Abstract:
We present a new pass-by-pass gain measurement
technique for a storage ring FEL. Typically, the FEL
oscillator gain is obtained by measuring the growth of
the envelope of an optical macropulse using a slow
photo-detector. While successfully used for low-gain
FEL operation at Duke FEL laboratory for many years,
this method was not adequate for measuring higher
gains of distributed optical klystron FELs. We have
developed a new gain measurement technique based
upon the direct measurement of the micropulse energy
from one pass to another using fast photo-detectors.
This technique provides a powerful tool to study the
entire FEL gain process, including the startup process
of the FEL lasing. In this work, we describe this new
gain measurement technique in detail and compare it
with the old technique. Using fast photo-detectors with
a picosecond time response, this new technique can be
extended to measure the gain of other oscillator FELs,
including those driven by superconducting linacs.