Page 491 - Tunable Lasers Handbook

9 Tunable Free-Electron Lasers 45 1

electron beamline. This range has been extended to a 10% wavelength range
(5% in energy) more recently by use of a computer control system [16].
The TRW/Stanford FEL collaboration was successful in achieving lasing
between 4.0 and 0.5 pm by varying the electron-beam energy [ 171. This laser uses
the same superconducting accelerator used for the first Compton regime FEL.
Tuning the wavelength via energy change has several advantages and disad-
vantages. One major advantage is in the undulator design. A fixed undulator is
simpler and less expensive to design and build than a tunable undulator. For an
undulator with more than around 80 periods it becomes extremely difficult ec
built a wiggler Ivhose field can be adjusted continuously. The wiggler parameter
K can also be smaller. Most designs for compact wigglers result in values of K
much less than unity [18-201. These designs must therefore rely on energy tun-
ing to achieve a broad tuning range.
Another advantage of energy tuning is that it can be exceedingly rapid. The
laser should be able to tune at a rate of one gain bandwidth per turn-on time.
This can lead to tuning across a range of 10% in tens of microseconds. The
TRW/Stanford collaboration has demonstrated tuning of 2%/ms during a
macropulse several milliseconds long. Researchers at LANL [21] and at the
FELIX facility [22] have also demonstrated fast wavelength tuning via energy
change. This feature might be quite useful in lidar applications.
The primary disadvantage to energy tuning is the need to readjust the entire
electron-beam transport line leading to the laser. In some lasers this can be a very
slow task. A good computer control system can. in principle, allow reasonably
rapid scanning of the electron-beam energy over a factor of 2 range as is done in
storage rings, but this has not been demonstrated in a FEL device to date.
The second disadvantage is that. if the beam current is fixed, the eiectron-
beam power decreases as the electron energy decreases. Thus. the power out of
the laser varies as the inverse square root of the wavelength. Because the gain
often increases as the energy decreases, it is possible to change the undulator
and increase the efficiency as the laser wavelength is increased. Just removing
periods would present severe mechanical design challenges. It has been shown
that introducing a taper to the wiggler field enhances the efficiency [23]. One can
change the taper. and therefore the efficiency, as the wavelength is increased. In
some accelerators, it is possible to reduce the energy by increasing the beam cur-
rent while holding the beam power constant. This could also be used to tune the
wavelength at constant laser power. 4 special case of energy tuning is that of a
storage ring FEL, whose power is proportional to the third power of the electron-
beam energy. The gain is not a steep function of electron-beam energy and taper-
ing is not usually an option due to the energy aperture of the storage ring so
energy tuning is not a good choice for storage-ring-based FELs.
Finally, in an energy recovery linac such as in the FEL planned for CEBAE
the efficiency for the overall system will decrease at lower electron-beam