EMI SOURCES, TRANS"TS RELATED TO LIGII'I'NING AND E m

BY

MAJOR ROHIT lFHUKLA ARMY EMC AGENCY, MHOW

Abstract

1. The electromagndic pulse (EMP) produced by tile detonation of a nuclear explosive is a large, fast rising transient generated by the interaction of gamma radiation and Compton recoil electrons with atmospheric air molecules arid the geomagnetic field.

Introductioq

2. Concem of the nuclear EMP as a source of interference has led to analysis of its generation mefhanisrti, interaction of structures with the EMP and the tolerance of communicatiodpower/ military systems to the EMP induced Gansients. Experimental techniques for evaluating interaction analysis and for determining system toletancis have also been developed.

3. Although the energy content of EMP is not very large because of its short duration, however very high voltage or current can be induced in components of electronic or weapon systems when EMP energy couples into such systems. These voltages and currents can in turn, cause temporary or permanent damages affecting system missions.

4. The intensity of EMP field is so large that often it does not need an antenna to collect the damaging energy. For example, it is not uniisual for the EMP energy to reach a circuit or equipment inside a shield or enclosure by a direct penetration through the shield or through cables leading to the equipment or through doors, windows and seams of a shielded structure. Very often, the exact or nearly exact magnitudes of voltugex, currents or fields induced by the EMP in a circuit or equipment is not of concern to the user or designer of the system, since their ultimate objective is to detennine the susceptibility and vulnerability impact of tlie EMP in their systems and possible remedies to eliminate, deter or circumvent such an impact, as and when needed.

EMP GENERATION FR OM SURFACE NUCLEM BURST

ComDton Process

5. The principal driving mechanism of EMF' generation is Compton electrons generated by nuclear detonation - induced ganuna rays interacting with matter which is usually the air in the atmosphere. These so-called Compton collisions knock electrons out of the air molecules, and the freed Comptbn (recoil) electrons move rapidly away from the centre of the burst and from their, now positively charged, parent moleculcs. This large scale separation of charges creates 8 strong non-radiating electric field between

the electroiin ai i t l parent ions. The movement of these charges produces tkw ('cimpton current.

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Time

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Frqvency

Radiated E M P Fleld Proiects as a Plane Wave

6. A iiidiatcd EMP field is generated by an unbalance in th is C'oiiiidon current caused by asymmetries in tlie snwmdliig wvitonment. For bursts on or near earth's surface, NIII 11 t q " e t r y is produced by the air earth

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fn case of exo-atmospheric burst (> 40 Ian) the Complon electrons interact with the geomagnetic field to produce Em.

Cbrsification of EMIP Environments

7. EMP considerations :-

(a) Deposition Region (Saurce R a o a ) . l'he deposition region is the space around the Byrst where the EMP is generated. It wntains intense electric and magnetic fields 88 well a~ highly conductiqg plasma (ionized gas). The deposition region is limited lo a radius of 3 to 6 km around a surface burst. (b) Radlatlon Region. Depending upon the burst height, the asymmetq between air and earth causes tHe same fields to radiate well beyond the deposition region. It can ?use charge to flow in very distant bM good conductors such as metallic

'Illis radiation containb somewhat less intense fields that have three general characteristics :-

Two regions surrounding the burnt are important in

structures, antemas arid electrical wiring.

(i) The direction of propagation is readily outward from the burst. (ii) The E & H field vectors are in a plane perpendicular to the direction of propagation (in other words field prqpagates locally as a plane wave). (iii) The fields have a far field-range dependence of 1/R.

EMP THREAT SCENARIO

Threat In Deposition Reglon.

8. The potential hazard to electric and electronic equipment from Eh4P will be greatest within and near the deposition region, extanding to 3 to 8 lun depending on the weapon yield.

Threat in Radhtloa Reglon

9. When the systems and equipment are not positioned in #e source region but are paced in the radiation region they will experience EM field of various dimensions. This field varies with the distance.

RMR Environment Interaction

10. A system undergoes following three types of electromagnetic @eraction with its environment in case of surface nuclear burst :-

(a) Electromagnetic field. (b) conductivity. (c) Direct interaction.

11. In case of elcctromagnetic interaction, the classical 'antenna' type problem predominates. This is the case outside the EMP source region where the EMP is a radiated field (plane wave).

12. 'Ikc conductivity interaction is obviously not independent of the electric field interaction, since there would be no conduction current without an electric field. However, it is clunniliod separately from electromagnetic field interactioii tuwuse of the signiticant changes which can occur in a systeiii's response when the medium is conducting (either naturally ccindiictiog or ionized by weapon radiation).

E W PROTECTION MEASURES

System €Mpoiise

13. EMI' e k t s in a system are classified as operational upset or funclioraal damage. Operational upset refers to the casewhere tho system rwponds to an EMP induced transient nR it wore norinel idormation andm erroneous output results. I'emlanent ptrametric changes or catastrophic failures are referred to an t*bnctional damage. Thc allowable degradation iii a piece of' equipment is dependent on the role which that equipment pluys in the success of a mission. The transient loss of inlbrinution on audio line for example, may have negligihle cllbct on communication channel. Whereas the operatioriul upsets of a missile computer may cause it to miss its intalcled (ergot.

14. I+xlionic circuits are generally the most sensitive components of a system to operational upset. The susceptibility of a circuit depends on the time signature of the EMP excilalioii and the terminals on which this excitation appears.

Design Giiitlcliner for EMP Protection

15 through Following three mechanisms :-

PMI' energy is going to penetrate inside a system

( t i ) Direct (hardware) penetration (I)) Aperture penetration, and (c) Diffusion

16. l'hc direct (hardware) penetration basically act like iuitenncis oii I[MP field. In general, in a system two types of lultetlnus ntnictures can be identified from EMF' field interaction ruid coupling point of view :-

( 1 1 ) Deliberate ant&a, and (I)) Nondeliberate antenna.

17. 'I'lic tlcliberate antennas in the frequency range of VLF to 1.11 II' will couple more EMP energy as compared to antenna ui the microwave frequencies, because of their dimensions. The non- deliberate antennas are used for some other purponen but they act as collectors of EMF' energy. Even source region Compton current and time varying conductivity is of greul Higiiilicance to calculate the induced currents and voltages.

Aprtunt I'ciietration.

18. 'l'hc "apiture penetration" is equally significant in cnse of 13MT' protection measures. In fact the main sources of coupling belwwn the EMF' field and vulnerable equipment we the lack ol' shielding and bonding. This lack of shielding occurs wtioro doors and hatches are left open or when the

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@dating seals round the doors and hatches are damaged or are not present.

Diffusion.

19. Diffusion is a h c t i o n of skin depth of the shield. It is mainly a low frequency phenomenon. The penetration through difhsion is not that SiBRifiCant as hardware and aprture penetrations.

Hardening of a Syrtem

20. effects of Ehap can be viewed from two a@es :-

In general, the hardening of a system against the

(a) Tho el~-c topological apprwch to hatdening. (b) EMP Mitigation teuhdques.

The Topological Approach to Hardening

21. In order to harden a system, it is usell and convenient to first identi@ its electromagnetic shield topology. Identification of this topology establishes zone boundaries or surfaces through which transient energy passes to reach sensitive electronics. The energy uin pass through these zonal boundaries as discussed above - by field diffusion, through apertures and by conducted transients. In the toplogical approach various shielding layers are idenMied which separate regions of space into distinct environments. For example, in case of communication shelter, the shield layers could be the metallic or non metallic structures of the shelter, equipment bays, and the individual shields of components. Also the topology of the ground scheme should be defined in the beginning. Between the shield layers are the zones for which environment^ can be established.

22. Once the zone cund the shielding layershave been identified, the next step is to determine how much harderung should be achieved at each layer. This process is called 'allocation'. The allocation must be reasonable in terms of c a t and reliability, thtlt is, the allocation for a g i ~ e n layer muat be able to be readily abtainable at a reasonable cost.

EMP Mitigation Techniques

23. The detailed description of the EMF' mitigation techniques, which may be applied in the zones or at the zonal boundaries may not be possible, but the technologies involved are briefly introduced :-

(a) Shielding. The outside skin (if highly conducting) of a system usually provides the &st layer of shielding. Additional shielding is provided by equipment bays and shielded boxes. Energy penetrates through volume shields by means of diffusion, through apemues, seams and conductive penetrations. For example, in case of metallic shelter, dimision is not the most significant penetration in the radi&tien region of a surface burst, except that at tho late times current - redistribution effects can produce iaternal magnetic fields which

i n q be of consequence. But for a non metallic wsluinu, dif€usions can provide a large penetration Iluld. Improving the shielding characteristics of cniipsite skins is usually accomplished by applying ti int.lullic layer on the outside surface. The following techniques are adopted :-

(I) Flamespray (ii) Metal foil (iii) Wiremesh,or (iv) Conductive cloth

All these measures have weight and cost penalties, un well as camsion problems associated with making good joints. Techniques which can be used to harden apertures inclkde the use of :-

(I) Wiremesh (ii) Conductive coating (iii) Honeycomb filters, or (iv) Waveguide below cutsff

(b) Terminal Protection Devices (TPDs). Thc TPDs used may be both linear and non linear devlwx. These devices are usually applied at the ahlc penetration of a zonal boundaiy :-

(I) Non-linear. TPm include metalsxide varistors (MOVs), spark gaps, zener diodes. When choosing these devices for hardening applications, one not only has to include their hc&d properties, but other properties such as reliability; insertion. It is important to Mte that non-linear TPDs do not typicat& absorb as much energy as they reflect, thus although the transient energy is preventud h d"g a circuit, it appears somewhere else which also has to be considered. Also because of their non- 'hear properties, the TPDs change die spectrum of the transient energy. Another problem some times occurs in RF circuits, where the presence of a nonlinear TPD will prodwe undesirable intermodulation products.

(ii) Software Techniques. There are numerous techniques which utilize computel s o h a r e to determine if an upset

(ea) Cireumven

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recycling stored pre-event dah or simply waiting for a new set of data. Requesting fm immediate retransmission involves a return link to the data originating system and a memory in that data system. The data receiver must have the capability to receive delayed data. This added complextty increases system costs and weight and decreases reliability. A separate external antenna or probe cfm be used to detect cable currents in electronic packages. (ab) Check Point Roll Bark It is a process where critical information is periodically arid routinely recorded on a backtip or a redundant storage medium. When an upset fault is detected, any loss of information is stored from the check pointed copy and a restart is performed. (ac) Plausibility Check. T h i s verifies that information being processed or which is the result of computations, falls within realistic bounds. (ad) Process Redundancy. It involves the multiple execution of a process and comparison of the results and it is therefore useful for fault detection. (ae) Time out. It is the time interval allotted for certain operations to occur, for example, response to polling, addressing, and performance of VO. A time out error occurs if the task takes more time thrm that of the allotted one.

CONCLUSION

24 IF I I vital system has to survive from the effects of blast, Ilicriiiiil und TREE in the source region of surface magnelic IluldH, it has also to survive from the strong electric and i~riyiiullt llelds present in that region. Moreover, the large tri~nirit~iiln will conduct on the long transmission and telephono IIIIUH uhich are passing through the deposition region oi rritl l i it ioii region of a surface burst to a larger distance iiiiit Iliosc conducted transients will be of sufficient amplitude to damage the connected equipment even if they are very far OR‘ from the ground zero level of the burst. Besides, 1hlh low frequency magnetic field also poses a formidablo Iliicul to the equipmentkystems present in the source regtoil I’hc severity of this magnetic field is not only of concoiii lo Ihc systems on the ground but also to Uie undergroiiiitl qdims. The complexities involved in devisiiig mitigu(ioii~irirdctiliig techniques against effects of EMP and lightening oii clech-ical and electronic components requires a co-ordinalcd reseiirch work by all agencies working in the same direction, 8s EMP can have far reaching effects, in the future bulllcl~cld.

Jleferenw

[l]. EMl’ liiluraction : Principles, Techniques & Reference Data.by K 8 I I IElL [2]. EMC’ Ilavironment and system hardness design by Dr Rabindra N lhosli. [3]. ‘Ellucllt (11’ E M P from surface Nuclear Burst’ by LRDE, Bangaloi U

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