could go up on the balcony and stand there,” says Wechsler, “and look down at this huge, complex drawing. It was neat, especially for the theoreticians — there it was at full scale and they could say, ‘Oh, what's that thing over there?’ ‘Well, don't you remember, that's part of what we've got to hold it up with.’ ‘Oh my God. Smart man, that Holloway.’”
A crucial decision concerned the internal shape of the test device. (Externally, it would be shaped like a giant capsule — a hollow steel cylinder with rounded ends.) Carson Mark's division worked overtime that autumn and winter to devise the best possible theoretical design. When the fission primary at one end of the test device fired, it would radiate equally in every direction. How much of that radiation could be channeled into the secondary? Originally, Panda thought of building a massively shielded end above the fission primary, thickly coated with lead, to try to contain as much radiation as possible. The radiation would flow down the interior of the big cylinder, which was essentially one big radiation channel — a big pipe. How would the radiation flow? Mark wondered if it flowed like a bucket of water thrown into a big tank. Where did it go? Did it splash along the walls? They would have to substitute calculation for experiment to decide, but the MANIAC would not be finished before spring 1952; hand calculation would have to suffice — hand calculation and instinct. Wechsler says it was Mark more than anyone whose instinct for the physical behavior of the radiation and whose judgment of the corresponding risks they could take with channel design carried the day. “Carson is par excellence a physicist and a theoretician, especially his appreciation for the mechanical aspects of a problem and his ability to assess whether something is really going to make any difference. I just thought that he was fabulous.” When Norris Bradbury argued that nobody is the father of the hydrogen bomb, that its development was a group effort, he was probably thinking of this collective work of imagining abstract concepts into physical reality, a work at which steady, reliable men like Mark, Wechsler and Holloway shone. (Edward Teller spent only two weeks at Los Alamos in the crucial six months between October 1951 and April 1952 when the equilibrium thermonuclear was designed; his main contribution seems to have been to kibitz. “Once Teller left Los Alamos,” Hans Bethe observes, “even though they were working on ‘his’ weapon, he found all sorts of reasons why it wouldn't work. He hated the project director, Marshall Holloway… So he had every reason, he tried to criticize it wherever possible.”)
The thermonuclear secondary, which would essentially be a bottle of liquid deuterium with a stick of plutonium mounted inside for a sparkplug, would hang in the center of the big steel casing below the fission primary, behind a heavy blast shield. The flux of soft X rays from the primary would flow down the inside walls of the casing several microseconds ahead of the material shock wave from the primary. The X-ray flux would be dense as solid metal, but it would not have time to exert much pressure directly to implode the secondary. The X rays were hot, however, so hot that they would ionize solid materials instantly and turn such materials into a plasma hot enough to radiate further X rays. (Materials are said to ionize when they are heated sufficiently to break up their atoms into electrons and nuclei — negative and positive ions. Plasma is a fourth state of matter — solids, liquids and gases are three more familiar states of matter — consisting of hot, ionized gas; the sun is a ball of plasma maintained by thermonuclear burning.) So the steel casing would need to be lined with some material that would absorb the radiation and ionize to a hot plasma which could radiate X rays to implode the secondary.
As the radiation flowed from the primary end of the casing around and past the secondary, it would start generating radiation pressure at the end nearer the primary sooner than at the end farther away. “If the pressure is real high at this end and real low at the other end,” Wechsler recreates the Panda discussions, “how is it going to work? That was one of the big questions.” They first thought to taper the channel, Wechsler says, “leave it as open as it could be near the primary with a minimum amount of material to generate pressure and then put more and more material in to narrow it” down the channel. From the spherical channel of their idealized calculations they evolved to a channel shaped like an inverted bowling pin they called the Schmoo after an imaginary creature of that shape in the
According to Wechsler, Mark led the way to a successful radiation-channel design. “Carson's feeling was that if the system is going to work at all, these pressures are going to be so high that the differences from one end to the other aren't going to be very much in this brief length of time. If it
Teller continued to find fault with the Mike design, Bethe remembers. “At one point he said, well, it may all work perfectly well, except that the radiation will go into the casing and then there will be Taylor instability. Now, I know a lot about Taylor instability, and I worked on this radiation penetration, and then came out with the conclusion that there would not be Taylor instability, and wrote it down and sent it to Teller. That was my main contribution. Certainly my work was not critical to success.”
Through late 1951 and early 1952, while the physicists pursued design questions, Wechsler and the other project managers and engineers organized the production of deuterium and the exotic equipment for storing and transporting it in liquid form.
Cryogenics had its substantial start in the work of the Scottish physicist James Dewar, who first produced liquid oxygen in quantity and in 1898 was the first person to liquefy hydrogen. (He was also the co-inventor of cordite, the smokeless explosive that propelled the deadly artillery of the First World War.) At Cambridge University, one of the two institutions where Dewar maintained joint appointments, his low-temperature achievements won celebration in raffish verse:
Dewar's most enduring invention — in 1892 — was a double-walled flask with the space between the walls pumped out to a good vacuum. A vacuum is an extremely effective insulator against heat convection; the double- walled vacuum flask, with the walls usually silvered to reduce the transport of radiant energy as well across the vacuum, became a standard container for the insulated storage of liquids. In its larger scientific and technical versions such a container is called a dewar; one smaller version adapted for home use is the familiar thermos bottle.
Liquefying gases requires more than straightforward refrigeration, but one important step in the process is incorporated into all home refrigerators: compressing a gas and then allowing it to expand, work which reduces its temperature. In a home refrigerator, compressing and expanding a coolant gas within an arrangement of coiled piping cools the piping, which takes up heat from the food being stored; for liquefying gases, the gas being liquefied is compressed and expanded through a nozzle or in a small piston engine or turbine to cool itself.
Until after the Second World War, the most ingenious and efficient liquefaction system was one developed in the 1930s by Peter Kapitza when he worked at Cambridge University. With John Cockcroft, Kapitza first developed a hydrogen liquefier (in 1932); then, on his own, devised a machine for liquefying helium, which has the lowest boiling point of all gases, only 4.2°K. Kapitza's helium liquefier first precooled the helium gas with liquid nitrogen, which
