Defense

Laser Technologies under Development

Three different types of high-energy laser (HEL) are currently

under development: the chemical laser, the solid-state laser, and the

free-electron laser (FEL), each of which uses a different principle to

produce a laser beam. The most developed concept, and the only one

yet to be scaled to HEL power levels, is the chemical laser, in which

energy release comes from a chemical reaction. This is the laser type

employed in the airborne laser (ABL) and in the U.S. Army/Israeli

Tactical High-Energy Laser (THEL). It is also the technology

employed in other HEL demonstrator systems such as the SpaceBased Laser (SBL) and the Mid-Infrared Advanced Chemical Laser

(MIRACL) high-energy laser at White Sands, New Mexico. The second type of laser, the electrically powered solid-state laser, could

provide benefits in propagation, lethality, and engineering design

(less complex, smaller size, less sensitive to shock). The third system, the free-electron laser, also electrically powered, is the most

complex, but is the only laser concept that is completely wavelengthselectable. For selected applications, such as transmission through

the atmosphere at sea level, this attribute is critical. While there is

no set power level threshold that defines a high-energy laser, average

powers of tens of kilowatts to megawatts are generally considered to

define high power in a weapons sense.

The HEL has the potential to address a range of applications

and missions from ground to space. Ground-based lasers have been

considered mostly for tactical air defense, which is the role of the

THEL, and also for antisatellite (ASAT) capability. Recently, lasers

Overview

After more than 30 years of technology development and many

billions of dollars in investment, the Department of Defense is

poised to operationally deploy its first high-energy laser (HEL)

weapon system, the airborne laser. The unique attributes of an

HEL—speed-of-light delivery of energy, surgical precision, variable lethality, and multiple target engagement—could significantly alter the balance between offensive and defensive weapons

or provide options for nonlethal weapons. On the other hand, the

long development period and large outlay of funding to date suggest the significant technical, operational, and policy challenges

of fielding such systems.

This paper considers the unique and promising attributes of

HEL weapon systems and examines the technical challenges, at

both the system and component level, that need to be overcome

for an HEL to be competitive against alternative weapon systems. Related operational and policy issues are also discussed.

The paper concludes that advances in HEL technology emphasizing speed, precision, and flexibility together with the ongoing

transformation of the military services have both indicated the

need and provided the opportunity for further HEL development

and testing.

High-Energy Lasers: Technical,

Operational, and Policy Issues

by Elihu Zimet

Center for Technology and National Security Policy

The National Defense University (NDU) established the Center for Technology and National Security Policy in June 2001 to study the implications of technological

innovation for U.S. national security policy and military planning. The center combines scientific and technical assessments with analyses of current strategic and

defense policy issues. Its major initial areas of focus include: (1) technologies and concepts that encourage and/or enable the transformation of the Armed Forces, 

(2) developments by defense laboratories, (3) investments in research, development, and acquisition and improvements to their processes, (4) relationships among

the Department of Defense, the industrial sector, and academe, and (5) social science techniques that enhance the detection and prevention of conflict. The staff is

led by two senior analysts who hold the Roosevelt Chair of National Security Policy and the Edison Chair of Science and Technology and who can call on the expertise

of the NDU community and colleagues at institutions nationwide. The papers published in the Defense Horizons series present key research and analysis conducted

by the center and its associate members.

A publication of the

Center for Technology and National Security Policy

National Defense University

OCTOBER 2002

Number 18

Horizons

October 2002 Defense Horizons 1
have been proposed for mobile ground-based systems to use against

air and ground targets. The Navy developed the MIRACL chemical

laser system for ship defense against cruise missiles in the early

1980s; however, it requires a different laser at a shorter wavelength

to solve propagation problems associated with self-defense in the

maritime environment. The current leading Department of Defense

(DOD) program is the ABL system for boost-phase ballistic missile

defense. Another airborne concept, this one addressing ground targets, is the Airborne Tactical Laser (ATL). Finally, the Space-Based

Laser is focused on high-altitude boost phase and mid-phase ballistic missile defense. All of these options illustrate the potential military value of laser weapons, but technological, operational, and policy challenges remain.

History

Enthusiasm for high-energy lasers and other directed energy

concepts such as particle beams and high-power microwaves has

waxed and waned during its 30 years of

technology development. In the early

1970s, the services were highly interested in HELs as an alternative to conventional guns and missiles. The Navy

mission of interest was, and still is, shipboard defense against maneuvering lowaltitude cruise missiles that could saturate the Phalanx gun system. The Air

Force in this period, using a multikilowatt carbon dioxide (CO2

) laser, built an

airborne demonstrator called the Airborne Laser Laboratory to experiment

with air superiority and missile defense. The Army also developed a

multikilowatt CO2 laser for a tracked vehicle called the Mobile Test

Unit to demonstrate effectiveness against light vehicles and sensors.

None of these early systems showed the potential to compete successfully against missile and gun evolution because their capability

did not match initial hopes and expectations. Since these initial

demonstrator programs, the Navy has developed a layered missile

system employing the rolling airframe missile, the enhanced Sea

Sparrow missile, and the Standard Missile coupled with enhanced

surveillance and targeting as well as electronic support measures.

The Air Force has developed the highly capable advanced mediumrange air-to-air missile for air superiority, while the Army has made

significant progress in penetrating-round technology for antiarmor.

In the early 1980s, the Reagan administration vision of a shield

against ballistic missile attack led to the formation of the Strategic

Defense Initiative Office (SDIO) and a new interest in and requirement for HEL. Service funding transferred to SDIO initiated a very

ambitious program in chemical and free-electron lasers. SDIO continued to support the MIRACL chemical laser at White Sands until

the late 1980s, when a series of tests against a head-on, low-altitude

target produced discouraging results, and Navy involvement in HEL

waned. While Army interest in tactical-vehicle-mounted lasers also

waned in the 1980s, the service assumed management of the SDIO

ground-based FEL program. The goal of this system was to defeat

strategic ballistic missiles in the boost phase by redirecting a powerful beam from the ground with a low orbit mirror system. The program objectives were well beyond the technology level of the FEL,

which was the laser system chosen because of its potential for optimized propagation at shorter wavelengths than the chemical laser.

Eventually, the program was terminated.

Recent advances in FEL technology have rekindled Navy interest, while advances in solid-state laser technology have prompted

the Air Force and Army to look again at tactical applications. The

Army is the lead service for the Advanced Concept Technology

Demonstration (ACTD) THEL program. During the past 2 years, the

Marine Corps has looked at the concept of employing an HEL (a

scaled-down version of the ABL chemical oxygen iodide laser

[COIL]) in a helicopter or V–22 as a gunship in an urban environment. This ACTD, which is managed by

the Air Force, is called the Airborne

Tactical Laser (ATL). Although the

Navy and Army have renewed interest

and the Marine Corps has become

active in HEL development, the Air

Force has been the principal developer

of HEL technology since the 1980s. The

ABL, begun in 1994, utilizes a COIL and

an adaptive optic beam control system

to address the ballistic missile threat in

the boost phase from an airborne platform. The Air Force continues to invest

in technology to enhance this concept and has even looked at the

requirements for a fighter laser weapon system.

Three years ago, individuals in Congress and the Office of the

Secretary of Defense became concerned that the investment in highenergy lasers was almost entirely in development of three chemical

laser systems (ABL, SBL, and THEL), with only a limited budget

addressing next-generation laser concepts and supporting technology areas such as optics. The result was the Floyd D. Spence National

Defense Authorization Act for fiscal year 2000, which directed DOD

to prepare a “detailed plan to develop and mature high-energy laser

technologies.” DOD has subsequently developed such a plan and created the DOD High-Energy Laser Joint Technology Office that has its

own funding line for science and technology. The formation of this

office and the potential to leverage its funding have stimulated new

investment by the services. 

Defense planners are taking a fresh look at HEL weapon systems. Longstanding missions such as ship self-defense are under

investigation to determine if advances in both HEL technology and

electric on-board power, compared to advances in the threat capability coupled with new threat scenarios, have changed the calculus

of ship defense to favor HEL weapons. In addition, new requirements

such as urban warfare, the small boat threat, and the projected utility of nonlethal weapons, in addition to the heightened interest in

missile defense, have spurred renewed interest in HEL. 

2 Defense Horizons October 2002

Elihu Zimet is a senior research fellow in the Center for Technology and

National Security Policy at the National Defense University. Dr. Zimet may be

contacted at zimete@ndu.edu or by phone at (202) 685–3586.

recent advances in FEL

technology have rekindled

Navy interest, while

advances in solid-state

laser technology have

prompted the Air Force

and Army to look again
Attributes

To reach deployment, a new weapon system must either show

significant capability or cost advantages over existing systems or

address a priority requirement that cannot be met by incremental

improvements to an existing system. This section considers key HEL

attributes, the principal attribute being the speed-of-light delivery of

energy. The HEL could be a weapon of choice for a transformational

military concerned with homeland defense, asymmetric threats from

rogue powers and terrorists, and urban warfare, as well as the more

traditional military missions on the battlefield and at sea.

Speed-of-light reaction time. A look at typical times available

for the engagement of a target compared with the flight time of projectiles and missiles highlights the continued interest in HEL

weapons. In a ballistic missile defense scenario, tactical considerations for an airborne defensive system lead to standoff distances of about 200 miles. A ballistic missile

is most vulnerable to tracking and killing

during the boost phase. For a Scud missile,

the boost phase is approximately 60 seconds. If a missile is destroyed early in the

boost phase, it will fall back on the area of

the launch, a critical concern for nuclear,

biological, and chemical warheads. If a defensive projectile or missile is fired from an aircraft at an average speed of Mach 5 (5 times

the speed of sound), it will take more than 3 minutes to travel the

200 miles. This is too long a flight time for an assured kill in a boost

phase engagement of a Scud missile in which the times for detection,

tracking, weapons coordination, and fire control are added. By comparison, a laser beam would travel the same 200 miles in less than a

millisecond. Although missiles have a finite fly-out time, their time

to kill is near instantaneous and the kill is catastrophic. Lasers have

essentially zero fly-out time, but they do require time, typically several seconds, to deposit enough energy into the target for a kill. The

assured lethality of an HEL remains an issue that will only be

resolved by convincing demonstrations.

Another situation in which the time to destroy a threat is short

is that of a ship defending itself against a supersonic cruise missile

flying only a few feet above the top of the waves (so-called sea skimmers). If the threat is detected as it comes over the horizon, about

15 miles out, and is traveling at Mach 3, it will travel from detection

to impact in approximately 24 seconds. If several simultaneous

high-speed missiles are fired at the ship, its defensive system could

be overwhelmed even if several defensive missiles are fired at the

same time.

Maneuvering and crossing targets. If the same cruise missile

scenario outlined above concerned a maneuvering target, the gun or

missile kill would be greatly complicated, since a defensive missile

typically needs a three-to-one gravity (G) advantage over the target.

A maneuvering target, however, is probably even more vulnerable to

an HEL. A weaving target traveling at Mach 3 and executing 20+Gs

of maneuver will present minimal problems to the laser beam director in holding the beam on the target. However, a target pulling

20+Gs will be much more susceptible to breakup after being damaged by the laser. A crossing target presents its profile rather than

just its nose to the HEL beam, increasing the target’s vulnerability to

a guidance or propellant kill. While the slewing laser beam can easily keep up with the target, the slewing greatly enhances the propagation of the beam through the air. Thus, more energy is deposited

on the target, and the odds of a kill are significantly increased.

Assured lethality. The magazine for an HEL is laser-reactant

fuel for a chemical laser, or electric generator fuel for both solidstate lasers and free-electron lasers. The lasers can run as long as

fuel is available, which can provide for a very deep magazine. Coincidentally, the power needed to run an electric HEL and the power

necessary to propel a surface combatant can be the same, particularly on the new all-electric ship concepts that generate the

needed electricity. This means that fuel is available for hours of

HEL run time, which in turn ensures that

a target or targets can be engaged until

they are destroyed (provided that enough

energy is propagated through the air and

deposited in the target and that the closing target does not hit the ship first). The

number of simultaneous targets that can

be engaged depends on the hardness of the

target and the irradiance time needed for

a kill, typically several seconds. In a ship

defense scenario, two or three simultaneous threats might be

engaged sequentially before engagement time runs out. If the

threats are spaced sufficiently, only fuel or electric power limits

the number of engagements.

Avoidance of collateral damage. The precision pointing and

tracking of the beam director leads to the absence of collateral damage. For example, consider air defense in an urban environment. An

air defense projectile or missile that does not engage a target, unless

it has the ability to self-destruct, provides a real threat to people and

dwellings on the ground. In contrast, an HEL does not produce unexpended rounds, and there is no additional ground threat. This attribute might make an HEL attractive for the defense of high-value

buildings and facilities.

Target identification. The HEL beam director is a high-quality

telescope using reflecting optics that can provide a detailed image of

the target. While the reflective coatings on the mirrors are tuned to

the frequency of the HEL, there is residual reflectivity at other wavelengths, including the visible. This imagery can be used to provide a

positive identification of the target, which could lead to rules of

engagement in operations with commercial and friendly aircraft, as

well as in urban operations. 

Lethal to nonlethal capability. The power output of a laser can

be modulated from destruction at full power, to disable at lower

power, to deny and dissuade at still lower power. This capability

would be of value to the Navy in the midst of small craft of unknown

intent or to an airborne laser “gunship” in an urban environment

aiming at ground targets. As an alternative, a separate low-power

laser could be deployed to warn away potential threats and to destroy

or blind threat sensors.

October 2002 Defense Horizons 3

the HEL could be a

weapon of choice for a

transformational

military concerned

with homeland defense
Technology Challenges

Technology challenges for HEL weapon systems include the

issues not only of the particular laser device employed but also of the

associated optics and beam director. In addition, a systems analysis

of an HEL includes an examination of weapon effectiveness, which

includes the propagation of the beam to the target and the lethality

of the beam on the target. An HEL system must also be engineered

to be operational in the field. Finally, the overall utility of an HEL

system depends on the environment in which it is placed (surface,

air, or space) and on the missions it is tasked to perform. This section considers the component and system level challenges of an HEL

weapon system. 

Laser Devices. The aforementioned classes of HEL currently

under development are chemical lasers, solid-state lasers, and freeelectron lasers. Work on other types of HELs, such as the gas

dynamic laser and the pulsed electric gas discharge laser, was discontinued years ago because the lasers produced beams at unfavorable wavelengths for propagation. Technology issues (as developed

in the DOD HEL Science and Technology Investment Strategy of

November 2001) are different for each type of HEL but include scalability, beam quality (ability to focus beam), and laser line (wavelength) selection for optimum beam propagation. Issues peculiar to

each type of laser include laser reactant and effluent handling

(chemical lasers); thermal management and low-cost laser diodes

(solid-state lasers); and high-current photo-injectors, high-damage

threshold resonator optics, and high-efficiency wigglers (free-electron lasers).

Chemical lasers are the only class of HEL currently able to

achieve megawatt power levels. The MIRACL is a deuterium fluoride

(DF) laser operating at a wavelength of 3.8 microns that has been in

continuous operation at the megawatt level since the mid 1980s at

the White Sands HEL Systems Test Facility. It was originally conceived as a test bed for ship self-defense but suffered from propagation losses at full power. The Navy discontinued development of

chemical lasers because of the inherent propagation losses in the

operational wavelengths. However, DF technology found a home in

the U.S. Army/Israeli THEL, where propagation losses are mitigated

by lower power levels and a crossing target. The THEL demonstrated

its capability against Russian 122-millimeter (Katyusha) rockets in

a multimissile engagement in late 2000. The THEL, however, is not a

mobile system and was designed specifically for Israeli border

defense. Concepts for a mobile THEL (MTHEL) are currently under

consideration. Questions remain as to how well the THEL would

operate in a battlefield with smoke and dust. 

The hydrogen fluoride (HF) laser is similar to the DF laser but

operates at a shorter wavelength (2.7 microns), which will not propagate well in the atmosphere but will perform well in space, potentially for ballistic missile defense. The Missile Defense Agency has

recently cut back the space-based laser program, and no date can be

set for potential deployment.

More near term is the Air Force/Missile Defense Agency ABL for

theater missile defense. The ABL employs a COIL that operates at a

still shorter wavelength of 1.3 microns. The shorter wavelength

allows the focusing of the beam to the required spot size with smaller

optics than with longer wavelengths. Chemical lasers have acceptable beam quality (ability to focus the beam) that can be improved

with adaptive optics. Reactants for the COIL include hydrogen peroxide, chlorine, and iodine; the HF and DF lasers use gasses containing hydrogen, deuterium, and fluorine, requiring special handling and disposal of the effluents. These lasers require pressure

recovery (ejectors) from the low-pressure laser cavity unless operated at high altitude or in space. Due to the gas handling, the laser

cavity, the large optics, and the ground-based pressure recovery,

these devices are relatively large. The ABL system, for example, will

fill a Boeing 747–400 series freighter aircraft.

Free-electron lasers could be designed to operate at any

desired frequency and are, to a degree, tunable in operation. At

ground level, they might have the best chance of producing a beam

that propagates successfully through the relatively dense or “thick”

air. Since the laser beam is produced in a vacuum cavity, the beam

quality is excellent. Significant technical challenges exist both to

scale the beam to megawatt powers and to engineer a laboratory

device into a weapon system. The principal scaling issue is in generating an electron beam (high-current photo-injectors) with about

a three-orders-of-magnitude increase of current over the existing

kilowatt-class laser. Another risk technology area is with the resonator optics needed to extract the light from the resonator electron beam in a practical length. The FEL is the most complex of the

three HEL alternatives and most likely is suited only for ground or

shipboard use.

Solid-state lasers have the most potential for a compact engineered weapon, although current devices are still large and complex.

The laser does not require flowing exotic gases or relativistic electron beams. The system could be inherently smaller and less complex than the other laser concepts. The lasers also operate at about

1.06 microns, lower wavelengths than the COIL, which further simplifies the optics. However, solid-state lasers have the largest challenge to scale up to megawatt power levels because of waste heat

removal. Waste heat from a chemical laser is carried off in the flowing gas medium. The unexpended energy in the electron beam of a

free-electron laser can be disposed of in a large heat sink. But with

a solid-state laser, the waste heat remains in the laser medium,

increasing the temperature in the medium until lasing with acceptable beam quality is impossible. Several techniques to combat this

problem are being studied. The current options are short pulses followed by cool-down, cooled geometries, including fiber-optic laser

bundles, and slab lasers. Increasing laser efficiency will also reduce

the amount of waste heat. Combining the power output of many

smaller lasers into a single high-power coherent beam has also been

investigated. With this concept, some of the system simplicity advantages would be lost. However, the fiber-optic laser successfully uses

the principle of beam combination. 

A final important technology area for solid-state lasers is the

development of affordable, high-power laser diodes to pump the

laser to the excited state necessary for lasing. For example, a halfmegawatt laser that is 20 percent efficient using 10-watt diodes will

require 250,000 pump diodes. With diodes costing more than $100

each, the price of the diodes alone would be more than $25 million.

4 Defense Horizons October 2002
At minimum, costs must be reduced by an order of magnitude. While

there is a large industrial market for laser diodes for commercial

application, there is as yet no other market for the high-power diodes

needed for an HEL. 

Optics and beam control. Significant progress in this technology area has led to optical pointing and tracking accuracies, which

can hold a laser beam on a given target at current ranges of interest

for the required kill time. In fact, technology transfer from HEL

adaptive optics development has contributed to the current generation of ground-based telescopes for astronomy. Principal future gains

to be made are in large lightweight deployable optics, particularly for

space; advanced adaptive optics to

remove beam aberrations both in the

laser cavity and in transmission through

the atmosphere; and windows, coatings,

and deformable mirrors to handle highpower beams and improve the ability to

correct for stressing atmospheric aberrations in the propagation of the beam.

Techniques for the pointing and tracking

of a laser beam from a moving platform to

a moving target through a fluctuating atmospheric medium need to

be developed and tested for precision engagements at extended

ranges. Real-time characterization tools for the atmosphere must

also be developed for HEL deployment. Utilizing the beam director

as an imaging telescope, algorithms need to be developed to process

the imagery for target identification and rules of engagement. 

Propagation through the atmosphere. Propagation of a highenergy beam at ground or sea level, where the air is relatively dense,

has been the single most limiting factor in the Navy’s ability to field

a shipborne HEL cruise missile defense system. In space, this problem does not exist and is significantly less for the ABL at altitude.

The principal loss mechanism the Navy has experienced is a nonlinear process called thermal blooming, in which absorbed energy in

the air creates a negative lens that defocuses the beam. Increasing

the power of the beam increases the energy absorbed and worsens

the problem. Thermal blooming is nonlinear; it evolves over a horizontal path that has atmosphere all the way to the target. Therefore,

closed loop adaptive optics techniques have not proven effective. To

address this problem, the Navy is developing free-electron lasers

that can generate wavelengths at which absorption and thermal

blooming are minimized.

Turbulence-induced defocusing causes another loss mechanism that is more severe at shorter wavelengths. This loss mechanism, however, has been amenable to partial cleanup by adaptive

optics employed on the ABL. A major issue for the utility of lasers,

particularly at ground and sea level, is that of propagation in

weather and battlefield dust. Aerosols significantly reduce propagation both by the mechanisms of absorption and by scattering. Lasers

operate in the near infrared rather than the visible frequencies, but

the laser generally will not propagate well in any environment of

limited visibility (such as in heavy fog). A requirement for allweather lethality could limit the utility of an HEL as a stand-alone

weapon. This drawback is only partially relieved by the fact that

missile seekers, including millimeter wave radar seekers, are also

degraded by weather.

Lethality. Several variables affect lethality from an HEL,

namely the spot size on the target, the energy transmitted to the target, the coupling of the energy into the target (as a function of both

the beam characteristics and the target characteristics as changed

over time by the damage), and the disruption and destruction of the

target due to the damage imposed by the beam. Chemical lasers produce a continuous rather than a pulsed beam. The damage mechanism to a target is due to thermal heating of the surface causing

melting and ablation. The lethality mechanism for the ABL and the

THEL is ignition of the rocket or booster propellant by irradiating

the thin case of the booster. The uncontrolled solid propellant burning out of the side of the motor destroys

the missile. For a liquid propellant under

pressure, the stress-induced crack propagation could lead to total failure. Other

lethality mechanisms include a guidance

and control kill, a warhead kill (the warhead is usually less vulnerable than the

propellant), or an aerodynamic kill, in

which the missile is damaged to the

point that aerodynamic forces tear it

apart. Solid-state lasers operate in a pulsed mode, in which additional damage mechanisms may augment a pure thermal kill. Very

high pulse energies can create shock and spallation in a target, and

interpulse intervals could allow time for laser debris to clear out of

the path of the beam. Free-electron lasers operate at a very high repetition rate effectively similar to that of a continuous beam. Lethality technology needs a predictive and validated modeling capability

as well as more studies using various beam pulse widths, pulse rates,

and irradiances. In addition, lethality modes other than ignition of

warhead or propellant, including guidance and control and aerodynamic kills, need to be investigated for high-speed missiles.

Engineering. High-energy lasers of the megawatt class provide

significant challenges in both design and integration into mobile

platforms that induce shock and vibration. Substantial advances

during the past 30 years of HEL development have resulted in fieldable laser systems such as the ABL. Among the advances are material development, computer aided design, computational fluid

mechanics, micro-machining, dynamic vibration control, large lightweight optics, and diamond-turned optics, windows, and coatings.

The current generation of HELs is chemical lasers. Despite all of the

advances, they are still large, complex, integrated fluid mechanical

and optical systems that require highly skilled operators and considerable maintenance. They also involve chemicals that need special

handling. Free-electron lasers, currently at the kilowatt level, are

even more complex. They operate at electron beam energies requiring shielding, high vacuum, and probably cryogenic cooling. Electron

beam control and vibration control are critical for high efficiency

operation. Even clever packaging most likely will result in a system

larger than the other laser concepts. Solid-state lasers show the

most promise for relative simplicity but are also still at the kilowatt

level with significant unresolved scale-up issues, both in the laser

October 2002 Defense Horizons 5

substantial advances

during the past 30 years

of HEL development have

resulted in fieldable laser

systems such as the ABL
design to handle the thermal management and the laser diodes. As

the laser kill range increases, the requirements on the optical systems increase. To maintain a spot with centimeter-level stability

over 100 kilometers requires submicroradian control.

HELs are at the beginning of their acquisition learning curve

and probably will not be manufactured in large enough numbers in

the foreseeable future to benefit from the reduced costs associated

with the purchase of hundreds or thousands of units. High acquisition, operational, and maintenance costs are to be

expected. The main exception to this is

the cost of expendables. The cost of the

fuel to create a laser beam is negligible

compared to the cost of a missile; therefore, the number of rounds fired over the

system lifetime is an important parameter. Because of the current

uncertainties in performing engineering system tradeoffs, enthusiasm for HELs remains driven by their special or unique attributes.

Operational Challenges

Space-based lasers. The SBL mission is boost phase in the

upper atmosphere and midcourse ballistic missile defense. Operationally, an SBL system would be deployed in a low Earth constellation to provide continuous coverage. The number of lasers in a constellation might be 20 to 35, depending on orbit, laser power, and

coverage. There are distinct advantages to operating a laser in space.

The vacuum of space allows beam propagation with no losses (the

principal shortcoming at sea level). A chemical laser, such as the

proposed hydrogen fluoride laser, needs to exhaust the laser cavity

at low pressures. The vacuum of space solves the pressure recovery

issue. Finally, freedom from gravitational forces allows for some

relief from structural considerations. Despite the advantages of

space, the operational challenges for an SBL are daunting. They

include lift to orbit and space assembly, stable and reliable operation

in space, and maintenance and resupply of fuels. The problems and

cost associated with the deployment of a constellation of megawattclass chemical lasers with multimeter diameter optics are challenging. The SBL program is still in the stage of developing a plan for a

subscale integrated flight experiment; a full deployment plan has not

been developed.

A number of HEL operational issues are more difficult in space

than on the ground. For example, the chemical laser has many of the

attributes of a rocket motor, to include thrust and combustion instability driven vibration. On the ground or even in the ABL, this issue

is handled by massive supports and by isolation of the laser flow system from the optical system. In space, thrust from the exhaust must

be carefully balanced to hold the laser system in orbit and to allow

the optical system to maintain track on its target. Dynamic vibration

control must also be employed to keep beam jitter to the very low

levels required from the beam director. Another potential issue is the

dispersion of the laser exhaust. If the laser beam were to pass

through the exhaust, it could be significantly attenuated and distorted. The exhaust, which is corrosive, could also damage the optical system. Due to the need for vacuum and zero gravity for a realistic test, eventually, the potential impact of these concerns will have

to be tested in space. 

Airborne lasers. Currently, two airborne operational missions

utilizing a laser weapon are under development: the ABL for boostphase missile defense and the ATL (a new program just under way)

for air-to-ground operations. The ABL, the HEL system closest to

operational test, is arguably the largest and most complex piece of

military hardware ever flown in an airplane, as well as being the

most expensive. Nonetheless, the operational issues of the ABL

appear to be tractable and are considerably less than those of the SBL. The platform is much larger, the system is

manned, and maintenance and refueling

are done on the ground. Due to the complexity of these HEL systems, field maintenance would require specialized equipment and highly trained maintenance

crews, possibly even dedicated support shelters. System readiness is

paramount if the primary mission of the HEL is missile defense.

From an operational point of view, the ABL system bears similarities

to the airborne warning and control system (AWACS). Both are large

systems required to be on station continuously at a high state of

readiness. For boost phase missile defense, it will be necessary to

forward-deploy the ABL. The Air Force plans to be able to deploy and

be on station within 24 hours of initial threat warning and to maintain two ABL aircraft in orbit continuously at 40,000 feet.

Another concept for an airborne laser is the ATL for surgical

strike or nonlethal operations, particularly in an urban environment.

While one would not consider using a megawatt-class weapon as an

antipersonnel weapon, devices of significantly lower power are still

classed as an HEL and form the basis of the ATL concept. The ATL

will investigate the use for a laser with power from the tens to a few

hundreds of kilowatts to disable ground targets such as wheeled

vehicles from an air platform such as a helicopter. The current plan

to investigate military utility and operations for the ATL is to build a

fieldable prototype and experiment with operational utility.

Ground-based lasers. The THEL system, designed for air

defense against rocket attack, is a fixed, nonmobile system. Operational considerations for this system, which has shown considerable

success in testing, would require the system to be mobile or at least

easily transportable unless used to defend a fixed site. The September 11, 2001, attacks on the United States have raised the issue of

systems to support homeland defense. Just as lasers have been evaluated for the defense of high-value ships at sea, the HEL could be a

strong candidate for defense of high-value facilities in the homeland.

A potential first application of a free-electron laser could be to

defend a nuclear power facility in which the real estate and prime

power for the laser are readily available. The advantage of an HEL

for homeland defense is that the laser does not provide a threat to

the area it is protecting. This may not be true for missile air defense

systems currently being installed. Projectiles used for homeland

defense need to be self-destructing before they reach and potentially

damage ground infrastructure. As it stands now, a launched air

defense missile could provide a hazard to residents and homes if it

failed to engage a target. Of course, a target brought down by an HEL

6 Defense Horizons October 2002

despite the advantages

of space, the operational

challenges for an SBL

are daunting
also would constitute a threat to the ground. Similarly, the placement of explosive missile warheads and missile propellants near a

power facility or other high-value facility creates its own peril. An

electric powered laser would have no reactive materials, and the

beam would not provide a hazard to the ground. A nearer-term HEL

system for homeland defense could be a derivative of the chemical

laser THEL. 

Although external target acquisition is still required, an HEL

beam director system serves as its own precision pointing and tracking system, providing azimuth, elevation, and range information.

The system also can provide imagery for target identification. In

engagements in which threats must be discriminated from friendly

and commercial aircraft, or in urban environments, the fire control

system must be programmed to shoot only at threats. Such rules of

engagement are already programmed

into the THEL system whereby it is

specifically programmed to shoot

only at Katyusha rockets. In general,

allowance must be made to accommodate different rules of engagement rapidly and to allow for human

intervention where time permits.

Ship-based systems. From an

operational point of view, a Navy ship

might seem an ideal platform for an

HEL. Ships are far larger than other

platforms, and ship defense against cruise missiles is a clearly

defined mission. The current lack of a Navy HEL development program is due to the technical issues of propagation and lethality

rather than to operational issues. At this stage, an HEL would have

to show advanced performance at an affordable cost through a tradeoff study of alternative systems and missions. Pop-up threats such as

small speedboats and jet ski watercraft, as well as cruise missile

defense, are valid missions. If an HEL system is compared to an

alternative system such as a missile or hypervelocity gun system, the

entire system must be considered on a mission-by-mission basis,

including threat characterization, raid size, and weather. The system

elements are target acquisition, target tracking, command and control, and the weapon system (or weapon systems in a layered

defense). A weapon system may also be an integrated HEL and missile system. 

Policy Issues

Space-based lasers. While the withdrawal from the Anti-Ballistic Missile Treaty has expanded the envelope for HEL employment,

significant policy issues (in addition to technical and operational

issues) remain for space-based lasers. To field an SBL is to put a

weapon system in space that other countries (particularly Russia

and China) would consider as escalating the U.S. strategic capability. While the concept of the SBL is for defense against intercontinental ballistic missiles (ICBMs) in boost and mid-phase, the SBL

also would have the capability to destroy or disable other spacebased systems such as surveillance satellites. Although the HF

chemical laser technology currently being planned would not produce a beam capable of propagating through the atmosphere, future

laser concepts could put airborne or even ground targets at risk. By

definition, a space-based defensive weapon is always armed, always

overhead, and in constant flight over other countries. This is true not

only for potential enemies but also for our allies and the U.S. homeland. To be effective, an SBL would require a high level of autonomy

so that it could react promptly to pop-up threats and reduce the risk

of jammed or subverted data links. This capability would require levels of reliability well beyond that of laser systems with manned operators, as well as periodic and frequent maintenance. The system

would also need to be survivable against a wide assortment of countermeasures that a country with space capability could employ.

Finally, the cost of a constellation of SBLs to achieve continuous coverage will be extreme (no reliable figures are available, but they

might approach $100 billion). This cost would need to be weighed

against funding other elements of the

missile defense system as well as

other DOD priorities.

Airborne lasers. The range of

the ABL system (on the order of 200

miles) and its mission of boost phase

intercept against Scud missiles,

rather than ICBMs, would greatly

limit its utility against Russia or

China, where missile silos are at considerably greater distances than 200

miles from their borders. An ABL

would not have continuous presence, its defense would be limited to

specific threat areas, and it would be a manned system. Therefore,

the ABL should not be perceived as a realistic threat to the strategic assets of other countries. However, the survivability and implications of an attack on an ABL are of some concern. If an HEL is

used for ballistic missile defense, an attack on that system becomes

an attack on a strategic asset that could presage a missile attack.

Defensive considerations for an ABL could include self-defense by

either the HEL weapon system itself or an auxiliary weapon system

used for escort support or deployment beyond the range of groundbased air defense. The defensive situation for the ABL is similar to

that of the AWACS surveillance aircraft, which relies on standoff

and escort as its principal tactics for survivability. Both policy and

operational considerations dictate the deployment of a strategic

asset. From a policy point of view, keeping an ABL out of harm’s way

is advisable, since an attack on it would tend to escalate a confrontation from tactical to strategic. On the other hand, from an

operational point of view, it is necessary to get an ABL within lethal

range of the boost-phase flight path of incoming missiles. Accordingly, deployment of an ABL against Russian or Chinese strategic

missiles, with its currently projected range, might be possible operationally but problematic in policy. 

The ATL is designed to disable ground targets in an urban environment. An overarching policy concern for this concept is that U.S.

policy does not permit the development of weapons primarily

designed to create permanent eye damage or blindness. While this is

not the objective of the ATL, the potential exists that reflected or

scattered light from an ATL could cause eye damage if the laser were

October 2002 Defense Horizons 7

to field an SBL is to put a

weapon system in space that

other countries (particularly

Russia and China) would

consider as escalating the

U.S. strategic capability
used near people. Retinal eye damage can occur at low levels of

intensity (tens of milliwatts) at wavelengths that pass through the

cornea. The cornea itself can be damaged by absorbed light at watt

levels of energy. Occupational health studies have set the wavelength at which light does not pass through the cornea at 1.5 microns

and above. DF and HF chemical lasers, CO2 lasers, and FELs operate

above 1.5 microns, while COIL (at 1.3 microns) and solid-state lasers

(generally at 1.06 microns) could cause concern for low-power-level

retinal damage. While these concerns exist,

the inherent precision of a laser and the

ability to tailor the energy required for

effect to specific target vulnerabilities

should greatly reduce the potential for collateral damage.

Ground- and sea-based lasers. Over

the years, the Army has looked at a number

of ground-based HEL concepts, including

missile defense and countersensor lasers.

The principal development from the Army is

a system jointly developed with Israel for

defense against Katyusha rockets. While there are no policy concerns with the THEL as an air defense system, a more general issue

is how this technology could create policy issues involving the export

of laser technology to other countries. For example, a ground-based

system scaled to higher powers than the THEL could be used to disable satellites. At lower powers, a laser beam at wide aperture

slewed rapidly could be used as a nonlethal weapon for crowd control (much like a long-range fire hose). The use of a millimeter wavelength beam is also being examined for nonlethal applications. The

Navy has looked at reducing the power of an HEL to use at sea as a

warning device to approaching small craft. At higher laser powers,

the craft, such as a speedboat or a jet ski watercraft, could be damaged or destroyed (a difficult task for a 5-inch gun). Of these potential uses, all raise the issue of employing antipersonnel weapons that

may cause blindness.

Conclusions

Currently, the HEL is a niche weapon only operationally competitive against other weapon systems where speed-of-light engagement is essential for defense of high-value assets, particularly

against ballistic missiles with weapons of mass destruction. The ABL

is the system closest to deployment, with difficult but tractable technical and operational challenges and a clear mission of boost-phase

missile defense against potential rogue state threats. If the range of

the ABL were to be significantly increased, perhaps to 700 miles, the

system also could be considered a threat to the strategic assets of

major powers, thereby creating diplomatic issues. Other missions for

HEL still require technology and engineering advances along with

convincing demonstrations that they can compete against more conventional missile and gun weapon systems. Ground-based lasers

have been effective against rockets in the THEL demonstrations, but

a mobile system has not yet been engineered. Similarly, space-based

lasers are nowhere near deployment. Significant technological, operational, and financial issues need resolution. There is the added possibility that a space-based system could attack not only ballistic missiles but also any other air or space target. A different laser concept

than the HF chemical laser, whose beam could propagate to the

ground, could attack ground targets. Introducing such a weapon system is a major policy decision with global and national ramifications.

On a smaller scale, while the Navy has a clear mission for ship

defense against cruise missiles, technology

issues still preclude a viable sea-based system. A laser such as the free-electron laser,

capable of producing a beam that will propagate through the maritime atmosphere,

needs further development to make this

concept viable. DOD is continuing to experiment with an airborne tactical laser to

address ground targets such as light vehicles. The antipersonnel aspects of such a

system will provide operational and policy

challenges. The potential for a fixed-site

HEL system to defend high-value facilities such as nuclear power

plants also warrants continued investigation. Advances in laser

devices (particularly in solid-state lasers), in optical components,

and in beam control would greatly expand the potential mission viability of HELs.

After 30 years of development, the revolutionary concept of the

high-energy laser is now proceeding through evolutionary advances.

While no fundamentally new laser concepts have emerged in the

past few years, significant progress has been made in scaling lasers

to higher powers, in creating adaptive optics, and in engineering the

laser system for applications. As the power and compactness of the

HEL improve, the potential missions and applications will increase.

A continued, measured investment in HEL research and development is certainly warranted.

8 Defense Horizons October 2002

Defense Horizons is published by the Center for Technology and National Security Policy through the

Publication Directorate of the Institute for National Strategic Studies, National Defense University.

Defense Horizons and other National Defense University publications are available online at

http://www.ndu.edu/inss/press/nduphp.html.

The opinions, conclusions, and recommendations expressed or implied within are those of the

contributors and do not necessarily reflect the views of the Department of Defense or any other

department or agency of the Federal Government.

Center for Technology and National Security Policy

Hans Binnendijk

Director

the ABL is the system

closest to deployment,

with difficult but

tractable technical and

operational challenges

and a clear mission