How can scientists turn the latest breakthrough into a new clean power source?


Laser energy is converted into X-rays in a canister that compresses a fuel capsule until it explodes. Credit: LLNL

Researchers in the US have finally achieved a goal set decades ago: achieving “ignition” using nuclear fusion – getting more energy than you put in.

Scientists at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF), where the experiment took place, are undoubtedly both excited and relieved that the promise implied by their facility’s name has finally been fulfilled. But how excited should the rest of us be? What does this really mean for the possibility of effectively creating an unlimited amount of clean energy, and what else does it take to achieve this?

While fusion reactions release more energy than is put into the target, this does not account for the much larger amounts of energy required to fire the laser used to run the experiment. Also, the burst of energy was not in the form of electricity, but in the form of a pulse of energetic particles. Using these particles to generate electricity and keeping a fusion reactor running all the time will require many hurdles to be overcome.

However, the firing is a remarkable achievement and promises to spark interest and possibly also strengthen funds to tackle these other challenges.

Experiment: How did it work and what did it achieve?

Let’s take a look at the details together. The researchers used a high-power laser to fire 2.05 million Joules of energy at a small target containing fusion fuel. This forced the lighter atomic nuclei in the fuel to form heavier nuclei, releasing 3.15 million Joules of energy in the process.

This corresponds to a gain of approximately 1.5 (2.05 x 1.5 = 3.1). It was such an intense burst of energy that for a moment, burning fusion fuel produced ten thousand times more power than the combined output of every power plant on Earth.

This is great science. The NIF building is not one, but consists of 192 individual laser beams that bounce back and forth over a distance of more than a kilometer before reaching the target. The building that houses all this technology is ten stories high and the size of three (American) football fields lined up side by side.

Fusion research is divided into two main branches: laser driven fusion and magnetic confinement fusion. Magnetic confinement involves levitating fusion fuel in the form of a plasma (charged gas) using a large magnetic field.

Instead, laser-powered fusion involves blasting tiny fusion fuel capsules to incredibly high densities; at this point the combustion will progress so rapidly that a significant amount of energy can be released before the fuel has a chance to dissipate.

In either case, the fuel must be raised to tens of millions of degrees Celsius for it to start burning. It is this requirement, more than any other, that makes fusion so difficult to achieve.

Laser-guided fusion still poses major challenges

Laser fusion is a pulsed technology, and the so-called laser repetition rate is a major hurdle. Energy is released in intense bursts that last much less than a billionth of a second, and this must be repeated several times per second to produce an average power output comparable to modern fossil fuel power plants.

By these standards, the NIF laser is very slow. It can only be fired twice a day. However, the purpose of NIF was not to emulate the requirements of a real power station, but to demonstrate that firing was possible in one shot.

Another reason the firing took so long is that it’s not the only mission of the NIF, but it also supports the US nuclear weapons program.

The physics of laser-guided fusion is so complex and versatile that computer simulations of it often take more time than real experiments. In the early days, modelers learned more from experiments, rather than telling experimenters what to do next. The increasing closeness between the model prediction and the experimental result has underpinned recent success in NIF and bodes well for future improvements in target design.

In the next few months, modelers and experimenters will need to demonstrate that a result that has proven difficult in the past can be reproduced.

There are also a number of other challenges to overcome. Considerable work has been done in designing and building lasers that can fire high energy pulses many times per second.

Another major limitation is that the NIF laser requires an electrical input of 300 million Joules to deliver two million Joules of laser light output; which is less than 1% efficiency. The target would therefore have to achieve an impossibly large gain to generate more than the energy used to power the laser used in this example.

However, the NIF laser is based on technologies that date back to the 1980s. It uses strobe lamps and amplifiers made of glass sheets doped with the rare earth element neodymium.

Modern high-power lasers using semiconductor technology can do much better, reaching efficiencies of around 20%. Given that laser-guided fusion targets are expected to achieve gains of more than 100 when operating optimally, using modern lasers will produce significant net energy output.

Building a working reactor is still far away

Another challenge for laser-guided fusion is to reduce the cost of targets. The manpower involved in creating NIF targets means that each costs as much as a brand new car.

A new target is required each time the laser is fired. For real power generation this means new power several times per second. The targets used in NIF also rely on a technique known as “indirect propulsion,” in which the target first converts laser energy into X-rays and then detonates the fusion fuel capsule inside the target. This increases both complexity and cost.

Many scientists think the way forward for laser-powered fusion energy will involve “direct-drive” ignition. Here, the laser directly illuminates a simple, spherical fuel capsule. However, this approach to ignition has yet to be proven.

NIF’s fuel (deuterium and tritium) gives off most of its energy in the form of high-energy neutrons (particles that make up the atomic nucleus along with protons). Neutrons interact with the materials in the reactor vessel, changing their composition and microscopic structure.

This can pose serious challenges for optical components that must transmit or reflect laser light efficiently. Some scientists consider using similar physics in alternative ways, perhaps using direct pulsed electrical power or focused beams of ions (charged atoms).

Magnetic confinement fusion research leads in many areas related to the construction of a power reactor. He had to overcome many of the same challenges to design and build the ITER facility, which was also aimed at profit and was nearing completion in the south of France. Scientists and engineers from the two research branches collaborate on aspects of reactor construction that are common in both fields.

For decades, fusion power seemed like an forever inaccessible reward. While significant challenges remain as researchers actively work to improve laser technology and reactor design, breakthroughs will inevitably lead to further progress towards nuclear fusion-based power plants. Some researchers working on fusion now feel that they can see fusion powering the grid for their lifetime.

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Quotation: Nuclear fusion: how scientists can turn the latest invention into a new source of clean power (2022, Dec. 15) on December 15, 2022 at received .html

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