Thursday, April 26, 2012

Directed Energy and Electric Weapons Systems (DEEWS Serial 2)

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. 

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