With a modification to Greenwald's Law, fusion reactors might produce more energy.
Future fusion reactions in tokamaks might create far more energy than previously believed, as a result of important new research demonstrating that a fundamental rule for such reactors is incorrect.
Physicists at the Swiss Plasma Center at École Polytechnique Fédérale de Lausanne (EFPL) have discovered that the maximum hydrogen fuel density is almost double the “Greenwald Limit” – an estimate obtained from studies conducted more than three decades ago.
Paolo Ricci, a physicist at the Swiss Plasma Center, stated that the discovery that fusion reactors can actually operate with hydrogen plasma densities much higher than the Greenwald Limit they were designed for will affect the operation of the massive ITER tokamak being constructed in southern France and the designs of ITER’s successors, the Demonstration power plant (DEMO) fusion reactors.
“The exact value depends on the power,” Ricci told Live Science. “But as a rough estimate, the increase is on the order of a factor of two in ITER.”
Ricci is one of the leaders of the research project, which combined theoretical work with the results of approximately a year of experiments at three different fusion reactors in Europe – EPFL’s Tokamak à Configuration Variable (TCV(opens in new tab)), the Joint European Torus (JET(opens in new tab)) at Culham in the United Kingdom, and the Axially Symmetric Divertor Experiment (ASDEX(opens in new tab)
He is also one of the primary authors of a research describing the discovery that appeared in Physical Review Letters on May 6.
Tokamaks, which resemble doughnuts, are among the most promising designs for nuclear fusion reactors that might one day be utilised to generate electricity for power systems.
In contrast to nuclear fission, which generates energy by shattering apart extremely big atomic nuclei, nuclear fusion might provide even more energy by fusing together very tiny nuclei.
Fusion generates far less radioactive waste than fission, and the neutron-rich hydrogen it utilises as fuel is very inexpensive to get.
The same process fuels stars like the sun, which is why controlled fusion is compared to a “star in a jar;” but, because the extremely high pressure at the centre of a star is not possible on Earth, fusion processes require temperatures hotter than the sun to function.
Temperatures within the TCV tokamak can reach more than 216 million degrees Fahrenheit (120 million degrees Celsius) – about 10 times the temperature of the fusion core of the sun, which is around 27 million F. (15 million C).
Several fusion power projects have reached an advanced level, and some experts believe the first tokamak that generates energy for the grid might be operational by 2030, according to a recent piece by Live Science.
The ITER tokamak (“Iter” means “way” in Latin), which is scheduled to create its first experimental plasmas in 2025, is also funded by more than thirty nations from across the world.
ITER is not planned to generate power; however, tokamaks based on ITER that will generate electricity, known as DEMO reactors, are now being constructed and might be operational by 2051.
The Greenwald Limit, which was established in 1988 by MIT physicist Martin Greenwald, is key to the new estimates.
Greenwald derived an experimental limit based on a tokamak’s minor radius (the size of the donut’s inner circle) and the amount of electric current passing through the plasma. Researchers were attempting to determine why their fusion plasmas effectively became uncontrollable (they expanded outside the magnetic fields they were contained by within the tokamak chamber) when they increased the fuel density past a certain point.
Ricci stated that the Greenwald Limit had been a pillar of fusion research for more than three decades, despite the fact that experts had long assumed it might be improved upon. It is a guiding idea of the ITER design, for instance.
However, the current work extends on both the tests and theory that Greenwald used to determine his limit, resulting in a significantly greater fuel density limit that will boost the capacity of ITER and influence the designs of subsequent DEMO reactors, he added.
The crucial discovery was that a plasma may maintain a higher fuel density as the power output of a fusion reaction rises, he explained.
Ricci stated that it is currently impossible to predict how such a substantial increase in fuel density would effect the power output of tokamaks, but it is expected to be considerable; and research indicates that higher fuel density will make fusion reactors simpler to run.
“It makes it easier to produce safe, sustainable fusion conditions,” he added. “It helps you to achieve the desired regime so that the fusion reactor can function correctly.”