Classical diffusion

Classical diffusion is a key concept in fusion power and other fields where a plasma is confined by a magnetic field within a vessel. 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 rate of diffusion scales with 1/B2, where B is the magnetic field strength, implies that confinement times can be greatly improved with small increases in field strength. In practice, the rates suggested by classical diffusion have not been found in real-world machines, where a host of previously unknown plasma instabilities caused the particles to leave confinement at rates closer to B, not B2, as had been seen in Bohm diffusion. 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. Over time, the instabilities were found and addressed, especially in the tokamak. This has led to a deeper understanding of the diffusion process, known as neoclassical transport. Diffusion is a random walk process that can be quantified by the two key parameters: Δx, the step size, and Δt, the time interval when the walker takes a step. Thus, the diffusion coefficient is defined as D≡(Δx)2/(Δt). 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. Temperature is a measure of the average velocity of particles, so high temperatures imply high speeds, and thus a plasma will quickly expand at rates that make it difficult to work with unless some form of "confinement" is applied. At the temperatures involved in nuclear fusion, no material container can hold a plasma. The most common solution to this problem is to use a magnetic field to provide confinement, sometimes known as a "magnetic bottle". When a charged particle is placed in a magnetic field, it will orbit the field lines while continuing to move along that line with whatever initial velocity it had.