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The Laser Directed Energy Weapons (DEWs) offer a transformational ‘ga me changer’ to counter asymmetric and disruptive threats, while facing increasingly sophisticated traditional challenges. Laser technology provides major advantages for military applications over kinetic weapons due to High precision and rapid on-target effect, precise and scalable effects, avoidance of collateral damage caused by fragmenting ammunition, Low logistics overhead and minimum costs per firing.
The development of laser weapons requires many critical technologies, first is development of lasers capable of generating powers in kilowatts to megawatts range to be able to produce useful damage effects on the target. For instance to destroy anti-ship cruise missiles would require a beam of 500 kilowatts and demand megawatts of power.
The first new generation laser weapon was a 30-kilowatt Laser Weapon System (LaWS) deployed on USS Ponce in 2014. It was first laser weapon to have attained Initial Operating Capability (IOC) by virtue of being deployed in a combat theater. And now it plans to deploy 150 KW weapon on the ship in next year. Boosting beam power further—to something like 200 kW or 300 kW—could permit a laser to counter at least some ASCMs. Even stronger beam powers—on the order of at least several hundred kW, if not one megawatt (MW) or more—could improve a laser’s effectiveness against ASCMs (Anti-Ship Cruise Missile) and enable it to counter ASBMs (Anti-Ship Ballistic Missile.
The next generation laser weapons will be very large multi-Mega Watt Strategic laser weapons which can be both endo or exo-atmospheric and will be deployed on high altitude Airborne, space based / satellite based platforms or space relayed (Endo-atmospheric with Orbiting Relay Mirrors) for countering Tactical Ballistic Missiles (TMD), boost phase defence, ICBMs ( NMD) – boost phase defence and Satellites (ASAT).
The subcommittee on Emerging Threats and capabilities met Dr. Michael Griffin undersecretary of defense for Research and Engineering on accelerating new technologies to meet emerging threats. Rapid technological advancements have the potential to change the very character of war. Michael Griffin said: “We need to have 100-kilowatt-class weapons on Army theater vehicles. We need to have 300-kilowatt-class weapons on Air Force tankers,” Griffin said. “We need to have megawatt-class directed energy weapons in space for space defense. These are things we can do over the next decade if we can maintain our focus.”
Defense officials want to test a neutral particle-beam in orbit in fiscal 2023 as part of a ramped-up effort to explore various types of space-based weaponry. They’ve asked for $304 million in the 2020 budget to develop such beams, more powerful lasers, and other new tech for next-generation missile defense. Such weapons are needed, they say, to counter new missiles from China, Russia, North Korea and Iran.
China and Japan develop petawatt lasers
Researchers from Shanghai have created the most powerful laser beam ever made with potentially wide-ranging applications in fields from nuclear physics to high-tech weaponry, according to their paper published in the issue of the journal Optics Letters. The beam reached a peak power of 5.13 petawatts (1 petawatt is equal to 1 billion millions watts), dwarfing the record set recently by Japanese scientists.
However It cannot sustain its peak power output for long, and lasted for less than 30 femtoseconds (30 quadrillionths of a second), according the to the team. Its poor power of endurance means that the total amount of energy generated by a single pulse was very low – 190 joules, The new record was generated at the State Key laboratory of High Field Laser Physics under the Shanghai Institute of Optics and Fine Mechanics. The team was led by Professor Li Ruxin.
“Conventional laser weapons take very different designs. The original purpose [of the Shanghai beam] is not to form part of a laser gun or cannon,” said Li, a researcher at Chinese Academy of Sciences’ Key Laboratory of Functional Crystals and Laser Technology in Beijing. Li was not involved in the research. Nonetheless, future military applications cannot be ruled out, Li said.
U.S. missile-defense researchers launch project to build a prototype megawatt-class laser weapon
The Ballistic Missile Defense System (BMDS) Laser Scaling project of the U.S. Missile Defense Agency (MDA) will develop a prototype laser weapon system that will weigh no more than about four tons, including the laser, electric power, and thermal management subsystems. The project’s focus is on reducing size and weight, and increasing power, electrical-to-optical efficiency, beam quality, and lasing runtime.
MDA officials issued a request for information (HQ0277-19-RFI-0001) for the BMDS Laser Scaling project in efforts to understand industry’s ability to demonstrate a 1,000-kilowatt electrically pumped laser sometime between 2025 and 2026. MDA officials envision a laser weapon able to shoot down incoming ballistic missiles with near diffraction-limited beam quality at 1 megawatt of laser power with a vertical beam quality of 1.1 at 0.25 lambda/D. It should have a laser wavelength shorter than one micron to offer high intensity on the target at long ranges.
Researchers also are interested in electrical power and thermal management subsystems for the prototype, and are not yet providing a specific platform or strategic mission. It is to be a ground demonstrator laser with technology maturation and light-weight engineering paths to potential future applications.
The system should have a mass efficiency of two to four kilograms per kilowatt, including electric power and thermal management. Early prototypes may have a lower mass efficiency as long as they have clearly defined paths to increase mass efficiency. The electrical-to-optical efficiency goal is at least 48 percent, and continuous laser shot durations must be from 2 to 60 seconds. The prototype must have an energy storage system able to supply power for two minutes at full power without recharging.
Challenges
Design and testing of high energy laser weapons require the generation of an adequate amount of power, preferably in megawatts, to cause considerable damage to the distant targets. Such high-power transmissions pose serious safety challenges during lab/field testing, where the performance of such systems is to be evaluated under a controlled environment. As this domain is gaining maturity, development, and configuration of safe, cost-effective and comprehensive testing facilities for high-energy laser-experiments has emerged as an important research challenge.
high-energy laser generation systems also require efficient cooling mechanisms, having capabilities to maintain stable temperatures during transmission cycles. Moreover, enhancing the accuracy of laser systems under intentional (jamming) or naturally-encountered unfavorable weather conditions, such as due to heavy smoke, humidity or dust, is another important research direction. Another important consideration while dealing with high-power laser transmission is to minimize collateral damage and threats to nearby friendly sensors and equipment. Research in this domain involves exploring potent methods to electronically and physically harden friendly electronics to ensure their optimal functionality and reliability.
Megawatt class laser technologies
Chemical lasers are the only systems that have produced megawatt-level outputs, however, they require special handling because of toxic chemicals hence fallen out of favor. Another reason is that they rely on what is essentially an external/independent power source, and thus lack the key strategic value of directed energy weapons: a virtually unlimited magazine.
The Boeing YAL-1 Airborne Laser Testbed (formerly Airborne Laser) weapons system was a megawatt-class chemical oxygen iodine laser (COIL) mounted inside a modified Boeing 747-400F. It was primarily designed as a missile defense system to destroy tactical ballistic missiles (TBMs) while in boost phase. The aircraft was designated YAL-1A in 2004 by the U.S. Department of Defense.
Solid state lasers are electrically powered, and they are separated into three types: Fiber solid-state lasers like LaWS, slab solid-state lasers, and free electron lasers. While they avoid the complicated logistics associated with chemical lasers, SSLs are generally not very efficient.Existing lasers generally dissipate two-thirds to three-quarters of the energy as heat, requiring still-bulky cooling equipment to avoid overheating damage. Air cooling can yield an unacceptable delay between shots.
Sustaining good beam quality at that power level is important. Mike Griffin, former undersecretary of defense for research and engineering, told Congress that current fiber laser technology could be scaled to 300 kilowatts to protect air force tankers. However, that may be pushing the upper limits of how many beams from separate fiber lasers emitting at closely spaced wavelengths can be combined coherently to generate a single high-energy laser beam of high quality.
Liquid Lasers Challenge Fiber Lasers as the Basis of Future High-Energy Weapons
But now aerospace giant Boeing has teamed with General Atomics—a defense contractor also known for research in nuclear fusion—to challenge fiber lasers in achieving the 250-kilowatt threshold that some believe will be essential for future generations of laser weapons. Higher laser powers would be needed for nuclear missile defense.
The challenging technology was developed to control crucial issues with high energy solid-state lasers: size, weight and power, and the problem of dissipating waste heat that could disrupt laser operation and beam quality. General Atomics “had a couple of completely new ideas, including a liquid laser. They were considered completely crazy at the time, but DARPA funded us,” said company vice president Mike Perry in a 2016 interview. Liquid lasers are similar to solid-state lasers, but they use a cooling liquid that flows through channels integrated into the solid-state laser material. A crucial trick was ensuring that the cooling liquid has a refractive index exactly the same as that of the solid laser material. A perfect match of the liquid and solid could avoid any refraction or reflection at the boundary between them. Avoiding reflection or refraction in the the cooling liquid also required making the fluid flow smoothly through the channels to prevent turbulence.
Free-Electron Laser
Because an FEL’s photons—the concentrated particles of light composing the laser beam—have the potential to be powerful enough to destroy cruise (and ballistic) missiles many miles away, FELs are called the Holy Grail of military lasers. In an FEL, the electrons are produced in an electron injector and injected into a particle accelerator, which kicks them up to fantastically high energy levels. Researchers at AOT recently built and successfully tested an advanced injector—a key FEL component—that produced a beam of electrons powerful enough for a megawatt-class (one million watts) antimissile FEL weapon.
The electrons are created by a photocathode inside an injector—a photoelectron injector—and are then injected into the particle accelerator. “The photoelectron injector was invented at Los Alamos,” explains Nguyen. “Electrons make a high-gain medium, which makes a powerful FEL possible. Using this technology, it becomes feasible to amplify 1 watt to 1 megawatt!” These waves of electrons, traveling at the speed of light inside the accelerator undulate between a series of alternating magnets, which causes the electrons to emit the powerful beams of photons.
“We accelerate the electrons through a series of radio frequency (RF) cavities, known as. RF accelerators, to almost the speed of light. The resultant energy of the electrons ranges from tens of millions to hundreds of millions of electronvolts,” says Dinh Nguyen, who co-leads the Laboratory’s FEL research team. These electrons are used to create the high-powered photons that make up the precise and concentrated beam of light of the FEL. “Our injector increased the electron beam current by a factor of 10 over what was previously demonstrated. A megawatt FEL is no longer theoretical.” This injector can operate continuously, meaning the FEL can fire continuously and destroy multiple targets
This is “a major leap forward for the [FEL] program,” says Quentin Saulter, the ONR’s FEL program manager. “You need megawatts of laser power to destroy a cruise missile,” says Nguyen. “The laser kills with heat. Extreme heat destroys the missile’s mechanics and electronic guidance systems, making it aerodynamically unstable so it tumbles wildly out of control. Extreme heat can also ignite the missile’s fuel, causing it to explode. But there’s not much time to heat up a missile. You need a tremendous amount of heat, like that from a megawatt laser, and a beam several feet in diameter to cook something like a missile that quickly.” He adds, “Imagine being able to use a ‘super blowtorch’ to destroy something that’s miles away…”
The FEL is an ideal countermeasure for ships because its beam can be optimized for varying atmospheric conditions at sea. For example, substances in the atmosphere— particularly water vapor, but also smoke, salt particles, dust, pollen, and other pollutants—absorb and scatter light. At sea, absorption by substantial amounts of water vapor is a particular problem for lasers. The problem of light absorption increases as the distance the light travels increases, reducing a laser’s effectiveness against distant targets.
Yet, there are wavelengths of light in the electromagnetic spectrum where light absorption by water vapor is markedly less, creating a window in the vapor for the light to pass through. These windows change along with atmospheric changes. FELs overcome these problems because they can be operated at different wavelengths. Indeed, FELs have the widest frequency range of any type of laser. This means FELs’ wavelengths are tunable—they can be changed, in essence, by the turn of a dial. If an FEL’s operators know the wavelengths that will become attenuated in the atmosphere, they can adjust the FEL’s wavelength to a different wavelength. By finding the window, the FEL’s beam of light travels longer distances.
Extreme Light Infrastructure ELI
Laser intensities have increased by several orders of magnitude in the last few decades, now reaching frontiers where the laws of light-matter interaction change fundamentally due to the dominance of relativistic effects in the dynamics of charged particles under the influence of laser light. Among the important by-products of this field there are novel mechanisms for the generation of highly energetic particles, x-rays and gamma-rays, and their applications in various disciplines ranging from fundamental physics to materials research and life sciences.
ELI will be the world’s first international laser research infrastructure, pursuing unique science and research applications for international users. ELI will be implemented as a distributed research infrastructure based initially on 3 specialised and complementary facilities located in the Czech Republic, Hungary and Romania. ELI is the first ESFRI project to be fully implemented in the newer EU Member States. ELI is pioneering a novel funding model combining the use of EU structural funds (ERDF) for the implementation, and member contributions to a yet to be established European Research Infrastructure Consortium ERIC for the operation.
ELI will increase the available laser power by at least one order of magnitude in its first three pillars, and by another order of magnitude in its fourth, ultra-high-intensity pillar. One important aspect of ELI is the possibility to produce ultra-short pulses of high energy photons, electrons, protons, neutrons, muons and neutrinos in the attosecond and possibly sub-attosecond regimes on demand. Time-domain studies will allow unraveling the attosecond dynamics in atomic, molecular and plasma physics.
Today, all three ELI institutes have operational laser systems now. They do not reach exawatt pulses, but they have and will continue to achieve world records in various parameters. And they are competing with a rapidly growing number of laser facilities pursuing ultrahigh intensities around the world. Pulse powers of 10 petawatts have been shown, 100 PW are proposed or even under construction. New records in focused intensity can be expected in 2021.
LLNL petawatt laser reported in March 2022
Over the next several years, LLNL’s Advanced Photon Technologies (APT) program will design and construct one of the world’s most powerful petawatt (quadrillion-watt) laser systems for installation in an upgraded Matter in Extreme Conditions (MEC) experimental facility at LCLS, funded by the Department of Energy’s Office of Science-Fusion Energy Sciences program.
The new laser will pair with the LCLS X-ray free-electron laser (XFEL) to advance the understanding of high-energy density (HED) physics, plasma physics, fusion energy, laser-plasma interactions, astrophysics, planetary science and other physical phenomena.
The existing MEC facility uses optical lasers coupled to X-ray laser pulses from LCLS to probe the characteristics of matter at extreme temperatures and pressures. MEC experiments have produced groundbreaking science, such as the first observations of “diamond rain” under conditions thought to exist deep inside giant icy planets like Uranus and Neptune.
The MEC-Upgrade (MEC-U) is motivated in part by increasing calls for the United States to re-establish world-class leadership in high-power laser technology, such as in the 2018 National Academies of Science, Engineering, and Medicine report, “Opportunities in Intense Ultrafast Lasers: Reaching for The Brightest Light.”
SLAC is partnering with LLNL and the University of Rochester’s Laboratory for Laser Energetics (LLE) to design and construct the MEC-U facility in a new underground cavern. LLNL’s rep-rated laser (RRL), able to fire at up to 10 Hz (10 pulses per second), and a high-energy kilojoule laser developed by LLE will feed into two new experimental areas containing a target chamber and a suite of dedicated diagnostics tailored for HED science.
The LCLS, part of SLAC’s two-mile-long linear particle accelerator in Menlo Park, Calif., is capable of delivering 120 X-ray pulses a second, each one lasting a few femtoseconds (quadrillionths of a second). A concurrent upgrade dubbed LCLS-II will deliver a million pulses a second in an almost continuous X-ray beam that, on average, will be 10,000 times brighter and will double the X-ray energy previously attainable.
“This architecture, originally dubbed the Scalable High-power Advanced Radiographic Capability, or SHARC, eliminates the lossy second (titanium-doped sapphire) stage of the HAPLS laser system,” Spinka said, “ultimately delivering about five times higher energy than HAPLS at the same peak power and repetition rate.”
LLNL’s RRL for the MEC-U facility will be developed in parallel with performance ramping of the HAPLS (now known as L3-HAPLS) laser at ELI-Beamlines to its full design specifications. It also will leverage additional advanced laser technologies being developed by APT, including a new high-energy Faraday rotator developed under a Cooperative Research and Development Agreement with Electro-Optics Technologies Inc.
Enabling new physics
“MEC-U is a core part of NIF&PS’s strategy for developing next-generation high-average-power ultrafast lasers and enabling high rep-rate HED science,” Tang said. “The new physics the MEC-U enables is broad-ranging and highly applicable to LLNL missions. It is an exciting opportunity for LLNL and the community.”
“Not only are we working with some of the leading laser laboratories in the world,” Fry added, “but we’re also working with world experts in experimental science, high-energy-density science, and the operation of DOE Office of Science user facilities, where scientists from all over the world can come to do experiments.”
Access to the facility will be facilitated in part by LaserNetUS, a research network that is boosting access to high-intensity laser facilities at labs and universities across the country.
LLNL’s Diffraction Gratings to Enable Most Powerful Laser
Petawatt and multipetawatt lasers rely on chirped pulse amplification to stretch, amplify, and then compress a high-energy laser pulse to avoid damaging optical components. Pulse-compression gratings must be sufficiently large, efficient, and robust to withstand the high fluence (energy density) of the laser pulses generated by petawatt-class lasers such as the National Ignition Facility’s (NIF) Advanced Radiographic Capability (ARC).
Researchers from Lawrence Livermore National Laboratory (LLNL) and their collaborators reported in Sep 2022, to have developed high-energy pulse compression gratings that will be installed in what will be the world’s most powerful laser system. The laser system, called L4-ATON, which is currently being developed, is designed to deliver up to 10 PW of peak power. One petawatt is about 1000× the capacity of the entire U.S. electrical grid.
The high-energy, low-dispersion (HELD) multilayer dielectric gratings will be installed in L4-ATON at the ELI-Beamlines Facility in the Czech Republic. L4-ATON can generate 1.5 kJ of energy in 150-fs pulses, equal to 10 PW of power, at a repetition rate of one shot per minute.
According to LLNL senior laser scientist Hoang Nguyen, the HELD gratings are advancements over the NIF ARC-like gratings and allow for significantly higher energy outputs. The 85- × 70-cm HELD gratings are configured at a Littrow angle (the angle of maximum grating efficiency) of 37º and allow for a larger beamwidth — 62.5 cm. “Increasing the beam height to produce a square beam and accounting for the difference in LIDT (laser-induced damage threshold) results in approximately 3.4× more total energy on the grating compared to the ARC high-dispersion, 76.5º-angle-of-incidence grating design,” Nguyen said.
Multipetawatt laser technology opens the door to research in areas such as plasma and high-energy-density physics, astrophysics, laser-driven particle acceleration, enhanced medical diagnostics, industrial processing techniques, and nuclear materials detection.
References and resources also include:
https://www.photonics.com/Articles/LLNLs_Diffraction_Gratings_to_Enable_Most/a68391
https://phys.org/news/2022-03-high-power-laser-experimental-facility.html
