30-06-2012, 06:03 PM
Electromagnetic Pulse
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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 nearsurface
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 nearsurface
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.