Lasers for Fusion Energy – The Basics

When most people think about lasers for fusion energy, their minds immediately jump to the topic of inertial confinement fusion (ICF) where a large number of laser beams are used to concurrently heat and compress fuel pellets to the point of fusion. An example of this is the Lawrence Livermore National Laboratory (LLNL) National Ignition Facility (NIF; illustrated below), where 128 lasers were focused on a centimeter-sized target containing deuterium and tritium (D-T). The total power delivered by the lasers to the target was 2.2 MJ.

Of course, NIF is just one example of an ICF system, and several different approaches and types of lasers have been used for various fusion applications, from the initial warming of plasma to containment. Let’s look at the different types of lasers that are used for fusion energy, their application, how they are constructed, and what the state of the art looks like today.

What is a Laser, and Why Use Them for Fusion Energy?

A laser (“Light Amplified by Stimulated Emission of Radiation”) is a device that can produce visible light that is coherent in frequency and spatially. Typical lasers emit light that is of a single frequency, or a very narrow band of frequencies. The first laser was built in in 1960 at the Hughes Research Laboratory by Theodore Maiman. Lasers were preceded by masers (microwave amplification by stimulated emission of radiation), a device used initially in long-distance microwave communications. As such, lasers were initially named “optical masers”. Today, “laser” refers to all devices emitting frequencies higher than 300 GHz, which includes infrared, optical, ultraviolet, X-rays, and gamma radiation.

Fusion is, among other things, an extremely fast process. The advantage of lasers for fusion is that a laser can emit a large amount of energy in a very short time. The lasers that are used for fusion can emit a pulse carrying 1 to 2 megajoules (MJ) of energy in 10 to 20 nanoseconds, a power of 100 megawatts (MW). The lasers used for ICF have typically been neodymium-doped phosphate glass lasers, but other materials such as Krypton Fluoride (KrF) are also being explored for use in fusion energy. Historically, these laser mediums (whether a solid material such as a glass, or a cylinder of gas) were pumped by flashlamps; however pumping these with solid-state diodes (so-called diode-pumped solid state lasers) is becoming more interesting due to their smaller size.

The Design of an Inertial Confinement Fusion Machine

Next let’s look at the design of the National Ignition Facility (NIF). NIF, while a “science experiment” at the Lawrence Livermore National Laboratory (LLNL), has the distinction of being the only fusion machine to date that achieved a Q of greater than 1 (i.e., the power released by the fusion reaction exceeded the power put into the fuel). A Q of 1 is known as a “scientific break-even”, but does not represent a commercially-viable reaction. The Q required to produce fusion energy commercially, also known as economic break-even (i.e., the energy produced can be sold to pay for the operating, maintenance, and amortized cost of the plant), depends heavily on the type of fusion utilized. 

For ICF, a Q of greater than 10 is believed to be required. While NIF was not built with the goal of achieving economic break-even, its overall design does illustrate what an ICF electricity plant might look like:

1. A weak laser pulse (~1 nanojoule, or nJ) is created and then split into 48 beams. These beams are carried by optical fibers to 48 preamplifiers.
2. The preamplifiers then increase the power of each of the 48 beams by a factor of roughly 10 billion, with each beam now having the power of a few joules. Each beam is then split into four parts and carried to the main laser amplifiers (the three stages on the far left above).
3. The main laser amplifiers (192 of them) further amplify the beam power, as well as “cleaning” the beam, increasing its coherence. Then the 192 beams enter the power amplifier, which increases each beam’s power to four million joules, or megajoules (MJ).
4. The beams then go through the final optics assembly, where they are transformed from infrared laser beams of 4MJ each into ultraviolet beams of 2MJ each; these beams are what impinge on the fusion target.

 When the ultraviolet beams hit the fusion target (frozen deuterium-tritium inside a gold-plated plastic cylinder, also known as a hohlraum), it heats the target to about 3M degrees Celsius. The hohlraum then emits X-rays, ablating the hohlraum covering and compressing the fuel and causing fusion.

Other Uses of Lasers in Fusion Machines

In addition as the primary mechanism for heating and compressing fusion fuel, there are several other uses for lasers in fusion machines:

• Tritium Monitoring: Measuring the tritium gas trapped in the inner wall of a tokamak is critical to ensuring a “good ignition”. The Joint European Torus (JET) did this by utilizing Laser-Induced Desorption Quadruple Mass Spectrometry (LID-QMS) for exactly this purpose. In LID-QMS, a high-power laser pulse is fired at the JET interior walls, releasing whatever tritium has been absorbed by the walls. This allows rapid diagnostics during machine operation.
• Plasma Diagnostics: Similar to the concept above, but instead of firing the laser on the interior walls of the tokamak, it is fired directly into the tokamak plasma. This capability, developed by Tokamak Energy, provides interferometry to measure the density of fusion fuel in an operating tokamak.

Conclusion – Lasers are Not Limited to Inertial Confinement

While high-power lasers are integral to ICF, it is not their only use in fusion machines. As fusion progresses towards commercialization, lasers provide a convenient way to measure conditions inside a fusion machine to provide diagnostics, “burn away” fusion waste, and in the case of ICF, to provide the power driving the fusion reaction itself. Additionally, lasers are a critical tool for the related industrial processes such as cutting material such as steel, welding, and even 3D printing of metals. It is highly likely that lasers will continue to be critically important to fusion energy, whether it is magnetic or inertial confinement.