In the previous articles Fusion energy is the reason to be excited about the future & Nuclear fusion, the perfect energy source to achieve net-zero, I have briefly examined the sustaining fusion via the Magnetic Confinement Fusion (MCF) approach, which utilises magnets to confine the plasma. In this note, we will be looking at the Inertial Confinement Fusion (ICF) and results from the National Ignition Facility (NIF). We will further study the current efforts at the Institute of Laser Engineering (ILE), Osaka University, Japan with the upcoming Hydrogen-production Plant and Energy Reactor of Inertial-fusion (HYPERION) project.
For the non-experts in the field, it is advised to read the two past published commentaries as some of the scientific jargon used here is investigated previously. Additionally, it is useful to note, that with these CGLN commentaries, I am taking a step further to brief the scientific backgrounds to help the readership to develop a more insightful knowledge of the discussed science.
Lets review some of the basics of ICF:
In Inertial Confinement Fusion (ICF), the fuel consists of a cryogenic shell of deuterium and tritium ice. In most of the ICF approaches high power lasers are used to implode the shell at extreme velocities (300-400 km per second), achieving less than 100 g/cc at the core with temperatures around 10 keV (100M degrees Celsius) surrounded by a higher density layer of 1000 g/cc at a much lower temperature and densities (orders of 1026 – 1027 cm-3).
It should be obvious now that at Magnetic Confinement Fusion tokamaks (discussed previously) [link 2], the plasma is confined at a very low density for several seconds, and on contrary, in ICF, the plasma presents at much higher densities with minimal time units (billionth of a second). The burning time is primarily related to the shell size and the speed at which the fuel is imploded.
There are two primary approaches to reach ignition in ICF, direct-drive and indirect-drive, and they simply refer to the configuration that lasers are used to heat and implode the capsules. In the direct approach, the lasers are simply focused on the capsule as shown in figure 1. Here as shown the fuel is heated, leading to the creation of plasma around the fuel before being compressed in the second stage. Later the ignition takes place while reaching high densities at 100M degrees Celsius, and at the final stage, the thermonuclear burn produces energies many times the input energy.
On the other hand, with the indirect approach, the laser drivers are converted into x-rays confined in a cavity. Their absorption on the fuel surface generates ablation pressure to drive the implosion. So far the experimental results confirmed the indirect approach to be the most viable route to achieve fusion.
Now let’s review the latest achievement by National Ignition Facility at LLNL using the indirect approach.
NIF achievement in Burning Plasma – Inertial Confinement fusion
National Ignition Facility (NIF) consists of 192 laser beamlines delivering 500 trillion watts on a nanosecond scale (one billionth of a second). The lasers are focused into a cavity known as hohlraum via holes at various angles, as shown in Figure 2. The hohlraum is gold lined depleted uranium that acts as an x-ray conversion cavity. The x-rays produced by the laser irradiation ablate the outer layer of the fuel, pushing towards the centre with velocities of 400 km per second. When the implosion reaches the maximum density (known as stagnation), the kinetic energy of the imploded fuel is consequently converted into internal energy, enveloping the lower density core (hot spot) and fusion occurs.
As the central hotspot is formed (approximately in micrometre in size and lifetime of 100s of a picosecond), the alpha particles (He) generated by the fusion reaction of deuterium and tritium :
heats the hotspot. Furthermore, due to the transport of these particles including electrons and neutrons, the temperature in the outer layer increases and as the pressure builds up (Giga bar), the fuel eventually blows apart and the cycle ends.
The reported achievement was the result of a new hohlraum design
As said previously, to sustain and increase the probability of fusion reaction occurring, high plasma temperatures are required, and typical experiments at NIF are showing 4-5 keV (40-50 million degrees centigrade) using the indirect approach in hohlraum.
In recent years, several types of hohlraums are designed and tested in trial-and-error experiments. In 2021 Hybrid E’ and ‘I-Raum’ types yielded the best result achieved so far. The result yielded a Q factor of 0.26 – 0.4 (Q factor discussed in the previous article, which is simply the ratio of energy gain versus the energy lost. NIF reported 1.3 Mega Joules of energy produced while using 1.9 Mega Joules to generate the x-rays in the hohlraum to accomplish the burning plasma.
Other efforts around the world
Japan, Institute of Laser Engineering (ILE)
The institute of laser engineering (ILE), located in Osaka, Japan is the house of the most powerful lasers, LFEX and GEKKO XII. Together they deliver the FIREX-1 (Fast Ignition Realization Experiment) program which will be a predecessor to the upcoming HYPERON (Hydrogen-production Plant and Energy Reactor of Inertial-fusion) project.
Fast Ignition is a scheme for igniting the fuel using a combination of lasers at different intensities and duration. In this case, during the ignition phase, an ultra-intense laser is used to heat the core. The heating is done using the relativistic electron beam (REB) produced by the heating laser.
In the second phase of the FIREX project, ILE has taken initiative and started developing a 100J laser with a 100Hz repetition rate in a project so-called J-EPoCH. The facility will bring together 160 beams providing 10 Kilo Jules (1.6 Mega Jules in total). The J-EPoCH laser produces similar energies to NIF using different technology but will deliver 100 pulses every second. NIF produces is one shot per day in a best-case scenario.
Prof. Kodama, the director of ILE and the HYPERION project comments further:
“in the HYPERION project, a small fusion test reactor with a hydrogen production system (hydrogen production target: >1.7t/h) will be realized around 2040 using a medium gain laser fusion target with a laser system (0.5MJ/2Hz/1MW/electric efficiency: 20%). In addition, by 2050, we hope to achieve a commercial reactor for hydrogen production that drives four fusion reactors (4Hz/ reactor) with an advanced laser system (0.5MJ/16Hz/8MW). This will result in a clean hydrogen production rate of >27 t/h or 130,000 t/y per plant (55% availability) and hydrogen production cost <2.3$/kg (current green hydrogen production cost: 3-8$/kg).”
Prof. Kodama believes the dream of making the fusion a reality is for us to (1) develop human resources by providing training and high-quality education while (2) sharing knowledge and (3) expanding our collaborative research. Of course, the development of the key technologies will bring us much closer to this goal.
This article concludes our current series on nuclear fusion. We will be further reviewing fusion using other approaches in short commentaries. The upcoming articles will review the more conventional method of power generation using fission reactors.