Explosive Flux Compression Generator

Explosive Flux Compression Generators Seminar Report

Abstract

Explosives flux compression devices have discussed frequently in recent years. An explosive flux compression generator (EFCG) is a device used to generate a high-power electromagnetic pulse by compressing magnetic flux using explosive. Chemical explosives are used to compress an initial magnetic flux by driving part, or all, of a conducting surface which contains the flux.Work done by the conductors moving against the magnetic fields results in an increase in the electromagnetic energy. The additional energy comes originally from the chemical energy stored in the explosives, a part of which is transmitted to the moving conductors. These devices are normally called flux-compression generators or, more simply, generators. The explosively driven conductors are frequently called armatures, and occasionally, non driven elements of the generators are called stators. Explosive flux compression generators are popular as power sources for electronic warfare devices known as transient electromagnetic devices that generate an electromagnetic pulse without the costs and side effects of a nuclear weapon. They also can be used to accelerate objects to extreme velocities, and compress objects to very high pressures and densities; this gives them a role as a physics research tool.

Dept. Of Electrical & Electronics Engg. Saintgits College Of Engg..


Chapter: 1

INTRODUCTION

An explosive flux compression generator (EFCG) is a device used to generate a high-power

electromagnetic pulse by compressing magnetic flux using high explosive. Magnetic Flux

Compression Generators (Magneto cumulative Generators, MCGs) were independently invented

by A. Sakharov in Russia and C.M. Fowler in the United States. They are the most powerful

Pulsed Power devices ever built.

An explosive flux compression generator is coupled to a high resistance load by a

pulse transformer. Chemical energy is converted to electromagnetic energy and is transferred by

means of the transformer to the high resistance load. The transformer allows coupling to a higher

resistive load than is achieved with a transformer less generator system

Explosively pumped flux compression generators are popular as power sources for

electronic warfare devices known as transient electromagnetic devices that generate an

electromagnetic pulse without the costs and side effects of a nuclear weapon. They also can be

used to accelerate objects to extreme velocities, and compress objects to very high pressures and

densities; this gives them a role as a physics research tool.



Chapter: 2

BASIC PRINCIPLES OF FUNCTION

Magneto-explosive generators use a technique called "magnetic flux

compression" The technique is made possible when the time scales over which the device

operates are sufficiently brief that resistive current loss is negligible, and the magnetic flux on

any surface surrounded by a conductor (copper wire, for example) remains constant, even though

the size and shape of the surface may change.

This flux conservation can be demonstrated from Maxwell's equations. The most

intuitive explanation of this conservation of enclosed flux follows from the principle that any

change in an electromagnetic system provokes an effect in order to oppose the change. For this

reason, reducing the area of the surface enclosed by a conductor, which would reduce the

magnetic flux, results in the induction of current in the electrical conductor, which tends to return

the enclosed flux to its original value. In magneto-explosive generators, this phenomenon is

obtained by various techniques which depend on powerful explosives. The compression process

allows the chemical energy of the explosives to be (partially) transformed into the energy of an

intense magnetic field surrounded by a correspondingly large electric current.



FIG 2.1 For a constant intensity magnetic field of magnitude B

traversing a surface S, the flux Φ is equal to B x S

2.1 Elementary description of flux compression

An external magnetic field (blue lines) threads a closed ring made

of a perfect conductor (with zero resistance). The nine field lines represent the magnetic flux

threading the ring.

Fig. 2.2: Original magnetic field lines.



After the ring's diameter is reduced, the magnetic flux threading the ring, represented by five

field lines, is reduced by the same ratio as the area of the ring. The variation of the magnetic flux

induces a current in the ring (red arrows), which in turn creates a new magnetic field, so that the

total flux in the interior of the ring is maintained (four green field lines added to the five blue

lines give the original nine field lines).

Fig. 2.3: Configuration after the ring's diameter has been reduced.

By adding together the external magnetic field and the induced field, the final configuration after

compression can be obtained; the total magnetic flux through the ring has been conserved (even

though the distribution of the magnetic flux has been modified), and a current has been created in

the conductive ring.



Fig. 2.4: Magnetic field lines after compression.

The following figure shows how a basic spiral generator works. The load coil has a fixed

inductance L1, which is to be powered by the generator. The generator itself consists of the

external spiral winding and the explosive loaded conductive cylinder. An initial magnetic flux is

supplied to the generator and the series connected load coil from an external source, usually a

capacitor bank, but in some cases a battery. The conductive cylinder, the armature serves as a

part of the circuit. When the explosive is detonated, the armature expands, causing a conical

FIG: 2.5: Spiral generator before explosive initiation



metal front to move with the explosive detonation velocity. The first contact between the

armature and stationary coil, stator, shorts out the initial field source removing it from the circuit

and trapping the initial current from the generator. After this closure of the current input, the

conical front proceeds down the armature, contacting the spiral turns in a continuous wiping

fashion. The fig depicts the generator after the explosive has been initiated and the armature has

closed the current input circuit. The inductance of the generator is roughly proportional to the

square of the number of turns in the spiral, and inversely proportional to the remaining length

over which the turns are spaced.

FIG: 2.6: Spiral generator after explosive initiation



2.2 Schematic of generator circuit

FIG:2.7

The generator inductance LG , thus varies continuously from its initial value L0 to zero after the armature has shorted out the last turn. The generator inductance may be taken as a function of time in most cases. If this is the case, FIG: 6 is a schematic of generator circuit. The generator inductance is a variable inductance LG(t), and the external load to be powered is L1.Allowance for source or waste inductance (from various external leads to the load, residual generator inductance, etc.) is shown as R. The equation governing the performance of the circuit

d¿dt

¿G + L0 +L1) I] + IR = 0; with I (0) =I0

Time is measured from the instant that the armature shorts out the input current source; at this

initial moment the current is I0 and the initial generator inductance is LG (0) = LG0 .

If the circuit is perfectly conducting (R = 0), the well known electrodynamic result of flux

(LI) conservation is obtained. Under this condition the conservation of flux leads to the result:

I =(LG 0+L0+ L1)(LG+LO+L1) I0



Under burnout condition, LG =0, the final current becomes

I = (LG 0+L0+ L1)

(LO+L1) I0



Chapter: 3

DIFFERENT TYPES OF GENERATORS

3.1 Hollow tube generators

In the spring of 1952, R.Z. Lyudaev, E.A. Feoktistova, G.A. Tsyrkov, and A.A. Chvileva

undertook the first experiment with this type of generator, with the goal of obtaining a very high

magnetic field.

FIG: 3.1 Hollow tube generators

The MK-1 generator functions as follows:

A longitudinal magnetic field is produced inside a hollow metallic conductor, by

discharging a bank of capacitors into the solenoid that surrounds the cylinder. To ensure a

rapid penetration of the field in the cylinder, there is a slot in the cylinder, which closes

rapidly as the cylinder deforms;

The explosive charge placed around the tube is detonated in a manner that ensures that

the compression of the cylinder commences when the current through the solenoid is at

its maximum;


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The convergent cylindrical shock wave unleashed by the explosion produces a rapid

contraction (greater than 1 km/s) of the central cylinder, compressing the magnetic field,

and creating an inductive current, as per the explanation above (the speed of contraction

permits, to first approximation, the neglect of Joule losses and the consideration of the

cylinder as a perfect conductor).

The first experiments were able to attain magnetic fields of millions of gauss (hundreds of teslas,

given an initial field of 30 kG (3 T).

3.2 Helical generators

Helical generators were principally conceived to deliver an intense current to a load situated at a

safe distance. They are frequently used as the first stage of a multi-stage generator, with the exit

current used to generate a very intense magnetic field in a second generator.



FIG: 3.2: Helical generator function

The MK-2 generators function as follows:

A longitudinal magnetic field is produced in between a metallic conductor and a

surrounding solenoid, by discharging a battery of capacitors into the solenoid;

After the charge is ignited, a detonation wave propagates in the explosive charge placed

in the interior of the central metallic tube (from left to right on the figure);

Under the effect of the pressure of the detonation wave, the tube deforms and becomes a

cone which contacts the helically wrapped coil, diminishing the number of turns not

short-circuited, compressing the magnetic field and creating an inductive current;





At the point of maximal flux compression, the load switch is opened, which then delivers

the maximal current to the load.

The MK-2 generator is particularly interesting for the production of intense currents, up to 10 8 A

(100 MA), as well as a very high energy magnetic field, as up to 20 % of the explosive energy

can be converted to magnetic energy, and the field strength can attain 2 × 106 gauss (200 T).

The practical realization of high performance MK-2 systems required the pursuit of fundamental

studies by a large team of researchers; this was effectively achieved by 1956, following the

production of the first MK-2 generator in 1952, and the achievement of currents over 100 mega

amperes from 1953.

3.3 Disc generators

FIG: 3.3 Disc generator function


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A DEMG generator functions as follows:

Conductive metallic discs, assembled in facing pairs to create hollow modules having the

form of a lined torus, with explosive packed between pairs of modules, are stacked inside

a cylinder; the number of modules can vary according to the desired power (the figure

shows a device of 15 modules), as well as the radius of the discs (of the order of 20 to 40

cm).

Current runs through the device, supplied by a MK-2 generator, and an intense magnetic

field is created inside each module.

When initiated, the explosion begins on the axis and propagates radially outwards,

deforming the disc shaped protuberances with triangular section and pushing them away

from the axis. The outward movement of this section of conductor plays the role of a

piston.

As the explosion proceeds, the magnetic field is compressed in the inside of each module

by the conductive piston and the simultaneous drawing together of the inner faces, also

creating an inductive current.

As the induced current attains its maximum, the fuse opening switch fuses and the load

switch simultaneously closes, allowing the current to be delivered to the load (the

mechanism for the operation of the load switch is not explained in available

documentation).

Systems using up to 25 modules have been developed at VNIIEF. Output of 100 MJ at 256 MA

have been produced by a generator a metre in diameter composed of 3 modules.



Chapter: 4

ENERGY SOURCES

Generators are flux-compression devices and almost any source that

can furnish the initial energy in magnetic form is a potential source for the initial generator

energy. Generators also work more efficiently against large magnetic fields and it is usually

advantageous to have the initial magnetic energy also at reasonably high fields.

4.1 Primary Energy Sources- Initial energy can be supplied to generators either by

passing currents through them directly, thereby creating the initial magnetic fields, or by

generating the fields in an external coil which surrounds the generator. Capacitor banks are used

almost exclusively as direct field energy sources, whereas external source coils may also be

superconducting or battery-powered.

4.1a. Direct Feed. The current discharge through the circuit upon switching in the capacitor bank follows the classical sinusoidal discharge represented by Eqs. (21):

I= V0 √C¿ sin ωt =√

2E0C¿ sin ωt (21 a)

P =√LTC; ω =1

√ LTC(21 b)

E0C = 12

CV02 (21 c)

Φ 0G = L0I = L0 √2E0C

¿ (21 d)

E0G =L0¿ E0C

Here, C is the capacitance of the bank, V its charging voltage, LT the total inductance of the circuit.



The period of the discharge is given by Eq.(21b) and maximum current occurs at the

quarter period. As described earlier, generator detonationsystems are timed to close off the

current input supply at, or near, peak current and Eqs. (21d) and (2 le) then give the initial flux

and energy of the generator. After this time the capacitor bank is out of the circuit and Eqs. (21)

no longer apply.

The presence of resistance in the circuit necessitates some modifications in Eqs. (21). Usually the

effects are relatively small. Currents are normally reduced by

10-15% and initial generator energies by 20-30%. However, in cases where the

generator inductance is small it is imperative to keep the waste inductance terms small, as seen

from Eq. (21e), otherwise much of the initial energy available from the capacitor bank will end

up in source inductances.

4.1b. Indirect or Inductive Feed. Generators are initially energized inductively when the following difficulties are experienced:

(1) Perturbations that arise from explosive closure of the current input openings

necessary for direct current feed is excessive,

(2) Initial capacitive storage is too heavy or bulky,

(3) Initial generator inductance is so low that the source inductance in the

capacitor circuit stores too much of the available energy,

(4) Forces on the generator or load are too great with direct feed.



Chapter: 5

TECHNICAL ASPECTS OF GENERATOR

TECHNOLOGY

5.1 Switching and Pulse Shaping.

Subject to conditions dealing with the nature of the single output pulse,

generator outputs can often be handled in ways customarily used with more conventional

electrical sources. The output of a spiral generator used in experiments is as shown below. The

generator burnout time is about 40 ps. Because of its relatively short length, the θ -pinch plasma

could be subjected to magnetic compression for only a few microseconds.

C D

A

B E

To make the plasma conducting, and therefore magnetically compressible, it is necessary to put a

high voltage pulse around the θ-pinch coil before subjecting it to the magnetic compression field.

This is accomplished by the combination of pulse shaping and switching indicated by elements

B, C, and D. Element C is a spark gap



switch set to break down at a specified voltage, determined by the pretested breakdown strength

of the dielectric insulator D. Element B is a carefully constructed resistive fuse designed with

low initial resistance and constructed in such a manner that it vaporized (thus leaving the circuit)

not only at the voltage necessary to break down the spark gap, but also at an appropriate

preselected time during energy generation. Owing to the nonlinear character of the fuse, the

generator output pulse is considerably sharpened but at a significant cost in energy.

5.2 Use of Transformers. In the generator examples considered so far, the loads to be

energized have been fed directly by the generators. Further, they have been essentially low

inductance loads. Circuit resistances; have been treated as unavoidable loss terms. When

transformers are used to couple to low inductance loads, into which generators operate more

efficiently, it becomes possible to energize a wide range of impedances with generators. These

impedances can include resistive loads as well as loads with inductances far greater than the

inductance of the generator itself. Figure shows schematically a generator system with

transformer coupling to an inductance load L3 which is to be energized. The primary circuit

consists of the generator, a source inductance, and primary load coil‘1”. The secondary circuit

consists of the load L3to be energized and a transformer coil of inductance L2.The mutual

inductance between L1 and L2 is denoted by M.

L1

LG M

T

L3



Chapter: 6

ADVANTAGES AND DISADVANTAGES OF

GENERATOR

Good generator technology is governed by a number of factors. Because explosives generators

destroy some components their use is limited to single shot or, at best, low repetition rate

applications. Also, in many cases they are expensive to construct. Finally, additional safety

precautions must be taken over those required for more conventional power sources to guard

against explosive hazards.

For these reasons there must be compelling advantages to justify their use.These advantages do

exist. Among them is the capability of producing enormous energy bursts in time scales not

normally available otherwise. Further, unlike capacitor or inductive storage units, generator

designs can be readily modified to allow them to meet changing load requirements. Again, unlike

capacitor discharges, generator output voltages and currents have large time regions where they

are approximately in phase. For some applications this can be a decisive advantage. Other

advantages arise from their extremely high ratio of energy output to weight and volume.

Ideally, the type of generator required is governed strongly by the type of load to be energized. If

the currents required are not excessive and the time scale of the experiment is not too short,

spiral generators are probably best because of their inherently large energy multiplication.

coaxial, strip, and spherical generators are well suited to power loads where large currents are

required but where the experimental time scale is not too short. Plate generators are capable of

carrying very large currents and can operate on substantially faster time scales. Cylindrical flux

compression systems are used when very intense magnetic fields are required. If they can be

used to drive transformers, they have the capability of powering external loads on still shorter

time scales. Economic factors also play a role in generator selection. Strip generators are



relatively inexpensive because the fabrication of both the metal components and the explosive

system are simple.

Plate generators and cylindrical implosion systems require expensive explosive initiation system,

but the rest of the explosive system and the metal fabrication are relatively simple. The helical

section of a spiral generator is expensive to fabricate but the explosive system is simple. A

number of other technical factors must also be considered. These include: minimizing source or

loss inductance terms; minimizing unwanted motion of system components by magnetic forces,

both during introduction of the initial energy and during the energy generation stage; minimizing

or eliminating metallic jets which are often formed at contacts between metal structures in

relative motion; keeping flux losses to a minimum during the generation stage; and preventing

internal voltage breakdown inside the generator. While these factors are important in any

generator application, three of them are of special significance in the present study in that they

restrict the classes of generators that can be used. These factors deal with flux losses, high

magnetic field effects and internal voltage breakdown.



REFERENCES

Texts

1. An Introduction to Explosive Flux Compression Generators by C.M Fowler, R.S Caird,

W.B Garn, University of California

2. Helical Explosive Flux Compression Generator research at the AIR FORCE

RESEARCH LABORATORY by M. Lehr, L. Bamertt, K. Bellt, T. Cavazosl, D.

Chama, S. Coffer', J. Degnan, D. Galet, G. Kiuttu, P. Pellitierl, and W.Sommars

Websites

www.wikipedia.com


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