Page 121 - High Power Laser Handbook

90 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s High-Power Fr ee-Electr on Lasers 91

5
4.5

4
3.5 3
Power (arb. units) 2.5 2

1.5
1
0.5

0
3000 3050 3100 3150 3200 3250 3300 3350 3400
Wavelength (nanometers)
Figure 4.6b The corresponding spectra of operation at three points in the detuning
curve. Narrowest (widest) spectrum is lowest (highest) power. At all times, the
output is Fourier transform-limited—that is, the micropulse length changes as a
function of detuning.

4.3.6 Energy Recovery
The low losses in superconducting cavities enhance the benefit of
another technology—energy recovery. As was discussed earlier, the
FEL interaction can remove approximately 1 percent of the electron
beam power as light, while increasing the energy spread to 6 percent
or more. That leaves 99 percent of the electron beam power available.
When FELs are built for small, low-power facilities, the electron beam
is typically dumped into a copper block after performing the lasing.
In a CW high-power system, however, this dumping is extremely
wasteful; therefore, the electron beam is reinserted into the linac
180 degrees out of phase with the RF fields, so that instead of being
accelerated, the beam decelerates back down to the injection energy.
The beam power is thus provided back to the RF fields. Because this
process takes place in a nearly lossless superconducting cavity, the
efficiency of such conversion is near perfect.
There are three benefits to such a procedure: (1) The electrical effi-
ciency is substantially enhanced, because the only RF power that must
be made up is the power lost to lasing and that beam power dumped
at the end; (2) the power losses on the dump are substantially reduced,
thereby simplifying the dump engineering design; and (3) because the
electron beam energy is reduced below the photoneutron threshold of
approximately 10 MeV, there is essentially no production of neutrons