Classical diffusion

It considers collisions between ions in the plasma that causes the particles to move to different paths and eventually leave the confinement volume and strike the sides of the vessel.

The failure of classical diffusion to predict real-world plasma behavior led to a period in the 1960s known as "the doldrums" where it appeared a practical fusion reactor would be impossible.

Plasma is a gas-like mixture of high-temperature particles, the electrons and ions that would normally be joined to form neutral atoms at lower temperatures.

Since the axial velocities will have a range of values, often based on the Maxwell-Boltzmann statistics, this means the particles in the plasma will pass by others as they overtake them or are overtaken.

When the topic of controlled fusion was first being studied, it was believed that the plasmas would follow the classical diffusion rate, and this suggested that useful confinement times would be relatively easy to achieve.

To examine this, the Model-B2 stellarator was run at a wide variety of field strengths and the resulting diffusion times were measured.

Further experiments demonstrated that the problem was not diffusion per se, but a host of previously unknown plasma instabilities caused by the magnetic and electric fields and the motion of the particles.

Over time, a number of new designs attacked these instabilities, and by the late 1960s there were several machines that were clearly beating the Bohm rule.