Magnetic confinement fusion is one of the two current methods being researched for the containment of the plasma. Within these machines, magnetic fields are used to contain the charged particles that compose the hot plasma and keep it away from the chamber walls. This method is used for containing the plasma for a relatively long time at a low density.

Magnetic confinement rests upon the property that charged particles, like those in a plasma, will travel along the lines of a magnetic field. By arranging magnetic fields in just the right way, scientists have been able to "trap" the plasma within the fields. While the plasma is held, it can be heated through a combination of microwaves, particle beams, and the heating generated from currents flowing through the plasma. The plasma density in a magnetically confined reactor is roughly 10¹⁵ particles/cm³, which is thousands of times less dense than that of air at room temperature. Currently there are two types of magnetic confinement systems: the mirror (open) and the toroidal (closed). The primary toroidal method we will be looking at is the tokamak, although there are other toroidal confinement techniques, including reversed-field pinch, sellarator, and others undergoing research.

Magnetic field lines:
"mirror"
Magnetic field lines in a "magnetic mirror". Image courtesy of General Atomics.

The Mirror

Older designs of magnetic confinement machines involved designs similar to what is known as the magnetic mirror. These are considered to be of the open type. The original idea was based on the fact that an electric current generates a magnetic field, and that the currents flowing in the plasma will essentially "pinch" the plasma, containing it within its own magnetic field. Magnetic fields are much stronger than gravitational forces, so there was hope that strong fields would contain the plasma. However, the magnetic force is two-dimensional, and acts only perpendicularly to the direction of the current. Thus, the plasma contained with these devices were cylindrical. To lessen the loss of plasma at the two ends, two coils placed a distance apart produce a stronger magnetic field near themselves, with the weaker field in the center. The coils essentially create a "bottleneck" to the plasma at each end, preventing the plasma from escaping. The plasma is reflected at the ends by the stronger fields.

Magnetic field lines:
Tokamak; width=
Magnetic field lines in a tokamak. Image courtesy of General Atomics.

The Tokamak

The other type of magnetic confinement device is called the tokamak, a word formed from the Russian words "TOroidalnaya KAmera ee MAgnitnaya Katushka," or "Toroidal Chamber and Magnetic Coil". Tokamaks were originally designed and used in Russia. In this design, the chamber is toroidal, or doughnut-shaped, thus having no open ends. The magnetic field is generated through the current running in the coils that are wrapped around the reactor. The field is stronger towards the center, causing the plasma to tend towards the outer wall. However, another magnetic field generated by a current going through the plasma itself combines with the coils' magnetic field to create magnetic lines that spiral around the torus. This spiralling counteracts the drifting effect on the plasma because of the strong inner field, and effectively traps the plasma.

Cross-Section of a Tokomak Reactor
Image of a
Tokamak cross-section

Image courtesy of the Lawrence Livermore National Laboratory

The initial heating of the tokamak can occur in multiple ways and combinations.
1. Ohmic Heating - The plasma can be heated to temperatures up to 20-30 million K through the current passing through the plasma. It is called ohmic or resistive heating; the heat generated depends on the resistance between the plasma and current. However, as temperature rises, resistance drops, making this form of heating less and less effective. Other methods are neccessary in addition in order to heat the plasma to required temperatures.
2. Neutral-Beam Injector - High energy, neutral atoms are shot into the plasma, and are immediately ionized. These ions then get trapped by the magnetic fields, and transfer some of their energy to the surrounding plasma particles through collisions, thus raising the overall temperature.
3. Magnetic Compression - The plasma can be heated through a rapid compression, which is possible by increasing the magnetic field. In the tokamak, this compression occurs by moving the plasma to an area of a higher magnetic field.
4. Radiofrequency Heating - High-frequency waves are launched into the plasma through the use of oscillators. If the waves have the right wavelength, their energy can be transferred into certain particles, which then transfer the energy through collisions with others.

Both the mirror design and the tokamak attempt to meet the Lawson criterion through a low density plasma with relatively long containment times. Now let us see what inertial confinement does.

Let's Review:

Briefly describe what a tokamak is, and how it confines the plasma.

List three possible ways to heat the plasma in a tokamak.