Commercial fusion energy using lasers: Direct drive and indirect drive

Following in the footsteps of the recent advancements at the US National Ignition Facility (NIF) at LLNL, several commercial fusion energy projects will be attempting to recreate the conditions found in the hot center of the sun using new, high-energy lasers. Each project has identified unique processes to be developed in order to implement fusion reactions in rapid succession.

Contrary to the devices utilizing magnetic confinement, such as tokamaks, which could result in a continuous fusion reaction, laser-driven devices are targeting a method called inertial confinement fusion that compresses the nuclear fuel to the point of fusion. Each operation completely burns the nuclear fuel, which is then replaced, after which the process is restarted. The goal is to do this ten times per second or more. NIF is currently able to support about 400 experiments per year.

At this time, the two most promising methods of inertial confinement fusion (ICF) are direct drive and indirect drive.

In the direct drive method, the nuclear fuel is compressed using lasers to directly heat the fuel. Problems arise when the fuel has a less-than-perfect shape or position, as the reaction relies on internal shockwaves to initiate the ignition. Implosion symmetry is crucial to achieving fusion.

The indirect-drive method uses lasers to heat a metal enclosure, which emits x-rays that in turn heat and compress the fuel. In addition to different problems posed by the shape and position of the enclosure and the fuel, the method also suffers from lowered efficiency due to the change in wavelength.

A successful commercial fusion reactor must produce more energy than it uses. Additionally, the fuel pellet may be complicated to manufacture, and implosion symmetry may be difficult to achieve.

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Some projects are experimenting with a hybrid direct drive, in which indirect drive is only used to initiate the compression, followed by several shots from UV lasers to achieve fusion. These designs may allow for simpler fuel pellet design and simpler reactor design: one design achieves symmetry in the initial stage, then uses asymmetric lasers to add energy to the symmetric implosion, thanks to the existing dense plasma absorbing energy uniformly.

NIF uses 192 lasers arranged around the fuel pellet to achieve fusion. During laser generation and amplification, laser energy must be monitored so the expected energy can be delivered to the fuel within the necessary tolerances. As the laser pulse is transmitted from the generation area to the target area, energy must be monitored during this transport phase as well. Data must be monitored over time to ensure consistency between experiments, and laser measurement instruments must be closely monitored to ensure the data can be trusted. Read more about how NIF uses our detectors here.

Gentec-EO laser measurement instruments serve to aid commercial and academic fusion research around the world by monitoring laser output at all stages. As fusion research speeds up, so do the lasers – as we go from projects like NIF that may perform one or two experiments per day to the commercial projects that may be aiming for several per second. Gentec-EO’s latest pyroelectric laser energy meter, the QE195, is the largest pyroelectric laser energy meter in the world, can measure the large-diameter beams used in fusion reactors, and works at the fast repetition rates required. It was designed specifically for the needs of ICF research and will help to advance the next revolution in energy production.

Feel free to get in touch with one of our physicists or engineers to talk about your own laser measurement needs, whether it be for fusion research or another way of transforming the world.