Conventional Fusion Bomb
Nuclear fusion bombs are the most powerful explosives available to modern science. The amount of energy released from fusing hydrogen nuclei into helium is truly awe-inspiring: E=mc2 at its most glorious moment, the same process that powers our own sun.
However, the amount of energy required to fuse hydrogen nuclei is also enormous. The best way to get that kind of energy is with a smaller nuclear reaction, so fusion bombs are triggered with fission bombs.
The fission bomb itself is triggered with a conventional explosive. Fission occurs when a supercritical mass of fissile material is collected in one place, so one method of triggering a fission reaction is accomplished by detonating a chemical explosive to slam two subcritical masses of fissile material together fast and hard to form a supercritical mass. Too lightly and the beginnings of the reaction will create enough heat and radiation to separate the pieces again and the reaction will abort prematurely.
This three-step chemical → fission → fusion process is complex and expensive, and the equipment necessary to produce the reaction is therefore very large. If the fission reaction could be eliminated from the process, the whole system would be greatly simplified and reduced in size.
If you believe Samuel Cohen's fear-mongering, this just may have happened.
Ballotechnic Fusion Bomb
There just might be another way to generate the incredible energy necessary to initiate a fusion reaction in a bomb. A class of chemical pseudo-explosives called ballotechnics have extremely high energy densities, far larger than any traditional chemical explosive. I say pseudo-explosive because although they generate incredible amounts of heat very quickly, they do not actually release rapidly expanding gasses, which is the technical definition of an explosion.
They may pack enough punch to directly initiate a fusion reaction without an intermediate fission reaction, resulting in "pure" fusion. Specifically, it may be able to trigger deuterium-tritium fusion — the easiest form of fusion to initiate. The process is very much unlike the fission approach. Rather than create a massive shockwave around the fuel, the ballotechnic material simply generates enormous heat and pressure when it is set off. The result would be a very small, very simple, relatively inexpensive fusion device.
The most famous ballotechnic substance right now seems to be red mercury, a mercury antimony oxide (Hg2Sb2O7) semi-liquid believed by some to have been created in Russia under difficult to reproduce circumstances. The components are kept under neutron bombardment under high pressures in a nuclear reactor for a long time, and when the process is over the result is a ballotechnic material. If red mercury exists, it may have been sold to rogue nations such as Iraq or North Korea for exorbitant prices.
The problem is, all sources of information on red mercury and its nuclear applications can be traced back to a single source — nuclear physicist Samuel Cohen — and, to put it gently, he has been wrong before. Complicating matters are counterfeit supplies of red mercury sold to gullible buyers, actually inert and benign materials designed to match the description of the "real" substance. Additionally, red mercury may have been used from time to time as a cover name to smuggle controlled and illegal substances, possibly even fissile material.
Should this prove to be more than a hoax or media overreaction, the consequences would be disastrous. Samuel Cohen estimates a fusion device made with red mercury could be made as small as a baseball. The "briefcase nuke" would be a lumbering dinosaur in comparison. The most difficult part, after acquiring a supply of red mercury, would be getting the deuterium and tritium fuel — a simple, if time consuming, process in comparison.
The obvious application is of course terrorism. Most devices designed to detect nuclear weapons rely on detecting the radiation emitted by uranium and plutonium; the materials necessary for a fission bomb. Since current fusion weapons contain fission weapons as a trigger, they too can be detected. However a red mercury triggered fusion device has no radioactive materials and emits no detectable radiation, and its small size would be easily hidden or disguised as a benign object. This angle is the likely reason the ballotechnic triggered fusion bomb is getting any attention at all given the lack of reliable information on red mercury.
Tactical nuclear devices had some attention a few decades ago, but they were eventually determined to be more trouble than they were worth. For one thing, a nuclear device has such an enormous blast radius that it would be difficult for a man-portable unit to fire the weapon far enough to keep the shooter out of harm's way. For another, the use of tactical nuclear devices could spark an escalation of hostilities to the point where strategic nuclear devices are being used. And nobody in their right mind wants that to happen. A red mercury triggered tactical nuclear device, however, is small enough and simple enough to possibly be deployed by the militaries of a rogue nation, whose leaders are notoriously not in their right minds.
A third application is the re-emergence of the neutron bomb idea. A neutron bomb is, basically, a low-yield fusion device designed to scatter neutrons as its primary destructive energy (the enormous mushroom cloud being an inconsequential side-effect). Neutron bombs are designed to kill living things with neutron radiation without causing much damage to buildings, vehicles, or other structures or leaving behind radioactive material with long decay times. The target of a neutron bombing would be inhabitable again within a week. The neutron bomb was, like the tactical nuclear weapon, determined to be more trouble than it was worth. However, a relatively simple device triggered by red mercury would warrant a serious re-evaluation.
It is worth mentioning that Samuel Cohen invented the neutron bomb.
Malarky? or Effective Way?
Of course all of this is dependent on the controversial idea that red mercury, or a similar powerful ballotechnic material, even exists, and that it can generate enough force to actually trigger a fusion bomb. This has not been tested in an actual nuclear device, and neither the Los Alamos National Laboratory or (officially anyway) the CIA are convinced of the existence of red mercury. The whole scare appears to have been fabricated by Samuel Cohen and exploited by smugglers and con artists.
The idea is scary enough and scientific sounding enough to generate some media attention though, and cries of "Can we afford to simply ignore the possibility?" and "Won't somebody please think of the children!?" carry a lot of weight in a post-September 11th world. Personally, I'm waiting on more information before I start building my bomb shelter.