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A 300-word article in the scientific journal Nature has changed our global technology forever. We are talking about the article on the first laser by Theodore H. Maiman. Lasers are used almost everywhere these days, whether at supermarket checkouts, for eye surgery, as rangefinders or for cutting and welding metals. Now they could also make a decisive contribution to our future energy production - by bringing the sun's energy to earth.1
In a fusion process, two light atoms combine to form a slightly heavier atom. To achieve this, a major energy hurdle must first be overcome - the Coulomb barrier - before the fusion process produces net energy. In the sun, this happens under extreme conditions of 15 million degrees Celsius and a pressure of 150 billion bar. One way of simulating this extreme process on Earth is inertial confinement fusion 2.
In inertial confinement fusion, a small, approximately 2 mm sphere (target) of frozen hydrogen isotopes is compressed very quickly and evenly by laser or particle radiation, for example. The density and temperature inside the target increase. Due to the inertia of the target, it cannot expand fast enough and the hydrogen in the center fuses. The fusion process generates additional heat, which leads to fusion in the outer part of the target. The energy released in the fusion process is then used to generate electricity.
The aim is to repeat this process several times a second in order to ensure a constant supply of energy2.
The lasers used in inertial confinement fusion generally deliver very short pulses with a very high energy. There are various ways in which the laser beam compresses the target.
Indirect drive
With indirect drive, the target is placed inside a small metal capsule (cavity). The laser pulses are directed onto the inside of the capsule, generating strong X-rays. This is then used to compress the target. Great progress has already been made in research with this technique, but it is not very efficient. In addition, the capsule is destroyed by the laser radiation and has to be replaced with each pass3.
Direct Drive
With direct drive, the shell of the target is irradiated directly. As a rule, this is not possible as uniformly and therefore leads to more irregularities and less compression. However, the energy coupling is five to six times higher and is therefore a preferred approach for an energy-efficient reactor3.
In addition to these two widely used "classic" approaches, there are various other implementations and ideas in laser inertial confinement fusion that are being investigated worldwide.
Inertial confinement fusion is a field with a wide variety of forms. While most approaches are based on lasers, there are also magnetized target fusion approaches that accelerate hot plasma rings or compress a combustion chamber using pistons. But even laser fusion offers other types of approach. Some of them are shown here:
Fast Ignition
In fast ignition, the target is first compressed, as in classic inertial confinement fusion. An electron or particle beam is then used for fusion. Here, the symmetry requirements for the incident radiation are lower while at the same time a higher energy gain can be expected3.
Shock ignition
Here too, the fusion process consists of various compression stages. In shock ignition, a series of shock waves is used to first compress and heat the target. The last shock is the strongest and is additionally amplified with the previous shock waves, which leads to the fusion of the target3.
Magneto inertial fusion
In laser inertial fusion, a magnetic field is used to reduce the compression requirements. At the same time, the heat loss is reduced, which leads to a better fusion yield3.
Innovation & progress: a selection of current projects in focus
NIF
In 2009, the first modern inertial confinement fusion experiments were launched at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California. In December 2022, the facility succeeded in generating more energy for the fusion process than the laser power used4. However, due to the low efficiency of the lasers, the overall efficiency is less than one percent3.
Sources:
1 DPMA | Laser
2 Fusion - BMBF
3 R. Betti, O.A. Huricane, Inertial-confinement fusion with lasers, Nature Physics (2016), 12, 435-448
4 Achieving Fusion Ignition | National Ignition Facility & Photon Science
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