High Energy Laser Weapons Systems Applications - The Black Vault

picosecond electron pulses over thousands of passes needed to reach
maximum power. Consequently, the resonator length must be aligned to
an accuracy of a few microns and the angular alignment of the mirrors to a
few microradians. In the new high-gain RAFEL, the combination of very
large single-pass amplification ratios (between a few hundred and a few
thousand) and a small optical feedback enables the FEL to saturate in a
few passes. The optical pulses only need to overlap with the electron
pulses over the picosecond duration, and the low-Q resonator only has to
be aligned to an accuracy of a few millimeters, instead of a few microns.
The large amplification ratio also offers an additional benefit: the electron
beam acts like a fiber amplifier and guides the optical beam through the
amplifier. Small angular misalignments of the low-Q resonator do not
affect the performance of the high-gain FEL because the optical axis is
determined not by the low-Q resonator but by the electron beam (the
optical beam is automatically aligned to the electron beam). The highgain
RAFEL design is thus rugged toward vibration, microphonics and
misalignments.
Extraction Efficiency
The third issue that the RAFEL design can potentially solve is the low
extraction efficiency of uniform wigglers—wigglers with constant periods
and magnetic fields. The extraction efficiency of a uniform wiggler is
inversely proportional to the number of oscillation periods in the wiggler.
In a typical oscillator FEL, the number of periods is about 100 and the
extraction efficiency is approximately 1%. The extraction efficiency can
be increased to 10-15% by tapering the wiggler either in wiggler periods
or in magnetic field to maintain the FEL resonance condition as the
electron beams lose energy. Using a tapered wiggler however requires
very high optical intensity that causes optical damage in an oscillator FEL.
In the RAFEL design, the large amplification ratio enables the optical
beam to reach sufficiently high intensities inside the tapered wiggler for
maximum extraction, and the low-Q resonator allows most of the
extracted power to exit as useful power. By extracting 10-15% of the
electron beam power, the RAFEL increases the overall system efficiency,
reduces the required electron beam power and removes the need for
energy recovery (for the megawatt power FEL, energy recovery may be
used to reduce the radiation created by the high-energy electron beams).
This leads to a smaller and simpler FEL.
HIGH-GAIN FEL AND REGENERATIVE AMPLIFIER FEL
DEMONSTRATIONS
The high-gain FEL is being developed as a mirror-less approach to the
generation of coherent light in the spectral regions where mirrors do not
exist, e.g. x-rays or extreme ultraviolet. Very high single-pass gains were
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