for generating high-altitude E1 electromagnetic pulse
Unlike what is written elsewhere on this site, what is written below is necessarily speculative since it relies on information that is claimed to be available in classified sources. The claims below, however, have been repeatedly made by sources that have otherwise proven to be reliable. Therefore, they deserve a serious, but skeptical, examination.
There have been many claims about the existence of enhanced-EMP or super-EMP nuclear weapons. The open scientific literature only describes the operation of first or second generation nuclear weapons which are capable of producing a maximum EMP field strength of about 50,000 volts per meter on the ground (slightly to the equatorial side of the detonation point). Maximum field strengths near the horizon would be limited to about half of this value, or 25,000 volts per meter. The reason that the maximum field strength is slightly to the equatorial side of the detonation point (in other words, south of the detonation in the northern hemisphere) is that this is where the high-energy Compton electrons move through the Earth's magnetic field at nearly a 90 degree angle.
Obtaining field strengths that are higher than this is difficult due to saturation effects that completely ionize the mid-stratosphere where the electromagnetic pulse is generated. Basically, the process of generating the EMP in the middle of the stratosphere very quickly causes this region to become a fairly good electrical conductor, and therefore incapable of generating any additional EMP.
The E1 EMP from a nuclear weapon is generated from gamma rays emitted by the weapons within the first microsecond after the nuclear detonation. One way of enhancing the EMP is simply to make sure that the weapon is constructed so that as much of the gamma radiation as possible escapes from the weapon and is radiated into the upper atmosphere in a wide area below the detonation. This can be done by making the weapon casing as transparent as possible to gamma radiation and by using chemical high-explosives that are also as transparent as possible to gamma rays after the high-explosive detonation. This means sacrificing some of the explosive energy of the weapon since the weapon would probably blow itself apart before reaching the maximum explosive power. The (relatively) gamma-ray-transparent casing only needs to be on the lower side of the weapon. The gamma radiation that is emitted into outer space is wasted.
The high-explosives on the earliest known nuclear weapons was quite thick, as was the very heavy outer casing. Even in the case of the 1952 super-oralloy fission weapon, which was quite sophisticated for its time, the chemical high-explosive surrounding the spherical shell of U-235 was 44 centimeters thick.
When most people talk about super-EMP weapons, though, they are generally talking about nuclear weapons that can generate field strengths of much more than 50,000 volts per meter. This would require a much different design than "ordinary" nuclear weapons. Staff members of the United States EMP Commission have stated that there are nuclear weapons in existence that can generate 200,000 volts per meter below the detonation and 100,000 volts per meter near the horizon. This would have to be done by generating gamma radiation with energy levels that are far in excess of the energy levels generated by nuclear weapons described in open publications -- and by also generating a pulse of these gamma rays very rapidly.
In first and second generation nuclear weapons, the prompt gamma radiation reaches its peak a few tens of nanoseconds after the nuclear detonation begins. In super-EMP weapons, it is likely that the gamma radiation reaches its peak output within a nanosecond or two of the beginning of the nuclear reaction. One consequence of this is that the frequency components in these super-EMP weapons would be much higher, making the problem of shielding and transient protection much more difficult than simply protecting against higher field strengths.
The references to open literature describing super-EMP weapons outside of the United States can be found in a publication called "The Emerging EMP Threat to the United States" by Dr. Mark Schneider of National Institute for Public Policy (November 2007). It can be downloaded at:
http://www.nipp.org/Publication/Downloads/Publication Archive PDF/EMP Paper Final November07.pdf
The problem with the important references in Dr. Schneider's paper is that they are available at the Open Source Center, at http://www.opensource.gov, which is only available to employees or contractors of the U.S. federal government who have a specific need to access these documents. The referenced documents are not secret or otherwise classified documents in the normal sense, but they are nevertheless very difficult for the average citizen to obtain.
Also available are a number of documents about the super-EMP weapons designed and built by the United States. The weapons produced by the United States are known as High-Power Radio Frequency (HPRF) nuclear weapons. (Adding to the confusion, though, the phrase "high-power radio frequency" weapons has also been used to describe certain kinds of non-nuclear weapons.) A number of documents about this nuclear HPRF program have actually been released; but, for obvious reasons, the bulk of the content has been deleted before the documents were released to the public. There is just enough content there to likely confirm the existence of super-EMP weapons produced by the United States, or at least a former program to produce such weapons. For example, see:
The most worrisome aspect of these super-EMP weapons is the possibility that the construction process may be rather straightforward. Beyond the rather complex industrial capacity that it takes to produce a basic nuclear weapon, it may be that the only additional thing required to produce a super-EMP weapon is the knowledge of how to do it. If this is the case, then any country with a nuclear weapons program may be able to produce a super-EMP weapon without too much additional difficulty.
Although no details about super-EMP weapons are given in open publications by anyone who knows about these weapons, there is a strong implication in the statements of those who have studied the reports about those weapons that the enhanced gamma radiation weapons would have a comparatively low total energy yield. This means that weapons with a large E1 output would not cause a large E3 output. If this is the case, then they would be very destructive to electronic devices, but would not produce the large DC-like currents that would be likely to destroy large numbers of the largest transformers in the electrical power grid.
It appears that there is a possibility of a separate class of super-EMP weapons that can be made using multi-stage thermonuclear techniques. In 1987, a former nuclear weapons designer wrote an article in Scientific American in which he stated that some types of thermonuclear weapons can be designed where up to 20 percent of the weapon yield would be in the form of gamma radiation. (Theodore B. Taylor, "Third-Generation Nuclear Weapons", Scientific American. April 1987. Vol. 256, No. 4. pages 30-39.) This Scientific American article implies that it is possible to make weapons that are capable of causing both a very large E1 and and a very large E3 component of the EMP.