Aug
19
Fusion Stellarator Device Efficiency Improvement Confirmed
August 19, 2021 | Leave a Comment
Max-Planck-Institut für Plasmaphysik (IPP) has now confirmed one of the most important optimization goals underlying the Wendelstein 7-X fusion device. The analysis shows that in the optimized magnetic field cage, the energy losses of the plasma are reduced in the desired way.
Wendelstein 7-X is intended to prove that the disadvantages of earlier stellarators can be overcome and that stellarator-type devices are suitable for power plants.
The Wendelstein 7-x Stellarator is smashing records. Image Credit: Michael Liebreich. Click image for the largest view.
The team’s study results have been published in Nature.
The optimized Wendelstein 7-X stellarator, which went into operation five years ago, is intended to demonstrate that stellarator-type fusion plants are suitable for power plants. The magnetic field, which encloses the hot plasma and keeps it away from the vessel walls, was planned with great theoretical and computational effort in such a way that the disadvantages of earlier stellarators are avoided.
One of the most important goals was to reduce the energy losses of the plasma, which are caused by the ripple of the magnetic field. This is responsible for plasma particles drifting outwards and being lost despite being bound to the magnetic field lines.
Schematic diagram of the superconducting stellarator Wendelstein 7-X. The last closed magnetic flux surface is indicated in magenta. The plasma vessel is shaped according to the magnetic field geometry. The 50 non-planar coils have an aluminum foil plated steel casing, the 20 planar coils a copper plated steel casing. The non-planar and the planar coils of each type are connected in series via superconducting bus bars. Coils, coil casings and the central support ring are supplied with liquid helium by means of a pipe system. The magnet system is operated in the evacuated cryostat volume between the plasma vessel and the outer vessel. Image Credit: Wikipedia. Click image for the largest view.
Unlike in the competing tokamak-type devices, for which this so-called “neo-classical” energy and particle loss is not a major problem, it is a serious weakness in conventional stellarators. It causes the losses to increase so much with rising plasma temperature that a power plant designed on this basis would be very large and thus very expensive.
In tokamaks, on the other hand – thanks to their symmetrical shape – the losses due to the magnetic field ripple are only small. The energy losses are mainly determined by small vortex movements in the plasma, by turbulence – which is also added as a loss channel in stellarators.
Therefore, in order to catch up with the good confinement properties of the tokamaks, lowering the neoclassical losses is an important task for stellarator optimization. Accordingly, the magnetic field of Wendelstein 7-X was designed to minimize those losses.
In a detailed analysis of the experimental results of Wendelstein 7-X, scientists led by Dr. Craig Beidler from IPP’s Stellarator Theory Division have now investigated whether this optimization leads to the desired effect. With the heating devices available so far, Wendelstein 7-X has already been able to generate high-temperature plasmas and set the stellarator world record for the “fusion product” at high temperature. This product of temperature, plasma density and energy confinement time indicates how close you get to the values for a burning plasma.
Such a record plasma has now been analyzed in detail. At high plasma temperatures and low turbulent losses, the neoclassical losses in the energy balance could be well detected here: they accounted for 30 percent of the heating power, a considerable part of the energy balance.
The effect of neoclassical optimization of Wendelstein 7-X is now shown by a thought experiment: It was assumed that the same plasma values and profiles that led to the record result in Wendelstein 7-X were also achieved in plants with a less optimized magnetic field.
Then the neoclassical losses to be expected there were calculated – with a clear result: they would be greater than the input heating power, which is a physical impossibility. “This shows,” said Professor Per Helander, head of the Stellarator Theory Division, “that the plasma profiles observed in Wendelstein 7-X are only conceivable in magnetic fields with low neoclassical losses. Conversely, this proves that optimizing the Wendelstein magnetic field successfully lowered the neoclassical losses.”
However, the plasma discharges have so far only been short. To test the performance of the Wendelstein concept in continuous operation, a water-cooled wall cladding is currently being installed. Equipped in this way, the researchers will gradually work their way up to 30-minute long plasmas. Then it will be possible to check whether Wendelstein 7-X can also fulfill its optimization goals in continuous operation – the main advantage of the stellarators.
The aim of fusion research is to develop a climate- and environmentally-friendly power plant. Similar to the sun, it is to generate energy from the fusion of atomic nuclei. Because the fusion fire only ignites at temperatures above 100 million degrees, the fuel – a low-density hydrogen plasma – must not come into contact with cold vessel walls. Held by magnetic fields, it floats almost contact-free inside a vacuum chamber.
The magnetic cage of Wendelstein 7-X is created by a ring of 50 superconducting magnetic coils. Their special shapes are the result of sophisticated optimization calculations. With their help, the quality of plasma confinement in a stellarator is to reach the level of competing tokamak-type facilities.
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The tokamak concept seems simple, a donut shape with the plasma circulating inside. Seems simple, but plasma is quite dynamic and exploits every possible exit, leading to decades of delays and billions of dollars in getting much progress with them. There remains billions more needing spent over likely more decades.
Stellarators on the other hand are quite a complex concept that seem to be far less apt to leak out the plasma. Leaking plasma is a very dangerous thing. Spilling 100 million degree things are quite a problem such that engineers really don’t want it to happen.
If the IPP team’s assertion proves correct, and one can be sure that will be tested thoroughly, the stellarator concept could be the first to get hot enough to get a fusion energy output result higher than the input energy cost. Plus the design is much more likely to run continuously – the critical design goal for commercial power. If it can be made to work . . . And that is looking more probable now at less billions of dollars and much sooner.
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