Sun in a Bottle - Charles Seife [54]
Imagine that you have a hunk of material—a whole lot of atoms—that you want to turn into a laser. The first step is to excite all the atoms. You do this by “pumping” the material full of energy. It doesn’t matter how. Some lasers pump a material with electricity. Some lasers do it with light, and some do it with chemical reactions. It might even be possible to use nuclear bombs to pump atoms into an excited state.46 Once the atoms in the material are excited, they are primed to get rid of their energy—they want to emit light of a particular color.
This is where the clever part happens. Send a photon of that specific color into the material. The photon encounters an excited atom, which then disgorges a second photon of the same color through stimulated emission. These two photons move in lockstep. They encounter another excited atom, which emits another photon of the same color: three photons now in lockstep. Another excited atom, another photon: four photons, all the same color, all moving in precisely the same way. As the photons move through the material, they encounter more and more excited atoms, which emit more and more photons. The beam snowballs, growing bigger as it travels through the material. By the time it finally emerges, the beam consists of an enormous collection of light particles. It is an intense beam, and all the photons have the exact same color and are moving in lockstep, almost like one enormous particle of light. This is the secret to the laser’s power. It is why the photons in a laser beam don’t zoom out in different directions and have all sorts of colors as a flashlight’s do. The photons in an ordinary beam of light are like are an unruly mob; the photons in a laser beam are an army marching together with a single mind.
The laser’s unusual properties make it an incredible scientific tool. The tight beam allows it to travel great distances—to the moon and back, even—without scattering and dissipating too much. Because the beam is made of photons of the exact same color, it provides a great way to measure very, very hot temperatures.
Shine a laser at a plasma. The photons in the beam will begin with the exact same color. But as the photons strike the fast-moving particles in the plasma, the plasma gives the photons a kick, adding a bit of energy to them, shortening their wavelengths and making them slightly bluer. By looking at the color of a laser beam after it hits a plasma, scientists can calculate the energies of the particles in the plasma, which, in turn, reveals the temperature.
When the British scientists shined a laser beam at Artsimovich’s tokamak plasma, they saw that the Russians were not exaggerating. Their plasma was tens of millions of degrees, dense, and relatively well confined. The tokamak was performing much better than the other forms of magnetic bottles. This was wonderful news for the fusion community in the West, even though the Russians, rather than the Americans or the British, had done it. (One Atomic Energy Commission worker reportedly danced on a table when he heard the news.) Sakharov’s invention showed a way to bypass the troubles of the pinch machines and the Stellarators. Practically overnight, plasma physicists across the world scrapped their old devices and built tokamaks. Even Spitzer succumbed to tokamania. By January 1970, the model-C Stellarator was scrap metal. In its place, a mere four months later, a tokamak sprang up. The fusion community had been pumped full of energy once more.
The laser measurements of Artsimovich