Deceleration of runaway electrons paves the way for fusion power

Fusion power has the potential to provide clean and safe energy that is free from carbon dioxide emissions. However, imitating the solar energy process is a difficult task to achieve. Two young plasma physicists at Chalmers University of Technology have now taken us one step closer to a functional fusion reactor. Their model could lead to better methods for decelerating the runaway electrons, which could destroy a future reactor without warning.

"It takes high pressure and temperatures of about 150 million degrees to get atoms to combine. As if that was not enough, runaway electrons are wreaking havoc in the fusion reactors that are currently being developed. In the promising reactor type tokamak, unwanted electric fields could jeopardise the entire process. Electrons with extremely high energy can suddenly accelerate to speeds so high that they destroy the reactor wall.

They have been able to show that it is possible to effectively decelerate runaway electrons by injecting so-called heavy ions in the form of gas or pellets. For example, neon or argon can be used as "brakes".

Trots att vakuumkammaren i den brittiska fusionsreaktorn JET har en vigg som ir gjord av solid metall, kan den smilta om den triffas av en strile av skenande elektroner.

When the electrons collide with the high charge in the nuclei of the ions, they encounter resistance and lose speed. The many collisions make the speed controllable and enable the fusion process to continue. Using mathematical descriptions and plasma simulations, it is possible to predict the electrons' energy - and how it changes under different conditions.

Facts: Fusion energy and runaway electrons

Fusion energy occurs when light atomic nuclei are combined using high pressure and extremely high temperatures of about 150 million degrees Celsius. The energy is created the same way as in the sun, and the process can also be called hydrogen power. Fusion power is a much safer alternative than nuclear power, which is based on the splitting (fission) of heavy atoms. If something goes wrong in a fusion reactor, the entire process stops and it grows cold. Unlike with a nuclear accident, there is no risk of the surrounding environment being affected. The fuel in a fusion reactor weighs no more than a stamp, and the raw materials come from ordinary seawater.

As yet, fusion reactors have not been able to produce more energy than they are supplied. There is also a problem with so-called runaway electrons. The most common method of preventing this damage is to inject heavy ions, such as argon or neon, which act like brakes due to their large charge. A new model developed by researchers at Chalmers describes how much the electrons are decelerated, paving the way to making these runaway electrons harmles

Abstract

We analyze the dynamics of fast electrons in plasmas containing partially ionized impurity atoms, where the screening effect of bound electrons must be included. We derive analytical expressions for the deflection and slowing-down frequencies, and show that they are increased significantly compared to the results obtained with complete screening, already at subrelativistic electron energies. Furthermore, we show that the modifications to the deflection and slowing down frequencies are of equal importance in describing the runaway current evolution. Our results greatly affect fast-electron dynamics and have important implications, e.g., for the efficacy of mitigation strategies for runaway electrons in tokamak devices, and energy loss during relativistic breakdown in atmospheric discharges.
"When we can effectively decelerate runaway electrons, we are one step closer to a functional fusion reactor. Considering there are so few options for solving the world's growing energy needs in a sustainable way, fusion energy is incredibly exciting since it takes its fuel from ordinary seawater," says Linnea Hesslow.