NSTX establishes early milestones
By Steven Schultz
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Martin Peng (l) and Masa Ono with NSTX (Photo by Elle Starkman) |
The successful tests are the first concrete indications that the new device is capable of carrying out the demanding series of experiments planned for the next several years, according to PPPL director Rob Goldston.
The machine, called NSTX, was first powered up in February. It is the start of a new generation of devices that are smaller and possibly more efficient than previous machines.
One million amperes
In the most important test, conducted on December 14, NSTX generated an electric current of one million amperes in the ultra-hot swarm of particles called plasma that is contained within the machine. This does not represent an output of power; rather, one million amps of current circulating in the device is the full current required to conduct additional tests that could ultimately lead to a working power plant, Goldston said.
Earlier machines at Princeton and other labs around the world have required a lot more preparation before achieving their rated current levels, and NSTX was not scheduled to reach this until August 2000. One million amps of plasma current is a world record for a device of NSTX's design.
"I want to caution about getting overexcited at this time, but these results have certainly made people feel very good about the prospects for NSTX," said Goldston.
Validation of two ideas
In a further boost for PPPL, a series of experiments in January gave preliminary validation of two ideas for adding more power and new ways to produce and control current in the machine. In one of these tests, scientists coupled the first two of what will eventually be six radio-frequency devices for heating the plasma to tens of millions of degrees (hotter than the center of the sun), using a technique not unlike the microwave systems in your kitchen.
In the other test, they generated a current in the plasma by putting a giant voltage across the machine (like touching wires to the two ends of a fluorescent bulb) and then depending on the plasma to organize itself into a proper configuration. There was skepticism among scientists overseeing the project that the idea would work, said Goldston. Nonetheless, the test generated a current of 130,000 amps. PPPL scientists hope this new technique, developed in cooperation with the University of Washington, will eventually generate half a million amps.
"We're a quarter of the way there. A lot of people were doubtful we could even get that far," said Goldston, "so it's a really nice scientific accomplishment."
Rethinking scientific approach
These achievements, and increases in the lab's funding this year, suggest that the lab has charted a good course for itself. Previous years saw congressional budget cuts that resulted in the shutdown of the previous fusion device, TFTR, and forced a nationwide rethinking of the strategy for achieving fusion as a clean, safe and practically limitless source of energy.
NSTX itself is not intended to produce amounts of fusion energy, as was the goal with TFTR. Rather, Goldston noted, NSTX is a proof-of-principle experiment in which scientists hope to verify that their revised ideas merit continued development to the scale of TFTR and beyond.
The central difference between the two machines is their shapes. While TFTR looked like a doughnut, NSTX looks like an apple with the core taken out. This squeezing of the device reflects a remarkable insight on the part of Martin Peng, a physicist at Oak Ridge National Laboratory who is now on long-term assignment at PPPL.
Although scientists knew that a compact design would be more efficient than a doughnut shape, most believed that large central opening was dictated by the need for powerful magnets in the center.
Peng theorized that reducing the center hole would bring so many efficiencies that the center magnets would no longer be necessary. Key results from TFTR confirmed his analysis. Minimizing the electro-magnets translates into a significant reduction in power requirements and construction costs. The design also reduces the size of magnets needed on the outside of the sphere.
Three times power of TFTR
Peng, who codirects NSTX with PPPL physicist Masa Ono, said an important goal for NSTX is to provide the scientific knowledge to build a machine that will put out three times the power of TFTR at one third the cost.
Goldston said he hopes to see such a next-generation device built in the space now occupied by the old TFTR machine. Good results there would put the technology on the road to widespread use, an event that he believes would have a profound effect on the world environment and economy.
But Goldston said there is no way to put a timetable on this outcome, even with the best of scientific results. "It depends strongly on how much society recognizes the need for fusion and on the resulting scale of the research effort," he observed.
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