This is a nuclear fusion reactor, scientists and engineers use theories and fundamental
laws of physics to ensure the reactor will not only work and run safely, but also operate
at its maximum potential with the overall aim of self-sustaining and commercial nuclear
fusion.
These theories and laws provide limits that the reactor must stick to.
However, a recent discovery has redefined one of these limits that was thought to be
set by fundamental laws of physics back in 1988.
This discovery is huge for nuclear fusion as it could double the energy in the fusion
reactor.
This video will explain this discovery and then show what this could mean for nuclear
fusion in the future.
When designing a nuclear reactor, we can simplify things down to one main objective.
To explain that, let's recap the basics of a nuclear reactor, which in this case will
be the common tokamak reactor.
Tokamaks use superconductive electromagnets to confine, or squeeze together, extremely
hot plasma into a donut shape.
In our case, this plasma contains deuterium and tritium ions, which are the fuel for nuclear
fusion.
The aim for fusion is to get these to collide with enough energy that they fuse together
to create a helium nucleus and in the process, release energy.
That main objective I just mentioned is then to optimise something known as the triple
product.
This triple product is a combination of the density of ions in the plasma, the plasma
temperature, and the energy confinement time.
The best temperature for our type of nuclear fusion between deuterium and tritium is about
100 millions degrees celsius, or 5 times the sun’s core.
With the temperature set, we now want to reach the best triple product we can by increasing
the ion or fuel density of the plasma, or by increasing the energy confinement time.
In this video, we will be focussing on the ion density.
So ideally we would increase this indefinitely.
However, there are laws of physics at play that mean there is a limit to the density
that can be achieved before the plasma collapses and fusion stops altogether.
In the 1980’s, this was a hot topic of research, and in 1988, Martin Greenwald provided an
equation which explained the limit of ion density in the plasma.
This limit is known as the Greenwald limit and has been used for designing the largest
and most complex fusion reactors to this day.
The Greenwald limit states that maximum ion density is equal to the current in the plasma
divided by the area inside the plasma ring.
But, this had a problem.
This law was derived solely from experimental data.
This means now that fusion reactors are getting more powerful than ever before, certain relationships
are at play that Martin Greenwald was unable to see.
This is where the new discovery that could have huge implications for nuclear fusion
comes in.
Published on the 6th of May by ‪Maurizio Giacomin and his team, a new equation describing
the limit for ion density has been proposed.
Instead of using experimental data to find this, the equation is based on first principles
and includes everything we know about tokamak fusion reactors.
While this equation looks pretty complex, the important finding is this.
This crucial part of the equation means that as more power is used to heat the plasma,
the density limit actually increases.
So for modern high powered tokamak reactors, the ion density can be much higher than previously
thought.
But why is this the case, and what are the impacts of this in practice?
The reason we have a density limit is because the plasma ring will collapse if it gets too
high.
As we increase the density the plasma on the outside cools down, and because cold plasma
is less dense than hot plasma, it causes pressure to build, disrupting the magnetically confined
plasma.
However, if we have a lot of power going into the reactor to heat the outer edge, we can
prevent the outer edge getting too cold, therefore minimising the outward pressure, allowing
us to increase the density much more than previously thought.
So, what does this mean in practice for fusion energy?
Is it finally less than 30 years away?
‪Maurizio and his team have applied this new density limit to the ITER reactor, an
international nuclear fusion research and engineering megaproject that will be the largest
tokamak ever made, and the impact is pretty incredible.
They found that due to the huge amount of power that will be used to heat the plasma,
the ion fuel density can actually be twice what Martin Greenwald's limit predicts, therefore
allowing 200% of the predicted energy to be held within the plasma.
If we look back to the triple product, we can plot a useful graph for comparison.
The y-axis is the triple product and the x-axis is our temperature.
With double the ion density, the ITER project could have double the triple product.
As the y-axis is logarithmic, this causes the red dot to shift from here to here.
This is a huge breakthrough as anything within the blue shaded area is supposedly viable
for a power plant using deuterium and tritium as fuel.
This means ITER would be further into this area and shows commercial power from fusion
is becoming ever closer to a reality.
However, as ITER has already been designed using the Greenwald limit, it is unlikely
to see the benefits from this breakthrough.
But thankfully there is a new big project being developed called DEMO, this is the planned
successor to ITER and it could still account for this new limit in its design.
DEMO will be even more powerful, so this new limit is highly relevant, and the plan is
to actually extract power from it to power homes for the first time using fusion power.
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nuclear lab.