EMP.pdf

The Electromagnetic Pulse (EMP)

This is an essay I wrote about EMP. The sources may not be cited as well as

they should, but some documentation was lost when converting to html. If

you want a better citation, please ask.

Introduction

Nuclear weapons can have devastating effects. Usually, one thinks only of

the blast, thermal, and radiation effects as they relate to the human body.

However, considering only these factors ignores some of the other

devastating effects. One such effect is that of the nuclear electromagnetic

pulse (EMP). The effects of the nuclear electromagnetic pulse must be

considered and calculated when preparing for a nuclear war.

This essay will try to describe what the electromagnetic pulse is. It will then

explore the types of bursts that produce different pulses, and the possible

effects of the pulses will be examined. Next, the ways to guard against EMP

will be examined. Finally, the policy issues concerning the vulnerability of the

United States will be explored. To achieve these goals, three basic sources

will be used to describe the technical aspects of the pulse. Once this has

been completed, several journal and magazine sources will be used to

consider the vulnerability and policy issues. This format will create a

technically based essay. From this science base, several observations of

vulnerability will be made to evaluate the United States’ policy and strategy.

EMP Physics

Early on in the development of nuclear weapons, the presence of the

electromagnetic pulse was known. Before the July 16, 1945 Trinity test,

Enrico Fermi had tried to calculate the possible electromagnetic fields that

would be produced. Unfortunately, the actual effects of the EMP were still

not truly known. It wasn’t until the mid-1960s that the true nature of the

EMP was better understood. However, even then, many of the possible

effects, like other nuclear weapon effects, were not well-known due to the

lack of data.1 The basic theory of EMP is now well understood.

In a nuclear detonation, gamma rays are produced. These gamma rays

interact with the surrounding air molecules by the Compton effect to

produce electrons. In this effect,

"...the gamma ray (primary) photon collides with an electron and some

of the energy of the photon is transferred to the electron. Another

(secondary) photon, with less energy, then moves off in a new direction

at an angle to the direction of motion of the primary photon.

Consequently, Compton interaction results in a change of direction (or

scattering) of the gamma-ray photon and degradation in its energy. The

electron which, after colliding with the primary photon, recoils in such a

manner as to conserve energy and momentum is called a Compton (recoil)

electron"(2)

These Compton-recoil electrons travel outward at a faster rate than the

remaining heavier, positively charged ions. This separation of charges

produces a strong electric field. The lower-energy electrons produced by

collisions with the Compton electrons are attracted to the positive ions.

These ions produce a conduction current. This current is directly related to

the strength of the Compton effect. Also, this conduction current flows in a

direction opposite to the electrical field produced by the Compton effect.

Because of this, the conduction current limits the electrical field and stops

it from increasing.(3-5)

Varieties of EMP Explosions

There are three main types of explosions to consider when examining the

effects of the electromagnetic pulse. These are near-surface busts,

medium-altitude bursts, and high-altitude bursts. Near-surface bursts are

those at altitudes up to 1.2 miles, medium-altitude bursts range from 1.2

miles to 19 miles, and high-altitude bursts are those above 19 miles. These

altitudes are only rough guidelines, but a better understanding of where

each occurs will be gained after examining each type of burst briefly.(6)

The greatest effect on surface bursts is caused by the ground. Unlike in the

air, the gamma rays cannot escape the blast in all directions. For this reason,

near-surface bursts are also in this category. Although they may not be on

the ground, they have similar effects. The ground absorbs many of the

gamma rays. This produces an asymmetric field. The resulting field is very

similar to that of a hemisphere that is radiating upward. The electrons also

are able to return to the burst point through the ground. This makes the

area near the center of the burst contain a high concentration of highly

ionized particles. This net movement of electrons creates current loops that

generate a magnetic field running around the burst point. This is the basic

model of a near-surface burst.(7)

When the nuclear explosion occurs in the medium-altitude range, the effects

of the ground are much. A medium-altitude range would be away from the

ground but below the upper atmosphere. As the height of the burst

increases, the asymmetry of the field produced decreases. However, the

asymmetry increases, after a point, with altitude due to changes in the

atmospheric density. This asymmetry can be seen in Figure One.

Figure One--Approximate variation of an asymmetry factor relative to a

surface burst as a function of altitude8

Since the ground is absent, the magnetic field produced in near-surface

bursts will be absent. The electric fields will be similar to those of near-

surface bursts.(9)

High-altitude electromagnetic pulses (HEMP) produced by high-altitude

bursts occur in an area of the atmosphere where the density of the air is

low. Because of this, the gamma rays can travel very far before they are

absorbed. These rays travel downward into the increasingly dense

atmosphere. Here, they interact with the air to form ions as previously

described. This region, called the deposition or source region, is roughly

circular. It is thick in the middle and thinner toward the edges. It extends

horizontally very far creating source regions that are over 1000 miles in

diameter.(10) The size of it depends on the height of the burst and the yield

of the weapon. The EMP in this source region gets deflected downward

towards the earth due to the earth’s magnetic field. Although the fields

produced from a high-altitude burst are not as great as those for a near-

surface burst, they affect a much larger area.(11) Because of this huge

potential, high-altitude bursts could be the most dangerous type of EMP.

EMP Effects

The electrical field produced by the EMP only lasts a very short time before

it quickly tails off. The electric field has a rise time of about 1

nanosecond.(12) Even with such a short pulse, the effects can be

tremendous. For a high altitude burst, the effects can also be far reaching.

By many calculations, one properly placed nuclear bomb detonated above the

center of the United States could produce huge electrical fields on the

surface of the earth. "The EMP from a single hydrogen bomb exploded 300

kilometers over the heart of the United States could set up electrical field

50 kV/m strong over nearly all of North America"(13). Since EMP is

electromagnetic radiation traveling at the speed of light, all of the area

could possibly be effected almost simultaneously.

With such a possible threat, it is important to consider what may be

affected. "Because of the intense electromagnetic fields (about 10 kV/m)

and wide area of coverage, the HEMP can induce large voltages and currents

in power lines, communication cables, radio towers, and other long conductors

serving a facility"(14). Some other notable collectors of EMP include railroad

tracks, large antennas, pipes, cables, wires in buildings, and metal fencing.

Although materials underground are partially shielded by the ground, they

are still collectors, and these collectors deliver the EMP energy to some

larger facility. This produces surges that can destroy the connected device,

such as, power generators or long distance telephone systems. An EMP could

destroy many services needed to survive a war.

"Society has entered the information age and is more dependent on

electronic systems that work with components that are very susceptible to

excessive electric currents and voltages."(15) Many systems needed are

controlled by a semiconductor in some way. Failure of semi-conductive chips

could destroy industrial processes, railway networks, power and phone

systems, and access to water supplies. Semiconductor devices fail when they

encounter an EMP because of the local heating that occurs. When a semi-

conductive device absorbs the EMP energy, it displaces the resulting heat

that is produced relatively slowly when compared to the time scale of the

EMP. Because the heat is not dissipated quickly, the semiconductor can

quickly heat up to temperatures near the melting point of the material. Soon

the device will short and fail. This type of failure is call thermal second-

breakdown failure.(16)

It is also important to realize how vulnerable the military is to EMP.

"Military systems often use the most sophisticated and therefore most

vulnerable, electronics available, and many of the systems that must operate

during a nuclear war cannot tolerate the temporary disturbances that EMP

may induce."(17) Furthermore, many military duties require information to be

communicated over long distances. This type of communication requires

external antennas, which are extremely susceptible to EMP. Also, some

military duties require information-gathering techniques. Many of these

techniques use electronic devices connected directly to antennas or radar.

Although the devices may be inside shielded buildings, the antennas bring

the EMP inside to the electronics. Therefore, the effectiveness of shielding

must be examined.

EMP Hardening

There are two things to consider when considering hardening targets against

EMP. The first question to answer is whether the hardened system will

become useless if shielded. The second question to be answered is whether

the target is economically worthwhile to harden. The answers to these two

questions are used to determine what devices should be shielded

To explain the first consideration, Makoff and Tsipis give the following

simple example. If there was a communication plane with many antennas used

to collect and transfer data, it would not be useful if its antennas were

removed. However, to harden the plane, the antennas would need to be

removed because they provide a direct path to the interior of the plane.(18)

It is important to understand how the hardening will affect the performance

of the hardened item.

The second consideration is very easy to understand. Some systems,

although important, may not seem worthwhile enough to harden due to the

high costs of shielding. "It may cost from 30% to 50% of the cost of a

ground based communication center…just to refit it to withstand EMP," and,

"as high as 10% of the cost for each plane."(19)

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