Researchers at MIT’s Plasma Science and Fusion Center have set a new world for plasma pressure inside a tokamak, creating over two atmospheres of pressure for the first time.
The record was set using the Institute’s Alcator C-Mod tokamak nuclear fusion reactor. Plasma pressure is a key ingredient to producing energy from nuclear fusion and the result is a major step towards developing it as a source of unlimited clean, safe, carbon-free energy.
Alcator C-Mod is a compact, high-magnetic-field fusion reactor with advanced shaping in a tokamak design, which confines the superheated plasma in a donut-shaped chamber. C-Mod has a high-intensity magnetic field of up to 8 tesla, 160,000 times the Earth’s magnetic field.
Its magnetic field is more than double what is typically used in other designs, which quadruples its ability to contain the plasma pressure and allows it to create dense, hot plasmas and keep them stable at more than 80 million degrees.
Dale Meade, former deputy director at the Princeton Plasma Physics Laboratory, said: “This is a remarkable achievement that highlights the highly successful Alcator C-Mod program at MIT. The record plasma pressure validates the high-magnetic-field approach as an attractive path to practical fusion energy.”
Nuclear fusion is the same process that powers the sun, and can be realised on Earth in reactors that simulate the conditions of ultra hot miniature “stars” of plasma — superheated gas — that are contained within a magnetic field.
Nuclear fusion research started more than 50 years ago and it was quickly established that to make fusion viable on the Earth’s surface, the plasma must be very hot, more than 50 million degrees, it must be stable under intense pressure, and it must be contained in a fixed volume.
Successful and sustainable fusion for power generation also requires the energy released in a reactor to exceed the energy required to keep the reaction going. Pressure, a product of density and temperature, accounts for about two thirds of the challenge. The amount of power produced increases with the square of the pressure — so doubling the pressure leads to a fourfold increase in energy production.
While setting the new record of 2.05 atmospheres, a 15% rise from the previous record also held by Alcator C-Mod, the temperature inside the reactor reached over 35 million degrees Celsius, around twice as hot as the center of the sun. The plasma produced 300 trillion fusion reactions per second and had a central magnetic field strength of 5.7 tesla. It carried 1.4 million amps of electrical current and was heated with over 4 million watts of power.
The reaction occurred in a volume of approximately 1 cubic metre and the plasma lasted for two full seconds.
Other fusion experiments conducted in reactors similar to Alcator have reached these temperatures, but at pressures closer to 1 atmosphere. MIT said its results exceeded the next highest pressure achieved in non-Alcator devices by approximately 70%.
Funding to run Alcator C-Mod ended last month, so that it can be used to help finance the construction of Iter at Caradarache in the South of France.
Iter will be approximately 800 times larger in volume than Alcator C-Mod, but will operate at a lower magnetic field. Iter is expected to reach 2.6 atmospheres when in full operation by 2032, according to a recent US Department of Energy report.
Unless a new device is announced and constructed, C-Mod’s pressure record will likely stand for the next 15 years.
Alcator C-Mod is similar in size and cost to non-tokamak magnetic fusion options being pursued by private fusion companies, though it can achieve pressures 50 times higher.
Dennis Whyte, director of the Plasma Science and Fusion Center, and head of the department of nuclear science and engineering at MIT, said: “Compact, high-field tokamaks provide another exciting opportunity for accelerating fusion energy development, so that it’s available soon enough to make a difference to problems like climate change and the future of clean energy — goals I think we all share.”
MIT’s fusion group is now working on adapting high-field, high-temperature superconductors to produce magnetic fields of even greater strength without consuming electricity or generating heat.
These superconductors are a key of a conceptual pilot plant called the Affordable Robust Compact (ARC) reactor which has been proposed at MIT, and could generate up to 500MW.
Dr David Kingham, chief executive of Tokamak Energy, which aims to develop a commercial fusion power plant, welcomed MIT’s latest world record. He said: “This emphasises the potential of small tokamaks by way of some very clever magnet engineering. It demonstrates that these extremely high magnetic field devices can be developed and that high levels of performance can be achieved on a small scale.”
Engineers at Oxfordshire-based Tokamak Energy are taking a similar approach to MIT’s, but with a slightly different compact design - a “spherical” tokamak. Kingham said the spherical design means they don’t have to achieve such high magnetic fields because the ratio of plasma pressure to magnetic field pressure is proven to be greater in a spherical tokamaks than it is in a conventional large aspect ratio device.
Tokamak Energy is developing two prototype fusion reactors at the same time to accelerate R&D, one to perfect the spherical design and another which will use high temperature superconducting magnets made of yttrium barium copper oxide. First plasma on their first reactor is planned for early next year and engineers plan to achieve a plasma temperature of 15 million degrees C before the middle of next year.
Kingham said: “Iter should be regarded as an exciting collaborative project in plasma physics, but fusion’s potential is so huge we need to keep up with other things too. It’s now about solving engineering challenges and our approach is different – to keep things small and iterate designs.”