Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have been working to develop various methods for managing plasma in order to use it to produce electricity through fusion. In this pursuit, they have demonstrated how to combine two old methods to increase flexibility.
Two methods for controlling plasma in fusion reactors—known as electron cyclotron current drive (ECCD) and applying resonant magnetic perturbations (RMP)—have been studied for a long time. However, this is the first instance where researchers have simulated the combined use of both methods to achieve improved plasma control.
“This is a relatively new concept,” said Qiming Hu, a staff research physicist at PPPL and the lead author of a new paper published in Nuclear Fusion about this work, which has also been experimentally demonstrated. “The full extent of its capabilities is still being determined, but our paper significantly advances our understanding of the potential advantages.”
In the end, scientists aim to utilize fusion to produce energy.e electricity. Initially, they must address various obstacles, such as refining techniques to minimize bursts of particles from the plasma known as edge-localized modes (ELMs).
“Periodically, these bursts release a little bit of pressure because it’s too much. But these bursts can be dangerous,” said Hu, who works for PPPL at the DIII-D National Fusion Facility, a DOE user facility hosted by General Atomics. DIII-D is a tokamak, a device that uses magnetic fields to confine a fusion plasma in a donut shape. ELMs can end a fusion reaction and even damage the tokamak, so researchers have developed many ways to try to avoid them.
The most effective method we have found to prevent these disruptions is by using resonant magnetic perturbations (RMPs) to produce extra magnetic fields,” explained PPPL Principal Research Physicist Alessandro Bortolon, who was one of the co-authors of the study.
Magnetic fields create islands, while microwaves adjust them
The initial magnetic fields produced by the tokamak wrap around the torus-shaped plasma in both the long direction – along the outer edge, and the short direction – from the outer edge to the center opening. The additional magnetic fields generated by the RMPs travel through the plasma, weaving in and out like.The fields created by the coils of the magnetic fusion device are similar to a sewer’s stitch. These fields form oval or circular magnetic fields in the plasma known as magnetic islands.
“Typically, islands in plasmas are very detrimental. If the islands become too large, they can disrupt the entire plasma,” stated a researcher.
However, it was already known from experiments that under specific conditions, the islands can have a positive effect. The challenge is to create Resonant Magnetic Perturbations (RMPs) large enough to create the islands. This is where Electron Cyclotron Current Drive (ECCD), a form of microwave beam injection, comes into play. The researchers discovered that adding ECCD to the edge of the plasma reduces the amount of current needed to generate the magnetic islands.the microwave beam injection also allowed the researchers to refine the size of the islands to achieve maximum plasma edge stability. In a metaphorical sense, the RMPs function as a basic light switch to activate the islands, while the ECCD serves as an additional dimmer switch to enable researchers to adjust the islands to the optimal size for manageable plasma. Hu stated that the simulation enhances our comprehension of the interactions at play. When the ECCD was introduced in the same direction as the current in the plasma, the width of the island decreased and the pedestal pressure increased.The results of applying ECCD in the opposite direction were different, with the width of the island increasing and the pressure of the pedestal dropping or helping to open the island.
The study is noteworthy because ECCD was introduced to the edge of the plasma rather than the core, where it is typically used.
“Typically, people believe that applying localized ECCD at the plasma edge is risky because the microwaves may cause damage to in-vessel components,” Hu said. “We have shown that it is feasible, and we have shown the adaptability of the approach. This could open up new possibilities for d
Reducing the current needed to create the RMPs could lower the cost of producing fusion energy in future commercial-scale fusion devices. The research received funding from the DOE under multiple award numbers, as well as support from the EUROfusion Enabling Research project for co-author Qingquan Yu.
The simulation work has the potential to impact the design of future fusion devices, making them more efficient and cost-effective. The funding from the DOE and the European Union has been instrumental in supporting this important research.
Journal Reference:
- Q.M. Hu, N.C. Logan, Q. Yu, A. Bortolon. Effects of edge-localized electron cyclotron current drive on edge-localized mode suppression by resonant magnetic perturbations in DIII-D. Nuclear Fusion, 2024; 64 (4): 046027 DOI: 10.1088/1741-4326/ad2ca8