Flux-conynession germrators

This article describes the flux-compression process for the production of a high-energy pulse, and identifies two classes of generator - one which produces high energy (MJ) and high current (MA) in an external load and the other which concentrates a magnetic field into a small volume at a high flux density (MG; 1 MG = 1 O’T) and a high energy density (1 Os MJ/m3). For each class, a representative generator is described and various possible applications are explained.

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

Introduction It is on occasions necessary to undertake high-risk, high-energy experiments at remote sites, or even in outer space, or to test new devices for which a high-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 multimegajoule 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 (I O-’$/J). 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.

illustrated by considerations of the flux and energychanges in a system in which the inductance is reduced from Lo to L1 with an accompanying increase in the current from I, to I , . It follows that, for an ideal system,

The principle of flux compression is readily

LIIl = Lo/,

and that the initial stored energy is

1 2 E - -L,Jo O - 2

The final stored energy is

E 1 r 2 - > L I Z - € 1 1 - o[;l] 2

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

L,/, = 0.1 5L010

so 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 specific applications.

A general statement of the action of a flux-compression generator could be ’any closed conducting cage, surrounding 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 sufficiently rapid (,us) to prevent massive magnetic diffusion through the cage and, wherever possible, the current density should be maintained below the level which would vaporise the metal as a consequence of the nonlinear 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 from a nuclear explosion, the process is sometimes more efficiently achieved by using external electromagnetic forces. The total energy of the system is increased by the work done as

POWER ENGINEERING JOURNAL APRIL 1995 97

Table 1 cornDression aenerators

The Megagauss Club: maximum magnetic field, energy and current achieved with flux-

Russia (USSR) USA France EURATOM (Pramti) UK Romania Japan Poland PR China Germany

1 7/25' 100 10'/14* 50

11 7 8 5 5 4/7* 2

5 10 517 5' 0 5

5 4 3 s 0 0

[3

>300 1952 320 1 9 M 24 1961 16 1961 20 1956 12 1982 L l 1970

0 8 1973 2('i 1967

1 2 ~ ) 1975

* obtained onlyonce r i data not available 7 much hiyherfiyiiri.8 probablyobtained inferred from X rays pictures and a numerical wnulation code

the cage moves against the internal magnetic field forces, and very high voltages and currents are induced with very rapid rates-of-change These, and the explosive environment, have provided the impetus for the development of a 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 83, many different geometries for the cage have been investigated and tested, and types which are now familiar 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 achieved by the different countries involved in this work is

given in Table 1 Included in this list are two quite different classes of device

0 those in which high energies (MJ) and currents (MA) are produced in an external load and which is the conventional usage of a flux-comaression aenerator

, End-initiated helical generator: (a) priorto initiation; (b) following initiation

those which toncentrite the magnetic field energy towards the centre of the device, in an attempt to obtain ultrahigh magnetic fields (MG; 1 MG = 1 02T).

io stator 1 windina armature

initial

explosive a

~ load

point

poin!

final current I

detonation products b

'./ armature

cone

In what follows, attention is concentrated on one e x m p l e from each of these classifications, the helical-cylindrical generator and the cylindrical implosion device It will be noted from Table 1 that in this area of technology very large units are involved MJ, MAand 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 ronverts the chemical energy contained in an explosive charge into electromagnetic energy, via an intermediate stage of kinetic energy In a helical generator, the detonation process accelerates part of the conducting contour (the armature) to a very high velocity, and as this h,ippens there is a corresponding decrease in the generator inductance

assembly of an end-initiated helical generator is shown in Fig 1 (a), where a priming current I , from the capacitor bank C is used to create an initial magnetic flux between the armature and the helical stator ainding When the explosive 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 progrewvely to the right as the explosive front 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 wbsequently being done against the magnetic field, the current in the load increases towards a final value /,as a substantial portion of the kinetic energy of the expanding armature is

converted into electromagnetic energy in the load coil

Since a typical explosive stores chemically about 5MJ of energy per 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 high-energy, high-power output In practice, of course, conditions at the site at

POWER ENGINEERING JOURNAL APRIL 1995

A general arrangement of t h r basic

98

which the generator is fired will 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 cornpression generator of the type shown unassembled in Fig 2 can typically take more than 1 Oops to complete its compression process, with the energy gain from the capacitor bank to the MJ, MA . output in the load being of 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, this gain can be raised to a few hundred For very high energy multiplication, of the order of 1 O6 to IO8, 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 techniques, such as the simultaneous initiation of the explosive charge at different points along the generator axis can be used to ensure a very high voltage for high- impedance loads

Among the important features of a flux 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 availability 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 tD be sharpened so that the current delivered rises to i ts maximurn value in a few nanoseconds, by the use of opening and closing switch techniques involving exploding metallic fuses and plasma devices

Cylindrical implosions Even though magnetic fields exceeding

2OMG (2000T) were claimed to be produced near shaped rods carrying very fast-rising currents, and fields of the order of 1 OOMG (1 04T) are believed to exist in both fast- moving high density hot magnetised plasmas and short pulse compression laser experiments. the only means known to date of producing ultra-high magnetic fields in volumes sufficiently large for prdctical application (more than 1 cm3 or 10 6m3) is t o use a converging implosion generator The imploding metallic 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 5MG (500T). the electromagnetic forces generated by discharging capacitor banks into the type ofZ pinch or @pinch loads shown in Fig 4 have been successfully used to collapse the copper liner For higher

magnetic fields, explosive rather than 2 Wire-wound electromagnetic effects are used to produce the implosive forces Although figures exceeding 2OMG (2000T) have been reported I:. 11) aluminium armature; exceptionally, mametic fields up to about

multiple-sedion 1MJ helical generator: I) helical coil;

(iii) end rinas; 10M'G (1 000T) can be produced consistently by the simple arrangement of Fig 5 for col la psi ng copper cy li nders accelerated to more than 4000m/s by external explosive

(iv) measurement probes, (v) assembled

~ ~ ~ ~ & ~ ~ ~ ( ~ ~ $ t e 2m. weiqht 160kq)

charges With advanced firing techniques used to ensure that the charges are detonated with a high degree of simultaneity, the time for the implosion is about 1 Ops and the maximum fields exists for the order of hundreds of nanoseconds A limitation to this technique lies in the melting

[photograph usea by courtesy of DRA (Fort Halstead)]

hm for msgnetic lieid. p5 time for current x 10, p

3 helical generator and magnetic field for a cylindrical implosion device (/ = I , exp (at). B = Bo exp (exp (ut)). where a and yare constants)

Typical flux-compression characteristics for the growth of current in a

POWER ENGINEERING JOURNAL APRIL 1995 99

Bo

C

return ccmdumr

a b

4 Electromagneticflux compnrsion devices for fields wp Po 9MG BOW): (a) 2 gaomrtry. Liner squeezed by the fidd 8, produd by the current lz. High le cuirnents (Rot shown) flow in the lhwr as the central field builds up; (b) 8 geometry. liner squeezed by 8, fidd produmd by currents and fluwing in the primary coil and the liner

Stager in the production of the coil windings: (below) winding of coil on mandrel; (right) enupsubtion of coil windings

and vaporisation of the inner surface of the liner and, as a consequence, the loss of stability af the metallidfield interface, with the onset of Rayleigh-Taylor instability as the cylindrical geomettyof the liner is lost; see Fig. 5 (a).

(1 OOOT) in volumes sufficiently large for useful application, the so-called cascade

liner of the cascade, this is melted and transformed into a homogenous copper line1 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 stability of the previously accelerated liner is lost. The maximum presently reported reproducible magnetic

To obtain magnetic fields exceeding 1 OMG

100 POWER ENGINEERING JOURNAL APRIL 1995

- .

proof-of-principle experiments and the development of high-energy technology. At the same time, about the same level o f effort was being expended in the USSR. However, in the present era of collaboration and reduced spending on weapons, both types of device described in this article have been used in controlled thermonuclear fusion research. Using disc type 'batteries' of generators as a power source, liners were electromagnetically accelerated to mor? than 50km/s and then made t o compress a pre- heated deuterium plasma which generated a high burst of neutrons.

A sharing of the high costs involved in this work (estimated at more than $1 billion) is one objective of the recent scientific collaboration between the USA and Russia, and a first successful experiment has already been conducted by a joint team A 1 GJ, 1 OOTW design has been produced for the power source for a proof-of-principle break- even fusion experiment.

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

If the next century sees man colonising the Moon, the flux compressor may well be used in the electromagnetic accelerators which will launch return payloads t o orbital earth stations. Furthermore, consideration is being given t o i ts use in high-energy particle physics, where the production of new particles is more easily studied in the presence o f ultra-high magnetic fields. Flux compressors may he developed as repetitive power sources with, instead of solid explosives, gas mixtures being used t o accelerate plasma liners in nondestructive arrangements. Many o f 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 t o be the only major obstacle t o 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 1965-1 992

1 Megagauss I ( 1 965). Frascati, Italy. Published as 'Megagauss magnetic field generation by explosive and related experiments'. Y . Knoepfel and F Herlach (Eds ), EUR 27500, Brussels, 1966 2 MegagaussII (1979). Washington DC, USA. Published as 'Megagauss physics and technology', P. J . Turchi (Ed 1, 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, 1983 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

POWER ENGINEERING JOURNAL APRIL 1995

London 1987 5 Explosive-initiated 5 Megagauss V 1989, Novosibirsk USSR implosive device Very Published as 'Megagauss fields ana pulsed power systems, V M Titov and G A Shvetsov (Eds ), Nova Science Publishers, NY, 1990 6 MegagaussVl(l992) Albuquerque, NM USA Published as 'Megagauss magnetic field generation and pulsed power applications', M Cowan and R B Speilman (Eds ), Nova Science Publishers NY 1994

Further valuable sources are the Proceedings of the nine IEEE Pulsed Power Conferences held between 1976 and 1993

C IEE 1995

Prof lvor Smith is Professor of Electrical Power Engineering in the Department of Electrical Engineering at Loughborough University of Technology He is an IEE Fellow Rod Stewardson is a Research Assistant in the same Department, supported tClrough 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 a t Loughborough, funded by the Royal Society, on leave from the Institute of Atomic Physics, IFTAR. Bucharest, Romania

high circular currents

~ ' ' ~ ~ ~ , ) ~ ~ ~ e during the implosion

101