-moving high- density hot magnetised plasmas and short pulse- compression laser experi- ments, the only means known to date of producing ultra-high magnetic fields in volumes sufficiently large for practical application (more than 1 cm3 or i04m3) is to use

very high voltage for hgh-impedance loads. Among the important features of a flu-compression

generator are its relatively low weight, compactness and total autonomy, since if necessary the initial priming source could even be a permanent magnet. The avdability of very large levels of output current and energy has led to their use in the thermonuclear fusion programme, outer-space experiments, electromagnetic launchers, X-rays or neutron pulsed sources and when developing powerful lasers. In a number of these applications, the generator output needs to be sharpened, so that the current delivered rises to its maximum value in a few nanoseconds, by the use of

a converging implosion generator. The imploding metalhc cylinder is termed a liner and the initial magnetic field is produced by an outer coil energised by either a capacitor bank or the type of helical generator described above.

For magnetic fields up to about 5 MG (500 T), the electromagnetic forces generated by discharging capacitor banks into the type of Zpinch or &pinch loads showri in Fig. 4 have been successfully used to collapse the copper liner. For hgher magnetic fields, explosive rather than electromagnetic effects are used to produce the implosive forces. Although figures exceedmg 20 MG (2000 T) have been reported

Fig. 3 Typical flux-compression characteristics for the growth of current in a helical generator and magnetic field for a cylindrical implosion device (I = lo exp (at), 6 = 6 0 exp (exp (ut)), where a and yare constants)

I 5 10 15 20

time tor magnetic field, &IS time for current x 10. ps

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I

I probe -+- Y liner

- insulator

-. ‘probe

return ’ conductor

b

coil

- _c_

IB

C F

Fig. 4 Electromagnetic fluxsompression devices for fields up to 5 MG (500 T): (a) Z geometry. Liner squeezed by the field Be produced by the current b. High le currents (not shown) flow in the liner as the central field builds up; (b) e geometry. Liner squeezed by &field produced by currents hand 1; flowing in the primary coil and the liner

exceptionally, magnetic fields up to about 10 MG (1000 T) can be produced consistently by the simple arrangement of Fig. 5 for collapsing copper cylinders accelerated to more than 4000 m/s by external explosive charges. With advanced firing techtuques used to ensure that the charges are detonated with a high degree of simultaneity, the time for the implosion is about 10 p and the maximum field exists for the order of hundreds of nanoseconds. A limitation to this t e c h q u e lies in the melting and vaporisation of the inner surface ofthe liner and, as a consequence, the loss ofstabhty ofthe metallic/field interface, with the onset of Rayleigh-Taylor instabhty as the cylindrical geometry of the liner is lost; see Fig. 5a.

To obtain magnetic fields exceeding 10 MG (1000 T) in volumes sufficiently large for useiul application, the so-called cascade system of liners shown in Fig. 5b is adopted. Each member of the internal system of coaxially positioned liners consists of an unconnected axial copper wire structure held in position by an epoxy resin and transparent to the magnetic field. When an implodmg liner hits the next liner of the cascade, ths is melted and transformed into a homogenous copper liner which can allow circling currents and so compress further the magnetic field. The design and positioning of the individual liners must be such that the contact is made before geometrical stabhty of the previously accelerated liner is lost. The maximum presently reported reproducible magnetic field of nearly 17 MG (1700 T) achievable with this technique corresponds to an energy demity of 10‘ MJ/m’, and is produced in about 20 p, as shown in Fig. 3.

The first reported application of the implotive flux- compression technique was in the 1940s, when it was used in the Y-Manhattan atomic bomb project to study

the cylindrical implosion of a metahc shell. More recent applications have included the study of materials under pressures of many d o n s of atmospheres, in particular in seeking the transition of hydrogen to the metallic state. For these experiments the probe of Fig. 5 consisted of a copper cylinder, squeezed by the magnetic field developed inside the flux-compression generator in a manner similar to the copper liner of Fig. 4b. Also, these implosive techniques were used for producing high magnetic fields for plasma research, for the magnetic acceleration of microspheres and for the study of a wide range of magneto-optic phenomena.

The way ahead

Between 1985 and 1990, many billions ofdollars were spent in the USA on developing energy weapons technology in the Star WadSDI project. Several of the concepts underlying flux-compression generators figured prominently in ths programme, where they were used as power sources in proof-of-principle experiments and the development of high-energy technology At the same time, about the same level of effort was being expended in the USSR. However, in the present era of collaboration and reduced spendmg on weapons, both types of device described in this article have been used in controlled thermonuclear iusion research. Using &sc type ‘batteries’ of generators as a power source, liners were electromagnetically accelerated to more than 50 h / s and then made to compress a preheated deuterium plasma which generated a high burst of neutrons.

A sharing of the high costs involved in this work (estimated at morc than $1 bfion) is one objective of the recent scientific collaboration between the USA

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probe

a initial position of liner

final position of liner

Flg. 5 (not shown) are developed in the liner during the implosion

Explosive-initiated implosive device. Very high circular currents

and Russia, and a first successhl experiment has already been conducted by ajoint team. A 1 GJ, 100 TW design ha? been produced for the power source for a proof-of- principle break-even hsion experiment.

In addtion, the moratorium on nuclear testing has meant that the 5ux compressor is being considered as the large, compact and inexpensive power source needed for the X-ray, neutron and EMP pulse generators used to provide strong pulsed radiation.

If the next century sees man colonising the Moon, the flu compressor may well be used in the electromagnetic accelerators which d launch return payloads to orbital earth stations. Furthermore, consideration is being given to its use in high-energy particle physics, where the production of new particles is more easily studied in the presence of ultra-hgh

magnetic fields. Flux compressors may be developed as repetitive power sources with, instead of solid explosives, gas mixtures being used to accelerate plasma liners in nondestructive arrangements. Many of the applications mentioned above have been considered seriously by different research groups and a number of preliminary experiments have been performed. Financial restrictions would appear to be the only mAjor obstacle to very big developments being seen in the coming years. Tomorrow has already begun.

Further reading

The main sources of information on flux- compression generators are the International Megagauss Conferences held between 1965 and 1992.

1 Megagauss I (1965). Frascati, Italy Published as ‘Megagauss magnetic field generation by explosive and related experiments’, H. Knoepfel and E Herlach (Eds.), EUK 27500, Brussels, 1966

2 Megagauss 11 (1979), Washington DC, USA. Published as ‘Megagauss physics and technology’, P. J. Turchi (Ed.), Plenum Press, NY and London, 1980

3 Megagauss 111 (1983), Novosibirsk, USSR. Published as ‘Ultrahigh magnetic fields- physics, techniques, applications’, V M. Titov and G. A. Shvetsov (Eds.), Nauka, Moscow, 1Y84

4 Megagauss IV (1986), Santa Fe, NM, USA. Published as ‘Megagauss technology and pulsed power applications’, C. M. Fowler, R. S. Caird and D. J. Erickson (Eds.), Plenum Press, NY and London, 1987

5 Megagauss V (1989), Novosibirsk, USSR. Published as ‘Mcgdgduss fields and pulsed power systems’, V M. Titov and G. A. Shvenov (Eh.), Nova Science Publishers, Nu, 1990

6 Megagauss VI (1992), Albuquerque, NM, USA. Published as ‘Megagauss magnetic field generation and pulsed power applications’, M.

Cowan and R . B. Speilman (Eds.), Nova Science Publishers, Nu, 1994

Furthrr valuable sources are the Proceeding of the nine IEEE Pulsed Power Conferences held between 1976 and 1913.

0 IEE: 1995

Prof. Ivor Smith is Professor ofElectrical Power Engineering in the Department of Electronic and Electrical Engineering, Loughborough University of Technolow, Loughborough, Leicestershire LE11 3TU, UK. He is an IEE Fellow. Rod Stewardson is a Research Assistant in the same Department, supported through a Research Contract with DRA (Fort Halstead). He is an IEE Member. While the article was being written Bucur Novac was a Senior Visiting Research Fellow at Loughborough, funded by the Royal Sociey, on leave &om the Institute ofAtomk Physics, IFTAR, Bucharest, Ron&.

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