A pass-by-pass gain measurement technique for oscillator FELs
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.
