Ions and electrons will collide with one another, but not like billiard balls because they have electrical charges. Like charges repel, so an ion approaching another ion will feel the repulsion well before they come close and will veer off. There is an occasional head-on fusion collision, of course, but these are very infrequent. The result of the more distant collisions is to form the most probable distribution of velocities; namely, the Maxwellian distribution shown in Fig. 3.3. Electrons will do the same thing, only faster because they are lighter and move faster at the same energy. So their velocities will also fall into a Maxwellian distribution. However, it does not have to be at the same temperature as the ion distribution. The way we heat a plasma usually heats one species preferentially. For instance, driving a current through a plasma will preferentially heat the electrons, so that the electron tempera­ture, called Te, will be higher than the ion temperature, T. A plasma can have two different temperatures, Te and T, at the same time, or even more if there are other species in the plasma. It may seem unusual that a plasma can have two temperatures at the same time, but imagine turning on the heat in a cold room. The air will get hot first, while the furniture stays cold. It will take some time for everything to come to the same temperature. Though ions and electrons in a plasma are inter­mixed, they exchange their heat comparatively slowly because they collide infre­quently and have vastly different masses. Plasma particles are always being regenerated as they leave the container, and usually they leave before they can come into equilibrium with other species, so it is normal for T to be different from T.

When an electron collides with an ion, their opposite charges attract, and the electron will orbit the ion the way a comet orbits the sun. These collisions will tend to equalize Te and T, but it takes much longer because an electron is so much lighter than an ion that very little energy is exchanged at each collision. Generally, parti­cles do not stay in the plasma long enough for Te and Ti to equalize, so the tempera­tures are usually different.

What do we mean by a collision when particles do not actually touch? The mag­nitude, so to speak, of a collision depends on how much the particles’ paths have been deflected or how much their energies have changed. In this type of collision at a dis­tance, each particle feels the electric field of the other particle during the time when they are close. This time becomes very short when the particles are moving fast. An electron with 10 keV of energy, for instance, will go past an ion so fast that there is hardly any time for the ion’s electric field to deflect the electron or change its energy. It makes sense, therefore, that a hot plasma, whose particles have large velocities, hardly makes any collisions at all; in other words, it is a superconductor. Even plasmas with only 100-eV temperature can act like superconductors. We call these collision­less plasmas. Being able to neglect collisions makes theory much simpler, and most of the early work concerned collisionless plasmas. In most cases, this was a good approximation, since an electron can travel around a torus many times before it makes an effective collision. Later in the development of magnetic confinement, people finally realized that these weak collisions cannot be neglected after all.