Spheromak - Creating Spheromaks

Creating Spheromaks

Spheromaks form naturally under a variety of conditions, enabling them to be generated in a number of ways. These devices are also sometimes referred to as spheromaks.

The most common modern device is the "Marshall gun" or "injector". The device consists of two closed cylinders, one inside the other. The inner cylinder is shorter, leaving an empty space at the bottom. An electromagnet inside the inner cylinder is used to set up an initial field. The field is similar to the one from a bar magnet, running vertically down the center of the inner cylinder and up the outside of the apparatus. The magnet is positioned so that the area where the field loops over from the center to outside, where the field lines are roughly horizontal, is aligned with the bottom of the inner cylinder.

A small puff of gas is introduced to the area between the cylinders. A large electric charge supplied by a capacitor bank is applied across the cylinders, ionizing the gas. Currents induced in the resulting plasma interact with the original magnetic field, generating a Lorentz force that pushes the plasma away from the inner cylinder, into the empty area. After a short period the plasma stabilizes into a spheromak.

Other common devices include open-ended or conical theta-pinch, where they were first researched in depth, and machines that generate them magnetically in a steady state.

Since the spheromak's magnetic confinement is self-generated, no external magnet coils are required. However, the spheromak does experience the "tilting perturbation" that allows it to rotate within the confinement area. This can be addressed with external magnets, but more often the confinement area is wrapped in a conductor, typically copper. When the edge of the spheromak torus approaches the concudtive surface, a current is induced into it that, through Lenz's law, reacts to push the spheromak back into the center of the chamber.

It is also possible to get the same effect with a single conductor running down the center of the chamber, through the "hole" in the center of the spheromak. As this conductor's currents are self-generated, it adds little complexity to the design. However, stability can be further improved by running an external current in the central conductor. As the current scales up it approaches the conditions of a traditional tokamak, but in a much smaller size and simpler form. This evolution led to considerable research on the spherical tokamak during the 1990s.