Researchers at the University of São Paulo (USP) Physics Institute and the São Paulo company Politron have developed and built a new radiofrequency amplifier that will operate in what is currently the most important controlled nuclear fusion laboratory, the Joint European Torus (JET), maintained by the European Union in Culham, UK. The name of the laboratory comes from the machine’s chamber, which has a toroidal shape similar to a doughnut with a hole in the middle.
The radiofrequency source is the responsibility of a partnership between researchers at USP, at the Federal Polytechnic School of Lausanne (EPFL), in Switzerland, and at the Massachusetts Institute of Technology (MIT). They found that a source with the wide frequency spectrum, flexibility and robustness required to meet the extremely stringent operating conditions at JET did not exist on the world market. Nuclear fusion power plants promise to be able to produce electricity without radioactive waste products and are less prone to accidents. The system is different from today’s nuclear fission plants, which are still cause for concern due to their fuel preparation and nuclear waste storage procedures. In fission, the nuclear reaction continues even when the reactor is shut down. To develop the technology needed to build a commercial fusion power plant by mid-century, several experiments are being conducted worldwide.
The radio wave amplifier is essential to this process and the only company contacted by this international partnership that was interested in developing the equipment was Politron, which is now preparing a patent application for the invention together with USP. “It is a paradigmatic case of collaboration between a university and a company to achieve innovation,” says Ricardo Galvão, professor and coordinator of the USP Plasma Physics Laboratory. In 2014, the most advanced JET measurement and training stage will begin, and the amplifiers are essential components. “Our unit operates under conditions not satisfied by commercial equipment,” says Galvão. Founded in 1950, Politron pioneered the development of radiofrequency wave generating machines in Brazil, which are used in the production lines of various sectors, from footwear to mining.
The company is mid-sized and exports throughout Latin America. According to Maria de Oliveira, managing director, the company has suffered in recent years with Chinese competition, which flooded the market with relatively lower quality machines sold for half the price. As it is impossible to compete on a global scale, recently Politron has tried to provide products to customers with specific needs. “We have already produced custom-made machines for laboratories at several Brazilian universities, such as UFRJ (Federal University of Rio de Janeiro), UFMG (Federal University of Minas Gerais), Unicamp (University of Campinas), USP and UFSCar (Federal University of São Carlos),” Oliveira says. “The challenge is to build an amplifier that always works within its specifications,” explains engineer Alessandro de Oliveira Santos, manager of research and development at the company, which embraced the project for two years. The amplifier is the result of an agreement signed in 2009 between Brazil and the European Atomic Energy Community (see Pesquisa FAPESP Issue No. 186).
In September and October 2013, the British engineer Margaret Graham, of JET, visited Brazil and worked with the team at USP and Politron in the final assessment of the industrial version of the amplifier. “The tests were very successful,” she said. “There are only a few small details remaining that we will be able to work out in England.” On October 29, 2013, Francesco Romanelli, Director of JET, confirmed in a memo that the English laboratory was ready to receive the amplifier and formally asked USP to send the unit. “The amplifier and seven other units will be sent to the European laboratory later,” says Galvão. Politron and the researchers from USP expect that, if everything goes well with the experiments in England, other fusion laboratories around the world will be interested in purchasing new amplifiers. The international thermonuclear experimental reactor (ITER), under construction since 2007 in Cadarache, France, which is eight times larger than JET, may be particularly interested. The project is funded by a consortium consisting of the European Union, China, South Korea, the United States, India and Japan. ITER is behind schedule, and the forecast is that it will be ready in 2020.
The fuel used in fusion reactors consists of two types of heavy hydrogen: deuterium, which can be extracted from seawater, and tritium, which is produced from lithium. A mixture of deuterium and tritium is injected into a tokamak—a machine invented by the Soviets in the 1960s that is being improved to become the reactor for new nuclear plants. Within the machine’s chamber, the hydrogen is heated to release its electrons from its atomic nuclei, forming an electrically charged gas, also known as a plasma. Magnetic fields trap the plasma, which circles the torus, preventing it from cooling and damaging the wall of the chamber by touching it. If the plasma’s temperature reaches 150 million degrees Celsius (10 times greater than the temperature at the center of the Sun), the deuterium and tritium nuclei will begin to merge after they collide, producing helium and neutrons, both of which will be highly energetic.
The helium nuclei remain within the plasma, helping to heat it and sustaining further fusion reactions, while the neutrons, which are unaffected by the magnetic field, escape, colliding with the walls of the tokamak and creating heat to move the turbines of an electric generator. Fusion would be safer than fission because, as the plasma cools very quickly, the reaction is stopped immediately when the magnetic fields are turned off. However, there are many technological challenges to be overcome to make nuclear fusion feasible.
JET has the largest tokamak ever built, capable of confining 80 cubic meters of plasma. In operation since 1983, it achieved the first controlled fusion reaction in history in 1991, which lasted only a few seconds. The fusion reaction still does not persist long enough to generate more electricity than it consumes. “It’s like making a fire with wet wood,” compares Galvão. “Just as you have to overcome the moisture so that the energy of the fire sustains combustion of the wood, the helium nuclei must remain in the plasma long enough so that the energy from the reaction keeps the plasma hot. For this to happen, you need to increase the size of the plasma “bonfire.”
Galvão’s team has been collaborating with JET on the research project since 2011. It was begun by the Swiss and by MIT to study a type of wave that propagates in the plasma used in fusion, called Alfvén waves. The greatest concern is the waves produced by the movement of the helium nuclei (alpha particles) created during fusion. No one knows for sure how long these waves last and how much they could disrupt the continuity of reactions in the torus. When exciting Alfvén waves, alpha particles lose energy, cooling the plasma and hindering the continuity of fusion reactions. Researchers produce Alfvén waves using radiofrequency waves in the plasma. Since 2009, the Swiss team from EFPL has been working on improvements in a system with eight radio antennas inside the torus that serve both to generate Alfvén waves and detect them. But the USP researchers noticed a serious problem with the system. The radiofrequency signal supplied to the antennas was generated by a commercial amplifier via a 100-meter-long cable. The simulations and tests performed by Galvão’s team found that, when varying the frequency, the natural resonances of the transmission line caused high-amplitude reflected signals that forced the amplifier to shut down.
“The old system had no way to feed the antennas,” explains Ukrainian physicist Leonid Ruchko who has worked with Galvão in Brazil since 1995. Based on solutions he created for Alfvén wave experiments the Brazilian team had been carrying out at the USP tokamak in São Paulo, Ruchko created a new amplifier concept—high-speed transistor-based and able to amplify a band of radio frequencies from 10 kHz to 1 MHz. The speed and robustness of the amplifier prevent it from being affected by high-voltage reflected pulses. “It has good protection against reflection,” says the physicist. Ruchko proposed that each of the eight antennas be supplied by an amplifier. Each would generate a short, precise wave pulse. Controlling the shape and duration of these pulses via computer, they could be combined to produce Alfvén waves with the desired properties within the plasma.
After Ruchko’s concept and the prototype built by the USP team were approved, they began to search for a company. The result was the approval of Politron and project funding from FAPESP and the Brazilian Innovation Agency (Finep) of the Ministry of Science, Technology and Innovation. The total cost of the equipment was R$150,000. “Making Leonid’s design operational, allowing us to operate the equipment correctly, immune to reflected signals, was exceptional. It is this robustness that’s worth a patent,” says Santos.
Center of excellence in plasma physics and applications–FAPESP-MCT/CNPq-pronex-2011 (nº 2011/50773-0); Grant Mechanism Thematic Project; Coord. Ricardo Galvão, USP; Investment R$ 1,633,433.66 and $ 705,552.82 (FAPESP).