Fast ions in fusion reactors andPlasma waves carry out complex energy transfer, in which resonance and collision impact play an important role. This understanding is critical to maintaining optimal plasma temperatures and advancing fusion energy technology.Just like there are waves in the ocean, waves are created in a charged gas made of electrons and ions, called a plasma. In the ocean, when people surf, the speed of their surfboard is almost the same as the speed of the waves. This matching condition, known as resonance, allows the waves to effectively propel the surfer through an exchange of energy.

Schematic illustration of the interaction of fast ions (black spirals) with plasma waves (color) in a nuclear fusion experiment. Source: Steve Allen (Lawrence Livermore National Laboratory), adapted by Mike Van Zeeland (General Atomics)

In plasmas, "surfers" may be very fast ions that may appear in nuclear fusion devices as a result of fusion reactions or other processes used to heat the plasma. These fast ions often do the opposite of what surfers do in the ocean - they provide energy to the waves, making them bigger. As the resonating particles exchange energy with the waves, they are also squeezed by other particles in the plasma through random collisions.

The type and frequency of these collisions determines the size of the waves and how much the particles shake. If the waves are too big or excessive, they can kick surf particles out of the device, potentially causing danger to the walls, while also reducing the amount of fusion energy produced.

Fusion reactor challenges

The plasma in a fusion reactor must be constantly heated to maintain the temperature needed to produce energy. However, the fast ions that heat the plasma also resonate with the waves in the plasma. This causes these waves to grow and potentially kick fast ions out of the device.

Researchers need to understand the resonant interactions between fast ions and plasma waves to predict and mitigate any adverse effects. The study combines mathematical calculations with computer simulations to reveal how different types of collisions compete to determine how energy is transferred between resonating particles and plasma waves.

Researchers are using this new understanding to develop models of how to keep plasma hot enough to sustain nuclear fusion reactions. The problem of resonant wave-particle plasma is also related to certain gravitational interactions in galaxies. This means the project's methods could be applied to astrophysics research, including dark matter research.

Understanding fast ion collisions

In nuclear fusion experiments, fast ions transfer their energy to the background plasma by colliding with electrons, thereby keeping the plasma hot enough for nuclear fusion. There are two different types of collisions: diffusive scattering and convective drag. Diffusion collisions are the same type of scattering from billiard balls on a pool table. At the same time, you feel the drag bump as you stick your hand out the window of a moving car.

Depending on the speed of the fast ions and the temperature of the plasma, each collision will have a greater impact on the behavior of the fast ions. Specifically, the greater the speed of fast ions, the greater the resistance, and the higher the plasma temperature, the more favorable diffusion is.

While fast ions heat the background plasma through collisions, they also resonate with plasma waves that dissipate their energy, potentially cooling the plasma. In the absence of any collisions, resonance between fast ions and waves only occurs when the speed of the particle exactly matches the speed of the wave.

Scientists have long known that diffusive collisions work to "erase" resonances, effectively exchanging energy with the wave even if the particles move slightly faster or slower than the wave. The new finding of this study is that when drag is present, this collision changes the speed at which resonance occurs, meaning that energy exchange is actually most efficient when the speeds of fast ions and plasma waves are very different.

The role of resonance function

In the study, the researchers characterized the strength of wave-particle interactions using a mathematical object called a resonance function, which depends on the difference between wave speed and particle speed.

When drag collisions occur more frequently than diffusion collisions, an even stranger phenomenon occurs - efficient energy transfer becomes possible at entirely new speeds. This phenomenon actually creates new resonances that would not exist at all without resistance, manifesting itself as new peaks in the resonance function and expanding the range of resonant interactions.

The resonance function, which is derived entirely from theory, determines how big the waves will become after free energy is harvested from the resonating fast ions, and also determines how these particles will be kicked around by the waves. Nonlinear computer simulation results are in excellent agreement with theoretical predictions, confirming that the derived resonance function is valid for any combination of the two collisions, and deepening our fundamental understanding of how collisions affect the interaction of resonant waves with particles in plasmas.

With the basic theory validated, it can now be confidently used to improve codes used to simulate the behavior of fast ions in fusion devices, a critical step on the road to developing commercial fusion power plants.

Compiled from /ScitechDaily