First Lasing of the Jefferson Lab IR Demo FEL Page: 4 of 10
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occurs for the shortest electron bunch length and the smallest longitudinal emittance as
expected. Sensitivities to variation in steering, focussing, cavity phases, and average
current are qualitatively similar to expectations. Note that the mirror sensitivity in table 1
is for pulsed or low power operation with large cavity losses. When the mirrors heat up
or when the output coupling is small, we have found that the sensitivity to mirror tilt is
In figure 2a we show the power as a function of cavity length as the micropulse
repetition rate is varied . To our knowledge this is the first time this has been reported
for a FEL. The total cavity loss is 11% per round trip so the threshold gain (defmed as
G = (1- r)-N -1 where - is the round trip cavity loss) is 12.4% for 18.8 MHz repetition
rate, 26.3% for 9.4 MHz, and 59.4% for 4.7 MHz. We see from the detuning width in
figure 2a that the gain must be well in excess of 59.4%. The electron beam in this case
was pulsed with a 1.2% duty cycle. The electron gun was run at 327 kV for this data so
the emittance is not as small as in Table 1. The other parameters are similar. Mirror
heating effects should have been negligible. In figure 2b we have scaled both the power
and the cavity length detuning by the reduction in the frequency to give an indication of
the extraction efficiency as a function of the optical delay from synchronism. Note that
the optical delay scales linearly with the number of round trips per gain pass. The curves
in figure 2b are remarkably similar to each other. Note, however, that the scaled cavity
length detuning curve is actually shorter for a smaller threshold gain (9.4 MHz case
compared to 18.8 MHz). This is a very puzzling feature which may be due to optical
In figure 3a we show the power vs. cavity length as a function of bunch charge. In
figure 3b the power is scaled to the bunch charge. As expected the laser power is
approximately proportional to the bunch charge. The length of the cavity detuning curve
is a very non-linear function of the charge, however. This is not entirely unexpected since
the emittance and energy spread are smaller when the charge is smaller. It is a bit
surprising, though, since the bunch length and transverse match are not optimized for the
low charge operation. If we assume that the bunch length is unchanged, the predicted gain
for 13 pC is around 25% and the length of the detuning curve according to supermode
theory  is 13 gm, very close to what is observed.
Note how the shape of the curves also changes from convex to concave as the charge
is reduced. Also note that the predicted detuning curve length for even 60% gain (and the
gain must be at least this large as shown in figure 2) is 31 pm. As noted above, the gain
caclulated from measured parameters is approximately 90% which leads to an expectation
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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/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.