Stellarator challenge: The magnetic field of a fusion device encloses the hot plasma and keeps it away from the vessel walls to avoid inefficient energy losses from plasma particles drifting outward. Ripples in the magnetic field can put a device at risk of energy loss. While tokamak devices avoid energy loss from ripples because of their symmetrical design, the design of a stellarator’s twisted coils must be optimized to minimize ripples and avoid energy losses, which tend to increase with plasma temperature, according to IPP. The goal for stellarator fusion is to achieve plasma confinement that reaches the level of competing tokamak facilities, while delivering the optimized continuous operation that makes stellarator designs attractive in the first place.
A “fivefold” symmetry: In the Wendelstein 7-X, 50 superconducting niobium-titanium magnet coils about 3.5 meters high are cooled with liquid helium. According to the IPP website, “They need hardly any energy after being switched on. Their bizarre shapes are the result of the optimization calculations: They are to produce a particularly stable, thermally insulating magnetic cage for the plasma.”
The resulting magnetic field has a fivefold symmetry, according to IPP. The structure, which resembles a pentagon, is made up of five identical modules. Each module contains 10 coils, two of which are arranged upside down. The entire coil structure is made up of only five different types of coils. A second set of 20 planar superconducting coils is superposed on the stellarator coils, and a ring-shaped support structure keeps the coils in position.
The IPP website hosts an interactive panoramic tour of the device, which produced its first plasma in December 2015.
Experimental success: The analysis of recent experiments indicates that with the heating devices available, Wendelstein 7-X has been able to generate high-temperature plasmas and set a stellarator “fusion product” record at high temperatures. (Fusion product is the product of temperature, plasma density, and energy confinement time.)
According to IPP, a “thought experiment” led to the conclusion that the optimization efforts were successful. The energy losses expected if the same plasma values and profiles were reached in a less optimized magnetic field were calculated and found to be greater than the input heating power. Since losses greater than input are physically impossible, the researchers concluded that the results seen in Wendelstein 7-X must have resulted from a successfully optimized magnetic field.
“This shows that the plasma profiles observed in Wendelstein 7-X are only conceivable in magnetic fields with low neoclassical losses,” said Per Helander, head of the Stellarator Theory Division. “Conversely, this proves that optimizing the Wendelstein magnetic field successfully lowered the neoclassical losses.”
Continuous operation of Wendelstein 7-X has yet to be achieved. A water-cooled wall cladding is currently being installed that will permit researchers to test the performance of the Wendelstein concept in continuous operation, eventually sustaining plasmas for 30 minutes.
