Understanding the fusion energy breakthrough announced by the US (GS Paper 3, Science and Tech)
Why in news?
Recently, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL), California, had conducted a fusion test that produced 153% as much energy as went into triggering it.
Details:
NIF uses powerful lasers to heat and compress hydrogen nuclei. When the nuclei fuse, they release heat. When this heat is equal to or greater than the heat delivered to the container, the event is called ignition. The ratio of the output energy to the input delivered to the container is the gain.
In 2021, NIF reported that it had achieved a gain of 0.72, taking a big step closer to 1.
Now, it has reportedly achieved ignition with a gain of 1.53 with a yield of 3 megajoules.
A gain of 1 is called ‘scientific breakeven’, “an important milestone in the development of fusion energy because it signifies that very significant (but not all) plasma-physics challenges have been retired”. A plasma is a gas of charged particles, ions in this case.
Magnetic Confinement:
Magnetic confinement and inertial confinement are two popular ways to achieve nuclear fusion.
Magnetic confinement uses a torus-shaped reactor called a tokamak, in which a hydrogen plasma is heated to a high temperature and the nuclei are guided by strong magnetic fields to fuse.
ITER is a famous example of an experiment trying to achieve fusion using magnetic confinement. An international collaboration, it is under construction at a site in France and is scheduled to be built by 2025.
It is considered to be more technologically mature than what NIF is attempting.
Inertial Confinement:
In NIF’s setup, 192 high-power lasers fire pulses at a 2-mm-wide capsule inside a 1-cm-long cylinder called a hohlraum, in less than 10 billionths of a second. The capsule holds deuterium and tritium atoms.
As the pulses strike the hohlraum’s insides, the latter heats up and releases X-rays, which heat the nuclei to millions of degrees centigrade and compress them to billions of Earth-atmospheres.
The high temperature is required to energise the positively charged nuclei to overcome their mutual repulsion. The technique is called inertial confinement because the nuclei’s inertia creates a short window between implosion and explosion in which the strong nuclear force dominates, fusing the nuclei.
The total mass of the final helium nucleus is lower than the masses of the fusing hydrogen nuclei.
The difference is released as energy according to the mass-energy equivalence (E=mc2). Specifically, when two hydrogen-2 nuclei fuse, they yield a helium-4 nucleus, a neutron and 17.6 MeV of energy.
Burning plasma:
For fusion reactions to be sustainable, the energy released by the initial reaction needs to set the stage for more reactions. To this end, NIF’s goal has been to create a “burning plasma”.
“Burning plasma” is created when nuclei are encouraged to fuse not by the external heat source but by the heat of other fusion reactions.
NIF achieved this in 2021. The gain was 0.72: 1.37 megajoules produced by the fusing nuclei versus 1.97 megajoules delivered by the lasers.
In August 2022, the facility reported it had produced a burning plasma that met the Lawson criterion: the heat generated was sufficient to potentially trigger other fusion reactions as well as offset heat loss during the reaction.
Now, the facility has reportedly achieved a burning plasma that meets the Lawson criterion as well as a gain greater than 1.
Challenges:
First:
After NIF achieved a gain of 0.72 in 2021, the people in charge of the experiment tried thrice to repeat their feat.
They failed because the NIF fusion facility is a highly sophisticated system with tiny moving parts. Even small changes in input conditions, like microscopic bumps on the capsule, can lead to large variations in output. So NIF will need to reproduce its new results.
Second:
For fusion to be truly gainful, the energy released by the reactions needs to be greater than the energy going into the lasers, about 300 megajoules, and not just the energy delivered to the hohlraum. This hasn’t yet been achieved.
Third:
The road to a power plant from the NIF’s current achievement isn’t well-understood.
For example, at NIF, lasers fire at a hohlraum, generating X-rays that heat the capsule, instead of hitting the capsule directly. This prevents the laser pulses from being pinpoint accurate and allows the capsule to be heated in a symmetric way, which is highly desirable. But the cost is lower gain.
Another issue is that some of the input energy is devoted to compressing the capsule instead of raising the temperature (unlike in magnetic confinement). This fraction will increase as the amount of fuel increases, creating another barrier to high gain.
