|Posted by Craig on July 4, 2011 at 9:48 AM|
This follows on from my fusion post.
After looking at some Polywell updates, I've come to a slightly better understanding of the physics and their progress rates. Also, I've come up with some environmental impacts of widespread fusion proliferation.
The Polywell, like I said in my previous post, started with Farnsworth-Hirsch Fusors. Now, Farnsworth was in fact one of the engineers credited with the invention of Television. His fusor design is very similar in concept to the old CRT televisions.
Now, for those who can't remember the times when TV's were large and pyramid-shaped (instead of the sleek, sexy flat-screens), CRT stands for Cathode-Ray Tube. In the 1890s, a British physicist named J. J. Thompson had an idea to suck the air out of a glass tube, and stick two electrodes in there to see if empty space could conduct electricity. What he ended up discovering was something he called Cathode Rays. It turns out, these were in fact electrons he had discovered. He determined they had mass and charge. Lo! and Behold! about thirty years later, we have Television.
CRTs work like this: one of the electrodes becomes highly negatively-charged when a current is applied to the tube (as electrons build up on it), while the other electrode becomes highly positively charged (as electrons are stripped away). Eventually, the difference in charge causes the electrons at the negatively-charged electrode to jump across the gap and become attracted to the positively-charged electrode. The former is called the cathode, the latter the anode. This is high-school level physics, and it's relevant to fusion as well.
Back to Farnsworth. The guy had the idea that if CRTs accelerated electrons, could a similar technique be used to accelerate positively-charged protons. Consider this: remember the equation to calculate the kinetic energy of a particle at a certain temperature? Now, if you know the mass of a particle, you can work out its kinetic energy from simple motion. If v is the particle's velocity, and m is its mass, then its kinetic energy Ek is given by:
Okay, now suppose that the kinetic energy from heating is the same as the energy from moving (a reasonable assumption, since when dealing with tiny particles, temperature is only the macroscopic effect of their high-speed movement). Solve the resulting equation, and you get:
Say you've got a proton travelling at 1 kilometre per second. Plug the proton mass and Boltzmann Constant into the equation, and you get an equivalent temperature of around 40 Kelvins. That's not much, but the temperature will rise quadratically with velocity. Check out the chart below. You get to around room temperature with a velocity of 2.5 km/s.
In a CRT tube, the electrons can get to huge speeds, since there's no air, and the particles are extremely light. If you calculate the necessary temperatures to fuse two protons from the equations in the previous post, you'll find that the necessary velocity is a verysmall fraction of the speed of light, which is bloody easy to achieve. And Farnsworth said to himself, "Why not do the same with protons, and fuse them?"
So he built a device that has an electrical grid designed to confine plasma inside its electric field. Then there's another grid inside that one, negatively charged, which serves as a cathode attracting positively charged particles. The protons are accelerated toward the grid, meet in the centre, and fuse. It's actually pretty easy to do this, and it's a common high-school project.
The main problem with Farnsworth's fusor is that the protons collide with the grid and lose far too much energy to ever achieve break-even energy production or ignition. The Polywell seeks to solve that problem. Instead of having a cathode in the form of a negatively-charged grid, the coils generate a magnetic field that causes electrons to pool in the centre. This creates something called a virtual cathode. In other words, it's negatively charged air. The protons are attracted to the core, but don't have a grid to collide with. And when they hit the electrons, they have so much energy that the electrons bounce off them (or more like glance off). This does result in a loss of energy, due to Bremsstralung (Breaking) Radiation, which is given off by the protons as they interact with electrons. But this isn't nearly as big a loss as grid collisions.
So, now we can see that there is merit to the Polywell. But it's a little early to be optimistic. Remember I said fusors lost too much energy to grid collisions? Well, while there may be much less energy loss due to Breaking Radiation, we still need to make sure that the gains from the reactions can make up the loss. It may be that the losses are still too great to achieve break-even production. That is one of the other reasons EMC2 and the U.S. Navy are taking it slow on a shoestring budget. ITER is run by the governments of the world, primarily (I believe) as a publicity stunt. That's why they're taking such a massive step in constructing a reactor the size of the Empire State Building. The Polywell, on the other hand, is focused on smaller-level applications, like powering submarines. There's no interest in publicity. Moreover, expending so much money on a concept that might not be fully developed (like the US$200M required for a test Polywell reactor) could end up being a waste. So they're taking their time. EMC2 is cautiously optimistic that it will work, stating "There's no reason why it shouldn't." Having just completed a finance subject, I can understand and appreciate that position. Even though I don't really have an advanced degree in Physics, or even do too well in the subject in my undergraduate, I do feel confident that the Polywell will work. All it'll take is time and money.
Having said that, one thing I seriously hope the proponents of fusion (and other green tech) are considering is environmental impacts. Just because it's green or has no carbon footprint, it doesn't mean it won't have some other impact that's just as serious as Global Warming (and all the GW naysayers reading this, you're idiots). I actually conceived of this idea while running a thought experiment of the industrial process of a Polywell Reactor's operation. Let's recap: You have your fusion fuel, Hydrogen. The reactor takes this, and pumps out Helium. It actually uses the product to generate electricity in a highly efficient manner. It's also very safe, in that if the reaction chamber is breached, the worst that could happen is a fire from the plasma (and it's not like that hasn't happened in the past anyway). There's no risk of fallout either. The Helium products can then be shipped away to be used in MRI machines, blimps, and kids' birthday parties. Here is where I detect an environmental drawback. It lies in the current wasteful usage of Helium. Since the element is so cheep and abundant - an abundance made even greater by the use of fusion reactors - society uses it frivolously. We breathe it in to make our voices funny, we give balloons full of it to kids who almost immediately let it fly into the sky, we might even accidentally let it leak out of a tank somewhere. Due to its non-toxicity, it is released into the atmosphere with little thought. It has no environmental impacts, and does not interfere with Ozone, like CFCs do. But when it reaches the atmosphere, it escapes into space and cannot be recovered.
Can you see the point I'm trying to make? We take Hydrogen from water, Boron from the earth, fuse it into Helium, and then let it waft away into space, never to be seen again. By this process, the Earth would quite literally evaporate. By not treating Helium carefully or finding a way to convert it into other useful compounds like carbon or oxygen (by further fusion reactions - that's possible), we will in fact do more harm to the planet than Fossil Fuels ever could.
I'm not saying that we should give up on this. Don't misunderstand me. The point I am trying to make is that we must anticipate these problems now and build not only infrastructure to solve them, but the correct attitudes as well. The problem we have with Fossil Fuels now is because we reached a point where we can't live without them, and only then did our science realise the negative impacts it was having on our planet. We are now in a very unique position to make a change. But we have to do everything in our power to make sure we handle these issues in the correct way. The way to counteract the Evaporating Earth Hypothesis - if it turns out to be correct - is to simply continue the fusion reactions further until we get other useful compounds out of it, like Lithium and Carbon.
There. I presented a problem and the solution, thereby circumventing the mistake made when the Industrial Revolution started. We need to challenge our imaginations to find all the possible problems and impacts associated with emerging technologies, and develop simple solutions. Then, we can enact the fruits of our labour, and get on track to a cleaner human society.