Informing Tokamak Design with State-of-the-Art Physics Modeling

A team of plasma physics researchers at Columbia provide essential information leading to refinements in the design of a next-generation tokamak.

The plasma physics group at Columbia University has recently published a series of three articles describing essential considerations for a new tokamak designed to demonstrate net fusion energy gain by producing 140MW of fusion power. Tokamaks are toroidal devices that confine plasma inside of them with strong magnetic fields. The recent work by Columbia concerns the design of a new machine called SPARC, will be operated by Commonwealth Fusion Systems (CFS) and is currently under construction in Devens, Massachusetts. This research highlights computational and collaborative strengths of the Columbia plasma program.

Measuring the Magnetic Field

New research, published by Columbia researcher Ian Stewart in collaboration with CFS and the Massachusetts Institute of Technology (MIT) has identified the key areas where magnetic sensors need to be placed in order to control the shape and geometry of the plasma in the future SPARC experiment [1]. These magnetic sensors are small loops of wire that track the change in flux to measure the magnetic field at various locations around the tokamak. A cross section of SPARC and the layout of the optimized magnetic sensor set from the publication are shown in Figure 1.

Figure 1: (a) Cross section of one half of the SPARC tokamak, (b) diagram of the magnetic sensor locations on the vacuum vessel wall with the contours of the plasma equilibrium shown in blue and (c) a three dimensional view of the flux loop sensors defined in the study.

Determining the plasma location and shape using magnetic sensors is fundamental to the success of tokamaks like SPARC that rely on magnetic control to maintain a stable, high-performance plasma. This study determined where sensors should be placed and how many sensors are necessary to measure the plasma shape via a process known as equilibrium reconstruction. These general methods can be extended for use in the design of other future fusion experiments, which will have limited space and budgets to construct and install these types of critical sensors.

Ensuring Vertical Stability

Tokamak plasmas are often stretched vertically in order to maximize the volume of plasma available for fusion power production. However, there is a delicate balance between how stretched a plasma can be before it becomes subject to a vertical instability which can lead to loss of plasma confinement.

In order to assess this phenomenon on the SPARC machine, Columbia researcher Oak Nelson worked closely with scientists at MIT and CFS to model the response of the magnetic equilibrium to active control coils. This analysis, recently published in Nuclear Fusion [2], devises strategies and constraints on the control system that will allow operators to ensure safe operation of SPARC as it pursues its performance goals.

Catching Relativistic Electrons

Relativistic electrons are a phenomenon unique to plasma physics. An electron moving through a hot plasma in the presence of a strong and sustained electric field can quickly be accelerated up to speeds approaching the speed of light. These conditions can occur during tokamak events characterized by a quick termination of the current flowing through the plasma. The resulting beams of relativistic electrons have the potential to severely damage the walls of future tokamaks if they are not properly handled. 

Figure 2: Mitigation coils designed to control relativistic electrons for the SPARC and DIII-D tokamaks.

To mitigate the effects of these electrons on SPARC, Columbia researchers Alexander Battey and Chris Hansen explored the potential addition of a mitigation coil to the SPARC design in a stand-alone work [3]. The coil (depicted in figure 2 alongside a similar design for the DIII-D tokamak in San Diego) is a helical conducting structure placed within the tokamak that conducts large currents in the presence of strong electric fields. This will create a large three dimensional magnetic field which is capable of quickly dispersing the relativistic electrons to prevent wall damage.

Dive Deeper

The work described in this article is the subject of three publications, listed below, appearing in the journal Nuclear Fusion. Columbia Plasma Lab members are working on further related studies, particularly focused on the pursuit of clean and sustainable fusion energy.

References

[1] https://iopscience.iop.org/article/10.1088/1741-4326/acf600/meta

[2] https://iopscience.iop.org/article/10.1088/1741-4326/ad58f6

[3] https://iopscience.iop.org/article/10.1088/1741-4326/ad0bcf/meta