Page 113 - High Power Laser Handbook

82 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 83

Figure 4.3 The Stanford Linear Accelerator Center with a layout of the Linac
Coherent Light Source x-ray FEL. The LCLS utilized the final third of the SLAC linac
with a new injector and undulator added. (Courtesy John Galayda)

this higher capability. It is feasible to operate the FEL as a self-amplified
spontaneous emission mode in which the signal grows from noise, but
this requires yet a longer wiggler; in addition, the output is likely to
exhibit the characteristics of amplified noise. In some regimes, how-
ever, this may be the only option possible. For example, the Linac
Coherent Light Source (LCLS) x-ray FEL at the Stanford Linear
Accelerator Center (SLAC) uses a 120-m wiggler to produce a 10-keV
x-ray photon pulse of extraordinary brightness and peak power grown
strictly from noise (Fig. 4.3). This is not likely to be the method of
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choice for a high-average-power system, however.
A hybrid design developed at Los Alamos National Laboratory
(LANL) and called a Regenerative-Amplifier FEL (RAFEL) is another
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option. The RAFEL is essentially a high-gain oscillator in which a small
amount of feedback from the output is used to sustain the lasing. The
system’s high gain relieves, to a great extent, the tight tolerances on the
optics and mitigates against thermal loading issues in the mirrors.

4.3 Hardware Implementation

4.3.1 Overview
To generate high-average-power FEL light, it is necessary to start with
a very high-average-power–accelerated electron beam. Luckily, this
technology has been extensively studied for decades because of its uses
in nuclear physics; high-energy physics; and materials research on
storage rings, neutron sources, and so on. There exist several gigawatt-
level (10 watts of continuous beam power) average-power electron
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beams and hundreds of others at lesser high average powers. The goal
of high-power FEL research has been to effectively harness such linear
accelerator approaches to produce beams suitable for FELs. Even 1 per-
cent energy extraction from such a beam would yield an incredible
photon source.