This was a private joke amongst the aforementioned fusion scientists, until Prof Ben McMillan shared it in the Birmingham Café Scientifique last week and very much brought it into the public arena. I admit that it is not necessarily rip-roaringly funny in an obvious way but the entire evening was definitely one of the most humorous scientific discussions that I have been party to. I will try to capture some of this as I continue, but I will never be able to catch the deadpan delivery that Ben was able to achieve.

But let's start at the beginning and look at the basics of fusion reactors. Nuclear power currently comes from fission reactors in which large, heavy atoms such as uranium, decay to form smaller, lighter atoms and release heat in the process. We collect this heat though turbines and generators to produce electricity. In a fusion reactor, two small, light atoms are forced together until they form a single larger, heavier atom. The mass of the single, heavier atom is a little bit less than the combined mass of the two lighter atoms and in line with possibly the most famous equation in science (E=mc2) this mass loss is converted to a large amount of energy. The atoms used in fusion reactions are often forms of hydrogen called deuterium and tritium, which are heavier than normal hydrogen but still at the very small end of the atomic spectrum. Once these have been slammed together, they will form helium and an awful lot of energy.

The fusion process is not anything new and is certainly not something that man has introduced per se; nuclear fusion is the same process that occurs within stars and as such sustains all life on this planet. The neutrinos that you might have heard mentioned in the media, are all produced in the fusion reactions within our own Sun. They are sent flying across Space at the speed of light passing through everything in their path - including you and I. Fusion is also the same process that takes place in a hydrogen bomb to such devastating effect. This is not scaremongering, just an example of our current understanding of fusion. You might argue that if we can already generate these fusion reactions, in a bomb, why is it going to take 40 years to convert this into a useful energy source? And that is a very good question!

The promise of nuclear fusion as a potential energy source does indeed seem like a pie in the sky idea but the potential benefits are world changing. Finding the Higgs boson recently is indeed a major step forward in the understanding that mankind has of the physical world around us, but fusion reactors would actually change the life of each and every one of us because every one in the world could have an almost limitless supply of electricity without the bad stuff that goes with it currently - like climate change and toxic waste.

If we lived in a world where fusion reactors were the norm, we would not only have lots of electricity - there may even be a chance that it is cheaper too, but don't quote me on that, there will no doubt be some sort of tax that removes this. One of the waste products from fusion reactors is helium and for those of you that attended last month's Café, read the blog or are just wise in the ways of international helium stores, will know that we are undergoing a massive helium shortage. I am not talking about a threat to the life-sustaining balloon industry although my heart goes out to those manufacturers and distributors obviously, but liquid helium is vital for many applications that involve superconductors and just one of these is brain scanners. Helium is a very small molecule and as such is very difficult store - a bit like trying to keep flour in a sieve during a hurricane. This means that all of the helium that we currently have in the world, is slowly leaking away and we don't really have a way of making any more.

Another benefit is that the steel framework of the reactors slowly decays as a result of the nuclear bombardment that it is subjected to and turns into gold! Imagine that, the dream of alchemists is only 40 years away!

I am being semi-flippant here because there are some very serious reasons as to why fusion reactors are such great potential options for future energy sources. You can't help but acknowledge the more ear-catching points as well though. In a nutshell, fusion reactors can provide far more energy per unit amount of fuel than fission reactors. Not only does this mean you use less fuel but more importantly, you produce less toxic waste as a result. The waste from current nuclear reactors has to be processed or stored for hundreds and thousands of years which means some major foresight and planning - often into areas where we cannot really be sure of what the future holds, simply creating issues for many future generations. The waste from a fusion reactor is only toxic for 50 - 100 years, which although still quite a long time is much more manageable in comparison.

The fuels of fusion reactors (deuterium and tritium) are actually much more readily available than those of fission reactors (uranium and plutonium) and pretty much put them in the realms of renewable energy forms. Extending the renewable energy comparison, solar, wind and tidal power are all very expensive per unit of energy produced and all suffer from the same problem - the weather! I hear critics crying out at this point that solar energy is not being suggested as an energy source for the UK, but more for countries who can remember what the sun looks like! Ah yes, I answer, but what about once those prime spots have been taken and we still need more energy? When you start looking at 'second best' sites, you are also looking at 'second best' power outputs. For the UK, if we ever start relying on Scotland to produce our energy through solar power, I am emigrating! With fusion reactors, you can potentially build them anywhere and start reaping the benefits. There is only one small problem, we don't know how to build them!

We can make the reactions work and see the potential, but we can't make them work in a way that produces more energy than we have to put in to make the fusion reaction take place to begin with - or at least not in a controllable way. There are two basic methods of doing this, one is to fire several of the most powerful lasers known to man, all at the same single tiny point and force the atoms there to fuse together. Lasers though tend to be fairly energy-hungry things and although this method works in principle, i.e. makes things fuse, it is not really a way of providing cities with energy.

The more standard approach is to use something called a tokomak. This is a doughnut shaped device covered in extremely powerful superconductors and magnets. The hydrogen fuel is injected into the ring (or torus) and heated. As the temperature inside gets hotter and hotter, the hydrogen breaks down from a gas state into a plasma and eventually moves so fast and at such high temperatures that the atoms fuse together releasing large amounts of energy as heat. This heat will not only sustain the reaction conditions, but can also be used to produce electricity. The plasma inside the torus reaches temperatures on the scale of millions of degrees Celsius and this obviously represents problems for the engineers - how do you build something that can withstand temperatures hot enough to boil all known materials? This is partly where the magnets and superconductors come into the story. The magnetic field can not only be used to contain the plasma - a DIY demonstration of this is to put a strong magnet near a now old fashioned CRT television and watch the image wobble and change colour - but it also helps to heat up the reaction to begin with. The trouble is that to generate these magnetic fields takes very strong magnets and superconductors and these only work well as very low temperatures. This represents something of a paradox - in order to generate some of the hottest conditions in the Universe, you need to use equipment that is running at some of the coldest temperatures in the Universe and this is where the materials scientists are trying to make their living, by producing insulating materials that are up to the job.

The fusion reactor does work in principle and tokomaks do exist and can produce energy, but we currently cannot make one that produces enough energy to offset the cost of building them and the amount of energy required to make them run in the first place. But these are the sorts of challenges that become just a little bit easier each year as further advances are made. Whether it will actually be 40 years until we see viable reactors, is anyone's guess but I suspect they will appear one day. The latest effort is a joint international project in France called ITER. This is not being built as a proper power station but as the next generation of prototype. It is the version after that, ITER 2.0 perhaps, that is hoped to be an operational reactor.

The final point that I would like to make is one of safety. This is top of everyone's agenda, especially when it comes to nuclear power and fusion reactors should be treated no differently. There is a fundamental difference however between fusion and fission reactors. A fission reactor (the sort we have already) is in many respects trying to run away with itself; Diana Ross used to refer to this as a Chain Reaction! Left to itself, the splitting of nuclei would become rampant and the safety measures in place are there to permanently slow down the process and keep it under control. With a fusion reactor however, it is extremely difficult to maintain the reaction conditions and without constant input to sustain it, it would just fizzle out. This means that we could never have a Chernobyl-style incident with a fusion reactor. Many years ago, I did some work experience in a veterinary surgery and I remember one of the vets describing the life ethic of sheep as 'give them any excuse and they will die'. At the time I thought this was rather a strange phrase to use, but I was reminded of it the other night whilst talking about the fusion reactors. The conditions needed to sustain fusion are so extreme that given the slightest excuse they will collapse and fizzle out.

Yes, a lot of money is going to be needed to make this happen and there are some huge challenges ahead of us, but as Ben so eloquently put it, given the potential benefits of limitless electricity, is it not worth investigating?