Perfecting laser defenses
From “Star Wars” to “Star Trek,” the laser has captured the popular imagination like few other high-tech devices.
Short for “light amplification by stimulated emission of radiation,” the laser has a storied history dating back to 1917 and Albert Einstein, who first suggested the concept of stimulated emission. Simply stated, radiation can stimulate electrons in an atom to jump from higher to lower energy levels, giving off light in the process.
First demonstrated in 1960 by Theodore Maiman, a researcher with the Hughes Aircraft Co., lasers have been harnessing Einstein’s idea ever since, most recently in compact disc and DVD players, fiber-optic telecommunications, bar code readers, laser printers, laser pointers, auto part welders — and weapon systems.
Today, so-called tactical high-energy lasers are keeping the United States safe from incoming threats like ballistic missiles, enemy aircraft, and even spy satellites. Assessing how well these high-precision devices perform against a variety of metal and composite targets is the object of a new $800,000 Department of Defense Test Resource Management Center (TRMC)-funded project headed by University of Missouri mechanical and aerospace engineering professors Yuwen Zhang, Frank Feng, and J. K. Chen, all Fellows of the American Society of Mechanical Engineering (ASME).
“The laser’s most attractive feature as a weapon is that energy can be delivered to the target at the speed of light,” explained Zhang. “Conventional weapons fly at a much lower speed and reach the target far more slowly.”
Tactical high-energy lasers are almost exclusively defensive weapons. Israel, for instance, has used the technology to shoot down artillery shells and Katyusha rockets. But those threats are relatively primitive — Katyusha rockets haven’t been advanced much since World War II.
Newer intercontinental ballistic and long-range missiles incorporate sophisticated countermeasures such as reflective coatings that deflect laser radiation and heat-resistant composites lasers can’t easily penetrate. Thwarting those countermeasures helps drive laser weapons research, which is all about how much energy must be delivered to destroy a target.
Teaming up with the aero propulsion research and engineering firm Taitech, Inc., Purdue University researchers and the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio, the MU team wants to “provide accurate heat measurements when the laser beam hits a surface, a very challenging task since the environment is hostile to temperature sensors, but necessary to develop suitable lasers and control strategies,” said Zhang.
To accomplish their Department of Defense-funded mission, Zhang and his group will build a new breed of ultra-small, ultra-durable heat sensors and develop an innovative computer program that analyzes heat and energy readings from laser-destroyed targets.
Despite the laser’s well-studied history, just what will happen when its concentrated light beam strikes a surface isn’t well understood, largely because a variety of possibilities exist. “A high energy laser could produce very high temperature in a target, softening, melting, or even vaporizing it,” said J. K. Chen.
The William and Nancy Thompson Professor of Mechanical and Aerospace Engineering since 2005, Chen has made a career of studying lasers, previously working for General Electric and the Air Force Research Laboratory.
Weapon system analysis, he explained, combines so-called target vulnerability with weapon lethality. To measure these factors in lasers, the researchers are studying surface irradiance — the amount of energy the laser delivers to its target — and temperature distribution, or how much heat the laser delivers.
“Since heat can destroy a target, temperature distribution is an important gauge of the lethality of a high-energy laser weapon,” Zhang explained.
In warfare, destruction is the name of the game, so getting accurate measurements from a sensor-studded target is difficult and sometimes impossible.
“Existing approaches attach a sensor on the front of the target or use an infrared camera that measures the temperature on an irradiated surface,” said Zhang. “Front sensors are easily destroyed, or smoke between the target and the camera blocks the signal, so that the infrared camera is measuring the temperature of the smoke and not the target.”
A new approach
In order to measure heat on a target, the MU team is making and calibrating a new line of high-temperature heat flux sensors, which will record the energy gained or lost as heat through a given material.
To measure laser lethality, Zhang’s group will place the sensors in cleverly located positions on targets made from different metallic and composite materials. When the laser beam hits the target, the sensors will send readings to a computer that describe how much heat the target absorbs or loses as it is altered.
With laser-irradiated surface temperatures that can top 3,700°C (6,700°F), sensors on a target have to be super durable, and retain their ability to deliver precise measurements under extreme conditions.
Made from ceramics with high melting points, the best heat flux sensors are thin — on the order of micrometers — and small, millimeters in diameter. Adding to the challenge of producing them is the fact that ceramic materials are very difficult to handle. A new liquid material — polymer derived ceramic — promises more malleability. The researchers can simply pour it into a mold.
“We have to use ceramics for the sensors, but they’re generally too fragile and difficult to make into the desired thin film,” said Feng, who has been with the MU College of Engineering since 1999. “Instead, we start with a polymer material which is easy to handle, and then bake it in a furnace so that it will turn into a hardened ceramic.”
Next, etching the thin polymer film using photolithography, a technique Feng said is “similar to old-fashioned photo development,” the team is ready to add tiny electronic components using MEMS, or micro-electromechanical system techniques.
MEMS refers to micrometer (1/1,000,000th of a meter)-sized devices, and can include tiny gears, motors, electronic components, and semiconductors. “MEMS devices are found in iPhones and Wii remotes, and gyroscopes in Segway transporters,” said Feng.
Putting it all together
No one has yet adequately integrated sensor measurements into a graphic, user-friendly computer software program, but Zhang’s group intends to do just that. Sponsored by the U.S. Army Program Executive Office for Simulation, Training, and Instrumentation (PEO STRI), the software they will develop “will be useful for any defense agency involved with high-energy laser weapons,” Zhang explained.
The team will be able to make indirect measurements with a unique computer modeling tool that will capture data about laser irradiance and the resulting temperature on the target, Feng said. The computer model will essentially extrapolate, or deduce the target’s surface temperature at the point of laser impact from sensors placed away from that point.
To measure a laser’s impact on a hypothetical missile, for instance, the researchers would place heat flux sensors toward the rear of the missile and fire the laser at the nose, or tip. The sensors would relay readings — free of smoke and the intense heat of direct impact — back to the computer program, which would deliver a three-dimensional graphic image of surface irradiance and temperature distribution on the target under the laser spot.
“This data can then be used to determine various laser parameters — power, beam size, and irradiation time — required to accomplish a mission,” Feng explained.
In a military field setting, for instance, users would input what they know about the geometric, physical and outdoor conditions of the target into the program to generate the most effective laser response.
Assuming all goes as planned, this trio of scientists will have made a profound contribution to state-of-the-art laser research that Chen sums up in two sentences.
“Current sensor technologies cannot accurately measure the irradiance and surface temperature of a target,” he said. “But now, industrial and military engineers can use the computer code we developed to recover both of these parameters.”
The authors would like to thank the Test Resource Management Center (TRMC) Test and Evaluation/Science & Technology (T&E/S&T) Program for their support. This work is funded by the T&E/S&T Program through the U.S. Army Program Executive Office for Simulation, Training and Instrumentation’s contract number W900KK-08-C-0002. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the TRMC or PEO STRI.