What is the recent excitement about nuclear fusion all about? 

On December 13, the National Ignition Facility at Lawrence Livermore National Laboratory, US, announced that it had achieved a ‘net energy gain’ — got out more energy than was put in — by fusion. The input energy was 2.05 megajoules and the output was 3.15 MJ. This is unprecedented, hence the excitement. 

What was the experiment and how did the energy gain happen? 

They put in a mixture of Deuterium (D) and Tritium (T), which are isotopes (variants) of hydrogen, in a capsule the size of a grain of wheat and bombarded it with light from 192 lasers. This caused the electrons of D and T to come off their parent atoms; electrons and nuclei of D and T were floating about as a soup of matter — called ‘plasma’, which, like solid, liquid and gas, is another state in which matter exists. This plasma continued to be bombarded with laser lights and in the ensuing commotion, some nuclei crashed with others, fused with each other. When atoms fuse, they release energy. 

Why does it take energy to fuse atoms together? 

Atoms have positively charged protons in the nucleus (along with electrically neutral neutrons). Negatively charged electrons exist around the nucleus, in multiple ringed pathways — somewhat like the Sun and the planets. Since the number of protons and electrons are the same, the atom is electrically neutral. Take out one or more electrons, it becomes a positively charged ion. Add more electrons, it turns into negatively charged ion. 

If you rip all the electrons from an atom, what remains is the positively charged nucleus. When two such nuclei come close to each other, they repel, or move away from each other, because fundamentally, like charges (positive-positive or negative-negative) repel and unlike charges (positive-negative) attract.  

So, to fuse two nuclei into one larger nucleus, you need to overcome the force of repulsion. It is like bringing together the north poles or south poles of two magnets. The closer you bring the positively charged nuclei, the more energy you need to bring them further closer.  

But at a particular point (called ‘Coloumb barrier’) the nuclei yield. If you bring them as close as a millionth of a billionth of a meter (or, one ‘femtometer’), the repulsive forces are overpowered by the attractive nuclear forces. At this distance, the nuclei rush into each other’s arms and fuse into one.  

The trick is to ride over the hump called the Coloumb barrier; the way to achieve this trick is to keep giving energy. 

How much energy is needed? 

100 million Kelvin, which is six times as hot as the core of the Sun. 

Why does only hydrogen, or its isotopes, figure in all nuclear fusion experiments? 

Because the hydrogen atom has only one proton — therefore, the lowest positive charge. The next element, helium, has two protons, the next, lithium, has three, and so on. It is easier to try to fuse two nuclei with the least charge. They use isotopes of hydrogen, D and T, because these nuclei have one and two neutrons, respectively. The presence of neutrons is helpful in fusion — they increase the nuclear forces of attraction, which come into play once the Coloumb barrier is crossed. 

In February 2022, the ITER experiment in France also announced some success in nuclear fusion. Is the National Ignition Facility’s achievement the same as ITER’s? 

No. While the heart of both is nuclear fusion, their approaches are different. The fundamental difference is in the way the plasma — the soup of electrons, protons and nuclei — is kept there, so that the particles could fuse. Plasma, with its high energy, tends to scatter away. You can’t hold them in a vessel, because no matter what the walls of the vessel are made of, the particles will pass right through them.  

In ITER (a multi-country effort in which India is involved), they held the plasma in the vessel by a method called ‘magnetic confinement’. Wherever the particles go, they encounter magnetic forces that push them right back inside. In the US experiment, they used another method called ‘inertial confinement’. The laser beams, 192 of them, coming from all around the capsule, denied an escape route for the particles in the plasma to shoot out. In fact, the laser energy compresses the particles. 

Most scientists say that magnetic confinement is a better way.  Another difference is that while ITER produced energy from fusion, there was no ‘net gain’. But in the case of the US experiment, an energy gain was reported. 

So, if an energy gain was achieved, does it mean that nuclear fusion is within reach? 

Not at all. The energy gain was in the capsule in which D and T were kept. More energy came out of the capsule than what went in. But to operate the lasers, a heck of a lot more energy — 300MJ — was consumed. Besides, these are extremely expensive, sophisticated lasers, whose costs will tell upon the cost of energy. 

What happened at NIF was a baby-step, but in science, baby steps are significant, hence the buzz is entirely justified.