Part 1 of 2 Parts
École Polytechnique Fédérale de Lausanne (EPFL) is a Swiss public research university. Operating in a large European collaboration, EPFL has managed to revise one of the fundamental laws that have been foundational to plasma and fusion research for over three decades. Their work has even been used in the design of megaprojects such as the huge ITER fusion reactor being constructed in France.
Fusion is considered by many to be the most promising source of future energy. It involves combining two light atomic nuclei into one heavier nucleus. Huge amounts of energy are released in the process.
ITER is an international fusion research megaproject. It is aimed at replicating the fusion processes of the sun to create energy on Earth. In order to accomplish that, it must produce the extreme temperatures and pressures necessary for fusion to occur.
A plasma is an ionized state of matter similar to a gas. It is made up of positively charged nuclei and negatively charged electrons. The plasmas used in fusion research are about a million times less dense than the air we breathe. Plasmas are created by subjecting the fusion fuel, usually isotopes of hydrogen, to extremely high temperatures that are ten times the temperature of the core of the sun. This forces the electrons to separate from their atomic nuclei. One of the most popular experimental fusion designs is called a tokamak. It is a donut-shaped chamber surrounded by magnets.
Paolo Ricci works at the Swiss Plasma Center, one of the leading research institutions in fusion located at EPLF. He said, “In order to create plasma for fusion, you have to consider three things: high temperature, high density of hydrogen fuel, and good confinement.” Working within a large European collaboration, Ricci’s team has just released a new study. Their work updates a foundational principle of plasma generation. They have shown that the ITER tokamak can actually operate with twice the amount of hydrogen fuel normally used. This means that it can generate more fusion energy that was previously assumed.
Ricci said, "One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it. Since the early days of fusion, we've known that if you try to increase the fuel density, at some point there would be what we call a 'disruption'—basically you totally lose the confinement, and plasma goes wherever. So in the eighties, people were trying to come up with some kind of law that could predict the maximum density of hydrogen that you can put inside a tokamak.”
In 1988, fusion scientist Martin Greenwald published a famous law that correlates fuel density to the minor radius of the tokamak and the electric current that flows in the plasma inside the tokamak. (Minor radius refers to the radius of the inner circle of the donut-shaped tokamak.) Ever since Greenwald stated his law, the “Greenwald limit” has been a foundational principle of fusion research. ITER’s tokamak-building strategy is based on it.
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