Alumni Linnea Hesslow
​​Photo: Anna-Lena Lundqvist

Accelerated start to research career

Invited as a speaker at renowned conferences, requests for collaboration and major international media coverage – after only one published article. Linnea Hesslow has kick-started her research career in a spectacular fashion. 
Imagine being able to produce large quantities of energy regardless of wind or weather, without either carbon dioxide emissions or hazardous radioactive waste. This dream of fusion power has attracted researchers since the 1940s. But technically it is an extremely difficult challenge. 
- “Runaway electrons inside the fusion reactor are one of five or six major challenges that have to be sorted out before fusion power can become a reality,” says Hesslow, who is a doctoral student in the Division of Subatomic and Plasma Physics. 

Theoretical physics keeps her very happy. It provides tricky problem-solving – which she has been attracted to since she was a child – combined with insights into how the world fundamentally works.
- “The eureka moment when you gain such an insight is hard to beat for me. It gives the feeling of understanding something deep and fundamental about the world,” she says. 
After completing her undergraduate studies Hesslow realised that it was essentially impossible for her to find a job in industry with high enough levels of theoretical physics for her taste. Instead she headed for a post as a doctoral student with Chalmers’ fusion researchers. This was where there was interesting physics to get her teeth into, and the opportunity to help solve the world’s energy problems provided an extra sense of purpose. 

Fusion power involves mimicking the energy processes of the sun and stars: small, light atomic nuclei are fused into heavier atomic nuclei, releasing considerable quantities of energy in the process. But it is difficult to create the right conditions on the earth, and no fusion plant has yet managed to produce more energy than is required to operate it. 
- “There has been too little investment for fusion to be able to solve the climate change issue here and now, but in the longer term I believe that fusion can become a commercial energy source,” says Hesslow.

In order for fusion to take place the fuel, usually various types of hydrogen, needs to be at a heat of more than a hundred million degrees. At such high temperatures the fuel forms what is known as a plasma in which electrons and atomic nuclei are no longer bound to one another. 

Designing containers that can withstand the high heat levels is a challenge. The most highly developed concept is the tokamak, a toroidal chamber in which the plasma is kept suspended by means of magnetic fields and a huge electric current inside the plasma. 
  But if something goes slightly wrong the plasma can become unstable and rapidly start to cool. The current in the plasma then gives rise to runaway electrons, which can melt the reactor walls with their high speeds. There is certainly no danger to the general public, but it can result in a shutdown for repair lasting several months. 
  The most promising method of decelerating the runaway electrons is to spray heavy substances such as argon or neon into the reactor. But precisely how much to decelerate the electrons was something that nobody had worked out. At least not until Hesslow and her colleagues got their teeth into the problem. 
  Using theory and methods from different branches of physics they succeeded in developing a model of the way in which the electrons were affected by the injected materials. 
- “The model can now be used to calculate how much argon or neon needs to be injected to decelerate the runaway electrons. This is important knowledge that will be needed in future large-scale fusion experiments,” explains her supervisor Tünde Fülöp, a professor of theoretical plasma physics. 

The work was published in the well-respected journal Physical Review Letters, and Chalmers sent out a press release. The attention which followed exceeded their expectations by a wide margin. The news spread like wildfire in both the Swedish and international media and, at her very first scientific conference ever, the renowned Sherwood Fusion Theory Conference in the USA, Hesslow was one of only twelve speakers invited from the entire world. 
- “This is incredibly good visibility for both her, the research team and our entire research field,” Fülöp says. 

Being invited as a speaker is a milestone that normally comes later in someone’s career. But then Hesslow seems to be a very talented young researcher. 
- “She stands out by being incredibly quick to see what is important, both in the research and in other connections. In addition, she is incredibly well organised and gets things done,” says Fülöp.

Hesslow has presented her work at a number of conferences and enjoys being in front of an audience. The response and interest from other researchers has been significant – many want to collaborate and compare the model with experiments.
- “At present we have visitors from the Czech tokamak Compass. We are comparing their measurement data with our model. It will be exciting to see if it corresponds,” Hesslow says.
She has obviously managed, after only her first publication, to get her name into the public consciousness – it is even possible that researchers will call the electron deceleration model the Hesslow model. And she finds it hard to imagine a more enjoyable job than research. 
- “It’s really exciting, I learn plenty of different things and have fun at work with great colleagues. It has the feel of luxury,” says Hesslow.​

Main challenges of fusion power
The large ITER tokamak, which should be capable of providing more energy than is supplied to it, is scheduled to be ready to go into operation in France in 2025. But there are still a number of problems to solve before fusion power can become a reality. According to Linnea Hesslow the greatest challenges are to:
Develop materials that are better able to withstand the strong thermal radiation from fusion plasma
Develop better superconducting magnets which will hold the fusion plasma in place
Produce the fusion fuel tritium by allowing neutrons from the reactor to bombard a lithium blanket
Reach a condition in which the reactor can be run continuously instead of in short pulses
Optimise the design and running of the reactor for better efficiency
Manage, or prevent, the plasma from collapsing – this includes decelerating runaway electrons.​

Text: Ingela Roos
Article from Chalmers Magasin issue 1, 2018.​

Published: Mon 20 Aug 2018.