High Energy Laser Weapons Systems Applications - The Black Vault

RF Input
Accelerator
Optical
Feedback
Bending
Magnet
Photocathode
Exchanger
Wiggler
FEL Output
Electron
Beam
Dump
Figure E-5. Schematic of the high-gain Regenerative Amplifier FEL at
Los Alamos National Laboratory.
REGENERATIVE AMPLIFIER FEL TECHNOLOGY
Los Alamos has a long history of high-power oscillator FEL
development dating back to the SDI era. Although the SDI efforts did not
produce high average power, the basic physics of the radio-frequency
linear-accelerator-driven FEL for high average power systems has been
studied extensively. Much of the oscillator FEL physics is well
understood. In addition, a number of advanced technologies such as highbrightness
electron injectors and grazing incidence optical resonators have
been demonstrated during this program. From the lessons learned with
high-power oscillator FEL development, Los Alamos has moved away
from the oscillator design, principally because the oscillator FEL presents
a rather long list of issues whose resolutions depend on electron beam
physics and optical engineering that are still underdeveloped. Los Alamos
has since developed a novel FEL design based on a high-gain amplifier
that we call the Regenerative Amplifier FEL (RAFEL). The new FEL
design presents a few issues that can be solved with existing engineering
practices. More importantly, the RAFEL design resolves some of the
most difficult issues associated with the oscillator FEL design. These
problems—optical damage, sensitivity to vibration and misalignment, and
extraction efficiency—will be discussed in detail below.
Optical Damage
The first issue the RAFEL design resolves is optical damage to the
resonator mirrors, a serious problem in the oscillator FEL design because
the optical power inside the resonator is typically ten times higher than the
FEL power. The RAFEL key idea is to use a low-Q optical resonator—
one that allows most of the power to exit and stores a small fraction of the
E-8