The big news recently as reported in Nature (13 December) was that researchers at the US National Ignition Facility carried out a reaction that made more energy than was put in. That looks great, right? The energy crisis is solved. Not so fast. What actually happened was that 192 lasers delivered 2.05 MJ of energy onto a pea-sized gold cylinder containing a frozen pellet of deuterium and tritium. The reason for all the lasers was to ensure that the input energy was symmetrical, and that caused the capsule to collapse under pressures and temperatures only seen in stars, and thermonuclear weapons. The hydrogen isotopes fused into helium, releasing additional energy and creating a cascade of fusion reactions. The laboratory claimed the reaction released 3.15 MJ of energy, roughly 54% more than was delivered by the lasers, and double the previous record of 1.3 MJ.
Unfortunately, the situation is a little less rosy than that might appear. While the actual reaction was a net energy producer based on the energy input to the hydrogen, the lasers were consuming power even when not firing at the hydrogen, and between start-up and shut-down they consumed 322 MJ of energy. So while more energy came out of the target than went in to compress it, if we count the additional energy consumed elsewhere but necessary to do the experiment, then slightly less than 1% of what went in came out. That is not such a roaring success. However, before we get critical, the setup was not designed to produce power. Rather it was designed to produce data to better understand what is required to achieve fusion. That is the platitudinal answer. The real reason was to help nuclear weapons scientists understand what happens with the intense heat and pressure of a fusion reaction. So the first question is, “What next?” Weapons research, or contribute towards fusion energy for peaceful purposes?
Ther next question is, will this approach contribute to an energy program. If we stop and think, the gold pellet of frozen deuterium had to be inserted, then everything line up for a concentrated burst. You get a burst of heat, but we still only got 3 MJ of heat. You may be quite fortunate to convert that to 1 MJ of electricity. Now, if it takes, say, a thousand second before you can fire up the next capsule, you have 1 kW of power. Would what you sell that for pay for the gold capsule consumption?
That raises the question, how do you convert the heat to electricity? The most common answer offered appears to be to use it to boil water and use the steam to drive a turbine. A smarter way might be to use magnetohydrodynamics. The concept is the hot gas is made to generate a high velocity plasma, and as that is slowed down, the kinetic energy of the plasma is converted to electricity. The Russians tried to make electricity this way by burning coal in oxygen to make a plasma at about 4000 degrees K. The theoretical maximum energy U is given by
U = (T – T*)/T
where T is the maximum temperature and T* is the temperature when the plasma degrades and the extraction of further electricity is impossible. As you can see, it was possible to get approximately 60% energy conversion. Ultimately, this power source failed, mainly because the cola produces a slag which damaged the electrodes. In theory, the energy could be drawn in almost 100 % efficiency.
Once the recovery of energy is solved, there remains then problem of increasing the burn rate. Waiting for everything to cool down then adding an additional pellet cannot work, but expecting a pellet of hydrogen to remain in the condensed form when inserted into a temperature of, say, a million degrees, is asking a lot.
This will be my last post for the year, so let me wish you all a Very Merry Christmas, and a prosperous and successful New Year. I shall post again in mid-January, after a summer vacation.
Meanwhile, for any who fell they have an interest in physics, in the Facebook Theoretical Physics group, I am posting a series that demonstrates why this year’s Nobel Prize was wrongly assigned as Alain Aspect did not demonstrate violations of Bell’s inequality. Your challenge, for the Christmas period, is to prove me wrong and stand up for the Swedish Academy. If it is too difficult to find, I may post the sequence here if there were interest.