free electron lasers 151
FIGURE 8.12
Multi-pass FEL.
8.4.2 Single-pass FEL
In a single-pass FEL, the radiation has to grow within a single
passage of the beam through the undulator (Fig. 8.13). Such
FELs can be either seeded or the SASE type.
The need to use single-pass systems is actually a necessity
dictated by an absence of good mirrors in the X-ray spectral
region. The single-pass system has to be a high-gain system,
which puts extreme constraints on the quality of the electron
beam, as well as on the accuracy of the undulator. A typical
permanent magnet undulator is shown in Fig. 8.14.
FIGURE 8.13
Single-pass FEL.