Nuclear Weapon EMP Effects
Source: Federation of American Scientists A high-altitude nuclear detonation produces an immediate
flux of gamma rays from the nuclear reactions within the device. These
photons in turn produce high energy free electrons by Compton
scattering at altitudes between (roughly) 20 and 40 km. These
electrons are then trapped in the Earth's magnetic field, giving rise
to an oscillating electric current. This current is asymmetric in
general and gives rise to a rapidly rising radiated electromagnetic
field called an electromagnetic pulse (EMP). Because the electrons are
trapped essentially simultaneously, a very large electromagnetic
source radiates coherently.
The pulse can easily span continent-sized areas, and
this radiation can affect systems on land, sea, and air. The first
recorded EMP incident accompanied a high-altitude nuclear test over
the South Pacific and resulted in power system failures as far away as
Hawaii. A large device detonated at 400-500 km over Kansas would
affect all of CONUS. The signal from such an event extends to the
visual horizon as seen from the burst point.
The EMP produced by the Compton electrons typically
lasts for about 1 microsecond, and this signal is called HEMP. In
addition to the prompt EMP, scattered gammas and inelastic gammas
produced by weapon neutrons produce an "intermediate time" signal from
about 1 microsecond to 1 second. The energetic debris entering the
ionosphere produces ionization and heating of the E-region. In turn,
this causes the geomagnetic field to "heave," producing a "late-time"
magnetohydrodynamic (MHD) EMP generally called a heave signal.
Initially, the plasma from the weapon is slightly
conducting; the geomagnetic field cannot penetrate this volume and is
displaced as a result. This impulsive distortion of the geomagnetic
field was observed worldwide in the case of the STARFISH test. To be
sure, the size of the signal from this process is not large, but
systems connected to long lines (e.g., power lines, telephone wires,
and tracking wire antennas) are at risk because of the large size of
the induced current. The additive effects of the MHD-EMP can cause
damage to unprotected civilian and military systems that depend on or
use long-line cables. Small, isolated, systems tend to be unaffected.
Military systems must survive all aspects of the EMP,
from the rapid spike of the early time events to the longer duration
heave signal. One of the principal problems in assuring such survival
is the lack of test data from actual high-altitude nuclear explosions.
Only a few such experiments were carried out before the LTBT took
effect, and at that time the theoretical understanding of the
phenomenon of HEMP was relatively poor. No high-altitude tests have
been conducted by the United States since 1963. In addition to the
more familiar high-yield tests mentioned above, three small devices
were exploded in the Van Allen belts as part of Project Argus. That
experiment was intended to explore the methods by which electrons were
trapped and traveled along magnetic field lines.
The "acid test" of the response of modern military
systems to EMP is their performance in simulators, particularly where
a large number of components are involved. So many cables, pins,
connectors, and devices are to be found in real hardware that
computation of the progress of the EMP signal cannot be predicted,
even conceptually, after the field enters a real system. System
failures or upsets will depend upon the most intricate details of
current paths and interior electrical connections, and one cannot
analyze these beforehand. Threat-level field illumination from
simulators combined with pulsed-current injection are used to evaluate
the survivability of a real system against an HEMP threat.
The technology to build simulators with risetimes on the
order of 10 ns is well known. This risetime is, however, longer than
that of a real HEMP signal. Since 1986 the United States has used a
new EMP standard which requires waveforms at threat levels having
risetimes under a few nanoseconds. Threat-level simulators provide the
best technique for establishing the hardness of systems against
early-time HEMP. They are, however, limited to finite volumes
(aircraft, tanks, communications nodes) and cannot encompass an
extended system. For these systems current injection must be used.
HEMP can pose a serious threat to military systems when
even a single high-altitude nuclear explosion occurs. In principle,
even a new nuclear proliferator could execute such a strike. In
practice, however, it seems unlikely that such a state would use one
of its scarce warheads to inflict damage which must be considered
secondary to the primary effects of blast, shock, and thermal pulse.
Furthermore, a HEMP attack must use a relatively large warhead to be
effective (perhaps on the order of one mega-ton), and new
proliferators are unlikely to be able to construct such a device, much
less make it small enough to be lofted to high altitude by a ballistic
missile or space launcher. Finally, in a tactical situation such as
was encountered in the Gulf War, an attack by Iraq against Coalition
forces would have also been an attack by Iraq against its own
communications, radar, missile, and power systems. EMP cannot be
confined to only one "side" of the burst.
Source Region Electro-magnetic Pulse [SREMP] is produced
by low-altitude nuclear bursts. An effective net vertical electron
current is formed by the asymmetric deposition of electrons in the
atmosphere and the ground, and the formation and decay of this current
emits a pulse of electromagnetic radiation in directions perpendicular
to the current. The asymmetry from a low-altitude explosion occurs
because some electrons emitted downward are trapped in the upper
millimeter of the Earth's surface while others, moving upward and
outward, can travel long distances in the atmosphere, producing
ionization and charge separation. A weaker asymmetry can exist for
higher altitude explosions due to the density gradient of the
atmosphere.
Within the source region, peak electric fields greater
than 10 5 V/m and peak magnetic fields greater than 4,000 A/m can
exist. These are much larger than those from HEMP and pose a
considerable threat to military or civilian systems in the affected
region. The ground is also a conductor of electricity and provides a
return path for electrons at the outer part of the deposition region
toward the burst point. Positive ions, which travel shorter distances
than electrons and at lower velocities, remain behind and recombine
with the electrons returning through the ground. Thus, strong magnetic
fields are produced in the region of ground zero. When the nuclear
detonation occurs near to the ground, the SREMP target may not be
located in the electromagnetic far field but may instead lie within
the electro-magnetic induction region. In this regime the electric and
magnetic fields of the radiation are no longer perpendicular to one
another, and many of the analytic tools with which we understand EM
coupling in the simple plane-wave case no longer apply. The radiated
EM field falls off rapidly with increasing distance from the
deposition region (near to the currents the EMP does not appear to
come from a point source).
As a result, the region where the greatest damage can be
produced is from about 3 to 8 km from ground zero. In this same region
structures housing electrical equipment are also likely to be severely
damaged by blast and shock. According to the third edition of The
Effects of Nuclear Weapons, by S. Glasstone and P. Dolan, "the threat
to electrical and electronic systems from a surface-burst EMP may
extend as far as the distance at which the peak overpressure from a
1-megaton burst is 2 pounds per square inch."
One of the unique features of SREMP is the high
late-time voltage which can be produced on long lines in the first 0.1
second. This stress can produce large late-time currents on the
exterior shields of systems, and shielding against the stress is very
difficult. Components sensitive to magnetic fields may have to be
specially hardened. SREMP effects are uniquely nuclear weapons
effects.
During the Cold War, SREMP was conceived primarily as a
threat to the electronic and electrical systems within hardened
targets such as missile launch facilities. Clearly, SREMP effects are
only important if the targeted systems are expected to survive the
primary damage-causing mechanisms of blast, shock, and thermal pulse.
Because SREMP is uniquely associated with nuclear strikes, technology
associated with SREMP generation has no commercial applications.
However, technologies associated with SREMP measurement and mitigation
are commercially interesting for lightning protection and
electromagnetic compatibility applications. Basic physics models of
SREMP generation and coupling to generic systems, as well as numerical
calculation, use unclassified and generic weapon and target
parameters. However, codes and coupling models which reveal the
response and vulnerability of current or future military systems are
militarily critical.
Sources and Methods:
Adapted from - Nuclear Weapons Effects Technology
Militarily Critical Technologies List (MCTL) Part II: Weapons of Mass
Destruction Technologies Engineering and Design - Electromagnetic
Pulse (EMP) and Tempest Protection for Facilities
NATO HANDBOOK ON THE MEDICAL ASPECTS OF NBC DEFENSIVE
OPERATIONS PART I - NUCLEAR
http://www.fas.org/nuke/intro/nuke/emp.htm