Fusion with Lasers
Nuclear Fusion Part 2: Inertial Confinement Fusion Technology
Key Takeaways:
- Inertial Confinement Fusion (ICF) systems produce energy by repeatedly igniting small amounts of fuel with lasers.
- ICF systems have three main components: the “target” containing the fuel, the “driver” (lasers) to ignite the fuel, and the “chamber” to capture the fusion energy and begin the process of converting it to electricity.
- Both achieving ignition and demonstrating a positive energy balance require careful energy accounting.
- Improvements in the efficiency of laser technologies, in how ignition is achieved (enabled by better simulations), and in the frequency that high-power lasers can be fired have significantly improved the math for fusion concepts.
Let’s take a closer look at the technology behind Inertial Confinement Fusion (ICF) — fusion with lasers. The basic idea sounds like science fiction: small fuel pellets are repeatedly launched into a chamber, where they are struck by powerful laser beams to initiate fusion. This releases an even more enormous quantity of energy, typically captured by the chamber as heat, which is converted into reliable, carbon-free electricity.
How does Inertial Confinement Fusion work?
There are three components of ICF fusion: the “target” containing the fuel, the “driver” (lasers) to ignite fusion in the target, and the chamber to contain and convert the energy released.
The Target: Fuel for Fusion
Most fusion contenders use a mixture of hydrogen isotopes, deuterium and tritium (DT) for fuel, because these atoms will fuse at the lowest temperatures. Deuterium is a hydrogen atom (a proton) with one extra neutron. Tritium is a hydrogen atom with two extra neutrons. When deuterium and tritium fuse, they produce a helium atom (also called an alpha particle), neutrons, and a large amount of energy. [1] Most of this energy (80%) is carried by the neutrons.
While deuterium is abundant in the ocean, tritium will be challenging to manage. Tritium is unstable, radioactive, and rare — it is currently made by bombarding lithium metal with radiation, or as a byproduct at some nuclear power plants. Luckily, the amount of tritium needed is very small — on the order of milligrams per fusion event, or perhaps 50–500 kg per year. Additionally, tritium can be produced at the fusion power plant where it is used by capturing the neutrons with lithium. So, deuterium can be harvested from the ocean, and tritium will be produced or “bred” in the fusion reactor.
An attractive alternative is to use “aneutronic” fuels rather than DT. Aneutronic fuels are combinations of atoms that produce few or no neutrons when they fuse, such as mixtures of hydrogen and boron. Aneutronic fuels have two big advantages: 1) they avoid the complexities of breeding and handling tritium, and 2) they do not produce neutrons, which induce radioactivity in materials and require a moderator. A third benefit is that the helium (He) particles released could produce electricity directly, eliminating the need for a steam turbine and other expensive equipment to convert heat into power.
The trade-off is that the ignition temperature for hydrogen and boron (H-B11) is ten times higher than the ignition temperature needed for DT fusion. Recent research has suggested that this barrier could be lowered significantly by exploiting new plasma physics. [2] Given that ignition of the easier DT fuel has yet to be demonstrated, and that energy losses increase dramatically with temperature, this is a big ask.
The Driver: Fire the Lasers!
A large amount of energy is needed for the atoms within the fuel to fuse — on the order of several megajoules (MJ) per target. In ICF, this energy is delivered by lasers. The lasers can either hit the fuel directly (“direct drive”), or hit a container with the fuel inside (“indirect drive”). In both approaches, fusion is initiated by focusing the laser energy on a small “hotspot”, which ignites the remaining fuel so quickly that its inertia prevents it from escaping.
In indirect drive fusion processes, the laser energy is focused on a small metal cylinder with the fuel inside. This cylinder is called the “hohlraum”. The energy from the laser is re-emitted as x-rays inside the cylinder. These x-rays heat the outer layers of the fuel pellet, causing compression and heating of the inner layers to achieve ignition.
The advantage of indirect drive is that the x-rays are more evenly distributed than the original laser beams, promoting even heating of the fuel. Instabilities that arise during heating are one of the key challenges for achieving fusion. The trade-off is that the energy transfer between the hohlraum and the fuel is poor, and a lot of energy is lost in this conversion step.
In direct drive fusion approaches, the lasers are focused directly on the fuel. The advantage of direct drive approaches is that they generally use laser energy more efficiently. Approaches to direct drive ICF differ by how many times they shoot the target with lasers, how long each laser shot lasts, what type of laser is used, and many other variables. A few types of direct drive fusion are described in the notes. [3]-[6]
The number of times they “fire the lasers!” in a given period is a critical performance parameter, called the repetition rate. The total amount of power generated by a fusion power plant is set by the net electricity produced per target, but also the number of targets burned per second or hour. Based on the amount of energy each fuel pellet is expected to release, a reasonable target for a commercial fusion facility is 1–10 Hz, or 1 to 10 fusion events per second.
The Chamber: From Particles to Power
Chambers used at research facilities and concepts for future fusion plants are typically a spherical metal chamber with ports (holes) for the laser beams to hit the target. If neutrons are produced, the chamber’s interior must be blanketed with something to stop the neutrons, called a moderator. Lithium is a great moderator because it reacts with neutrons to make tritium, which can then be used as fuel. [7] One design option is a liquid lithium “waterfall”, where molten lithium flowing down around the chamber picks up the neutrons and heat produced by fusion.
Most designs to date capture the fusion energy as heat, and then use this heat to generate steam as in conventional power plants. The steam is converted to electricity in large turbines — the efficiency depends on the temperature of the steam. Designs that produce electricity directly from fusion products — avoiding the need for steam generators and turbines — would significantly increase efficiency and cut costs.
Fusion as Energy Accounting
Achieving net power production with fusion is essentially an energy accounting problem on multiple scales. Overcoming the repulsive forces between atoms so that they can get close enough to fuse takes a lot of energy. Once a small part of the fuel has started to fuse, the fusion reactions generate a significant amount of heat energy, which can start more fusion reactions. However, anyone who has tried to heat a poorly insulated home knows that not all the energy put into the house actually raises its temperature.
The key to achieving and maintaining fusion is to design a scheme where as much fusion energy as possible contributes to more fusion, instead of being lost to the environment. If the fusion reactions keep going long enough to generate more energy than is put in, it could be the basis for a power plant. This is measured by the energy gain “Q”, or the ratio of energy produced to the driver energy used to achieve ignition. Ignition is the point at which the energy given off in the fusion reactions is high enough to maintain the temperature of the fuel and produce additional fusion reactions — to keep the fuel “burning”.
The larger engineering challenge is producing ignition without “breaking the bank” in terms of energy. Energy is lost at each time it is transferred from one form to another, which happens several times between the laser’s power supply and the fuel. Some places where energy can be lost include:
- Powering the laser: the efficiency of converting electricity to laser light might be 1–20% depending on the technology
- Modifying the laser light, e.g. converting the laser light from an infrared to a shorter wavelength: efficiencies of 50%-100%
- Laser to x-rays (indirect drive): ~85% of the laser energy is converted to x-rays
- X-rays to target (indirect drive): ~15% of the energy from the x-rays is deposited in the target
- Laser to proton beam (in some direct drive approaches): ~10% laser energy is converted to protons
This article on NIF provides a great visual example. A rule of thumb is that the laser efficiency x Q needs to be at least 10 for commercial fusion to be viable.
The bottom line is that both demonstrating ignition and demonstrating a positive energy balance are key to a successful fusion energy plant.
How Technology is Changing the Balance Sheet
Laser Technology. Most ICF simulations suggest that lasers with some combination of high power, short pulse length, accuracy over a micrometer area, and advanced pulse shaping are needed. These capabilities are all now within reach. Large laser systems (multiple lasers focused on one spot) that deliver high peak powers exceeding one Petawatt (PW) have been constructed at dozens of research facilities worldwide. Lasers are available that can deliver kilojoules of energy in microseconds (a millionth of a second), or fire pulses as short as femtoseconds (one 1,000,000,000,000th of a second).
Fusion research facilities built prior to roughly 2010 relied on flash lamp pumped lasers, which can only be fired a few times a day at best. New diode-pumped solid state lasers can reach higher repetition rates (up to 1–10 Hz for some lasers) due to advances in cooling. The efficiency is also dramatically increased: glass flash-lamp pumped lasers convert about 1% of electricity “from the wall” into laser light, while diode-pumped solid state lasers can achieve up to 20% efficiency.
The precision of laser optics has increased to heat a target smaller than a pinhead. Delivering the same amount of power to a smaller area increases the local temperature to achieve ignition.
One reason that relevant laser technologies have advanced so rapidly is their applications beyond the fusion community. Associated research on laser-matter interactions has also enabled generation of radioisotopes for positron emission tomography (PET), targeted cancer therapy, medical imaging, and the transmutation of radioactive waste. Each of these promising applications requires lasers with peak power of hundreds of terawatts (TW) to petawatts (PW) and with average power of tens to hundreds of kilowatts. [8]
Simulation and Controls. The other challenges that have prevented ignition from being achieved are subtle effects that lead to energy escaping the system: 1) non-symmetric laser illumination, 2) laser-plasma instabilities (LPIs), and 3) hydrodynamic instabilities during fuel compression. Advanced simulations and increased computing power have enabled scientists to model and predict more complicated interactions.
Technology Cost. This is the last key consideration — how have the costs of various components come down, and how might a fusion power plant be built to deliver energy at a competitive rate? With this overview of ICF technologies, we are now prepared to cover this topic in a future post…
I gratefully acknowledge the help and input of several brilliant fusion scientists and physicists who informed this blog post. Any mistakes are my own, and I would welcome the opportunity to correct them. I also highly recommend “The Fusion Quest” by Professor T. Kenneth Fowler to anyone seeking a deeper understanding of fusion science.
The first post in this series was a high-level overview of the fusion space. The next post will cover magnetic confinement fusion.
Notes
- Fusion of a target containing 10 milligrams of DT fuel would release 3,400 MJ of energy assuming 100% “burn”. If the lasers fire and fuse one target per second (a commonly assumed rate), this fusion plant would produce 3.4 gigawatts (GW) of heat.
- Specifically a phenomenon called plasma-block ignition, described in Hora et al. 2017. When a hydrogen and boron-11 fuse, three charged helium atoms (alpha particles) are produced. This takes advantage of the “avalanche” production of alpha particles to produce additional fusion events.
- The simplest way to achieve fusion is to blast the fuel with enough laser power to reach a temperature and density that satisfies the Lawson Criterion. The challenge is to deliver this energy quickly, before the fuel flies apart. This would require a very short (~picosecond) but VERY powerful laser pulse.
- “Fast ignition” strategies have been researched extensively, particularly at the National Ignition Facility (NIF) in Livermore, CA and the LFEX facility in Osaka, Japan. This approach uses a combination of 2 laser pulses. First, a long laser pulse causes an implosion and compression of the fuel — compressing the fuel reduces the amount of heat needed. Next, a shorter (fast) laser pulse induces ignition. This reduces the amount of energy delivered in each of the two steps. The longer pulse can be “shaped” to be more efficient, reducing the overall energy needed (smaller lasers). Pulse-shaping is like pushing a friend on the swing- in mid-swing, you start and stop pushing gradually, to give them maximum additional energy.
- Even with pulse shaping and other laser beam manipulations, achieving uniform energy deposition is hard. Some approaches convert the laser energy (photons) into protons or electrons for the second fast ignition pulse. However, there is an efficiency penalty to pay for the increased beam uniformity; experiments on converting laser light to proton beams have shown efficiencies of roughly 10%.
- The shock ignition concept achieves ignition by accelerating the pellet shell to sub-ignition velocity, then igniting it with a converging shock produced by a high intensity spike in the laser pulse. A brief description can be found here.
- For more information about tritium breeding and the effects of neutrons on chamber materials, see this 2018 paper by Marek Rubel.
- Source: SPIE Conference Proceedings abstract
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