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Directed Energy Weapons

A Directed Energy Weapon is a system using directed-energy primarily as a direct means to damage or destroy enemy equipment, facilities, and personnel. Directed energy offers promise as a transformational "game changer" in military operations, able to augment and improve operational capabilities in many areas. Yet despite this potential, years of investment have not resulted in any operational systems with high energy laser capability. The lack of progress is a result of many factors from unexpected technological challenges to a lack of understanding of the costs and benefits of such systems. Ultimately, as a result of these circumstances, interest in such systems has declined over the years.

Initially, the American projects were developed as separate entities, with a relatively loose interservice coordination. In 1978, however, the Department of Defense organized an Office of Directed Energy Technology to coordinate the development of beam weapons, and the Pentagon established the Particle Beam Technology Study Group, composed of 53 Defense Department and US scientific community personnel.

Through fiscal year 1993, SDIO spent about $4.9 billion over 9 years of a planned $6.7 billion for directed energy research and development, or about $800 million less than the 1984 plan specified was needed over 6 years. SDIO said that this was nearly all of the national effort in high-power directed energy weapons. SDIO also said that this under funding becomes larger if it is recognized that stretched-out programs cost more than efficiently funded programs and that dollars spent in years following the planned years had been degraded by inflation. In the early years of SDI, the directed energy funding made up nearly a quarter of SDI's total funding. Annual funding peaked at $827 million in fiscal year 1983 and subsequently decreased to $162 million in fiscal year 1993.

A particle beam transmits a stream of high-energy atomic particles which can destroy or neutralize a target. The particles may have a positive or negative charge or may be neutral. In each case the particles are injected into some type of medium, normally an electron beam, and accelerated to near-light (relativistic) velocities. The medium, called a plasma when combined with the particles, can then be aimed at the desired target. For example, a negatively charged electron beam similar to those in a television picture tube can be fired through a gas or other source of positive atomic particles such as protons. These particles are swept along by the oppositely charged electron beam. Since the electron beam is relativistic, the positive particles are accelerated to relativistic velocities. The almost massless electrons can then be removed from the beam, leaving a stream of relatively heavy atomic particles.

This stream of particles traveling at a relativistic velocity is a tremendous energy emission. Einstein's famous formula, E = mc2, shows the relationship between energy, mass, and the velocity of light. For example, it demonstrates why a very small object, such as an atomic particle, moving at a relativistic velocity will have a very high energy potential and why it will impart an enormous amount of energy to whatever it strikes.

Particle beams are not a steady stream of energy but rather are a series of pulses. Like a bolt of lightning, each pulse is only a few millionths of a second long and discharges great quantities of energy which can have a variety of effects on a target, depending on the level of energy. For example, a beam of five seconds' duration with an energy of 25 megajoules would have the explosive equivalent of 50 pounds of TNT. Such an explosive force could have devastating effects on an intercontinental ballistic missile's (ICBM) reentry vehicle or its booster during the powered portion of flight. Additionally a selected target could be totally disintegrated, by making its molecular structure unstable through the enormous energy transfer. Similarly, a target could become super heated and vaporize. A beam with a lower energy level could pass through a target, such as an ICBM reentry vehicle, causing electrical and magnetic disruptions in its electronic 5components. The lethality and relativistic nature of beam weapons make them especially suitable for antiballistic missile (ABM) applications.

A Neutral Particle Beam (NPB) weapon produces a beam of near-light-speed-neutral atomic particles by subjecting hydrogen or deuterium gas to an enormous electrical charge. The electrical charge produces negatively charged ions that are accelerated through a long vacuum tunnel by an electrical potential in the hundreds-of-megavolt range. At the end of the tunnel, electrons are stripped from the negative ions, forming the high-speed-neutral atomic particles that are the neutral particle beam. The NPB delivers its kinetic energy directly into the atomic and subatomic structure of the target, literally heating the target from deep within. Charged particle beams (CPB) can be produced in a similar fashion, but they are easily deflected by the earth's magnetic field and their strong electrical charge causes the CPB to diffuse and break apart uncontrollably. Weapons-class NPBs require energies in the hundreds of millions of electron volts and beam powers in the tens of megawatts. Modern devices have not yet reached this level.

In 1951, Charles Townes and co-workers discovered stimulated emission in ammonia, which led to the development of a maser (microwave amplification by stimulated emission). They were encouraged by AFOSR to extend the work to shorter wavelength all the way to the optical region. Ten years later, in 1961, Ted Maiman (Hughes Aircraft Company) announced the development of a laser (light amplification by stimulated emission). The use of lasers in numerous applications eventually led to more powerful devices.

The long-cited advantages of high-energy lasers [HEL] include speed of light response, precision effects, limiting collateral damage, deep magazines, and low cost per kill. High-energy lasers have two characteristics that make them particularly valuable for effects-based application: they are extremely fast and extremely precise. The laser begins its attack within seconds of detecting its target and completes its destruction a few seconds later. This means the operator has time for multiple shots if needed to destroy the target or engage multiple targets.

Lasers can be built as either continuous wave (CW) or pulsed devices. CW laser effects are generally described in terms of power density on target in W/m2; pulsed laser effects are described in terms of energy density on target in J/m. For the space-earth geometry multimegawatt power is required for a CW weapons laser and hundreds to thousands of joules of energy per pulse is required for a pulsed weapons laser (depends on pulse length and pulse repetition frequency).

The laser beam delivers its energy to a relatively small spot on the target-typically a few inches in diameter. The incident intensity is sufficient to melt steel. Typical melt-through times for missile bodies are about 10 seconds. But if the heated area is under stress from aerodynamic or static pressure loads, catastrophic failure can occur more quickly. The beam can attack specific aim points on a missile that are known to be vulnerable; for example, pressurized fuel tanks or aerodynamic control surfaces. The laser weapon design, therefore, must include the ability to "see" and identify specific aim points, to put the beam on that aim point and hold it for a few seconds, and finally, to determine when the desired effect on the target has been achieved.

There are four fundamental approaches to high- and medium-power laser energy: chemical lasers, solid-state lasers, fiber lasers, and free electron lasers.

  • Chemical lasers can achieve continuous wave output with power reaching to multi-megawatt levels. Examples of chemical lasers include the chemical oxygen iodine laser (COIL), the hydrogen fluoride (HF) laser, and the deuterium fluoride (DF) laser. There is also a DF-CO 2 (deuterium fluoride-carbon dioxide) laser.
  • Diode-pumped solid-state (DPSS) lasers operate by pumping a solid gain medium (for example, a ruby or a neodymium-doped YAG crystal) with a laser diode.
  • Combining the outputs of many fiber lasers (100 to 10,000) is a possible way to achieve a highly efficient HEL. Fiber-laser technology continues to advance. At 1 ??m, 200 W amplifiers are available commercially, and > 500 W has been demonstrated in the lab.
  • Free-electron lasers (FELs) are unique lasers in that they do not use bound molecular or atomic states for the lasing medium. FELs use a relativistic electron beam (e-beam) as the lasing medium. Generating the e-beam energy requires the creation of an e-beam (typically in a vacuum) and an e-beam accelerator. This accelerated e-beam is then injected into a periodic, transverse magnetic field (undulator). By synchronizing the e-beam/electromagnetic field wavelengths, an amplified electromagnetic output wave is created.

Years of major investment in chemical lasers produced megawatt-class systems that could have a wide range of applications. However, size, weight, and logistics issues limit them to integration on large platforms, such as the 747 used for the ABL program, or fixed ground applications such as the Ground-Based Laser for Space Control. As a consequence, interest in these systems and expectations of progress has significantly decreased.

The laser weapon delivery system is a complicated one, consisting of many elements grouped as subsystems. Two parts to the system involve equipment used in the operation of the laser weapon: (1) beam generation or laser source; and (2) supporting technologies such as acquisition, pointing, and tracking (including fast beam-slewing equipment, adaptive optics, and reflective mirrors). To develop an effective laser system two other important areas have to be considered: (1) beam-target interaction/lethality science and validation; and (2) modeling and simulation including theoretical calculations and/or computer models.

Key issues that have an impact on all initiatives include pointing and tracking accuracy, beam control, and beam propagation in a battlefield environment or during poor weather conditions. In the case of laser weapons, lethality effects against a variety of targets must also be clearly understood.

Pointing and tracking accuracy is the ability to point the laser beam to the desired aimpoint and to maintain that aimpoint on the target. To achieve the status of a precision-aimed weapon, laser weapon systems will require pointing and tracking accuracies in the 10 to 100 nanoradian range for systems in low earth orbit.

Beam control refers to forming and shaping the beam. Depending on the nature of the specific laser, beam control can include initial processing of the beam to shape it and eliminate unwanted off-axis energy, or can include wavefront shaping and/or phase control. For visible and near-infrared lasers, the frequencies under study for use at long range, optics in the four to 20 meter diameter should suffice for a system in low earth orbit.

Beam propagation describes the effects on the beam after it leaves the HEL output aperture and travels through the battlefield environment to the target. Optical stability of the platform and beam interactions with the atmosphere, both molecules and aerosol particles, primarily determine the laser beam quality at the target. Beam quality is a measure of how effective the HEL is in putting its light into a desired spot size on the target.

Atmospheric and propagation effects on HEL performance requires expanded efforts to measure and understand low-altitude, "thick-air" atmospheric effects. Primary concerns include the effects of atmospheric turbulence and aerosol scattering on the HEL beam. Nonlinear propagation effects such as thermal blooming can also have important effects for many applications. Technical remedies are available to deal with atmospheric turbulence, but much more understanding is needed, as is the ability to predict and measure atmospheric turbulence. Non-linear propagation effects require detailed analyses and experiments. They also require beam control concepts to ameliorate the negative effects. No such analyses or experiments exist for multi-pulse systems.

Lethality defines the total energy and/or fluence level required to defeat specific targets. The laser energy must couple efficiently to the target, and it must exceed a failure threshold that is both rate dependent and target-specific. Laser output power and beam quality are two key factors for determining whether an HEL has sufficient fluence to negate a specific target.

By 2007 the focus was on solid state lasers with the promise of providing for smaller, lighter systems with deep magazines. However, the current goal for solid state laser development would provide a power level more than an order of magnitude lower than current chemical lasers. While beam quality and other factors can compensate for some of the difference in power level, there is currently little investment in those aspects. Further, these cannot make up the delta in power of chemical vs. solid state lasers. The near-term projection for solid state lasers is a power level closer to two orders of magnitude below that of chemical lasers.

Sources and Resources

Directed Energy/Space-Based Laser: Ballistic Missile Defense Organization (BMDO) The Directed Energy (DE) program continues the process of integrating high power chemical laser components and technologies developed over the past 10 years specifically for the ballistic missile defense boost phase intercept mission.

High Energy Laser Systems Test Facility The High Energy Laser Systems Test Facility (HELSTF) is located at White Sands Missile Range, New Mexico. HELSTF has been managed by the U.S. Army Space and Strategic Defense Command (USASSDC) since October 1990. HELSTF is designated as the Department of Defense (DoD) National Test Facility for high energy laser test and evaluation. HELSTF is the home of the Mid Infrared Advanced Chemical Laser (MIRACL), the United States' most powerful laser.

SEALITE Beam Director (SLBD) The SEALITE Beam Director (SLBD) is mounted on top of Test Cell 1. It consists of a large aperture (1.8 meter) gimbaled telescope and optics to point the MIRACL or other laser beam onto a target. The high power clear aperture is 1.5 meters. The remaining 0.3 meters is normally reserved for a tracker using the outer annulus of the primary mirror. The system is extremely agile and capable of high rotation and acceleration rates




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