Question about Electromagnetic Pulses (EMP)

[UncleBeer]

And when it comes to car electronics the body of the car itself would be a faraday cage.

Nope. You’ve got some pretty big holes on that cage - such as the glass windows. Not to mention that the underside of the car is entirely open to the engine compartment. An electronic device can be shielded effective by an enclosure with holes in it, but they have to be sized and spaced properly - and the material in which you are placing the holes has to be of the correct thickness and have certain conductive properties - to block/absorb the EM waves.

I used to work for a military contractor designing and manufacturing AC Gas Plasma displays - long before anyone began using them for televisions. Nearly all of display units were “hardened” against EMI and EMP. Pretty tricky engineering to get what was essentially a screen in front of the display tube that would permit light of sufficient strength to pass out of it and yet block any destructive radiation from entering - or exiting. Not to mention shedding the excess heat and accommodating various connectors and operational switches on several sides of the unit. We also had to shield our displays so that they didn’t emit any radiation which might interfere with other nearby electronic devices. In most cases we used a 220 or 260 mesh copper screen that had been coated with blackened silver. The screen was sandwiched between two thin pieces of glass which would incorporate other required optical characteristics - such as anti-reflection coatings. For certain models we’d also incorporate a touch screen into all of this - most often using GAW (Guided Acoustic Wave) technology.

[Bookkeeper]

And with regards to the OP, I should point out that a static electricity spark from your fingertip is enough to fry vital parts inside your computer but wouldn’t even be noticed by your fridge.

[Chronos]

A flashlight would have two advantages over a computer in surviving an EMP (assuming no particular shielding or hardening for either). First, the wires in a flashlight can handle much higher currents. Second, if the flashlight were turned off at the time, there would not be a complete metallic circuit, so any current would have to jump the gap at the switch.

And if you want to shield against both EMP and hard radiation, you could always use gold. That is, if weight is no object, and for some reason you don’t just want to use a few meters of concrete/rock/dirt.

[Desmostylus]

@Chronos

And if you want to shield against both EMP and hard radiation, you could always use gold. That is, if weight is no object, and for some reason you don’t just want to use a few meters of concrete/rock/dirt.

The effectiveness of a material as an EMP shield depends on a property called skin depth.

skin depth = 1 / sqrt( pi . f . mu[sub]0[/sub] . mu[sub]r[/sub] . sigma ) [m], where

pi = 3.14159… [dimensionless, constant]
f = frequency of the radiation [Hz]
mu[sub]0[/sub] = permeability of free space = 4 . pi . 10[sup]-7[/sup] [Hm[sup]-1[/sup], constant]
mu[sub]r[/sub] = relative permability of the material [dimensionless]
sigma = conductivity of the material [Sm[sup]-1[/sup]]

The smaller the skin depth, the better the material is as a shield. Field strength in a material declines exponentially with depth into the material. The skin depth is that depth at which the field strength is 1/e times the strength at the surface, and where e = 2.718… (dimensionless, constant).

Now, the variables for a particular material are mu[sub]r[/sub] and sigma. The bigger mu[sub]r[/sub] and sigma are, the better the material will be as an EMP shield.

The values for some of the elements we’ve been talking about in this thread are:

Gold: mu[sub]r[/sub] = 1, sigma = 45x10[sup]6[/sup] Sm[sup]-1[/sup]
Lead: mu[sub]r[/sub] = 1, sigma = 4.8x10[sup]6[/sup] Sm[sup]-1[/sup]

Gold has a conductivity about 9.4 times higher than lead, so its skin depth is about sqrt(9.4) = 3.2 times smaller than lead. We can conclude that gold is 3.2 times more effective than lead as an EMP shield. But, gold costs about 15,000 times as much as lead, so it’s an expensive trade-off.

What about some other, cheaper metals which are better conductors than gold?

Silver: mu[sub]r[/sub] = 1, sigma = 60x10[sup]6[/sup] Sm[sup]-1[/sup]

Silver is sqrt(60/4.8) = sqrt(12.5) = 3.5 times more effective than lead as an EMP shield, but it’s still pretty expensive at about 250 times the price of lead.

Copper: mu[sub]r[/sub] = 1, sigma = 58x10[sup]6[/sup] Sm[sup]-1[/sup]

Copper is sqrt(58/4.8) = sqrt(12.1) = 3.5 times more effective than lead, same as silver. And copper is only 3 times the price of lead.

How about playing with the mu[sub]r[/sub] factor, rather than just the sigma factor? Ferromagnetic materials have relative permeabilities greater than 1.

Iron: mu[sub]r[/sub] = 250, sigma = 9.6x10[sup]6[/sup] Sm[sup]-1[/sup]

Iron is sqrt(250x9.6/4.8) = sqrt(500) = 22 times more effective than lead, but is one sixth the price. Getting better, no?

Superpermalloy (79% Ni, 16% Fe, 5% Mo): mu[sub]r[/sub] = 100,000, sigma = 13x10[sup]6[/sup] Sm[sup]-1[/sup]

Superpermalloy is sqrt(100,000x13/4.8) = sqrt(270,000) = 520 times more effective than lead, but about 20 times the price.

The biggest “bang for the buck” comes from iron. The best shielding in terms of weight comes from Superpermalloy.

I’ll try to address gamma shielding in a subsequent post.

[Chronos]

I hadn’t realized that ferromagnetism was so advantageous in electromagnetic shielding. I had thought that conductivity was the only (or at least primary) factor, in which case gold is a strong contender. I realize that silver, copper, and a few exotic materials. are better conductors, but gold is also very dense, making it good for shielding hard radiation as well. And I also realize that gold is much more expensive, of course, but that part was intended to be somewhat tongue-in-cheek.

Even as a combination shield, though, one could probably make a shield out of layers of Super-M and lead, or other materials, which would be thinner and cheaper than gold, but just as effective.

But of course, if you’re looking for bang-per-buck, dirt has a terrible skin depth (I assume), but it’s free :). Bury anything deep enough, and you’re not going to have to worry about EMP at all.

[Stranger_On_A_Train]

@SpaceDog

Related, but possibly stupid, question: Would a bulb in an EMP light up briefly ? Would you be able to track the path of an EMP through the lights in a city or a warehouse of light bulbs ? What other noticable physical effects might there be ?

SD

A pulse could cause a fluorescent tube to light up if the pulse strength were high enough to create a potential across the posts. Also, the wavelength of some of the incoming radiation will undoubtedly cause the phosphorous coating to flouresce.

I don’t think you’d get a incandescent bulb to light up unless the circuit were already closed. You might have enough energy to get it to briefly flare a small bulb (flashlight or peanut) by inductance through the loop but the wattage requirements are much higher. If you are that close, you’re not going to be worried about flashlights, believe me.

Stranger

[gotpasswords]

@Chronos

And if you want to shield against both EMP and hard radiation, you could always use gold. That is, if weight is no object, and for some reason you don’t just want to use a few meters of concrete/rock/dirt.

Fine. Encase your device in gold. Layer it in lead, and for good measure, bury it in concrete. What good is in inside of all that? It’s going to need power and inputs/outputs. How do you protect it from anything coming in via the wiring?

@UncleBeer

Nope. You’ve got some pretty big holes on that cage - such as the glass windows. Not to mention that the underside of the car is entirely open to the engine compartment.

Yep. The underside is so open that law enforcement has been dabbling in the idea of using portable EMP generators to disable cars as an alternate to using spike strips, which are powerless against run-flat tires.

@UncleBeer

I used to work for a military contractor designing and manufacturing AC Gas Plasma displays

(fairly OT aside) I used to know a guy at a company called Lucitron - way, way back in the days when they were just trying to get a square foot of flat glass not to implode and to get monochrome CGA-type resolution without 75 or so dead pixels. Obviously, the technology has been fantastically improved in those 20 years.

[Desmostylus]

@Chronos

I hadn’t realized that ferromagnetism was so advantageous in electromagnetic shielding. I had thought that conductivity was the only (or at least primary) factor, in which case gold is a strong contender.

That’s quite possibly the fault of some of the physics texts. In discussing Faraday cages, it’s usual to talk about “perfectly conducting” cages. Real materials just don’t get close enough to perfect at EMP frequencies.

@Chronos

But of course, if you’re looking for bang-per-buck, dirt has a terrible skin depth (I assume), but it’s free :). Bury anything deep enough, and you’re not going to have to worry about EMP at all.

The conductivity of soil is about 1x10[sup]-2[/sup] Sm[sup]-1[/sup], so it’s something like sqrt(1x10[sup]-2[/sup]/4.8x10[sup]6[/sup]) = sqrt(2.1x10[sup]-9[/sup]) = 4.6x10[sup]-5[/sup] times as effective as lead as an EMP shield.

Let’s lay it out. Let’s take the frequency of the EMP as 300 Hz (the geometric mean of 3 HZ and 30 kHz). The skin depth of lead at that frequency is:

1 / sqrt ( 3.1416 x 300 x 4 x 3.1416 x 10[sup]-7[/sup] x 1 x 4.8x10[sup]6[/sup])
= 1.3x10[sup]-2[/sup] m = 13 mm.

We want to attenuate to much less than 1/e (=37%), however. Let’s attenuate to 1%. That requires about 4.6 skin depths.

If we use lead, we’d require about 13 x 4.6 = 60 mm.

If we used iron instead, we’d require 60 / 22 = 2.7 mm.

If we used Superpermalloy, we’d require 60 / 520 = 0.1 mm.

If we used soil, we’d require 60 / 4.6x10[sup]-5[/sup] = 1,300,000 mm = 1,300 m = 1.3 km.