These machines that will save us
or destroy us
June 11, 2006
I would like to tell you a story
that is unfolding before our eyes. Something has just been born,
a newborn cry echoing from the guts of Sandia’s Z-machine, a laboratory in New Mexico. The child’s first cry came in May 2005. It burst forth suddenly from the void.
For half a century, humans have chased the mirage of fusion. While the demon of fission was relatively easily tamed—back in the early 1940s, giving rise to the first atomic bomb, which exploded at Alamogordo, United States—
The first atomic bomb explosion at Alamogordo
the second child of humanity, the Promethean child, emerged only as a child of the first. As soon as humans realized they could unlock the fantastic energy hidden within matter itself, they began envisioning reactions not of fission, but of fusion.
At first, the "atomists," later called "nuclear physicists," were merely chemists (like Ernest Rutherford, the New Zealander who discovered the atom). To a chemist, fission is nothing more than a dissociation reaction, highly exo-energetic, involving
a self-catalytic process.
Uranium-235 is eager to split into fragments of comparable mass, releasing a few neutrons in the process. These neutrons, striking neighboring nuclei, trigger the "chain reaction," provided the amount of atoms gathered is large enough for these neutrons to collide with another U-235 nucleus. Rather than speaking of a "critical mass," we should speak of a "critical volume." See
http://www.savoir-sans-frontieres.com/JPP/telechargeables/Francais/energetiquement_votre.htm
After exploiting the instability of uranium-235, which exists in nature only in trace amounts (0.7%), humans went on to exploit that of plutonium-239, an element that does not exist naturally but can be created by bombarding uranium-238 with a "fast neutron" released during the fission of U-235. Plutonium is also "fissile," has a "critical mass," and lends itself to bomb production. This second bomb was "tested" at Nagasaki, Japan.
Still viewing things through the chemist’s lens, fusion seemed much more like the mundane chemical reactions we already knew. On the left, the "reaction products." On the right, the result of the reaction. Schematically:
A combined with B produces C plus energy
The fusion reaction occurring at the lowest temperature involves two isotopes of hydrogen: deuterium and tritium (together forming what is called "heavy hydrogen"), the second having the drawback of being unstable, "radioactive" (half-life: 12 years). This temperature is one hundred million degrees. The Americans attempted to trigger this reaction by using X-rays produced by the explosion of a fission bomb, simply placing a deuterium-tritium mixture "next to" an "A-bomb," a fission device. This experiment was named "Greenhouse," the "greenhouse." Hydrogen had the disadvantage of needing to be in a condensed, liquid form—thus requiring very low temperatures. Equipped with a large auxiliary cryogenic installation, this first "H-bomb" was therefore not operational.
There was a second reaction allowing the use of an explosive in solid form: lithium hydride (LiH). But the required temperature was five times higher: 500 million degrees instead of 100 million. In Russia, the young Andrei Sakharov had the idea of placing the (small) fission bomb at the focus of an ellipsoid shaped like an elongated egg, with a hollow shell serving as a reflector for X-rays. All optical experts in the world had known this for centuries. Placing a radiation source at the first focus of this ellipsoidal reflector concentrates the radiation at the second focus. It was therefore sufficient to place the end of a lithium hydride charge shaped like a "loaf of sugar" there.
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Sakharov-Teller-Ulam configuration
In the United States, two young Jewish scientists—one of Ukrainian origin, Stanislaw Ulam:
Stanislaw Ulam
the other of Hungarian nationality, Edward Teller—had this idea at the same time. The Anglo-Saxons dubbed it "technically sweet."
Edward Teller in 1958, who served as the model for the character "Dr. Strangelove"
and who affectionately called the H-bomb "my baby" ("my baby")
(For memory’s sake, let’s note the rather dubious humor of some scientists who wrote books about the birth of such weapons—Antoine Schwerer, for example, titled his 1990 book "Beside My Bomb" [120 pages])
Antoine Schwerer: "Beside My Bomb" ...
While the military worked to develop their bombs, civilians, on their side, tried to ignite a deuterium-tritium mixture inside toroidal chambers, the "tokamaks," invented by Russian physicist Artsimovitch (incredibly imaginative, these Russians, indeed...).
Most readers now know the layout of such machines, where the gaseous mixture, heated to extremely high temperatures, is "magnetically confined," meaning it is held at the center of a coil shaped like a torus (or today more precisely like the letter "D," rotating around an axis). This idea lies at the heart of this cathedral for engineers bearing the name: ITER.
The ITER machine
In the illustration above, one can see, in cross-section, this closed, revolving chamber surrounded by all its peripheral devices. This machine will take over, at the French site of Cada...