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Z.30VT ACCCSSIGN NO. S. RCCIPICMT”S CATALOO HuU8KR

TITLE (d UUo) 5.Twc OF REPORTk ●cmoo Covcnco

Electromagnetic Aspects of EMP Testing Technical Memorandum

& ●CRFORNINO ORO. REPORT NuU8CU

‘. AllTHOR(q) & CONTRACT OR ORANTNUMBCw9j

Egon Marx PRON-WJ4C068 O01A1A9MIPR-74-597

I.PCRFORMINO ORGANIZATION NtiCANOAOORCSS 10. ●ROORAMELEMCNT.PROJCCT, TASK

Harry Diamond Laboratories ARCA& WORK UNIT NUMBCRSPgm Element: 6.27.1O.H

2800 Powder Mill Road I_. DNA Subtask: L37EAXEX431Adelphi, MD 20783 04; Task Areaz 04

!.CONTRGLLINO O?FICCNANC ANOAOORCSS *2. RCPORTOATR

Defense Nuclear Agency October 1975ATTN : VLIS !,.NUUBCRO??AOES

Washington DC 20305 28h.UONITORINO AOCNCY NANE4 ADORSSS(li *fbtk Crnedltn# Olflem) lS: SCCURITY CLAS&(afti#O~)

UNCLASSIFIEDih.OCCLASSIFICATION/OOWNORAOINO

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& OISTRISUTION STATWMCNT @lUaiaRm

Approved for public release; distribution unlimited.

7. OISTRIDUTION STATCMCNT (dthotitrut MM{nMeoA 20,tf&ltitkR~)

8. SUPPLEMCNTARYNOTCS

This research was sponsored by the Defense Nuclear Agency underSubtask L37EAXEX431, Work Unit 04, Work Unit Title “AUTOVONSwitch Center Response Prediction.” HDL Project No. E144E6

B.KCYWOROS(C& tbm9911nurn altillnumaw -Ititll?h Uotimmbu)

Electromagnetic pulse .-.-- .....-.:EMP simulatorsEMP testing.

O. A8STRACT(Cuti=wWm dbit —Vtiltitlfy eunk~)

General considerations related to testing of complicatedexposed

SyStiFSto E2!P by means of a sirulator are presented, with

special er.phasis on the similarities and differences between thepulse generated by a hiqh=altitude burst and the one generated bya simulator. Other aspects of a testing program are alsodiscussed.

mlD——.

VU~~~nW cmnoworlwov sStso~s7 f UNCLASSIFIEDSCCURITY CLASSIFICATION OPTMISPAOC ~L?um&t&

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1. REPORT NUMBER 2. .soVT ACCESSION NO. 3. REClpl ENT’S CATALOG NUMBERHDL-TM-75-14

1.TITLIS (and SubffN.) 5. TYPE OF REPORT & PERIoD COVEREO

Electromagnetic Aspects of EMP Testing Technical Memorandum

6. PERFORMING ORG. REPOR7 NUMBER

7. AUTHOR(S) 0. CONTRACTOR GRANT tiUMBER(#)—

Egon Marx PRON-WJ4C068001A1A9MIPR-74-597

).PERFORMING ORGANIZATION NANE ANO AOORESS 10. PROGRAM ELEMENT,PROJECT, TASK

Harry Diamond Laboratories AREA& WORK UNIT NUMSERSPgm Element: 6.27.1O.H

2800 Powder F4ill Road DNA Subtask: L37EAxEX431Adelphi, MD 20783 04; Task Area: 04

1.CONTROLLING OFFICE NAME ANOADORES6 12. REPoRT OATE

Defense Nuclear Agency October 1975ATTN : VLIS 13. NUMBEROF PAGES

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8. SUPPLEMENTARY NOTES

This research was sponsored by the Defense Nuclear Agency underSubtask L37EAxEX431, Work Unit 04, Work Unit Title “AUTOVONSwitch Center Response Prediction.” Hl)L Project No. ?3144E6

B.KEY WORDS(&mtfnuom~

Electromagnetic pulseEMP simulatorsEMP testing.

>.ABSTRACT (ContiimmMr.wae ●!& ffn.e*aauy mdldmtf~~ blook~b?)

General considerations related to testin~ of con,plicated systemexposed to E?!P by means of a simulator are presented, withspecial emphasis on the similarities and differences between thepulse generated by a high-altitude burst and the one generated bya simulator. Other aspects of a testinq pro~ram are alsodiscussed.

Lll**e —-( UV73~R~”~3 EOITiONOF lNOV661sOB60LETIE1

UNCLASSIFIED6CCURITY CLASSIFICATION OF THISPAGE@On D=talhiq

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CONTENTS

Section Title

1. INTERACTION OF THE SYSTEM AND THE EMP

1.1 Introduction . . . . . . . . . .1.2 Direct Energy Transfer at a Node1.3 Currents Induced in Long Cables1.4 Effects of the Ground . . . . .1.5 Effects of the Structures . . .1.6 Response of a Node . . . . . . .

1.6.1 The Ground Network . . .1.6.2 The Signal Network . . .

1.7 Critical Components of a Node .

2. LARGE-SCALE SIMULATORS . . . . . . .

2.1

;:;

2.4

;::

Purpose . . . . . . . . . . . .Desiqn Considerations . . . . .

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.Prop&ties of the Electromagnetic Fields

2.3.1 Free-Space Fields . . . .2.3.2 Local Modifications . . .

Emitting Antennas . . . , . . .

2.4.1 TEMPS. . . . . . . . . .2.4.2 Continuous-Wave Radiators

Current Injectors . . . . . . .Voltage Pulsers . . . . . . . .

3. COMPLEMENTARY TESTING . . . . . . . .

3.1 Bench Tests . . . . . . . . . .3.2 Scale Modelling . . . . . . . .3.3 Subsystem Testing . . . . . . .

4. RESPONSE OF A NODE TO A SIMULATOR . .

4.1 Introduction . . . . . . . . . .

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.4.2 Location and Orientation of the Simulator4.3 Level of the Test . . . . . . . . . . . .4.4 Connectivity Effects . . . . . . . . . . .4.5 Degradation Effects . . . . . . . . . . .4.6 Synchronization of Simulators . . . . . .4.7 Resonant Modes . . . . . . . . . . . . . .

4.7.1 Direct Effects of the Incident Wave4.7.2 Late-Time Oscillations . . . . . .

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25

MEASUREMENTS . . . . .

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5.1 Design of Probes . . . .5.2 Location of Probes . . .5.3 Qualitative Observations5.4 Functional Evaluation . .

ANALYTIC, MODELLING . . . . . .

6.1 Introduction . . . . . .

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6.2 Re?Jresentation of the Pulse6.3 Re~resentation of Components6.4 Calculations for Long Cables6.5 Response of Small Subsystems6.6 Singularities Expansion Method6.7 Modelling of a Node6.8 Complex Systems

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L1. INTERACTION OF THE SYSTEM AI?O THE EMP

1.1 Introduction

The detonation of a nuclear weapon above the atmosphere produces anelectromagnetic pulse (EMP) that can affect electronic equipment overlarge areas on the ground.

One way to determine the vulnerability of a communications system toEMP is to subject the components and even whole nodes to a pulseproduced by a simulator. The properties of pulses generated bydifferent simulators and compromises that have to be accepted in atesting program are discussed in this report.

FOr a system to be tested for upset due to an EMP, it isto have a

importantgood understanding of the interaction that has to be

simulated. To a large extent, this undertaking has to be theoretical,due to the prohibition of high-altitude nuclear tests.

It is necessary to use an accurate description of the EMP producedby a high-altitude nuclear burst. The dimension of the region coveredby the pulse due to a single burst can be as much as several thousandmiles in diameter. The waveform, intensity, angle of incidence, andpolarization of the wave vary greatly over such an area, and therelationships among them are not simple. codes have been developed tocompute the waveforms at different locations; their validity islimited by the number of approximations that have to be used to dealwith the complicated processes brought about in the atmospheric burst.

c The other consideration of importance is the interaction of the EMPwith the system. The understanding of the coupling mechanism allowsone to evaluate the importance of the differences between the test andan actual event.

1..2 Direct Energy Transfer at a Node

A node in a communications network acts as a receiving antenna tothe EMP that sweeps over it. Although sometimes it is an actualantenna, most times it is a large conglomerate of conductors. Eachelement can be considered as an individual antenna, since 10caleffects dominate at the initial portion of the EMP when the wave frontfirst enters into contact with that particular element. At latertimes, the currents induced in the conductors flow from one banother, and the configuration of the whole node becomes important.

The propagation of electromagnetic effects is limited by the speedof light, according to the special theory of relativity. Since thewave front is traveling precisely at this speed, no induced currentsor reflected waves can posibly get ahead of the original pulse.

Most of the studies of energy pickup by an antenna have beencarried out for monochromatic or nearly monochromatic waves. Forthem, the coupling is most efficient when the size of the element isof the order of A/2, where X is the wavelength of the incidentradiation. FOr a pulse, the square of the modulus of the Fouriertransform of the time-amplitude gives the energy content in the pulsefor a particular frequency range.

c

5

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,,

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For a pulse, the square of the modulus of the Fourier transform of thetime-amplitude gives the energy content in the pulse for afrequency range.

particular

The coupling of a receiving antenna with an EMP depends also onthe direction of the incidenk pulse, that is, on the altitude of thenuclear burst, its distance from the node. and the orientation of thisseparation. For one configuration, the effect on a particular elementis maximal, as can be determined in principle either theoretically orexperimentally. In fact, such a determination for all the elements ofinterest appears very difficult for a complicated system, but somegeneral guidelines might be forthcoming from such a study.

1.3 Currents Induced in Lonq Cables

A particularly efficient coupling mechanism for a communicationsnode involves the long cables that frequently are connected to such anode. Large currents of the order of hundreds or even thousands ofamperes can be induced in a cable, and they then go into (or come outof) the system. These effects are especially important for wide areaillumination, in which the current rises quickly to a peak and thendecays slowly as contributions from remote sections of the conductorarrive at the node.

It is relatively easy to determine the current induced in infiniteconductors, but the-effe&s of its termination atto be taken into account.

Some of these cables are buried, and othersthe effects of’the ground considerably influencein the conductor.

1.4 Effects of the Ground

the node also have

travel overhead, andthe current induced

lln important factor always present in a problem involving El@ isthe earth under (or over) the system being studied. In a calculation,the earth is normally represented by a semi-infinite medium with aplane boundary and given dielectric constant and conductivity. Inmost real circumstances, these are crude approximations. The earthis not flat, even locally, and its physical properties vary oversizable ranges both from one point to another and in time, due to itscooqmsition and water content, for instance.

The field above the earth consists of the incident wave and thereflected wave. Their relative phase, orientation, and magnitude canbe determined for monochromatic waves through the well-known Fresnelformulas; the resultant pulse can then be obtained by integration overthe frequency spectrum.

The transmitted wave also is determined by the Fresnel formulas,but another factor to be taken into account is the attenuation thatthe wave suffers propagating in this conducting medium. The distance6 in which a monochromatic wave of frequency u/27ris reduced to l/e,that is, 37 percent of its intial strength, is

6 = (1/a)/{(2&/u)[l+/(l+u2E-2w-’)]],

6

(1)

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where G is the conductivity, E the permittivity, and P thepermeability of the ground. Generally, P is very close to LID, itsvalue for the vacuum. In the low-frequency limit, U<<a/c , equation(1) reduces to

(2)

That is, the penetration depth decreases with increasing frequency.In the high-frequency limit, O>>IS/E, equation (1) becomes

0: (2/u)J(E/11), (3)

an expression that has no explicit frequency dependence.

If one uses as typical values for the ground a conductivity of5 x 10-3 ohm-l ml and a dielectric constant of 17, the transitionregion between these two limits centers on a frequency of 5 MHz, whichcorresponds to u = a/c This frequency is well within the spectrum ofthe EMP.

The overall frequency dependence of the reflection andtransmission or refraction coefficients is quite complicated,their

andimplications for pulses depend stronqlv on the sDectrum of the

incident field..- .

1.5 Effects of Structures

The field reaching the components of a communications system isfurther affected by the structures in which they are housed. Oneextreme is equipment in an especially shielded room, where essentiallyno external radiation can penetrate. The extent to which an incidentwave is attenuated by the metal that is part of the building can varybetween wide limits and has to be taken into account. The field ismodified further when it is scattered by the conducting material thatforms part of the equipment and that can be considered as part of thereceiving antenna that absorbs the incident radiation.

The fields that affect directly the conductors that carry thesignals in the system are very different from the field that wouldexist in the absence of these structures.

1.6 Response of a Node

The EMP on a node causes currents in the conductors that normallycarry the communications signals and potential differences betweendifferent parts of the node. These currents can disrupt the systemin two ways. By t~upset,llthe operation of the equipment or the natureof the signal being transmitted changes temporarily, so that a message

7

.. . . _.. .

,,. . . .,., . . .

,9does not reach its destination in the oriqinal form. The otherdisruption, ~tdamage~ to the equipment, is reflected by physicalchanges in the components that require replacement or repair.

Many complicated systems can be conceptually separated into twoparts, the ground and signal networks. This simplificationthe scope

reducesof calculations that have to be performed to predict the

respnse of a given node.

1.6.1 The Ground Network

The ground network is formed by the shielding of cables andconductors, the cabinets, bars, and other supporting structures forthe equipment, which are connected to each other and usually to earthground. The currents induced in these elements are large, up tothousands of amperes. Long cables and large ground loops areparticularly effective in their coupling to the EMP.

1.6.2 The Siqnal Network

The signal network consists of the conductors and devices thatcarry and process the signals that travel through the communicationssystem. The current induced in these elements is much smaller,they

sinceare usually shielded by the ground network. They can be

associated with transfer functions between the shielding of cables andthe interior conductors, and similar descriptions suffice for the moreor less local effects for other elements. The different transfermechanisms can be considered for many configurations as a problemindependent of interactions with other parts of the node. Thereradiation effects of these small currents can be neglected. It isimportant to detect any unshielded ~rtion of this network, sincelarge currents can be introduced through them into other components.

1.7 Critical Components of a Node

The currents and voltages induced in the signal network affectdifferent components in different ways. Some components are upset ordamaged more easily than others, and they respond to a particularfeature of the pulse more strongly. It &s* then, of primaryimportance to determine the currents or voltages induced at theparticular locations of these most sensitive components. On the otherhand, redundancy built into a specific node might make damage to agiven component relatively unimportant, whereas other components areessential to the functioning of the system. Those other componentsthat are most sensitive to the EMP are the critical components thathave to be hardened and protected with special care. as well asmonitored during

2. LARGE-SCtiE

2.1 Purpose ,

In view of’communicationsdifferent nodesfield produced

any testing program.

‘)’ ●

SIMULATORS.: ‘/“i%

the restrictions on realistic.

testing of a “system in an EMP environment, the response of the 1

has to be either calculated or measured by use of the:

by a simulator. The extent to which the EMP is

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Gfaithfully reproduced depends on the purpose of a particular test. Asimulator that tests the whole node or a large subsystem presentsdifferent design problems from a field generator used in testinqcomponents or small subsystems.

2.2 Desiqn Considerations

The general characteristics of the fields, as well as the relativemagnitude and orientation of the electric and magnetic vectors, differfrom the induction zone to the radiation zone.

For determination of the response of a node to a wave, that is, apulse of electromagnetic radiation, an antenna should be used at arelatively large distance from the node, so that the system can beaffected mainly by the radiation fields.

It is necessary to determine also what characteristics of the EMPhave to be reproduced so that the upset or damage is comparable tothat in a real-threat situation, characteristics such as the peak am-plitude, rise time, and total energy.

The distinguishing aspects of a pulse are its shape, itsamplitude, its polarization, its direction of propagation, and thearea that it covers.

Two aspects in the shape of the pulse have to be considered: thetime dependence at a fixed location and the spatial configuration.The time dependence has frequently been idealized, in the EMPdialogue, to the double exponential

c

f(t) = a[exp(-at) - exp(-~t)], (4)

where 1/6 is the short rise time and l/a is the much longer decaytime; the field of the simulator is then compared to this waveform. Thespatial dependence of the plane wave is a result of the approximationof a spherical wave front to a plane wave when the region of interestis small compared to the radius, and the direction of propagation isonly a matter of the relative location of the source and theThe EMP is

system.expected to be linearly polarized, which is relatively

simple to achieve for the fields of an antenna.

The properties of a threat-level EMP impose contradictory demandson the simulator. For instance, a source far from the installation isrequired to reproduce a plane wave, but this requirement severelyrestricts the level of intensity that can be obtained at the site.This restriction might be unimportant when linear effects are studiedin low-level tests, but most systems contain nonlinear elements,- anddamage is intrinsically nonlinear. A short rise time can be readilyobtained from a small antenna, but this problem is major for a largesimulator required to produce threat-level fields. Consequently, thespecific objectives of the test and the nature of the node will have

9

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.~to determine the emphasis to be placed on the achievement of thedifferent desiderata.

2.3 Properties of the Electromagnetic Fields

All electromagnetic fields are solutions of ~xwell~s equationsand are determined by the sources and the boundary conditions. Thesefacts are true for both the EMP produced by a nuclear burst and thefields of all simulators. In some simple cases, it is possible tocalculate these fields accurately, but in general, they have to bemeasured or estimated from models.

2.3.1 Free-space Fields

When the currents and charges that give rise to a field are known,and the presence of conducting and dielectric bodies in their vicinitycan be neglected, it is possible ko find the fields in terms ofintegrals over the sources.

hlaxwell~s equations in free space, written in rationalized MKSAunits, are

I

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V“ii’=0,

(5)

(6)

(7)

(8)

Equations (6) and (7) imply that these fields can be obtained from avector potential and a scalar potential by means of the equations

g=vxz,! (9)

ii= -3&at-v9; (10)

“-).‘o

10

,.

#..

, ,.

.

These potentials then satisfy d~Alembert Qs equations

(1/c2)a22/at2- V2X s IJOJ,

(1/c2)a20/at2 - v2@ = p/E~

in a Lorentz gauge, where the potentials satisfy

(11)

(12)

veZ+(l/c2)aa/at= o (13)

of light in vacuum. Integration of theseGreen functions yields

and c = /(EOUO) is the speedequations by use of retarded

J@(i,t) = * p(z’,t-\2-x’l/c) dsxl

.1:-2’/(14)

I

f

[P]d3x’Z& R’

(15)

(14’)

(15’)

11

—.- ,.. — —— . .

where

and khe bracketstime t - R/c.

The fields;(9) and (10), are

.,

. . . . ..-

(16)

indicate that the sources are taken at the retarded I

derived from the above potentials through equations

i

= I-JoJp]

a? Rx ~2 d3x’ + ~I

REC t

[~]x ~~ d3x’, (17)

The vector 1’ ranges over the region of the sources and can be ~imitedby a magnitude Po’,the size of the s~urce. For a field point x suchthat r>>PO, the terms containing [aj/~t] or [3p/at] decrease Iil$e I/rwith the distance from the source, whereas those derived from ~ ~] or[P] decrease like I/ra. The first ones then dominate at largedistances, give a f“nite contribution independent of r when thePoynting vector fix~ is integrated over a sphere, and are calledllra~ationll fields. The other terms are labelled ?rinductionSZfields.

The Poynting vector represents the power+ flow ~radiated by thesources. In the radiation zone, the fields E and CB are perpendicularto each other “and have the same magnitude, and the Poynting vector isradial.

For some idea of the location ro of the ill-defined boundarybetween the radiation and induction zones, the magnitudes of theintegrands are set equal, whence

I

I1

12

(19)

.,.

-)“o

>.

,.. ..”

. .

which gives

The quantity

(20)

(21)

represents the rate of change of the sources and is independent of theamplitudes, and T is of the order of magnitude of the rise time ordecay !time of the pulse. If the simulator has a rise time of 3 nsec,ro z 1 m, whereas for a decay time of 1 psec, it increases to rO ~300 m.

Purther simplifications are possible when the wave is nearlymonochromatic (the EMP is not) . When it is, one can distinguish threezones for a distance r from the source, such that

Po<<r<<A near (static) zone,

po<<r%i intermediate (induction)

Po<<A<<r far (radiation) zone,

where the size of the sourceoo was assumed

zone,

to be much smaller than thewavelength X. For a pulse with a broad bandwidth, only the radiationzone is clearly defined.

The fields in the induction zone do not resemble those of a planewave, and the flow of energy includes not only that to be dissipatedas ra~iation, but also a significant amount that is basically goingback and forth near the sources. This field can be important if thereceiving ~antennaw is close t~ the emitter. Also, calculations underthe assumptions that fields E and c~ are perpendicular to each otherand of equal maqnitude have to be reexamined for locations in theinduction zone.

2.3.2 Local Modifications

The fields obtained abovestarting point for furtherthere are significant effects

can serve as an approximation or as acalculations. In a realistic situation,of the uround and of structures. if anv.

The ground e~fects suffer from uncert~inties due to varying ‘&oil aid

13

. . ...- . . . —..—.—--

. .I

.’. . . . .- .-,.

weather conditions and thus can be only estimated. They are ofimportance in calculations related to measurements of the field of anantenna and to the currents induced in long cables by the simulator.The effects of a structure with complicated configurations ofconductors are much more complicated, and the multiple reflections canchanqe the nature of the field in a radical manner. For the fieldinside a building, the magnitude due to an incident pulse depends onthe amount of shielding provided by the construction materials andinternal structures. The magnitude can vary greatly and havecomparable effects for the EMP and for the pulse of a simulator.

2.4 Emittinq Antennas

. .

)

It is possible to use a large var~et~ of antennas to generate apulse of radiation, and each conflffuraticm has advantages anddrawbacks. These can be Studiefi theoretically for simpleconfigurations, which ,can later be rndifiei!, and the changec? fieldscan he determined experimentally. The rac?iator can vary from a simpledipole, represented by a t!hinwire, tn bicones, cylinders, tcvmi$s,and their combinations. The generators can provi$e either a singlepulse or a continuous wave type of operation. The size of the antennais related to the level of the fields that it produces anfl the area ofillumination, and the desiqn is lirited by conflicting theoretical andpractical considerations.

2.4.1 TEMPS

An example of a pulse generator is the transportable EF?P simulator(TEMPS). The pulse is produced by a spark in a Marx generator andradiated by a two-part antenna. It has a high-frequency launcher inthe form of a b$conic antenna, which is responsible for the early partof the pulse with a rise t3.re fro~ 3 to 10 nsec, and a low-frequencyradiator consisting of a tapered dipole formed by a number of Ionqwires and terminated through resistive loading to earth grounc?. Thetotal length is approximately 300 mt and, this cylinder is effectivelater in the pulse, which has a decay tire of about 800 nsec. Theinitial peak Is relatively well defined, but the later period j.s

characterized by a considerable amount of oscillations; the overallshape, then, has to be comparer? to the double exponential assumed forthe EF@.

The amplitude of the field can be varied within certain limits byregulation of the voltage and the pressure in the spark gape but low-level fields have to be obtained by location of the TEMPS far from thetest area. The maximum field reaches threat level in an area onplane

theof symmetry of the TEMPS about 50 m from the bicone, a location

where the field is highly inhomogeneous and which is well within theinduction zone for the decaying part of the pulse.

The free-space fields can be calculated either from the currentsor from the measured fields above the ground. The peak values of theamplitudes show that the fields originate near the center of thebicone for the early times, and ~he magnitude decays roughly as theinverse of the distancefields.

to this point, as expected for radiation‘.

,.

. . . ,. .. - . --- . . .-.. “..=~=,,

. .. .“. ‘...

,

2,4.2 Continuous-Wave Radiators

Another possibility for an emitting antennaproduces a pulse that is repeated at a high ratealmost monochromatic wave. The response of thefrequencies can be measured easier and results arethe other hand, the frequency content of the Pulse

is a source thatand that emits ansystem for highmore precise. Oncan ranqe over a

wider spectrun”than is ~eadiiy attainable with ‘a CW sourc=, and thestrength of the wave emitted at a single discharge is generallyhigher. Furthermore, so that how a node responds to a pulse from CWmeasurements can be inferred, computation is necessary by use of theFourier transform of the signal, which not only demands a knowledgeof the response to a sufficiently large part of the spectrum, but alsoassumes that the system is linear in its interaction with the EMP.

2.5 Current Iniectors

The field of a pulse generator such as the TEMPS effectivelycovers a relatively small area, which is not adequate to simulate theeffects of currents induced by the EMP on long cables, known to bean important coupling mechanism between the pulse and the node. Thus,the pulse generated by the simulator has to be combined with the pulsefrom a device that injects currents on the shielding of long cables,which in turn will transfer in the appropriate manner to the internalconductors. Some current injectors are basically charged capacitorscoupled to the shielding of the cable so that a current pulse of theright magnitude and shape is produced when they are discharged. Otherdesigns with inductive couplings are possible, also.

The current pulse induced in a long cable can be determined to areasonable approximation from analytical models, scale modelling, andpast experience. The pulse is different for overhead and for buriedcables, especially in rise time; it is much larger in buried cables.Current injectors can reproduce either shape, regardless of the actualtype of cable they are attached to.

A problem that arises is the synchronization of the simulator withthe current injectors on different cables. It does not appear to befeasible to preset the firing time of the TEMPS, for instance, withina few nanoseconds, which would be required to coordinate the peaks ofthe corresponding pulses. The pulse of file simulator itself probablyhas to be used to fire the current injectors, and it is questionableto what extent this procedure reproduces the effects of the peak ofthe fields. If the rise time of the injected current pulse were muchlonger than several nanoseconds, it would not contribute significantlyto the evaluation of the effects of the initial peak, but only ofthose of the ringdown of the system at later times.

2.6 Voltaqe Pulsers

Voltage pulsers use some type of waveguide, such as parallelplates or wires, in the immediate vicinity of the equipment to betested, coupled to a voltage generator. It is a matter of interest todetermine to what extent the fields of a wave propagating in freespace are reproduced by those of a wave propagating in atype of

particularwaveguide, especially in the presence of other conductors

inside the waveguide. They could be used also with an emittingantenna to produce a particular local effect.

15

- T-- —-=- ——.——-—---— — .

.’ .’,,

,,.,

. .

3. COMPLEMENTARY TESTING

3.1 Bench Tests

The node as a whole should be subjected to pulses so that, as faras possible, upset and damage are limited to a minimum. The readingsof currents, voltages, and fields then have to be correlated to thepossibility of damage to the critical components.

One way to test a component is to remove it (or another with thesame characteristics) from the node and subject it to bench tests. Inthis way, it can be subjected to pulses of current, voltage, orelectromagnetic energy, and the failure rate under given conditions canbe determined. The values of the parameters that cause damage thenare compared to those expected in the different locations in thesystem where this component is found.

One major difficulty is the determination of the relevantcharacteristics of the pulse that cause the upset or damage. Itcould be the rise time, the frequency of oscillation,the power, the total energy, and so on. If this aspectis neglected, results obtained from the wrong kindCW can be irrelevant to the actual EMP threat.

Another important observation on such a testingdetermine the uniformity among different units of thecomponent.

the amplitude,of the testingof pulse or

program shouldsame type of

Some type of bench testing can be extended to also include smallsubsystems of a node.

_}o

) ,0...-.3.2 Scale Modelli.nq

It is possible to set up a model of a system to be tested in alaboratory, so it can be subjected to the effects of pulses from acorrespondingly reduced antenna.

The wave equation in a conducting medium is

(22)I ~2jj- cpa2Z/3t2 - p0ai3/at = o ,

I 1 .,

1 :: ;“’”s. .,

which is invariant under d transformation

.

That is, if the model is of a size 1:100 of the oriainal,the rise timet

of the pulser has to be 1/100 of.

the site, andthe conductivity (of the ground,

. . .

the one to be used-onfor instance) has to

16

.,.. . ..

be increased by

-)‘o

~,.-!. . .

.’

,= ~.

., . . ...= .-~--—. ~..----

,. .,’ 1. . . .

. . . ,. . ..’.

● La factor of 100. The permittivity c and the permeability ?-Iwereassumed to remain constant, and in practice, they are more difficultto change than the conductivity. In a real situation, v is nearlyequal to the constant Vo’, but c varies significantly with thecomposition of the surface (for instance, the water content of theground) and with frequency. Strictly speakinq, Maxwell~s equations inthe form of equations (5) to (8) are no longer valid when c dependson frequency.

Such scale modelling could be useful for experiments in which thelocation of the pulser is changed for finding maximum coupling and thecurrents induced in long cables and for similar testing that isdifficult on the original site. The question of the detail. of themodel limits the validity of the answers that are obtained in thismanner.

3.3 Subsystem Testinq

Experiments can be designed to test a subsystem of the node that isparticularly sensitive or a replica of such a subsystem--for example,another computer.

This approach can be useful also for validation of analyticalestimates of upset and damage, which become progressively lesspractical as the size and complexity of the object of the testincrease.

Results for different subsystems can then be combined so that howthe EMP affects the node can be predicted as a whole.

Cn a different scale, a similar synthesis has to be carried out, sothat how a HEMP affects a communications network can be evaluatedafter the responses of the nodes have been determined.

4. RESPONSE OF A NODE TO A SIMULATOR

4.1 Introduction

It is quite obvious from the foregoing discussion that thesimulator produces a pulse that differs significantly from the actualHEMP . The consideration of these differences is important for a validassessment of the hardness of the node.

One approach is the simulation of the worst possible case of EMP,increased by an additional safety factor against errors andmiscalculations. Such a requirement would be in all likelihooduneconomical in both the testing and the demands on the hardness ofthe system. Thus, a more modest view of a test program should sufficeto ascertain the hardness of the system with a sufficiently largeprobability.

Within this framework, an understanding of the interaction betweenthe node and the simulator allows the most efficient test program tobe followed. Scale modelling can be of great help in thedetermination of optimal coupling between the system and tie pulse.Also, the design of the simulator is determined to some extent by the

17

. .

,,. . .

relative importance of the parameters of the pulse that it produces,with respect to the coupling to the node.

I

4.2 Location and Orientation of the Simulator

In each test, a choice has to be made as to where the simulatorwill be located with respect to the site, how it will be oriented,and, to some extent, what shape it will have if the simulator can bedeformed. The currenk or voltage induced at a particular componentdepends on these variables and is maximal for values that can bedetermined. In general, though, this configuration differs for eachcomponent and many times depends further on the internal connectionsof a node, such as a switch, at the time of the test. someconfigurations induce a larger response throughout most of the compo-nents, and others most strongly affect some critical subsystem; amongall these configurations, a reasonable number has to be selected for avalid assessment of the hardness of the system. They should bedetermined beforehand from theoretical analysis, scale modelling,andpast experience; if this timing is not possible, they should beevaluated by a change in the configuration of the simulator, until apattern emerges.

The simultaneous use of current injectors gives more flexibilityto the testing, and their input can be calibrated to give a reliableupper bound.

4.3 Level of the Test

It is desirable that, at least in some of the testing~ theintensity of the pulse be of the order of magnitude of that from areal threat situation, so that the ever-present nonlinear effects canbe observed. Once the characteristics of a certain type of equipmentare understood, other similar units can be tesked at low level, todetermine the coupling to the EMP.

There is a lower limit to the intensity of the pulse produced by asimulator such as the TEMPS. Positioning the simulator farther awaydecreases the level at the test site, but such action may berestricted by terrain. Under difficulty, a different and simplersimulator can be used for low-level tests, which can also be used tocollect data that are easily gathered with a small device, but becometoo costly with one like the TEMPS. This procedure could includeeffects of the direction of propagation and polarization of thepulse.

4.4 Connectivity Effects

Zn a node such as a switch, the particular interconnection in thesignal network is always one of a large number of possible ones. Thevery, one is determined partly by the number and type of calls beingprocessed and partly at random. It is not practical to test allpossible configurations of the switch, and they have to be groupedinto states that show small dispersion of the measurements within agroup. If the system is sufficiently complicated, a statisticalapproach to the failure of a type of compnent is suggested.

18

~.”.-

, .,

.. . .. . ..-.=~.-.

..— -.. .>. . .!. ,,

. .,

,’. . .;’, . . ‘,

,.. ,.’.> .,”

,

,,, , ,,..;

.,, ..-.-..

o’ L4.5 Degradation Effects

At low-level testing, the facility is not changed by such tests,and successive pulses find the same physical system. On the otherhand, in threat-level testing or in a multiburst EMP environment,damage to a number of noncritical components can change significantlythe coupling characteristics of the node. A careful determination ofthe most sensitive components can indicate the pssible changes in thenode, but in general, such a prediction would depend on too manycircumstances to be reliable.

It is not clear to what extent it is desirable to perform such apartially destructive test. Possibly, tests on replicas of subsystemscould indicate the importance of the changes introduced in the systemby the failure of some of the components.

4.6 Synchronization of Simulators

e ‘c

When a pulse sweeps over a facility, it reaches different parts ofthe building in a well-defined sequence of time intervals, whichdepends on the direction of propagation of the pulse. It alsodetermines the characteristics of currents induced in long cables asfunctions of angles of incidence.

A decision has to be made about the degree of accuracy with whichthe current injectors, for instance, are going to reproduce thedifference between the effects of the simulated pulse and the EMP. Ifthe current injectors are triggered by the pulse from the mainsimulator, their main contribution to the testing is limited to powereffects of the latetime part of the pulse and the nngdown of thesystem.

4.7 Resonant Modes

Experience with simulators and EMP testing indicate that thegeneral characteristics of a time-amplitude trace are an initial peakand a subsequent oscillatory part with a slowly decreasing amplitude.Which part of the pulse affects a particular component depends on thenature of the upset- or damage-causing characteristic.

4.7.1 Direct Effects of the Incident Wave

The incident wave results in a peak in the current or voltage curvethat roughly corresponds to the peak of the incident field. For asimulator, the wave is generally characterized by a short rise timeofa few nanoseconds, a fairly high amplitude (possibly up to a maximumfor threat-level tests) , and a similar decay time. Components thatare affected by such a type of signal, even for a fairly low level,might be upset. For instance, digital information transmitted byessentially square pulses can be contaminated by such a peak.

~is wave might b-e contrasted to the double exponential waveformassumed for the EMP. The EMP waveform has a similar rise time, butthen decays slowly from the maximum without oscillations. It is notclear to what extent this double exponential waveform represents anoveridealization of the actual waveform.

19

—— .. .., #

., . .

I

Q.7.2 Late-Time Oscillations

The response of the node tothe time pattern of the pulse,

the incident pulsebut for later times,

)

initially followsoscillations build

up that can exceed in amplitude that of the initial peak. Thatoscillations build up represents a resonance phenomenon in which thebehavior of the perturbation induced in the node is no longerdetermined by the incident pulse, but obeys the constraints imposedsolely by the internal structure of the node. The energy absorbedfrom the pulse might in this manner be concentrated on certaincomponents and damage them quite independently from the direct pulseeffects.

5. MEASUREME~S

5.1 Desiqn of Probes

Some of the quantities to k measured at appropriate locations inthe facility are currents, voltaqes, and components of the electricand magnetic fields. The signals are picked up by probes andtransmitted through dielectric waveguides to oscilloscopes, whichdisplay the time-amplitude traces.

Many considerations should go into the design of these probes fora reliable measurement. The probe itself should not distortappreciably the quantity to be measured. This effect might beimportant for ,the measurement of the electric field nat a pointlf bythe placement of a sizable metallic box. The signal transmitted fromthe probe alsO can differ signi.f%cantly from the quantity that isbeing measured by intrinsic characteristics of the probe, such aSlimitations at the high-frequency end of the spectrum.

for

5.2

Physical size of the probes, too, can limit the choice of locationthese probes.

Location of Probes

The decision on where to place a probe acquires a specialimportance when the number is small and limits the collection of thedata. The time between firings of the simulator and that neededfor relocation of the probes can easily be 10 min.

It is desirable to place a probe as close as possible to acritical” component of the system. If it is not possible to do so dueto the physical configuration of the facility or to the size of theprobe, how the pulse affects the component might have to beextrapolated by analysis.

Other general guidelines obtained from past experience in EMPtesting suggest thak those components closest to the entrance paths oflong cables are more exposed to failure than those removed from these

1

I

o

20

. .\... ,

,.,,.. .,. -~, -- ..’. ,“

,,‘1

. ... ,’. - -,, .

,,

.,

0 L

Q ‘(

entry points. HOWeVer, this statement is not necessarily true for acomplicated system where other points of concentration of energy mightoccur. In general, high currents on the shielding of a cable indicatea correspondingly large (on a different scale) current transferred tointerior conductors.

5.3 Qualitative Observations

In any record of a test, the testing personnel need a means torecord observations that cannot be quantified or even predicted inadvance. These might be visual observations on the equipment ordamaged components or circumstances that could have affected theoutcome of certain tests.

In general, a description of the damage to a component, beyond thespecification of its identity, would require such an entry.

5.4 Functional Evaluation

Facility degradation due to upset of _components or subsystemsgenerally is not described by the results of measurement with probesor by qualitative observation, although such observations might beuseful to record gross malfunctions of the equipment. Detection of asmall-scale upset of the facility might be difficult, and detailedchecking must be programmed so that no fault is overlooked. When acomputer is part of the system, a program that finds any changes inthe stored information is a valuable part of the test measurement.

Also, it is desirable to test all the different modes of operationof a facility, so that damage or upset in a seldom-used, butpotentially important, function is not overlooked.

6. ANALYTIC MODELLING

6.1 Introduction

The theory of the propagation of electromagnetic waves and theirinteraction with matter on a macroscopic scale is well established andexpressed in a set of partial differential equations, Maxwell’sequations, which apply widely. Difficulties arise when systemresponse to EMP is evaluated. These difficulties are thus not basicin nature, but brought about by the complexity of the system and whethera node can be successfully modelled in this manner thus depends on itssize and complexity.

On the other hand, the generation of the EMP and the damage ofcomponents are much more complicated in principle. An analytic modelcannot be relied on for correct results, even if carried out withgreat precision.

6.2 Representation of the Pulse

There are relatively little experimental data on the EMP generatedby an exoatmospheric nuclear burst, and most models of the pulse havebeen generated analytically.

21

.!

,, ,, ,.

The order of magnitude of the amplitudes at different locationscan be calculated, but the shape of the pulse probably depends morecritically on the detail of complex processes in the interaction ofelectrons and. ions with the primary products of the burst. In such asituation, it is just as useful to use a convenient analyticalexpression for the wave shape, such as the double exponential, as amore complicated shape tl-mt is not likely to be closer to the actualEm. In other calculations, it might be more convenient to use aslightly different analytical formula that gives a curve of roughlythe same characteristics.

The pulse produced by a simulator is much better known and couldin principle be calculated from the known configuration of the pulsegenerator and the emitting antenna. Uncertainties associated with agaseous discharge and complications due to the actual structure of thesimulator do not make this a very attractive alternative to themeasurement of the pulse shape. On the other hand, such calculationsare important in the design of new and better types of simulators.

6.3 Representation of Components

Single components such ~S transistors can be reasonably wellrepresented by circuits with a small number of parameters. Thesecircuits can then be used to model the functioning of much morecomplicated devices, but a limit is soon reached when the number ofcomponents goes into the thousands.

These equivalent circuits also are designed to represent thenormal behavior of a component and might give a completely uselessextrapolation under conditions leading to damage. Another limitationof such an approach is the failure to take into account the absorptionand emission of radiation, which is important for pulses orsufficiently high-frequency signals.

6.4 Calculations for Lonq Cables

It is relatively simple to model long cables by infinitecylinders, and also a large body of experience with linear antennas isuseful for short segments of a line.

AS is generally true in calculations of this nature, the presenceof the ground complicates significantly the boundary conditions in theproblem.

Approximate results should be applied carefully in extreme cases.For instance, the current induced in an infinitely long, perfectlyconducting cylinder by a pulse in the shape of a double exponentialwas found~ to decay like I/(log t) for late times. This very slowrate of decay is unphysical and is due to the neglect of theresistivity of the cylinder. If it is taken into account,~ theinduced current decays exponentially for late times proportional toamplitude of the pulse. Another example is furnished by the currentinduced in such a cylinder by a wave when the direction of propagationis almost parallel to the axis; then the current tends to infinitywhen the angle “tends to zero for a perfect conductor, but the currenttends to zero when the resistivity is properly taken into account.

1P. 1?. Barnes, EMP Interaction Notes, Note 64, Air Force Weapons Laboratory,Kirtland AFB, NM [March 1971).

2E. Marx, Harry Dianvnd Laboratories, TR-1617 (January 1973).

22

...

.

,.

., . . . . ..,

,,,. . . ,,

0 LThe importance of the energy that is coupled into the system

through long cables increases as the shielding of the node itself isimproved. Accurate calculations of the currents induced on theshielding of the cable and those transferred to the internal conductorare of great help..

6.5 Res ponse of Small Subsystems

Analytic modelling of Prts of a system should be developed sothat a test can be evaluated on components that are not directlyaccessible to the probes, and critical parts of a node can be studiedwhen the inputs can be determined.

If the node is small and simple, an analytical model might sufficefor evaluation of its response to EMP, especially if it is wellshielded against the direct effect of radiation. If so, reliablemethods are needed so that the energy picked up by cables can becalculated.

6.6 Singularities Expansion Method

For several different configurations of antennas, the response atlate times is a combination of damped exponential.3 This generalbehavior has been interpreted as a manifestation of the dominance inthe Fourier transform of poles that are close b the real axis, whencethe transfer function is approximated by an expansion of the form

F(i))z Ziai/(u-ui) , (24)

where the ui are the locations of complex poles. Such anapproximation dominates the behavior of the time amplitude after theincident pulse has passed over the antenna.

These concepts can be extended to more complex systems, especiallybecause the expansion in the singular terms has given good results forantennas of quite different shape. Furthermore, measurements of EMPeffects on fairly complicated systems show the presence of dampedsinusoids for late times.

Thus , it would be possible to describe the late-time ringdown of anode by such an expansion, and the locations and strengths of thepoles could be determined by analysis or, more plausibly, frommeasurements. This transfer function is independent of the pulse usedto excite the system.

6.7 Modellinq of a Node

For a relatively simple node, analytical methods discussed abovecan give a fairly accurate prediction of its response to EMP. cm theother hand, as a node grows more complicated, more general principlesare needed to avoid detailed calculations, which become impracticaland unreliable. As in other problems, a large degree of complexity

r

3C. E. am, EMP Interaction Notes, Note 88, Air Force Weapons Laboratory,KirtlandAFB, NM (December1971).

23

.

eventually means a simplification, where the statistical laws becomemore important than meaningless microscopic predictions. It is thennecessary to find the relevant parameters that describe the system anddetermine the reliability of predictions obtained in this manner.

6.8 Complex Systems

The EMP covers areas of millions of square miles, and no simulatorcan reproduce the simultan-us effects on many different nodes. It ispossible to use several simulators, but the synchronization wouldpresent great difficulty. Because the links between nodes in acommunications system are not particularly sensitive to EMP, it isreasonable for researchers to rely on the experimental data on thenodes and calculate through an analytical model the overal~reliability of the communications network.

I

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