Carving a new path forward for compact fusion vessels.
Some experts believe that the future of fusion in the U.S. may be found in compact, spherical fusion vessels. A smaller tokamak is seen as a potentially more economical solution for fusion energy. The challenge lies in fitting all necessary components into a limited space. Recent research indicates that removing one key component used to heat the plasma could create the additional space required.
Scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), the private company Tokamak Energy, and Kyushu University in Japan have proposed a design for a compact, spherical fusion pilot plant that heats the plasma using only microwaves. Typically, spherical tokamaks also use a massive coil of copper wire called a solenoid, located near the center of the vessel, to heat the plasma. Neutral beam injection, which involves applying beams of uncharged particles to the plasma, is often used as well. But much like a tiny kitchen is easier to design if it has fewer appliances, it would be simpler and more economical to make a compact tokamak if it has fewer heating systems.
The new approach eliminates ohmic heating, which is the same heating that happens in a toaster and is standard in tokamaks. “A compact, spherical tokamak plasma looks like a cored apple with a relatively small core, so one does not have the space for an ohmic heating coil,” said Masayuki Ono, a principal research physicist at PPPL and lead author of the paper detailing the new research. “If we don’t have to include an ohmic heating coil, we can probably design a machine that is easier and cheaper to build.”
Identifying the ideal beam angle and heating mode
Microwaves are a form of electromagnetic radiation that can be generated using a device known as a gyrotron. The gyrotrons would sit on the outside of the tokamak — metaphorically speaking, just outside the apple skin — aimed toward the core. As the gyrotrons emitted powerful waves into the plasma, they would generate a current by moving negatively charged particles known as electrons. This process, known as electron cyclotron current drive (ECCD), both drives a current in and heats up the plasma. The heating process is not as simple as just turning on some gyrotrons, however. The researchers need to model different scenarios and determine various details, such as the best angle to aim the gyrotrons so the microwaves penetrate the plasma properly.
Using a computer code called TORAY coupled with one called TRANSP, the team scanned the aiming angles and saw what gave the highest efficiency. The goal is to use as little power as possible to drive the necessary current. “Also, you have to try to avoid any of the power that you’re putting into the plasma coming back out,” said Jack Berkery, a co-author on the paper and the deputy director of research for the National Spherical Torus Experiment-Upgrade (NSTX-U). This can happen when the microwaves are reflected off the plasma or when they enter the plasma but exit without changing the plasma’s current or temperature. “There were a lot of scans of different parameters to find the best solution,” Berkery said.
The research team also determined which mode of ECCD would work best for each phase of the heating process. There are two modes: ordinary mode, known as O mode, and extraordinary mode, known as X mode. The researchers see X mode as the best fit for ramping up the temperature and current of the plasma, while O mode is the best choice after the ramp-up, when the plasma temperature and current simply need to be maintained.
“O mode is good for a high-temperature, high-density plasma. But we found that O mode efficiency becomes very poor at lower temperatures, so you need something else to take care of the low-temperature regime,” said Ono.
Considering the impact of impurities
The authors, including postdoctoral researcher Kajal Shah, also investigated how power would radiate away from the plasma. Such radiation could be significant in a plasma as big as one needed for commercial fusion. Luis Delgado-Aparicio, the Lab’s head of the Advanced Projects Department and a co-author on the paper, notes that it will be particularly important to minimize the number of impurities from elements with a high atomic number, which is also known as a Z number, in the periodic table. Those are the elements with many positively charged particles, known as protons. The more protons an element has, the higher its Z number and the more it can contribute to heat loss. Tungsten and molybdenum, for example, have Z numbers, so their use inside a compact spherical tokamak would need to be carefully considered with an eye toward running the reactor in ways that would reduce impurity transfer into the plasma.
While the strong magnetic fields largely confine the plasma inside a tokamak in a particular shape, sometimes plasma can come close to the interior walls of the tokamak. “When this happens, atoms from the walls can sputter off and enter the plasma, cooling it,” said Delgado-Aparicio. “Even a relatively small amount of an element with a high Z number can cause the temperature of the plasma to cool significantly.” So, it is particularly important to keep impurities out of the plasma — as much as possible — particularly while the temperature is still ramping up.
Private-public partnerships: The future of fusion
The heating simulations are part of a design project known as the Spherical Tokamak Advanced Reactor or STAR. The project is a strategic initiative to develop plans for a pilot power plant. Berkery said the project provides PPPL researchers with an opportunity to apply their expertise in physics, engineering, and working with the computer codes for fusion simulations while working in partnership with private firms on their plans for fusion power plants with a spherical tokamak design.
Vladimir Shevchenko, a co-author on the paper and a senior technical adviser at Tokamak Energy, said he plans to run experiments at the end of next year in the company’s fusion vessel, ST40, to compare to the simulation results presented in the paper. “Other heating systems have very, very serious problems,” Shevchenko said. “I see this as the future for tokamak heating systems.”
Shevchenko thinks the project benefits from the public-private partnership between PPPL and Tokamak Energy, one of the companies selected for the DOE milestone-based fusion development program. “PPPL has a lot of experienced specialists in different areas related to plasma physics and tokamak technologies. Their contribution in terms of modeling and advising is very valuable for a private company like Tokamak Energy,” he said.
Reference: “Efficient ECCD non-inductive plasma current start-up, ramp-up, and sustainment for an ST fusion reactor” by M. Ono, J.W. Berkery, N. Bertelli, S. Shiraiwa, L. Delgado-Aparicio, J.E. Menard, Á. Sánchez-Villar, K. Shah, V. Shevchenko, H. Idei and K. Hanada, 19 June 2024, Nuclear Fusion.
DOI: 10.1088/1741-4326/ad556f
Other PPPL researchers on this project include Nicola Bertelli, Syun’ichi Shiraiwa, Jon Menard and Álvaro Sánchez Villar. This research was completed with funding from the DOE under contract number DE-AC02-09CH11466.
3 Comments
Is the existing energy in the universe insufficient? Is the ubiquitous flow of energy in spacetime unusable? What is the purpose of fusion?
The formation of the sun does not require human nuclear fusion. All things in the universe have been circulating in the spin cycle of topological vortices. We can personally experience the endless cycle of spring, summer, autumn, winter, and the passing of cold and heat.
The perpetually swirling topological vortices hah defied traditional physics’ expectations, and also changed people’s understanding of nature. Fusion is a severely outdated and backward theory, and a huge waste of social resources.
Radioactive elements and stars in the cosmos are one of the most active topological nodes in spacetime. Reasonably utilizing the synchronous effect of the spin of these topological vortex nodes is the direction of future energy research in human society. For example, intelligent batteries based on topological vortex spin.
Topological spin dominates the evolution of all things in the universe. Whoever masters topological intelligence holds the future of human society.
Based on the zero viscosity and absolute incompressibility of space, topological phase transitions occur at any point in space to form vortices, which is not difficult to understand mathematically. Once a topological spacetime vortex is formed, it will continue to rotate until it interacts with other vortices to cancel out, annihilate, or change. This is the synchronous effect of topological vortices.
The rotation of topological vortices is spin. Spin generates gravitation. Spin generates energy. Spin generates evolution. Their interaction via synchronous effect is very interesting. Their superposition includes both logical superposition in space-time and factual superposition in reality. Their entanglement includes both surface entanglement and body entanglement. Their deflection methods are endless. In the interaction and balance of topological vortex fractal structures, spin creates everything in the world.
The universe does not make algebra, formulas, or fractions. The universe is a superposition, deflection, and entanglement of geometric shapes, is the interaction and balance of countless topological vortex fractal structures. In these interaction and balance, the past is difficult to change. For the future, some predictable, some unpredictable. But, the present moment is real, certain, and actionable. Physics should not ignore that low dimensional topological fractal structures are the material basis of high-dimensional spacetime.
Scientific research guided by correct theories can help humanity avoid detours, failures, and pomposity. Please witness the exemplary collaboration between theoretical physicists and experimentalists (https://scitechdaily.com/microscope-spacecrafts-most-precise-test-of-key-component-of-the-theory-of-general-relativity/#comment-854286). Some people in contemporary physics has always lived in a self righteous children’s story world. Whose values have been overturned by such a comical and ridiculous reality?
Misguided by the pseudo-scientific theory of Physical Review Letters (PRL), many researchers do not consider the similarities and differences between geometric shapes and physical reality in physics research, but indulge in imagination, and some scholars’ physics research seriously deviates from science, and they are almost unaware of the dirtiness and ugliness. Although mathematics is the language of science, it must be understood correctly.
I hope researchers are not fooled by the pseudoscientific theories of the Physical Review, and hope more people dare to stand up and fight against rampant pseudoscience.
The so-called academic journals (such as Physical Review Letters, Nature, Science, etc.) firmly believe that two high-dimensional spacetime objects (such as two sets of cobalt-60) rotating in opposite directions can be transformed into two objects that mirror each other, is a typical case of pseudoscience rampant.
Scientific research should be respected, but some people in contemporary physics are not. Please witness the actions of certain individuals and publications. If researchers are really interested in Science and Physics, you can browse https://zhuanlan.zhihu.com/p/643404671 and https://zhuanlan.zhihu.com/p/595280873.
The Physical Review journals, encompassing both hybrid and open-access journals, features 17 peer-reviewed publications including Physical Review Letters, Physical Review X, and Reviews of Modern Physics. CP violation was published in Physics Review in 1956. All so-called peer-reviewed publications of Physical Review family are responsible for clarifying this.
CP violation opened the dirtiest and ugliest era in the history of physics.
this is going to sound wild but since governments have limited money and I was told that this whole environmental problem is like super pressing like we need to have solved it yesterday, we should probably not split the money between research on fusion which we don’t have a viable way of using at the moment, and vision which we absolutely do so maybe we should just get ourselves to the point where we can actually use nuclear power and then we can start spending money on improving it later because the improvement already will be so drastic that it’s worth it to just do it. But we don’t have the money to do both and if we keep researching the next big great thing when we haven’t even done the previous big great thing then we’re all going to die right?
Wrong. We can print all the money we need. MOney is just a concept anyway. This is why we have a national debt of $35 TRILLION. So print another $1-2 trillion for fusion research. What’;s the difference between $35 trillion and $37 trillion? [shrug]