Two weeks ago I explored the first of the two major approaches to nuclear fusion, magnetic confinement fusion. Today I return with the second approach, arguably less technologically impressive – inertial confinement fusion. It uses lasers though. Who doesn’t love lasers?
The reason I say it’s less technologically impressive is because there isn’t a massive grand tokamak which accommodates 150 million degrees Celsius plasma. Instead ICF fires extremely high power laser beams at the target fuel in the form of a tiny pellet, typically a few millimetres in diameter. Don’t be fooled by the minute size though, because the power of ICF lies in the delicacy of the process.
When the laser light strikes the surface of the pellet, the outer layer of the pellet explodes and generates shockwaves inwards towards the core, a consequence of Newton’s third law. These shockwaves force the inner core to compress, achieving densities approximately twenty times that of lead and temperatures of over 100 million degrees Celsius, and this readily increases the likelihood of fusion reactions occurring.
From this ICF appears to be much more straightforward approach than MCF, but there are many issues to consider. One is the Rayleigh-Taylor instability (RTI), an instability that arises when a lighter fluid supports a heavier liquid. Picture a layer of water atop a layer of oil. Minor disturbances are able to effortlessly disrupt this equilibrium.
RTI occurs in ICF because the less dense outer layer must withstand forces exerted by the more dense inner core. This leads to an unstable equilibrium which could prevent fusion reactions from occurring.
Mitigating RTI is quite a challenge as every component involved in the process must be near perfect. The laser beams must be timed to strike the pellet from all directions at precisely the same time with a constant intensity. The pellet itself must be perfectly spherical with no aberrations of over a few micrometres. ICF is indeed a delicate process.
ICF continues to have more difficulty than MCF mainly due to inadequacy in laser technology. The problematic aspect is not in generating the conditions required for fusion, but rather in obtaining a net power output. Lasers tend to have such a low efficiency that ICF can release hundredfold power outputs yet still have used up more energy in powering the laser.
Although ICF may fall behind its more extravagant partner, that’s not to say that ICF won’t suddenly uncover a breakthrough. However in the grand scheme of things, significant progress is being made in delegating nuclear fusion as the solution to humanity’s future energy demands. At least that’s going for us.