Directed Energy Weapons (DEWs) utilize concentrated electromagnetic energy focused on a target to cause damage, versus conventional kinetic energy weapons which hit the target with a solid projectile. Lasers, microwaves, and particle beams are examples of DEWs. While DEWs are not a new concept, they are gaining increasing interest as the technologies to concentrate energy to damaging levels, to track the targets accurately, and to manage the massive amounts of waste energy produced are reaching higher levels of maturity. At the same time, the price tag for conventional ammunition and conventional weapons continues to climb, placing increasing funding burdens on the government. Compared to conventional weapons, the advantages of DEWs include: (1) speed of light target engagement; (2) low cost per shot; and (3) deep magazine capability, limited only by the available power and thermal management capacity.

Thermally Managing Directed Energy Weapons

Thermal management is a critical component of DEW systems. Without robust cooling solutions, the massive amount of waste heat generated per high-powered shot will damage the weapon, and support systems, and likely cause acute failure. This is the case with any high-powered electronic device. The higher the power, the more waste heat these devices generate. Additionally, the temperature of the waste heat is relatively low, which makes it more difficult to manage with conventional methods.

The primary challenge is how to efficiently remove substantial amounts of low-grade waste heat, owed to the inefficiency of these devices (i.e., high energy lasers, microwave emitters, etc.) while simultaneously maintaining relatively tight thermal stability. For example, lasers are highly temperature dependent to achieve high efficiency and stable optical wavelength. If the heat rejection and temperature stability are not adequately managed, laser DEW performance degrades from the beginning of the shot to the end. Therefore, the ultimate thermal management systems must be compact, lightweight, and low power.

Cooling solutions are platform dependent (ground-based, airborne, shipboard) taking into consideration availability of resources and the ultimate heat sink (air, water, etc). For both ground-based and airborne DEW, Advanced Cooling Technologies (ACT), together with DoD partners have developed, prototyped, and tested several low size, weight, and power (SWaP) cooling systems. An integrated, fully functional thermal management prototype was delivered to the Navy in 2018 to support the Ground Based Air Defense (GBAD) program.

Thermal Storage Using Phase Change Material

Figure 2. Visual representation of the standard operation of the PCM heat sink.

Most thermal solutions are developed to handle the maximum heat load at the time of generation. In other words, a 5 kW load requires a 5 kW solution. While this is true for electronics operating in a steady state mode, it is not necessarily the case for pulsed technology such as DEW. In a DEW application, the heat load may only be “on” for a period of time and then “off” for a recovery, recharge, regenerate period of time. In these applications, the thermal management solution has additional time to dissipate the waste heat generated during the “on” period. The thermal solution can be smaller than the load generated during the “on” period. A 5 kW “on” load, that is on for one out of every five time periods, can use a 1 kW solution over the five time periods.

To accomplish this without overheating during the “on” period, thermal storage, phase change materials (PCM) can be utilized. Typically, the phase change is from solid to liquid and back. The material is selected such that it melts close to, but not in excess of, the maximum allowable temperature of the electronics being cooled. During the high power “on” portion of the shot, some of the waste heat is dissipated by the cooling system, while the balance of the waste heat melts the PCM from a solid to a liquid. During the “off” period, the traditional cooling system continues to dissipate the stored energy in the PCM, freezing it back to a solid, so that it is ready to absorb another pulse of energy when the weapon is fired again. One other benefit of the PCM is that it will provide a relatively high level of survivability should the smaller steady state cooling solution become incapacitated.

Figure 3a. Manifold Cold Plate Design. Heat from the source is transferred to a single-phase liquid coolant circulating through internal channels of the plate and subsequently rejected downstream to a radiator.
Figure 3b. Mini Channel Cold Plate

Thermal storage is an effective way to absorb and store the waste heat until engagement is again necessary. Thermal storage works by storing thermal energy in a phase change material (PCM). The phase change from a solid to liquid occurs through energy absorption. The latent heat from melting or freezing is at least 1 to 2 orders of magnitude higher than the energy stored by the specific heat.

ACT has conducted an abundance of research, development, and prototyping of PCM assisted cooling solutions. Efficiently delivering and extracting heat energy from the PCM reservoir is the key to a well-designed system. ACT's thermal experience with enhanced surfaces (fins), high conductivity (Hi-KTM) components, heat pipes, pumped loops, etc. allows us to design and develop PCM systems that are small, lightweight, and low power. As a reference, systems sized for the peak heat load typically weigh 40-60% more than those with thermal storage, although this is highly dependent on the duty cycle.

Pumped Single-Phase Cooling

Single-phase cooling is another option for managing waste heat produced by DEW. At a minimum, single-phase cooling requires a reservoir, or fluid source at the prescribed temperature, and a pump. This method of cooling traditionally involves cold plates with pumped water or water/glycol mixtures running through channels to absorb the waste heat. This thermal solution is typically used in systems with low to moderate heat fluxes.

ACT has also shown that single-phase systems can be used for heat spreading, and to remove very high heat fluxes over small areas. Researchers at ACT have removed heat fluxes of over 10,000W/cm2 over several square centimeters. The disadvantages of this technology for DEW are size issues. Many DEW thermal solutions will require a very small footprint and will not have space for a large pump and cold plate.

Pumped Two-Phase Cooling

Figure 4. ACT's Pumped Two-Phase Cooling System

In contrast, pumped two-phase (P2P) cooling systems use a non-corrosive, non-electrically conductive (dielectric) fluid, which vaporizes during the absorption of the waste heat. A much smaller amount of coolant, relative to pumped single-phase, is pumped into the cold plate, where the coolant changes phase from liquid to vapor. Because the latent heat of vaporization is many times the specific heat of the coolant, the coolant flow rate can be significantly reduced, resulting in smaller pumps and lower power usage.

High heat flux applications often use sintered wick materials in the cold plate channels to provide ample and uniform nucleation sites for boiling. These nucleation sites prevent flow maldistribution during transients and help to provide lower evaporator thermal resistances. P2P systems offer an attractive way to reduce SWaP, since they take advantage of the latent heat of vaporization of the coolant; they also allow for a much higher degree of temperature uniformity for electronic components and laser diodes that reach heat fluxes of up to 500 W/cm2.

Figure 5. Pumped Two-Phase Cooling Benefits

By combining a non-conductive dielectric refrigerant with the science of heat dissipation through vaporization, ACT has found that well-designed pumped two-phase cooling products can manage power densities more than 2x over traditional water / glycol systems for high-power electronics, while eliminating the dangerous consequences of a fluid leak. Although the systems are leak-tight, should an accidental leak occur, the non-conductive fluid is inherently safe even when in direct contact with sensitive electronics.

ACT has developed reliable P2P systems for thermal control of optoelectronics and other applications that need both low SWaP, high power removal, and tight temperature uniformity. Key components in these systems have also been scaled, prototyped and tested for large-scale DEW implementations.

Thermal Management is Key

Advanced thermal management is one of the keys to bringing directed energy weapons online and to allow them to operate at peak performance. Both single-phase and pumped two- phase cooling systems, in conjunction with thermal storage, have demonstrated significant SWaP benefits. These technologies are mature and ready for field test implementation. They can also be rapidly deployed for acquisition programs.

This article was written by Pete Dussinger, Chief Engineer, Products, Advanced Cooling Technologies, Inc., (Lancaster, PA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the September, 2020 issue of Aerospace & Defense Technology Magazine.

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