Tuesday, 21 March 2017

What's happening in Fusion? Part 2

Written by Sam Ha

In case people missed part 1, these posts are a small collection of questions and answers about what's happening in the world of fusion, especially looking to the future. Everyone is welcome to ask questions below, and I'll put together a Q&A post if there are enough questions and answers. Without further ado, the first question:

Q: So Why Is It Taking So Long?
This one is a classic and there are so many bits of the answer, but I'm going to try and keep this answer short.

The systems that we expect to be able to generate significant amounts of fusion power reliably are (by and large) magnetically confined fusion devices (MCF). What this means is that we turn a small amount of gas (generally two types of hydrogen) into the 4th state of matter: Plasma.

Plasma is a completely ionised gas and as such, can be controlled by magnetic fields. Early experiments in nuclear fusion (like these!) used simple magnetic fields to confine plasma.

Several problems that hinder our plans of fusion electricity have been discovered, but some of the most difficult have been due to confining plasma.

The plasma that we generate stores a huge amount of energy that must be well contained. The problem is that the plasma becomes very difficult to contain and over the years many types of plasma instabilities have been discovered. These instabilities cause the plasma to escape our magnetic trap and collide with the walls. When this happens there is a considerable amount of damage. To the plasma, of course. The walls can also be damaged, but there's several hundred kg's of the wall and only a couple of grams of plasma, and the plasma always comes off worst in a fight.

Therein lies one of the problems with fusion: plasma is very difficult to control. The physics behind these instabilities are also very complicated, but there are a number of experiments being carried out around the world that are investigating how these instabilities arise and what we could do to get rid of them.

However, we have made some good progress in the meantime: a common measure of how effective a fusion device is, is something called the Triple Product. This is the go-to way of measuring the performance of fusion devices and we've been able to improve the triple product of machines at an astonishing rate, even outpacing the rapid development of computer chips (often called Moore's Law)

Q: Tokamaks are cool and all, but what about Stellarators or Laser Fusion?
Stellarators were initially researched alongside tokamaks in the early days, mainly in America. The design of a stellarator was a strike of brilliance to counteract particles drifting out of their orbit around the torus, by way of some clever arrangement of the magnetic fields. 

In comparison, tokamaks require the plasma to carry a current, which is most commonly achieved by one, large magnet in the centre: the central solenoid. In many cases, this is a limitation that forces tokamaks to be pulsed machines, requiring regular stops and starts. For fusion research to this point, the limitations of short pulses (<1 minute) hasn't stopped developments, but in the future, continuous operation will be much more valuable. Regularly stopping and starting powerful machinery is a good way to break it quickly!

The initial trouble of designing stellarators without powerful computers and the good results from tokamaks led to the majority of effort being placed on tokamaks. Now, stellarators (like Wendelstein 7-X) are seeing a resurgence as the design of large, complex reactors has become possible thanks to significant increases in computational power.
And now to talk about Laser Fusion! This is a form of Inertial Confinement Fusion (ICF) and fuses hydrogen pellets by imploding them on all sides using really powerful lasers (really cool, all around). The American National Ignition Facility (NIF) excels at this.

NIF recently announced they had reached an important milestone never before seen: they achieved breakeven conditions during a controlled reaction. The main goal of ITER is to reach breakeven conditions in an MCF device, so one thing is definitely clear: this is a monumental achievement. 

What does it mean for producing a fusion power plant? Well, that's a little harder to answer. Firstly, the breakeven conditions are considering the energy absorbed by the small, hydrogen pellet, which is <1% of the total laser pulse energy. Secondly, to generate significant amounts of energy, a regular reaction rate is required, while NIF aims to have 400 pulses in the entire year. 

The road to a fusion power plant using lasers isn't a smooth one and I'm eager to see the achievements from NIF. I'm not expecting to see a laser powered fusion power plant for a similarly long time to a tokamak power plant.

Q: What about after DEMO?
DEMO is the Demonstration Power Plant, designed to prove the viability of a tokamak power plant putting Fusion electricity on the grid (see Part 1 for more details). DEMO isn't the end of the road for fusion research, in the same way that the first fission power plant wasn't the end of the road for fission research. There are advanced reactor designs that have gaps that need to be filled in by ITER and DEMO, such as radiation tolerant materials.

One area that I'm very excited about is the design of so-called hybrid reactors. These reactors find a harmony between fusion and fission, to make a fusion reactor that can get rid of waste from and make new fuel for fission reactors, while reducing the complexity and difficulty involved in making a fusion reactor. Designs of these types of reactors can't be properly evaluated and developed for manufacture until a fusion reactor is up and running, so that they can be demonstrated to be safe to a regulatory body. Hybrid reactors are not a focus for UKAEA, or in the UK in general, but are being seriously investigated in other countries.

Next Time
Questions from readers!

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