This is the second in my series of posts devoted to
DEEWS.
Though often confused and intermingled in the
press and literature, Directed Energy (DE) weapons and Electric Weapons are not
one and the same. As an example, there
are electrically powered lasers and there are lasers which depend on the
chemical combustion of liquid fuels for lasing.
Likewise, not all directed energy weapons are lasers. High Power Microwave (HPM) and Millimeter
Wave (MMW) devices (unlike lasers) operate in the non-optical portion of the
electromagnetic spectrum. And finally,
not all electric weapons are directed energy weapons. Electromagnetic Rail-guns (EMRG’s),
Electromagnetic Coil Accelerators (ECA’s), and Linear Electric Motor
Accelerators (LEMA’s), though all electrically driven are not in the true
sense, directed energy weapons as they accelerate mass and utilize kinetic
energy as their lethal mechanism.
With respect to lasers, there are three broad
categories currently under development:
chemical, solid state, and free electron.
Chemical lasers are capable of achieving
continuous wave output in the multi-megawatt range. Examples of chemical lasers
include chemical oxygen iodine lasers (COIL), hydrogen fluoride (HF) lasers,
and deuterium fluoride (DF) lasers. The COIL laser used in the Air Force’s
now-cancelled Airborne Laser (YAL-1A) was fed gaseous chlorine, molecular
iodine, and a liquid mixture of hydrogen peroxide and potassium hydroxide. Even
though the laser operates at relatively low gas pressures, the gas flow is near
the speed of sound at the reaction time. The fast flow facilitates heat removal
from the lasing medium in contrast with high-power solid-state lasers. The
principal reaction products include potassium salt, water, and oxygen.
Diode-pumped solid-state (DPSS) lasers operate by
using a laser diode to “pump” a solid medium (for example, a ruby or a
neodymium-doped crystal). High-power
lasers use many laser diodes, arranged in strips. This diode grid can be imaged
onto the crystal by means of a lens. Higher brightness (leading to better beam
profile and longer diode lifetime) is achieved by optically removing the dark
areas between the diodes, which are needed for cooling and delivering the
current. Combining the outputs of
multiple slabs is the primary means of achieving higher energy levels.
The beams from multiple diodes can also be
combined by coupling each diode into an optical fiber, which is placed
precisely over the diode. At the other
end of the fiber bundle, the fibers are fused together to form a uniform
beam. Combining the outputs of many
fiber lasers (100’s to 10,000’s) is one means of achieving high energy levels
Free-electron lasers (FELs) are unique as they
don’t use molecular or atomic states for the lasing medium. FELs employ a
relativistic electron beam (e-beam) as the lasing medium. The e-beam is generated in an electron
accelerator and then injected into a periodic, transverse magnetic field
(undulator). An amplified electromagnetic output wave is created by
synchronizing the e-beam and electromagnetic field wavelengths. The wavelength of the output is determined by
the e-beam energy and the periodicity of the transverse magnetic field in the
undulator. FELs can thus be designed to
a wider range of frequencies/wavelengths than other laser types.
Non-laser RF based DEEWS include the following
technologies:
Unlike lasers that have yet to be fielded
operationally as weapons, micro-wave (MW) based systems have achieved advanced
prototype levels of maturity and have been deployed. MW devices (of which millimeter-wave (MMW)
are a subset) operate in the non-optical range of the electromagnetic spectrum
just beyond the Far-IR. Because of this,
they are much less susceptible to attenuation due to aerosol and particulate
matter in the atmosphere, which plague optical systems such as lasers. Below a wavelength of one centimeter though
there can be considerable absorption of their energy by water molecules (in
particular at 0.1, 0.2, and 0.5 centimeters) and this can significantly affect
their range when operating at these wavelengths. Millimeter-wave technology has been developed
by the Air Force for this very reason.
The Active Denial System (ADS) takes advantage of the transmission
window between two and five millimeters and transmits high-frequency waves at
95 GHz (a wavelength of 3.2 mm). Much as a microwave oven heats food,
the millimeter waves excite water and fat molecules in the body, instantly
heating them and causing intense pain.
While higher frequency microwaves would penetrate human tissue and cause
considerable tissue damage, the millimeter waves used in ADS are blocked by
cell density and in general only penetrate the top layers of skin. This system was made available for use in
Iraq for personnel control at prisons but has not yet been employed due to
public perception concerns over its use. Aside from the specific frequency of
operation, an ADS system is very similar in physical design, construction, and
operation to the wide range of radar systems currently employed on naval
vessels. Incorporation of ADS on surface
vessels could provide significant capability in preventing adversaries from
approaching within several hundred meters.
While the actual effective range of the current ADS is classified, basic
physics allows us to determine that with sufficient power available and a
properly designed aperture, the effective range could be considerably extended.
Although the application of laser technology for
lethal effects has steadily advanced, the employment of High Powered Microwaves
(HPM) for soft or hard kill has developed less evenly. In general HPM refers to
a specific range of radar frequencies that can be used to couple large amounts
of electromagnetic energy to conductive objects at a distance. Analogous to the Electromagnetic Pulse (EMP)
effect caused by the high altitude detonation of a nuclear weapon, a HPM weapon
generates and transmits a focused microwave beam of sufficient energy to couple
electromagnetic energy to distant electrical and or electronic devices. Depending on the range and energy level of
the HPM weapon, the energy can be sufficient to temporarily disrupt or even
destroy the target devices. In 2005, the
Navy fielded a HPM system specifically designed to counter Improvised Explosive
Devises (IEDs). The system was named “Neutralizing IEDs with RF” (NIRF) and
consisted of a HPM source, control system, and aperture mounted within and on a
Buffalo Armored vehicle. The system
worked by sweeping a HPM beam along the path in front of the vehicle which
could couple enough energy into the fuse of IEDs to cause them to detonate. HPM
weapons can also be designed as single use devices consisting of an explosively
driven electromagnetic flux compressor and an antenna (feed horn). These can then be used in bombs or artillery
shells for generating localized EMP effects.
Tactical issues facing HPM weapons arise from the difficulty in finding
a frequency that can be adequately focused to retain sufficient energy flux
density at range to cause damage while also being able to electromagnetically
couple to the electronic devices being targeted. Use of HPM devices aboard naval vessels to
confuse or destroy the electronics in cruise missiles or the fusing devices of
other weapons has several advantages.
Ships have large amounts of power available to generate the HPMs and
sufficient space for the very large apertures necessary to focus their beams.
The final category of DEEWS use electromagnetic
forces to impart kinetic energy to a projectile:
Though it
received high levels of funding in the late 1980’s, research into weapons
applications for the full spectrum of electromagnetic launchers dropped to a fairly
low level following the demise of the Soviet Union. The U.S. Navy decision to pursue hybrid and
all electric ship topologies in the DDG-1000 and other classes of future
combatants triggered the recent resurgence in work. Though not technically a weapon, the progress
made in the development of the Electromagnetic Aircraft Launch System (EMALS),
to be installed in CVN-78, has also benefited rail-gun development as much of
the technology in the ancillary components is very similar. After tests on a proof of concept system, the
Office of Naval Research has begun testing to evaluate the barrel life and structural integrity of
prototype systems separately designed by BAE Systems and General Atomics. In the near term, the U.S. Navy aims to
develop a 20–32 megajoule (MJ) weapon with a range of 80–160 km. In contrast, conventional five-inch naval
guns have a range of about 25 km. On future all-electric combatants and or
modified versions of existing cruisers and destroyers, these guns would
eliminate the need for the powder magazines associated with conventional gun
systems. With an EM launcher there is no
propellant charge required and the projectiles, though perhaps containing small
fragment dispense charges, would be essentially inert.
That
pretty much covers the main technologies in the DEEWS space. As we go forward in this series, a number of
questions will arise about technical maturity, operational use cases,
cost/schedule and the like. We will not
cover everything, but we will cover a lot of ground. As we linked in the first of the series, when ONR’s main guy on Directed Energy starts
talking about fielding using numbers of years able to be counted on one hand, good things are happening.
Posts
