First Lasing of the Jefferson Lab IR Demo FEL Page: 5 of 10
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of a 46 pm long cavity length detuning curve. The length of the curve seems to be shorter
than possible assuming a one-dimensional supermode theory.
Spectral measurements have also confirmed that we have quite high gain. When
lasing near the synchronous detuning a strong sideband is observed. This should not be
the case unless the gain to loss ratio is greater than around 4 which implies at least 50%
small signal gain. As expected the spectra become much narrower as the cavity length is
shortened. The spectral brigthness varies little over the central part of the detuning curve.
With pulsed operation at 60 Hz and a duty cycle of 1.2% , 4.1 W of laser power
was detected in the optical diagnostic lab. The losses in the transport are at least 10% so
this means that the laser power was at least 380 W during the macropulse and may have
been as much as 480 W. During cw operation with a similar electron beam setup, the
power did not exceed 311 W despite repeated optimization. This indicates that mirror
heating reduced the laser output by 18 to 35%. Calculations of mirror distortion 
indicated that the mirror absorption had to be less than 0.1% to achieve over 300 W of
laser power. Measurements of the mirrors at the Naval Advanced Warfare Center in
China Lake CA put an upper limit of 0.1% on the absorption.
In general this laser behaves according to expectations. One exception to this
observation is the cavity length detuning curve. There seems to be some phenomenon
which is keeping the detuning curves short when the gain is high. One possibility is
guiding. Simulations have hinted that guiding effects can shorten a cavity length detuning
curve even though the gain is increased. This might explain the very slow change in the
cavity length detuning curve length with pulse charge.
With high gain and a 40 period wiggler it is not unreasonable to expect that one
might extract over 1% of the electron beam power. Since the electron beam power is in
excess of 40 kW, this would indicate that over 400 W of laser power is reasonable to
achieve. Mirror distortion has limited operation to only 311 W but operation at 3 pm
with sapphire mirrors should allow us to increase the power in "first light" mode to over
400 W. Recirculation can raise the electron beam power to 240 kW but may limit the
extraction efficiency to no more than 0.75%. This setup should allow much higher power
(close to 2 kW) if the quality of the sapphire mirror coatings is similar to those on the
CaF2 mirrors now in use.
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Benson, Stephen; Biallas, George; Bohn, Court; Douglas, David; Dylla, H.F.; Evans, R. et al. First Lasing of the Jefferson Lab IR Demo FEL, article, May 1, 1999; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc742655/m1/5/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.