Heating plasma to the ultra-high temperatures needed for fusion reactions new techniques. Researchers have considered multiple methods, one of which involves injecting electromagnetic heating waves into the plasma. This is basically the same process that heats food in microwave ovens. However, when they produce one type of heating wave, they can sometimes simultaneously create another type of wave that does not heat the plasma. This is a waste of energy.
To solve this problem, scientists at the U.S. Department of Energy’s (DoE) Princeton Plasma Physics Laboratory (PPPL) have performed computer simulations. They have developed a new technique that prevents the production of the unhelpful waves, known as slow modes. This boosts the heat put into the plasma and increases the efficiency of the fusion reactions.
Eun-Hwa Kim is a PPPL principal research physicist and lead author of the paper reporting the results in Physics of Plasmas. He said, “This is the first time scientists have used 2D computer simulations to explore how to reduce slow modes. The results could lead to more efficient plasma heating and possibly an easier path to fusion energy.”
The team included researchers from General Atomics who use the DIII-D tokamak fusion facility. They determined that positioning a metal grate known as a Faraday screen at a slight five-degree slant with respect to the antenna producing the heating waves (which are also known as helicon waves) stops the production of the slow modes. Researchers want to avoid creating slow modes because they cannot penetrate the magnetic field lines confining the plasma to heat the core. This is where most fusion reactions occur. In addition, the slow modes are easily damped or cancelled out by the plasma itself. Any energy used to create slow modes is energy that is not used to heat the plasma and foster fusion reactions.
The researchers simulated the production of helicon waves and slow modes using the Petra-M computer code. This is a powerful and versatile program used to model electromagnetic waves in fusion devices and space plasmas. The simulations replicated conditions in the DIII-D tokamak which is a doughnut-shaped plasma device operated by General Atomics for the DoE.
The team carried out a series of virtual experiments to test which of the following methods had the greatest effect on the production of slow modes: the antenna’s alignment, the Faraday screen’s alignment or the density of electrons in front of the antenna. The simulations confirmed that when the Faraday screen was aligned at an angle of five degrees or less from the orientation of the antenna, the screen, in effect, short-circuits the slow modes, making them dissipate before they propagate into the plasma. The suppression of the slow modes depends mainly on how much the Faraday screen leans to the side.
Masayuki Ono is a PPPL Principal Research Physicist and one of the paper’s authors. He said, “We found that when the screen’s orientation exceeds five degrees by only a little bit, the slow modes grow by a great deal. We were surprised by how sensitive the development of slow modes was to the screen alignment.” Scientists will be able to use this information to refine the design of new fusion facilities to make their heating more powerful and efficient.
In the future, the scientists plan to improve their understanding of how to prevent slow modes by running computer simulations that consider more of the plasma’s properties and factor in more information about the antenna.