First Lasing of the Jefferson Lab IR Demo FEL Page: 3 of 10
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effects or CSR induced emittance growth. Sensitivity to the phase of the cryomodule
cavities has prevented good measurements of energy spread when the laser is optimized.
Once first light was achieved the emphasis in the accelerator development shifted to
commissioning the recirculation system . To date both pulsed and cw beam have been
transported with essentially no loss to the energy recovery dump. The aperture of the
system is sufficient that the exhaust beam of the laser can also be propagated with no
losses through the recirculation loop back to the accelerating module. There are losses in
the final transport to the energy recovery dump at 9.5 MeV, however, leading to a limit
on average power. This loss is being investigated so that higher laser power can be
achieved with energy recovery.
Once it was determined that the beam quality was sufficient for lasing , the
wiggler and optical cavity mirrors were installed . For first light we chose to use a high
reflector and a 97.6% reflective output coupler. Using macropulses 200 jsec in duration
the electron beam was aligned with the wiggler viewers and the cavity length was scanned.
The laser power output immediately rose to saturation. Lasing at 4.9 microns was
achieved over a 10 micron range of cavity length with no optimization. The laser was
then operated cw. Using a power meter at the exit of the laser, 155 W of average power
was outcoupled and over 110 W of power delivered to the optical diagnostic room.
After changing to a 90% output coupler, 311 W of cw laser power was measured on
a power meter at the laser exit. Lasing weakly with energy recovery was possible but, as
noted above, limited due to beam loss at 9.5 MeV. Once the recovered beam is better
matched into the accelerating structure we expect to lase at high power with energy
In general the IR Demo laser has been very predictable and easy to run. When the
electron beam and optical cavity are set up carefully the laser lases very strongly before
any optimization. If the laser power is subsequently optimized, the resulting electron
beam configuration is not much different from the original setup. Using measured values
for the beam parameters, one ideally expects a small signal gain of approximately 90%.
When setting up the system the gain should not be reduced by more than a factor of 3 due
to electron beam and optical misalignments. This still allows sufficient gain to lase to
saturation. Variation of the electron bunch parameters indicates that the highest power
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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/m1/3/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.