Page 119 - High Power Laser Handbook

88 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 89

and pulse length. In recent years, it has been extremely successful as a
user facility, producing IR light for a number of two-photon experi-
ments, as well as continuing to investigate the physics of the FEL inter-
action. It has since been removed from the Stanford campus and
relocated at the Naval Postgraduate School in Monterey, California.
4.3.4 Wigglers
The wiggler represents a mature commercial technology. Wigglers
have been constructed with both helical and planar symmetry, as well
as with normal and superconducting electromagnets, permanent
magnets, or hybrid combinations of the two. Ferrite elements are also
used to concentrate the field. The commercial success of these devices
has been due not so much to the market drive from the FEL commu-
nity but rather to the second- and third-generation synchrotron light
sources, which can have many insertion devices and for which the
required quality of the magnetic field is very high.
The technology of choice is wiggler-period dependent, and for
long-wavelength applications, electromagnetic wigglers prevail. For
wiggler periods of 6 cm down to 2 cm or less, permanent magnets
with hybrid wiggler technology take over. These systems use SmCo
5
or NdFeB permanent magnets with flux channeled by vanadium per-
mendur, or similar materials, to produce K ≈ 1 for approximately
1-cm gaps. Originally developed by Halbach, these devices can pro-
24
duce significant gain in the infrared and visible spectra. The Jefferson
Lab IR Demo wiggler, manufactured by STI Optronics, has K = 1 at a
12-mm gap with a 2.7-cm wavelength and 40.5 effective periods.
High-power applications demand that the wiggler gap be significant
to avoid impingement of stray electrons into the radiation-sensitive mate-
rial. Tunability is achieved by varying either the electron beam energy or
the field strength. If the wiggler is adjustable, then it is much easier to tune
the wavelength, because electron transport systems are chromatic and
require retuning if the beam energy is adjusted outside a narrow range.
Tuning hybrid wigglers is performed by adjusting the pole gap.

4.3.5 The Optical Cavity
An FEL’s optical cavity is often more difficult to engineer than are those for
conventional lasers. The FEL requires excellent overlap between the elec-
trons and the optical mode in order to achieve high optical gain. The elec-
tron beam’s dimensions are small, which implies that the mode must also
remain small, with a relatively short Rayleigh range but modest mode size
variations within the wiggler. A broad performance optimum occurs with
a Rayleigh range of around 1/p of the wiggler length. Angular alignment
tolerances can be very tight—on the order of microradians. If the electron
beam is several hundred micrometers in diameter, one might expect that
overlap must be held to a few tens of micrometers out of, say, a 10-m cavity
length. In addition, the cavity length must match a subharmonic of the