The geometry of a free-electron laser (FEL) resonator differs from a conventional high energy laser (HEW. In most high powered lasers, the gain is provided by atoms or molecules that are excited into a higher energy level. It is usually possible to produce this type of excited state over a relatively large volume. This provides the cavity designer of more conventional HELs with both opportunities and problems. On the plus side, it is easy for a designer to use the large cross section of the active (lasing) material to spread the power of the laser beam over a larger surface of the cavity mirrors. On the negative side, the larger extent of this relatively dense active media produces variations in both the optical phase and intensity of the beam, which need to be controlled if the beam is to be focused effectively on a target. In contrast, the active medium of a free-electron laser consists of a beam of high speed electrons passing through a series of small magnets. The magnets make the electrons wiggle as they move, and this wiggling makes the electrons give off light. From an optics point of view, the main difference is that it is not practical, at this time, to produce very thick beams of high energy, high quality electrons so that the active area in an FEL has an overall shape of a long, very thin rod. In order for the laser to work effectively, the beam of light must also take on this shape so that the laser light and the electron beam overlap. This presents the cavity designer with the problem of dealing with a beam of light of very high power, concentrated into a very small cross-sectional area. This light can be too concentrated for a normal mirror, which makes it necessary to use some unusual cavity designs.