Electromagnetic Pulse Testing

The effects of electromagnetic pulses (EMP) on electronics can be severe, but pose an even more devastating threat to the processes and infrastructures that they support. Designing equipment and systems to withstand the effects of EMPs now will reduce the impacts of potential EMP attacks on our electronics in the future.

Direct Energy Weapons (DEW) and Security

Highly effective non-nuclear EMP technologies are progressively being developed worldwide. These technologies are classified as “Direct Energy Weapons” and are currently being used by U.S. armed forces, state and local police departments. Direct energy weapons travel to the target at the speed of light much like that of a conventional EMP, and are capable of graduated effects on electronics ranging from disrupting operation, to permanent damage or complete destruction.

A prime example of this technology is the arc discharge EMP generator. These devices use high voltage and massive energy storage of capacitors which is released across a thin under rated conductor to a low impedance load or short circuit. The wire acts like a fuse opening at the peak of the high current discharge of the capacitor resulting in a massive release of broadband electromag

netic pulse of energy similar to a conventional HNEMP. These generators typically integrate a small parabolic reflector to direct and focus the pulsed energy towards a target.

Another example of Non-nuclear EMP technology is the Flux Compression Generator (FCG). The FCG was first demonstrated by Clarence Fowler at Los Alamos National Laboratories (LANL) in the late fifties. This technology injects a high energy pulse into a large conductive coil. At the point of peak pulse current, a small explosive charge is deployed which quickly compresses the coil to one end of the generator creating massive amounts of electromagnetic energy. The first designs were several feet in length, but through technological advances, are now reported to be roughly the size of a soda can.

With the creation of non-nuclear direct energy weapons, and the existing use of the devices on the battle field, as well as civilian non-combat environments, the need to protect electronic equipment is at an all-time high. The U.S. Military has been evaluating the effects of electromagnetic pulses on equipment for the past 50 years, and have developed protective design guidelines and hardening techniques currently used today.

MIL-STD-461, RS105

MIL-STD-461G provides test methodology and screening levels for determining a device’s immunity to EMP from a radiated and conducted standpoint. The coupling modes onto the equipment enclosure and its interconnecting cabling can be complex, therefore are evaluated separately.

The RS105 test method specified in MIL-STD-461G addresses the risk of radiated exposure to an EMP event. RS105 testing is generally applicable for equipment installed in exposed and partially exposed environments. The U.S. Navy requires RS105 testing for nearly every installation platform, surface ships, submarines, and aircraft, to ground applications.

The RS105 pulse characteristics consist of a fast rise time, short pulse duration, and high amplitude which resemble those of an actual EMP. Peak field strengths of 50 kV/m are specified for exposed equipment. However, tailoring the peak field levels are often required for partially exposed installations due to the attenuated effects provided by enclosures such as the deckhouse structure, or hangar doors. For example, equipment installed near deckhouse apertures are required to meet the external stress reduced by the shielding effectiveness of that specific aperture or by the 40 dB of electromagnetic shielding provided by the deckhouse structure, whichever is less.

RS105 testing performed with a transmission line connected to a transient pulse generator. The generator and the far end of the transmission circuit are commonly bonded to reference ground. This connection provides a return path allowing current flow allowing for the generation of electromagnetic fields. The equipment under test is then installed underneath the transmission line within the predetermined uniform field area.

The field developed between the transmission line and the ground plane consists of large differential voltage and current fields. To ensure a proper uniform field distribution area, RS105 requires that transmission line length, and width are at least twice that of the equipment being tested and at least three times the height.

Prior to testing the uniform field is verified along a 5 point vertical grid. The results taken at each point are verified to be within 6dB (in terms of voltage) of each other, and greater than the specified test limit (no less than 50,000 v/m).

The purpose of RS105 testing is not to damage the equipment, but to determine its immunity threshold to the electromagnetic pulse. This is performed by starting at 10% of the peak field level and gradually increasing field until susceptibility is determined or the specified peak field level is reached. It is important to note that RS105 evaluates the equipment enclosure’s ability to attenuate and withstand the effects of an EMP, not its cabling. The RS105 test setup requires that all metallic interconnecting cabling including power input lines are routed in shielded conduit and/or underneath the ground plane to minimize coupling.

MIL-STD-461, CS116

The MIL-STD-461 CS116 test method evaluates the coupling effects of EMP on metallic interconnecting lines. The intent of this test is to ensure the equipment’s ability to withstand conducted damped sinusoidal transients, excited by platform switching operations, indirect effects of lightning, and EMP. The minimum set of test frequencies includes 10 kHz, 100 kHz, 1 MHz, 10 MHz, 30 MHz, and 100 MHz. In accordance with MIL-STD-461F, CS116 testing is applicable for all installation platforms and procurement agencies with limited applicability for submarines. Similar to RS105, CS116 testing is not to damage the equipment, but to determine its immunity threshold to the electromagnetic pulse. This is performed by starting at 10% of the peak field level and gradually increasing field until susceptibility is determined or the specified peak field level is reached. One important aspect to note about the testing method is that the transient signals are inductively coupled to each line. The amount of voltage and current induced onto each line is dependent on its impedance. Higher impedance lines will allow for greater voltages to be achieved at lower currents, where low impedance lines such as shielded cabling, will achieve greater currents at lower voltages. To avoid excessive over testing, pre calibration of the injected currents into a 100 ohm loop impedance is performed, and the currents induced onto each line are monitored. As mentioned, test levels are gradually increased until equipment susceptibility is detected, the current limit is achieved, or the generator setting determined during the 100 ohm calibration are reached.

In summary, the effects of electromagnetic pulses on electronics can be severe, but poses an even more devastating threat to the processes and infrastructures that they support. Designing equipment and systems to withstand the effects of EMPs now will reduce the impacts of potential EMP attacks on our electronics in the future.

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