As Era Of Laser Weapons Dawns, Tech Challenges Remain

Steve Trimble

As the U.S. Air Force comes within weeks of the first operational laser weapons, the Defense Department is hatching new concepts to address the power and thermal management limits of the state-of-the-art in the directed energy field.

In a largely secret dress rehearsal staged last week at Fort Sill, Oklahoma, the Air Force performed another round of tests of the deploying Raytheon High Energy Laser Weapon System (HEL-WS), as well as other directed energy options, such as the Air Force Research Laboratory's Tactical High Power Microwave Operational Responder (THOR), says Kelly Hammett, director of AFRL's Directed Energy Directorate.

“All I can say is there were multiple systems. From my reading of the reports, it looked like a very successful exercise,” says Hammett, who addressed the Association of Old Crows annual symposium Oct. 29.

The Fort Sill experiment was intended to put the weapons through their paces in a realistic operational environment. AFRL's Strategic Development, Planning and Experimentation (SDPE, which, despite its spelling, is pronounced “Speedy”) office called on the HEL-WS and THOR to engage swarms of small unmanned aircraft systems (UAS). The experiments also demonstrated new diagnostic tools, allowing AFRL testers to understand the atmosphere's effect on energy propagation in real time.

SDPE awarded Raytheon a contract in August to deliver a “handful” of systems to the Air Force for a one-year deployment scheduled to conclude in November 2020. The HEL-WS will be used to defend Air Force bases from attacks by swarming, small UAS and cruise missiles, Hammett says. The Air Force is not releasing the location of the deployed sites for the HEL-WS.

AFRL also is grooming THOR for an operational debut. Instead of blasting a UAS with a high-energy optical beam, THOR sends powerful pulses of radio frequency energy at a target to disable its electronics. Hammett describes THOR as a second-generation directed energy weapon. It is designed to be rugged for operational duty and compact enough to be transported inside a single container loaded into a Lockheed Martin C-130. Upon unloading from the aircraft, THOR can be activated within a couple hours, or broken down and moved within the same period, he says.

Despite decades of basic research on directed energy systems, such operational capabilities have evolved fairly rapidly. The Air Force finally consolidated its strategy for developing directed energy weapons in the 2017 flight plan, Hemmett said. The document narrowed a once-fragmented research organization that attempted to address too many missions.

“Directed energy zealots like myself have been blamed, rightly so, of saying directed energy can do almost anything you want it to do. And we pursued multiple applications to the effect that we were diffusing some of our efforts,” he says.

The 2017 flight plan selected three initial use cases: Air base defense, precision strike and self-protect.

The HEL-WS and THOR are addressing the first mission. The Joint Navy-Air Force High Power Electromagnetic Non-Kinetic Strike (Hijenks) program is developing a missile to address the precision strike requirement, as a follow-on to the Counter-electronics High Power Microwave Advanced Missile Project (Champ) that concluded five years ago. In the long-term, AFRL also plans to demonstrate the Self-Protect High Energy Laser Demonstrator (Shield), a podded defensive weapon for aircraft.

Although such technology has come far, researchers are still grappling with fundamental issues to make them practical. Namely, the power generation and thermal management requirement for high-energy lasers and high-power microwaves remains a challenge.

“If you're willing to have very limited duty-cycle, very limited magazine, the power and thermal management aren't very challenging,” Hemmett says. “Of course, that's not what we want from directed energy weapons. We want deep magazines. We want to be able to handle wave attacks as favorably or more favorably that kinetic weapons.”

The “rule of thumb” for a high-energy laser is an efficiency of about one-third, meaning a 300-kW generator is necessary to create a 100-kW laser beam, resulting in 200 kW of waste heat that must be dealt with in some way, says Frank Peterkin, a senior technologist on directed energy for the U.S. Navy who spoke at the same event. On Navy ships, that puts the laser in competition with the electronic warfare and radar subsystems for power and thermal management loads, he adds.

“The challenge for the directed energy community is we don't really own the solution,” Peterkin says. “It does need to be a more holistic solution for the Navy. We are a customer, but we're not driving the solution, per se.”

Although directed energy researchers cannot design the power grids for bases, ships and aircraft, they can help the requirement in other ways, says Lawrence Grimes, director of the Directed Energy Joint Transition Office within the Defense, Research and Engineering directorate of the Office of the Secretary of Defense.

The development of special amplifier diodes for fiber optic lasers are breaking the “rule of thumb” for high-energy systems, Grimes says. “They actually operate at higher temperatures and higher efficiency, so they can reduce the requirement necessary for the prime power and thermal management, and we're not throwing away 200 kW.”

Other Defense Department organizations are pursuing more ambitious options. The Strategic Capabilities Office is selecting suppliers to demonstrate small, 10 MW-size nuclear reactors, as a power generation option for directed energy weapons at austere forward operating bases.

Meanwhile, AFRL also is considering space-based power generation. Under the Space Solar Power Incremental Demonstrations and Research program, AFRL will investigate using high-efficiency solar cells on a spacecraft to absorb the solar energy. The spacecraft then would convert the solar energy into a radio frequency transmission and beam it to a base to supply energy. AFRL has awarded Northrop Grumman a $100 million contract to begin developing the technology.

If those seem like long-term options, the Air Force is not immediately concerned. The HEL-WS and THOR are designed to use “wall-plug” power or the military's standard electric generators, Hammett says.

https://aviationweek.com/defense/era-laser-weapons-dawns-tech-challenges-remain