Sun in a Bottle - Charles Seife [52]
The tokamak has multiple sets of coils. One group of coils sets up a magnetic field that constrains and stiffens the plasma; it’s an external magnetic bottle, somewhat similar to the Stellarator’s, although not quite as sturdy. What gives the tokamak an extra bit of oomph is another set of coils that pinches the plasma. When scientists send a current through those coils, it induces a corresponding pinching current in the plasma circulating in the torus. This one-two punch of the external magnetic fields and internal current gave scientists a tool that, they hoped, would keep a hot, dense plasma stable for a long time.
Of course, the tokamak design had drawbacks as well. Unlike the Stellarator, which doesn’t require a plasma current at all, a tokamak absolutely needs one; its external magnetic bottle can’t by itself contain the plasma cloud for very long. But reducing this plasma current adds layers of complexity to the plasma, making it more unpredictable.
In some sense, the tokamak is something like a bicycle. Just as a bicycle is not stable until it is going relatively quickly, a tokamak’s plasma is not stable until the plasma current is up and running. The Stellarator is more like a tricycle. Just as a tricycle can be stationary, or can move forward or backward without any threat to its basic stability, a Stellarator can either have no plasma current or have one in either direction and still, theoretically, be stable.
Unfortunately, the current in a tokamak’s plasma is just one more thing that can fail. If an instability causes the current to drop momentarily, things get very bad very quickly. The plasma suddenly loses its pinch and explodes in all directions. This event is called a disruption, and it can be extraordinarily violent. It can even damage the machine. (One disruption at a modern British tokamak made the whole thing, all 120 tons of it, jump a centimeter into the air.) However, the disadvantages of the tokamak soon seemed small compared to the advantages of the design.
Sakharov was too busy working on nuclear weapons to spend a lot of effort on fusion reactors. But other Russian scientists, particularly the physicist Lev Artsimovich, took Sakharov’s design and put it to the test. By the mid-1960s, he was reporting spectacular results. His tokamak was confining a plasma at a given temperature and density ten times longer than could any other machine. Though confinement times were still on the order of milliseconds, Artsimovich’s results, if they were to be believed, indicated that Sakharov’s tokamak was blowing its competition away.
When Spitzer and the Americans first heard the Russian claims, they were skeptical, in part owing to American arrogance. The Stellarator was performing quite poorly, so they concluded that the problems they were encountering were likely due to a universal problem with magnetic confinement. If they weren’t succeeding, nobody was. The Americans were dubious that the Russians could do much better with their tokamak. Furthermore, Artsimovich’s measurements of the temperature of the plasma were rather crude. American scientists were relatively quick to disbelieve them. In the mid-1960s, Spitzer’s skepticism led him to conclude that tokamak performance was roughly the same as the Stellarator’s—underwhelming.
This conclusion was bad news for American fusion research. The enthusiasm of the 1950s had brought a downpour of funding from Congress. Since Project Sherwood’s inception, its budget had skyrocketed from almost