Fusion Power - The Stellarator

The Stellarator

Being one of the earliest devices that was designed to study fusion power, the Stellarator was invented by Lyman Spitzer in 1951. Similar to the Tokamak, the Stellarator relies on magnetic confinement to trap the plasma in the torus. The image below displays how both designs follow the same foundation, yet have differences in features to make them stand apart.

Fun fact: the name Stellarator refers to the possibility of harnessing the power source of the stars.

Due to my explanation of how Fusion Reactors work in the Tokamak post, I will be going over the highlighting features that differentiate the Tokamak and the Stellarator.

Magnetic Field Differences

The most notable difference of the Stellarator is its external non-axisymmetric coils. According to Spitzer, the three different ways to twist a magnetic field (to contain plasma) is:

[i] creating a poloidal magnetic field by sending current through the toroidal field.

[ii] rotating the poloidal cross-section of stretched flux surfaces around the torus (this is why Stellarators look like it's being twisted).

[iii] making the magnetic axis non-planar.

Above is a photo of how the magnetic coils on the Stellarator create a non-planar magnetic axis.

While the Tokamak uses [i], the Stellarator uses [ii] and [iii].

Each of these methods have there own upsides and downsides:

[i] the benefits of using a current based twist is that confining the plasma particles is easy. However, adding a current also adds current instabilities, overall making the whole system difficult to operate in a steady state.

[ii/iii] due to the lack of a current, it is easier for Stellarators to operate in a steady state. However, plasma particles go through a lot of neoclassical transport (stray from the orbit), hence reducing energy efficiency.

Geometrical Differences

R and a represent the major and minor radii.

Tokamaks typically have an aspect ratio of 2.5 - 4.

Stellarators typically have an aspect ratio of 5 - 12. The aspect ratio is much large in efforts to have small rotational transforms per period.

Therefore, Tokamaks can effectively hold a higher volume of plasma.

Wendelstein 7-X

This Stellarator was built by Max Planck Institute of Plasma Physics in Germany. It was made as an experiment, and not to generate electricity. The shining feature of this experiment was its continuous operation of 30 minutes of continuous plasma discharge. I believe this to be the exemplary Stellarator as of now. It is currently still being worked on and is definitely worth to keep up to date with!