A new way to measure the stability of next-generation magnetic fusion devices

Scientists seeking to bring the fusion that powers the sun and stars to Earth must control the hot, charged plasma – the state of matter composed of floating electrons and atomic nuclei, or ions – that feeds the fusion reactions . For scientists who confine plasma in magnetic fields, a key task is to map the shape of the fields, a process called measure of plasma balance or stability. At the Princeton Plasma Physics Laboratory (PPPL), researchers have proposed a new measurement technique to avoid the problems expected when mapping fields on large and powerful future tokamaks, or magnetic fusion devices, that host the reactions. .

Neutron bombardment

These tokamaks, including ITER, the great international experiment under construction in France, will produce neutron bombardments that could damage the interior diagnostics currently used to map the fields of current installations. PPPL therefore proposes to use another diagnostic system that can operate in high neutron environments.

The system, a type of plasma diagnostic called "Electronic Cyclotron Emission (ECE)", measures the temperature of electrons cycling around the field lines. "By using an EPE system, we can learn more about plasma temperature and fluctuations in the plasma," said Andrew "Oak" Nelson, a graduate student in plasma physics at PPPL and senior author of a article on plasma physics and controlled fusion that reports research. "This method could be turned into a stand-alone mapping tool or used with existing tools."

The method combines ECE data with a fast camera image used to measure the plasma limit. The combination provides "diagnostics that can be robustly designed in high neutron environments," according to the document. The process works as follows:

• Researchers observe the radiation emitted by the cycling electrons.
• Radiation provides data on the temperature and modes, or instabilities, that develop in the plasma;
• Data makes it possible to measure the "q profile" – the helicity or the spiral of the magnetic field;
• The measurement of the helicity allows tokamak operators to map and control the plasma equilibrium.

Reverse a process

This technique, which the researchers tested on a simulated discharge of the National Spherical Torus Experiment (NSTX) experiment at PPPL before it was upgraded, reverses the process normally used in fusion research. "People usually get the q profile of balance," said Nelson, "but our article shows that you can also get balance by knowing the profile q."

Egemen Kolemen, a PPPL physicist and assistant professor in the Department of Mechanical and Aerospace Engineering at Princeton University, worked closely with Nelson. "Oak is an extremely talented student," said Kolemen. "The method that he developed makes it possible to build the state of the fusion plasma using a single diagnosis, the ECE.This will be useful for many tokamaks, including ITER, because the combination of many diagnoses is problematic and a source of errors. "

Researchers are now considering testing the ECE technique on a wide variety of plasma discharges. A proven and fully developed technique could provide a valuable system for mapping crucial magnetic fields in ITER and next-generation tokamaks.