Improved code reveals source of damaging fusion disturbances

Destructive magnetic disturbances create a complex 3-D structure of magnetic field lines that randomly roam inside the tokamak. The red line shows the 3-D trajectory of an example field line, and each field line can have a very different trajectory. The colors in the cross section represent the length of the field line path through each area, from short (black) to long (yellow) lengths. Credit: Min-Gu Yoo

Researchers at the Princeton Plasma Physics Laboratory (PPPL) of the US Department of Energy and the Los Alamos National Laboratory have discovered a key process behind a major challenge called thermal quenching, the rapid heat loss in hot plasmas that can occur in donut-shaped tokamak melting devices. Such quenchings are sudden drops of electronic heat in the plasma that feed the fusion reactions, drops that can create damaging disturbances inside the tokamak. Understanding the physics behind these quenches, caused by powerful disturbances in the magnetic fields that confine plasma in tokamaks, could lead to methods to mitigate or prevent them.

Researchers have now mapped out a complete mechanism for thermal quenching up to turbulent particle transport. Using the lab’s tokamak gyrokinetic simulation (GTS) code, physicists explored how hot plasma, made up of free electrons and atomic nuclei, or ions, generates the electric field and turbulent transport of particles at the start of quenching.

The GTS code was originally developed at PPPL to simulate turbulence and transport physics in hot-core plasmas which are confined by magnetic fields in tokamaks. Recently, the GTS code has been extended to study more complex plasmas and magnetic fields, such as destructive magnetic disturbances that break the magnetic field cage and create chaotic 3D magnetic field lines (Figure 1). The introduction of new digital algorithms and the acceleration of graphics processing units have made this powerful new capability possible. This upgrade enables the consistent simulation of plasma transport during thermal quenching at lower computational costs, providing important new insight into the physics of disturbances.

The GTS code traced the plasma transport mechanism to the evolution of a self-generated electric field in chaotic 3D magnetic fields, the complexity of which had previously made the extinction mechanisms difficult to understand. The improved code unraveled the controversy and laid bare the physics behind the mechanism.

The self-generated field mixes the plasma, causing high-energy electrons to escape from the nucleus and fly towards the wall. This improved heat transport produces a rapid and continuous drop in the temperature of the electrons, leading to thermal quenching.

From the results of the simulation and comparison with experimental observations, the researchers discovered that this new mechanism could be a major contributor to steep quenching. The researchers proposed an analytical model of plasma transport that provides new physical knowledge to understand the complex topology of 3D magnetic field lines. These groundbreaking discoveries could lead to new steps in tackling damaging disruption.

Neutral particles slow down disruptive plasma blobs

More information:
Abstract: TI02.00001. Collision-free transport mechanisms for thermal quenching in stochastic magnetic fields open at the wall boundary

Provided by the American Physical Society

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