Physicists at the Princeton Plasma Physics Laboratory (PPPL) have recently proposed that the formation of “hills and valleys” in magnetic field lines could be the origin of sudden collapses of heat ahead of disruptions that can damage donut-shaped tokamak fusion facilities.
This research was published in the journal Physics of Plasma last July. It traced the collapse to the 3D disordering of the strong magnetic fields used to confine the hot, charged plasma gas. Min-Gu Yoo is a post doctorial researcher at PPPL and lead author of the paper. He said, “We proposed a novel way to understand the [disordered] field lines, which was usually ignored or poorly modelled in the previous studies.”
Fusion is the process that powers the Sun and other stars as hydrogen atoms fuse together to form helium. During the process a great deal of energy is produced. If scientists could capture the process on Earth, they could create a clean, carbon-free and almost inexhaustible source of power to generate electricity. However, this task faces many serious challenges. In stars, massive gravitational forces create the proper conditions for fusion. On Earth, those conditions are much more difficult to achieve.
Instead of the immense gravity that holds fusion reactions in place in celestial bodies, strong magnetic fields can be utilized in a device known as a tokamak to confine the hot plasma in which the fusion reactions takes place. However, in laboratory experiments, when magnetic fields lines become disordered due to plasma instability, the superhot plasma heat can rapidly escape confinement which results in damage to the tokamak vessel.
Weixing Wang is a research physicist and a co-author of the paper. He said, “In the major disruption case, field lines become totally [disordered] like spaghetti and connect fast to the wall with very different lengths. That brings enormous plasma thermal energy against the wall.”
The PPPL team has studied the 3D topology of the disarrayed field lines caused by turbulent instability to find that these form tiny “hills” and “valleys”. Some of the particles become trapped in the “valleys” and are unable to escape confinement. Others roll down the “hills” ad impact the walls of the tokamak vessel.
Yoo said, “The existence of these hills is responsible for the fast temperature collapse, the so-called thermal quench, as they allow more particles to escape to the tokamak wall. What we showed in the paper is how to draw a good map for understanding the topology of the field lines. Without magnetic hills, most electrons would have been trapped and could not produce the thermal quench observed in experiments.”
The PPPL research provided new physical insights into how the plasma loses its energy towards the wall when there are open magnetic lines. It will assist in finding innovative ways to mitigate or avoid thermal quenches and plasma disruptions in the future.
From my own research, I believe that it may be necessary to develop real time control of chaotic turbulence in magnetically confined plasmas to achieve fusion for power generation.