Page 114 - High Power Laser Handbook

84 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 85

An FEL system comprises an electron source (injector), an accel-
erator or linac with electron beam transport magnets, the wiggler, an
optical system, perhaps an energy recovery system, and the dump.
These are all supported by a number of auxiliary systems, such as
power sources, cooling, alignment, controls, and so forth. A disad-
vantage of FELs is that all these systems are needed, even for low-
power output. An advantage is that they do not get much bigger for
high-average-power output. The discussion that follows covers tech-
nologies of these main subsystems as considered for high-average-
power operation in the infrared to visible region. Other technologies
may be more appropriate for other wavelength regions or for use at
low average power.

4.3.2 Injectors
The injector is the most critical component in the entire FEL system,
because the electron beam’s quality can only degrade once the beam
is formed. Because it is difficult to make high-quality continuous
electron beams, the performance of most FELs is set by the injector’s
ability. In the search for suitable injectors, many approaches have
been, and are being, adopted, but no clear winner for continuous
operation has arisen from the group. Present candidates include high-
voltage direct current (dc) guns with thermionic cathodes or photo-
cathodes, copper radio frequency (RF) cavities with a photocathode,
and superconducting RF guns with photocathodes.
The major issue that high-average current injector designers have
is the continuous production of bunch charges that are so high that
nonlinear space charge forces play a significant role in their control.
Other, low-power FELs have dealt with this issue through several
strategies: imposition of compensatory solenoid fields in a manner
pioneered by Sheffield and Carlsten at LANL, high initial cavity
10
gradients, and tailoring of the density profiles longitudinally and
transversely to linearize the forces. This design approach permits the
production of electron bunches with 1 nanocoulomb (nC) of charge at
a normalized emittance less than 1 mm·mrad. Such performance has
11
yielded UV lasing using electron beam energies of only 45.2 MeV
and is presently driving the operation of the world’s first hard x-ray
laser, the LCLS at SLAC (Fig. 4.3). High brightnesses at high bunch
8
charge are significantly dependent on the high-cavity electric field
gradients achievable in pulsed structures, because these gradients can
accelerate the beam before space charge forces can work to degrade it.
Typically a minimum of 20 to 40 MV/m is desired on the photocathode
surface, although operating gradients of up to 125 MV/m at the cath-
12
ode have been reported. Unfortunately such high gradients cannot
be maintained continuously—or even at high-duty factor—because of
enormous associated ohmic losses in the RF cavities. Neither can such
gradients be maintained in direct current fields, which are typically