by Barry Parker
His job was to look into the feasibility of using either a D-D reaction or a D-T reaction to trigger the fusion reaction needed for the bomb, and to come up with an appropriate design. Various designs had been tried, but nothing seemed to work. At this point it was known both that a tremendous amount of heat (twenty to thirty million degrees) was needed to trigger a fusion reaction and that an atomic bomb could be used to create such heat. But everything Ulam tried appeared to have problems. In December 1950, however, he stumbled on an idea that he was sure would work. Basically what was needed was a way to increase the compression of the hydrogen in the bomb by several magnitudes. An atomic explosion could be used to create an implosion that would compress the hydrogen, but a simple implosion didn't appear to be enough. Ulam decided that several explosions were needed. In essence, one bomb would be used to set off a second bomb, and the second bomb would set off a third. This was referred to as staging. He was sure the idea would work, but he kept it to himself for many months while he developed and perfected it.2
He finally decided to tell Teller about it, even though he didn't have a good relationship with Teller and was worried about Teller's reaction. Teller was not immediately convinced that it would work, but as he continued to study the idea he realized that it was an important step forward. Ulam suggested that the hydrodynamic shock, or possibly the neutrons from the fission explosion, could be used to create an implosion that would compress the hydrogen sufficiently. After studying the possibility for a time, Teller realized that the x-ray radiation would reach the hydrogen before the shockwave or the neutrons, and that it could be used to create the implosion that would be needed to trigger the thermonuclear explosion. And indeed it appeared to be the best solution. Teller and Ulam submitted a joint paper on what soon became known as the Ulam-Teller Design. For several years, however, Teller tried to play down Ulam's contribution, and there was considerable friction between the two men.3
THE FIRST TEST: MIKE
The next step was to build a bomb based on the Ulam-Teller Design to see if it would work. And indeed, work began on the project relatively soon. In reality this first bomb was not a bomb, as we know it; it was far too large to carry in an airplane. The basic parts of the device would be manufactured in United States and taken to a remote location in the Pacific Ocean, about three thousand miles west of Hawaii. The test, codenamed Ivy Mike, was conducted at Enewetak Atoll, a ring of forty small islands about forty miles long and ten miles across.4
A committee called the Panda Committee had been set up to look into the development and testing of the bomb. Its members were given a year to design and deliver the bomb, even though there were still many problems to overcome. One of the major problems was deciding which fusion reaction to use: D-D or D-T. It was finally decided that the D-D reaction would be both easier and more economical. But there was a problem in relation to how the deuterium would be stored. Deuterium has a boiling point of 417 degrees below zero Fahrenheit, so it had to be kept in a liquid state at extremely low temperatures. This required that it be stored in a cryogenic system—a large Dewar (or vacuum flask) that would keep it at a very low temperature. In addition, the device would require a fission bomb to trigger the hydrogen fusion, and at this time fission bombs were still relatively large. The radiation from this explosion would then be channeled into a secondary that contained liquid deuterium. The overall bomb was in the form of a cylinder, with a stick of plutonium at its center that would act as a “spark plug” for initiating the fusion reaction.
Formal assembly of the system, called “Mike,” began in September 1952. The bomb itself was placed at one point along the atoll, and several monitoring stations were set up at other points for measuring the energy output of the blast. In addition, a large number of ships were stationed around the atoll, and a number of aircraft were in the air, loaded with measuring equipment. In all, there were over four hundred scientific stations with measuring instruments of various types around the blast site.
By September 25, everything was ready; zero hour was to be 7:15 a.m., November 1. The “firing room” was actually about ten miles away, aboard a ship called the Estes. The power of the blast amazed almost everyone; again, as in the case of Trinity, no one was certain how powerful it would be. As it turned out, it was considerably more powerful than anticipated. Almost immediately a blinding, white-hot fireball formed on the horizon. It was three miles across, compared with the fireball of the Hiroshima blast, which was only a tenth of a mile across. Within two and a half minutes the cloud caused by the shock wave had reached an altitude of one hundred thousand feet, and it continued to billow out, eventually forming a huge canopy thirty miles across. The blast literally vaporized the entire island on which Mike had been staged, leaving a crater two hundred feet deep and more than a mile across. The energy of the blast was determined to be equivalent to 10.4 megatons of TNT. This was by far the largest man-made explosion ever to occur on earth.
PHYSICS OF THE HYDROGEN BOMB
Let's look now at how and why the hydrogen bomb works. In many ways it is much more complex than the atomic bomb. But without an atomic bomb it wouldn't work, so the atomic bomb had to come first. As we saw in the above section, it is, in effect, a staged radiation implosion that provides the required temperature (about 50,000,000°) for fusion reactions to occur.
For fusion reactions we need deuterium or tritium, and, as we saw earlier, they are relatively rare and must be separated from natural water. Reactions using both deuterium (D) and tritium (T) can be used, but tritium is much more expensive to produce, so scientists tried to avoid using it directly. But even though deuterium is much more plentiful, it is difficult to store and must be in a liquid state at very low temperatures, as was done in the case of Mike. This was eventually overcome by combining deuterium with lithium to produce lithium deuteride, which is a stable solid that is much easier to handle than deuterium. All modern hydrogen bombs now use lithium deuteride.
Basically, what is needed is an implosion of tremendous power that is able to compress the fusion fuel to densities high enough so that the fusion reaction can occur. The required density is at least a thousand times the fuel's normal density.
The simplest hydrogen bomb is a two-stage device, and this is the only type we will discuss here. It is possible to go to three stages, but most bombs use the two-stage configuration. The Soviets built a three-stage device, but little is known about it. As we saw earlier, the Ulam-Teller Design utilizes x-rays for compression because they move particularly fast (the speed of light) after the primary (the atomic bomb) is ignited. Shockwaves and neutrons are also emitted in the explosion, but they are too slow to use.
One of the critical aspects of the hydrogen bomb is precise sequencing of the stages. If anything is out of sequence, the bomb will not work. So timing is crucial. The overall bomb is in the shape of a cylinder (later bombs had a more elliptical shape). The primary (trigger) is at one end, and the secondary (the fusion device) is at the other end. The secondary generally takes up more space than the primary. The secondary is also in the form of a cylinder; it is smaller, however, than the outer cylinder, so there is a space between the two cylinders. This space is called the radiation channel. The fusion fuel, which is lithium deuteride, takes up most of the space in the secondary. The outer edge of the secondary is made up of U-238, and it is referred to as the pusher/tamper. When triggered, it pushes inward on the secondary fuel. At the center of the secondary, running down the axis, is a rod about one inch across that is made either from plutonium-239 or U-235. It is referred to as the spark plug.5
The area between the secondary and the outer cylinder is filled with plastic foam. And there is a large curved shield in the front of the secondary to prevent the fusion material from being triggered prematurely. When the primary (an atomic bomb) explodes, x-rays from it fill the radiation channel. This area is filled with plastic foam that becomes ionized after the initial explosion; it helps to control the explosion. It is import
ant that the tamper/pusher on the outer section of the secondary is not heated unevenly or too fast. An “equilibrium” condition is needed so that the energy throughout the region is uniform.
Internal structure of the hydrogen bomb.
As the explosion proceeds, the outer layer of uranium on the secondary finally fissions and an implosion occurs. The implosion compresses the fusion material, producing neutrons in the process. These neutrons trigger the uranium (or plutonium) rod at the center of the secondary, and it explodes. As a result, the fusion fuel is compressed from the top and the bottom. It therefore quickly reaches a temperature high enough for fusion reactions to occur. At this point the fuel has a density more than one thousand times its original density. Some tritium is generated in the fusion reaction, so in practice both D-D and D-T reactions occur.
It's easy to see from this that the hydrogen bomb derives its energy, or explosive power, from both fission and fusion. So the overall blast can be considered to be both a fission and a fusion blast, which may seem to be unimportant, but there is a significant difference in the two blasts. The radioactivity that spreads out after a bomb is exploded comes from the fission blast, whereas the fusion blast is “clean” in this respect. So when someone talks about producing a clean, or radiation free, bomb, they are referring to one that only has a small fission blast. It is, in fact, possible to build a relatively clean nuclear bomb.
The largest American hydrogen bomb had an explosive power of about fifty megatons of TNT. The Soviets exploded one that was even more powerful than this. Bombs can, in fact, be made more powerful by adding additional stages to them. And, as discussed previously, three-stage bombs are believed to have been built by the Soviets. What is particularly important in relation to the power of a hydrogen bomb is that, in theory, there is no limit to how powerful it can be built. There is a limit in the case of an atomic bomb.
LONG-RANGE MISSILES
Soon after hydrogen bombs were developed, it was realized that a better delivery system was needed. At first, long-range bombers were used, and at that time the United States had overwhelming superiority in long-range bombers. But as rockets became more sophisticated and their range was expanded, it became obvious that they would be much more appropriate as a delivery system.
As we saw earlier, the Germans developed the first ballistic missiles near the end of World War II. The most successful was the V-2, which was developed by Wernher von Braun and his group. Although it never got as much publicity after the war, von Braun was also working on a missile that could hit the United States. It was called “Project America.” With a far greater range, Hitler had hoped to use it on centers in America. Fortunately, it was never developed and used.
When the war was over von Braun and many of the other German rocket scientists came to United States, but some went to the Soviet Union. And very quickly the Cold War developed, with both nations stockpiling large numbers of nuclear weapons along with large numbers of long-range missiles to deliver them. Several projects were, in fact, initiated. At first they were merely extensions of the German V-2 program, but improvements came quickly, with the Soviets soon gaining an extensive lead. In August 1957, the Soviets launched the first intercontinental ballistic missile, which they called the R-7. And within a short time they also launched the first orbiting satellite, Sputnik—much to the shock of the Americans. This was followed by the launching of the first human into space, cosmonaut Yuri Gagarin.
The United States immediately developed a “crash” program in an attempt to catch up, and when the Russians detonated their first hydrogen bomb in 1953, the urgency increased. Plans for the development of the Atlas rocket were initiated in 1954, but it was not until 1958 that it launched successfully.
Soon there were two programs. One was for the development of intercontinental ballistic missiles (ICBMs) that could be used to carry nuclear weapons.6 The other, initiated by President Kennedy about the same time, was called the Apollo program, and it used Saturn rockets. The Apollo program had the goal of taking a man to the moon. Many of the earlier rockets, such as the Atlas, Redstone, and Titan, formed the basis of both this program and the ICBM program.
An ICBM is a ballistic missile with a range of more than three thousand five hundred miles, and ICBMs were usually designed to carry nuclear warheads. Many now have a range of up to twelve thousand miles. Modern ICBMs usually carry multiple independent re-entry vehicles, or MIRVs, each of which carries a separate nuclear warhead. This makes a single ICBM much more effective and deadly because it can hit several targets at once. MIRVs have become possible because nuclear warheads (hydrogen bombs) have become much smaller over the years. In addition, the rockets themselves have become smaller and now have a much greater range.
All early ICBMs were launched from very vulnerable, fixed, above-ground sites that could be easily attacked. This changed significantly over the course of the Cold War. Many were put in protected silos, mostly in northern states. In addition, they were now small enough that they could be launched from heavy trucks and railroad cars, which made them quite maneuverable. The most effective launch sites, however, are nuclear submarines. Once nuclear reactors were developed and perfected, they were soon used in submarines, and they proved to be particularly effective in them. Where early diesel-electric submarines had to surface frequently, nuclear submarines could stay submerged for months on end. And there was almost no need to refuel them; enough fuel for a sub's reactor for up to thirty years could easily be carried aboard the submarine. In some cases the reactor generates electricity that is used to power the propeller, and in other cases a reactor creates steam that drives turbines. Nuclear submarines are, however, very expensive to build, and because of this, only a few nations have them.
All American nuclear submarines are now equipped with ballistic missiles that have an intercontinental range. The biggest advantage of the submarine in this respect, however, is that it is highly maneuverable, relatively difficult to detect (although subs can be detected with sonar), and big enough to carry several MIRVs.
Soon after ICBMs with warheads were developed, several nations began to consider how they could be countered. In particular, could a missile be developed that had the ability to shoot down an incoming ICBM? Such systems were referred to as anti-ballistic missile (ABM) systems. The first study into the possibility of such a system was actually made as early as the later part of World War II by Bell Labs. The British had been bombarded by V-1 rockets, and later by V-2s, and they were looking for a defense. The V-1s were not ballistic, and British fighter planes and land-based artillery were able to shoot some of them down. But when the V-2 ballistic missiles appeared, there appeared to be no defense because of their high velocity and altitude. The Bell Labs study concluded, in fact, that it was not possible to shoot down a V-2 rocket. But that was before the advent of high-speed computers, and by the mid-1950s several nations were, indeed, considering the possibility of ABM systems.
Such systems are now divided into two classes: those that are directed against ICBMs and those that are directed against smaller rockets. At the present there are only two systems that can intercept ICBMs, since they are a much greater challenge than smaller rockets. The United States has developed what is called the ground-based midcourse defense (GMD). It consists of interception missiles along with a radar system to detect the incoming ICBMs. This system has been tested extensively over the years, with a mixture of successes and failures. It is still being worked on. The United States now has several smaller, short-ranged tactical systems that are more effective.
OTHER WEAPONS: LASERS
Another important modern weapon of war is the laser. When the first lasers were developed in the 1960s it was thought that they would soon become serious weapons, perhaps replacing guns. After all, Buck Rogers and many other early science fiction characters used “ray guns,” and it was believed that they would soon become reality. As it turned out, though, they have not replaced traditional guns, but recently they
have been used to knock down drones and possibly disable small ships or boats. They have also been used extensively for marking targets to determine their range.7
Despite their limited use as weapons of war so far, lasers do have considerable potential in that area, and they have been used extensively in everyday devices. They are used in DVD players and laser printers, as barcode scanners in stores, and their use in medicine has created a revolution in surgery. Furthermore, they are being used extensively in industry for cutting and welding.
The origin of the laser can be traced back to an early paper by Einstein. In an even earlier paper, Niels Bohr of Denmark postulated that atoms consisted of nuclei (protons) with electrons whirling around them in various discrete orbits, corresponding to various energies. We can, in fact, draw a simple energy diagram for an atom. Bohr mentioned the possibility of electrons jumping back and forth between these energy levels, but it was Einstein who put the idea on a firm basis.
Basic structure of an atom.
In the diagram we see several energy levels with electrons in some of the levels. When an electron absorbs a photon of light it moves to an upper, or excited, level; that is, to an orbit more distant from the nucleus. Usually it only stays there for a short time before it jumps back down to the original level (called its ground level). When it moves from an excited state to a lower state it emits a photon of light. This is referred to as spontaneous emission. Einstein also introduced the idea of stimulated emission; in this case the electron is already in an excited state. If a photon is directed at this electron, it can be stimulated to fall to a lower energy state, but it does not absorb the photon. In fact, it emits another photon as it falls down to the lower state so that we have two photons coming out in the process. And of particular importance, they both have the same wavelength, and they are in phase.