Researchers at UCSD and San Diego-based General Atomics have reported an improved control method for a type of nuclear fusion technology that confines a cloud of ionized hydrogen in a doughnut-shaped tokamak reactor. Unlike fission reactors, which generate energy by splitting atoms of uranium or plutonium, tokamak (TOE-ka mack) fusion devices create energy with almost no radioactive byproducts by combining two heavy atoms of hydrogen into helium. Researchers at UCSD, General Atomics, and dozens of university and government laboratories around the world are collaborating on a variety of fronts to improve the efficiency of the current generation of tokamaks, which use magnetic fields to confine the ionized hydrogen fuel, or plasma, in a circular cloud called a torus.
In a paper published in the July issue of Automatica, a group that includes UCSD professor of mechanical and aerospace engineering Miroslav Krstic and his former graduate student Eugenio Schuster, now an engineering professor at Lehigh University, described an improved control technique developed with General Atomics scientists Michael L. Walker and David A. Humphreys. The new mathematical approach was designed to be incorporated into existing General Atomics software to more effectively fine tune electrical currents flowing through tokamak control circuits. These currents produce magnetic fields that dampen the vertical instabilities and unwanted oscillations of the torus.
“A significant fraction of the hurdles faced by tokamaks are control problems, and vertical control is only one of them.” said Krstic. “The better we are able to control all these parameters, the more efficiently we will be able to run fusion reactors.”
Nuclear fusion occurs in tokamaks when a mixture of deuterium, and tritium -- isotopes of hydrogen with two and three times the mass, respectively, of ordinary hydrogen atoms -- fuse into helium. The common goal of an international team that includes the People's Republic of China, the European Union, Japan, the Republic of Korea, the Russian Federation, and the United States is to build by 2015 the next generation of fusion reactor based on the experience gained with tokamaks. The group’s planned $5 billion International Thermonuclear Experimental Reactor (ITER), which is slated for construction in France, will heat a diffuse cloud of ionized hydrogen to roughly 100 million degrees Celsius, fusing isotopes of hydrogen into helium in a process that will generate about 10 times more energy than the device will consume.
Several years ago tokamaks demonstrated the ability to produce more power than they consume, an important milestone, but the first commercial fusion reactors are estimated to be 30 years away. “Additional increases in efficiency will only be possible with more precise control of the vertical position, shape, and other parameters of the torus,” said Krstic. “Fusion is one of the hardest technological problems on the planet to solve, however, tokamak control is an area where tremendous strides are being made.”
Advanced tokamaks heat nuclei of hydrogen to temperatures hotter than those at the center of the sun. “This is one reason plasma is so unstable in these machines,” said Walker, a scientist with General Atomics and co-author of the study. “However, we’re better able to control the instabilities, which is why this study and others like it are signs of the steady progress that the fusion community has made in the past several years.”
One limitation of tokamak devices relates to the upper limit of current that can pass through their control circuits. These circuits behave like garden hoses capable of carrying only a limited flow of water, and once they reach their maximum carrying capacity they are said to be saturated. If more electrical current is needed to generate a stronger magnetic push on wayward plasma, vertical-control software may call for more current than the control circuits can possibly deliver, a process known in the control field as “winding up.” “In this case, the controllers are telling the actuators to work harder and harder, but they’re already maxed out,” said Krstic. “As a consequence, the vertical position of the plasma torus can potentially hit the interior wall of the tokamak and cause structural damage.”
The technique invented by Krstic and Schuster includes “anti-windup” features, including one dubbed “watch dog” and another called “rate limiter” that are designed, respectively, to monitor coil voltage demands and prevent the controller from asking for a response that can’t possibly be delivered.
“We’re starting from a disturbed plasma flow and we’re trying to return it to equilibrium,” said Krstic. “The General Atomics scientists designed a sophisticated vertical stabilization controller that we were asked to improve. We’ve had to push the envelope beyond the existing theory of anti-windup to accomplish this.