Finally, a Practical Use for Nuclear Fusion

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on 7th December 1995, a NASA probe entered Jupiter’s atmosphere and immediately started burning. It was hatched by the orbiting Galileo mission six months ago, and now, 80 million miles later, it was ready to sample the thick layers of hydrogen and helium around the Solar System’s largest planet.

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The spacecraft, called the Jupiter Atmospheric Probe, was carefully designed so that it could withstand the rising temperatures that would be exposed to Jovian air. It contained a massive carbon-based heat shield, comprising about 50 percent of the total weight of the probe, designed to dissipate heat as the probe descended. This controlled process, called ablation, was carefully modeled back on Earth—NASA even built a special test lab called giant planet feature In an effort to recreate the conditions and test the design.

As the probe descended through the clouds at more than 100,000 mph, friction heated the air around it by more than 28,000 degrees Fahrenheit — splitting the atoms into charged particles and creating an electrical soup known as plasma. known as. Plasma is responsible for natural phenomena such as lightning or aurora; The Sun is a huge burning ball of it. It is often referred to as the fourth state of matter, but it is actually the first: in the moments after the Big Bang, plasma was everything.


Plasma ate through the Jupiter probe’s heat shield much faster than anyone at NASA predicted. When agency engineers analyzed data from sensors mounted in the heat shield, they realized that their careful models were way off the mark. The gradient dissipated much more than expected in some areas and much less in others. The probe barely survived, and the only reason for this was that they had created a margin for error in the design by making it extra thick. “It was left as an open question,” says Auburn University plasma specialist Eva Kostadinova. “But if you want to design new missions, you have to be able to model what’s going on.”

After the Galileo mission, scientists used data from the probe to change their model of separation, but they still faced a major problem: precisely reconstructing the conditions for high-speed penetration into the dense atmosphere. is difficult, so it is hard to test those models for accuracy. It has also become a barrier for new heat shield materials that may be lighter or better than the carbon-based ones used now. If you can’t test them, it’s pretty hard to believe they’ll work when attached to a billion dollar spacecraft.

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Previous test attempts have used lasers, plasma jets and high-speed projectiles to simulate the heat of penetration, but none of them are perfect. “No aerospace facility on Earth can reach the high heat conditions that you experience during atmospheric entry into something like Jupiter,” Kostadinova says.

Now, new research from Kostadinova and UC San Diego collaborator Dmitry Orlov has demonstrated a possible alternative – the fiery innards of an experimental nuclear fusion reactor.

There are a few hundred such reactors in state-funded research facilities around the world, known as tokamaks, including united european torus in the United Kingdom, and ITER, the International Thermonuclear Experimental Reactor, a 35-nation collaboration in southern France. For decades, researchers have been using them to tackle the challenges of nuclear fusion, a potentially revolutionary technology that could deliver essentially unlimited power. Inside a tokamak, powerful magnets are used to hold the swirling plasma at high pressure, enabling it to reach the tens of millions of degrees necessary for atoms to fuse together and release energy. Cynics argue that nuclear fusion is doomed to remain the energy source of the future forever – right now, fusion experiments still consume more electricity than they generate.

But Kostadinova and her colleague Dmitry Orlov were more interested in the plasma inside these reactors, which they realized might be an ideal environment to simulate a spacecraft entering the atmosphere of a gas giant. Orlov works on the DIII-D fusion reactor, an experimental tokamak at the US Department of Energy in San Diego, but has a background in aerospace engineering.

Together, they used the DIII-D facilities to run a series of experiments on isolation. Using a port on the bottom of the tokamak, they inserted a series of carbon rods into the plasma flow, and used high-speed and infrared cameras and spectrometers to track how did they break up, Orlov and Kostadinova also fired minuscule carbon pellets In the reactor at high speed, a smaller scale mimicking the heat shield on the Galileo probe would have encountered Jupiter’s atmosphere.

The conditions inside the tokamak were remarkably similar in terms of the temperature of the plasma, the speed at which the material was flowing, and even its composition: the Jovian atmosphere is mostly hydrogen and helium, the DIII-D tokamak uses deuterium, Which is one isotope of hydrogen. “Instead of launching something at very high speed, we instead launch a stationary object into a very fast current,” Orlov says.

Experiments presented this month at a meeting of the American Physical Society in Pittsburgh helped validate that model of separation Which were developed by NASA scientists using data sent back from the Galileo probe. But they also serve as proofs of concept for a new type of test. “We are opening up this new area of ​​research,” Orlov says. “Nobody’s done it before.”

This is something that is highly needed in the industry. “There have been gaps in the new testing procedures,” says Yani Barghouti, founder of the company. Cosmic Preservation Corporation, a startup building radiation shields for spacecraft. “It allows you to prototype much faster and more cheaply – there’s a feedback loop.”

Whether nuclear fusion reactors will be a practical test base remains to be seen – they are incredibly sensitive devices designed for some other purpose entirely. Orlov and Kostadinov were timed at DIII-D as part of a special effort to use the reactor to expand scientific knowledge, using the port built at Tokamak for the purpose of safely testing new materials. To be. But it is an expensive process. His day on the machine was worth half a million dollars. As a result, such experiments are least likely to be performed in the future, when the opportunity arises, to try and improve computer simulations.

With further experiments, Orlov and Kostadinova hope that the models can be improved and used to optimize heat shield design for future missions—putting more material where it is needed, But removing it where it isn’t. NASA’s DAVINCI+ mission, projecting towards Venus at the end of the decade, may be the first to take advantage. It consists of an orbiter and a descent probe, which will require powerful shielding as it passes through the hot, dense Venusian atmosphere. The Galileo probe taught scientists a lot about the formation of the solar system, but with an improved heat shield, it could have done a lot more. “Half the payload is something that’s just going to burn,” Kostadinova says. “You’re limiting the number of scientific instruments you can actually fit in.”

In addition, the technique could be used to test new materials, such as silicon carbide, or new forms of heat shields that use a mixture of inert materials and other components that do not. Engineers will need them for future missions – the Galileo probe took the slowest, flattest trajectory to limit separation, and still extend the range of what was possible.

The research could also help with the design of the fusion reactors themselves. So far, most research has focused on the main plasma reactions inside tokamak. But as nuclear fusion moves toward commercialization, more attention will need to be paid to building reactors and designing materials in which a fusion reaction can take place and safely dissipate energy if things go wrong. .

Kostadinova and Orlov are calling for more collaboration among the fusion and space research communities, who are interested in both understanding and developing plasma reactions—and in developing the substances that can contain them. “The future is to create better materials and new materials,” Kostadinova says.

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