NUCLEAR FUSION DREAM MAKES A GREAT LEAP

Since mid-’50s, nuclear fusion (the same process that powers the stars) has allowed the dream of clean and virtually limitless energy, fuelled by two easily available forms of hydrogen, and producing almost no radioactive waste and no emissions of greenhouse gases. That would save a world threatened by climate change and an expected three-fold increase in global energy demand. However, it has always presented huge technical challenges, such as the management of a massive infrastructure or the design of ways of extracting net energy from it. Despite this, nuclear fusion benefits are so vast that it’s no wonder that scientists have spent half a century (and billions of dollars) into developing the right technology.

An important leap forward was made a week ago when researchers with the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, in California, announced that they reached a long-sought milestone. For the first time, the fuel used to trigger fusion in their reactor was capable of producing more energy than required to initiate fusion. This work, published in Nature magazine, crosses a crucial threshold on the ongoing pursuit toward wielding the power of nuclear fusion. Although the experiment did not yield a big amount of energy (around 1%), it brings us one step closer to a controlled and sustainable fusion reaction.

The nuclear power plants around the world today use fission, which works by splitting heavy atoms (such as those of uranium) and extracting the energy released. On the contrary, nuclear fusion produces a big deal of energy by “joining” two light atoms (hydrogen) together. This reaction takes place every day in the stars and the Sun which, deep at their core, contain a natural fusion reactor. Indeed, one of the challenges that makes fusion extremely difficult to control is the harnessing of the plasma that this process generates, as it reaches temperatures of millions of degrees.

The Livermore reactor is complex, and the way it works can be summarized into two fundamental steps:

1. Approaching hydrogen atoms: The two forms of the hydrogen fuel (deuterium and tritium) sit inside a plastic pellet in a cylinder. A huge amount of energy must be injected into the fuel to drive the hydrogen nuclei close together to overcome the electrical repulsion that keeps them apart. At the Livermore reactor this energy is provided by 192 laser beams.

2. Trying to trigger ignition: The lasers then heat up the cylinder, which re-emits the energy as X-rays. That causes the outer plastic shell to explode, raising the density of the fuel inside high enough to trigger fusion. The more energy the hydrogen atoms receive, the more fusion happens, and, ultimately, a chain reaction takes place and the fusion becomes self-sustaining. This is known as ignition. Although scientists have not yet achieved ignition (because only a very small amount of the laser’s power makes it to the hydrogen atoms), this work details how to obtain a net gain of energy within the fuel itself, which is a very important step on the way to ignition.

Plenty of work still remains for fusion researchers. According to them, only 1/200th of the energy that the lasers generate is delivered to the hydrogen fuel, which is not enough to set off a chain reaction. In that sense, scientists have to time their laser pulses to give the hydrogen the right kick. Moreover, scientists found it was extremely difficult to generate the right pressure and temperatures inside the hydrogen gas required for fusion. In order to do so, the plastic shell has to collapse perfectly symmetrically, and its design should be flawless.

The most ambitious scientific venture ever is starting to look more possible. The international nuclear fusion project, known as ITER, under construction in France, will also attempt to generate fusion energy by trapping the hydrogen plasma in a donut-shaped magnetic chamber (a completely different approach than what has been achieved at Livermore laboratories). Meanwhile, the NIF team is cautiously optimistic of their achievements and future prospects. Fusion-energy generation still remains a distant goal (first commercial nuclear fusion power plants are expected for 2050s), but it has never been worked on so enthusiastically.

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source: https://lasers.llnl.gov/science/energy-for-the-future

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