Safety at 300 kW: Considerations for Laser Weapons Designers

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Use of any laser technology requires special safety considerations. But the continuous-wave laser systems designed to output beam powers between 60 and 500 kW — primarily for defense applications — belong in a category of their own. These systems, which could be termed Class 5 or 4L (“L” for lethal), require additional physical considerations and controls for the space in which they are developed and tested. The typical laser lab is no longer a feasible workspace, nor does it provide sufficient containment.

Room walls made only of standard drywall material, for example, will not contain a direct beam or possible stray reflection from these lasers. Layering drywall with graphite is a superior approach. Graphite may crack if exposed to high-intensity laser light, but the light will not burn through. Cinderblocks with hard concrete fill is another option.

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Since the aim with directed-energy weapon systems is to design a beam that is able to burn through rapidly traveling rocket or mortar casings from a great distance, the question of room infrastructure cannot be overlooked. Cable trays, pipes, lab equipment, and coolant delivery systems that share the room with these high-power lasers are all at risk and must be positioned or shielded in some manner. Not everything can be hidden under an optical table.

Unique coolants are often part of the safety measures for these development labs. In such cases, the risk assessment needs to consider how the coolant is stored and shielded, as well as its path of delivery, if deployed.

Clear the room

It is questionable whether anyone should be allowed in the room during operation of the laser, since the diffuse reflection hazard can be as large as the room. As with all laser setups, the best solution for removing the potential hazard is to enclose the beam. Currently, the only ANSI standard that comes close to covering the topic is the organization’s newly published ANSI Z136.8 Laser Safety in Research, Development, or Testing, Appendix F.

Before these laser systems run at full power, however, the best practice would be to evacuate the space and operate the system remotely from a control room, with help from cameras, motorized mounts, and similar equipment.

Researchers often fight such arrangements. The common refrain is, “There are things I can determine by being in the room that I could not determine from a control room.” Such things may include the scent of smoke or unexpected sounds. But these arguments do not stand up against alternatives, such as using video cameras, microphones, smoke detectors, and stray-light detection systems. The other alternative is to place barriers in the laser lab, behind which staff members can stand during operation. Importantly, today’s protective eyewear is inadequate to protect the user against serious ocular exposure from the laser beams in question. The use of such eyewear can actually provide a false sense of protection.

Many researchers also argue that they will only remain in the room during low- power operation. But “low power” is a relative term, and it is effectively meaningless when the laser in question is designed to emit powers at or above 300 kW. Some researchers have argued that 100 W qualifies as low power, while the typical ANSI Z136 standard would define low as somewhere between 5 and 25 mW of output. Of these, which beam would you put your hand in?

Prepare the room

Linking stray-light detectors to room interlocks or alarms is a worthy consideration. This is usually achieved by equipping panels with either integrated circuits or photodiode detectors that, when activated, trigger an alarm or door interlock system. The door interlock system must be composed of reliable safety components, though this is not a standard ANSI requirement. Due to the power outputs in question, the speed at which the interlock system blocks or redirects the laser beam is a critical performance parameter.

Because of the extreme irradiance that high-intensity lasers emit, the chance of damaging optical components is another possibility. Such damage is not limited to catastrophic failure (i.e., cracking), but can also include the generation of defects in the optics. These defects are a concern because they can affect the beam path. Various devices and sensors can help to monitor the integrity of optical components, as can a routine inspection frequency.

Another special safety consideration for high-power laser labs is the need to monitor the temperature of liquid-cooled beam blocks and dumps, since they are the primary methods for dissipating waste heat. Particular attention should be given to the setup of and the information provided on graphic user interfaces to ensure that power on/off set points are easily found and set by laser operators. Additional training on the information that is signified by specific colors or patterns is also recommended.

In fact, training cannot be left out of any discussion about laser safety. The presence of workers in laser weapon labs demands a level of safety training beyond the fundamentals. Various approaches and categories of training need to be developed based on the activities that each staff member is approved to perform.

Additional considerations should be given to equipment grounding and relevant protocols defined by government bodies, such as OSHA’s Nationally Recognized Testing Laboratory.

Work in these labs requires a level of detail that is rarely applied in the typical laser lab — and such attention to detail is required for good reasons. Daily pre-work meetings (similar to construction tailgate meetings) are important to make sure everything and everyone one is ready for a safe day ahead.

Meet the author

Ken Barat, operator of Laser Safety Solutions, is a certified laser safety officer (CLSO); email: [email protected].

Published: October 2021
columnsLasers In UseLasersLaser Safety

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