Oscillator Configurations

The gain mechanism can also be used as an oscillator by feedback of a portion of the output signal. An oscillator may be self-excited in the sense that radiation will spontaneously grow from noise if the gain on traversing the wiggler exceeds the losses. An oscillator may also be mode-locked by the injection of a large-amplitude signal with a frequency within the gain band. Since the gain in the interaction region increases with beam current, there is a threshold value of current (known as the start current) below which the gain is exceeded by the losses in the cavity and the radiation will not grow. If the beam current exceeds this threshold, spontaneous noise in the electron beam will be amplified and will grow exponentially in time until the radiation in the oscillator reaches saturation. There are a number of reasons for constructing an oscillator as opposed to an amplifier. First, it may be that no source is available to drive an amplifier. Second, the amplification in one pass through the wiggler may be too small. An amplifier with a small gain is of little practical value. However, if most of the output signal can be fed back to the input, a useful oscillator can be constructed. As a general rule, one finds that shorter wavelength devices which rely on relatively high-energy but low-current electron beam sources fall into this category and tend to be oscillators because of their inherent low gain. In this chapter, we discuss the general theory underlying free-electron laser oscillators.