IR-FEL lasing over kilowatt average power has been demonstrated at JAERI-FEL driven by a superconducting linac. The lasing is distinguished from other FEL oscillators by its high extraction-efficiency, 6%, and unique cavity-length detuning curve. The lasing at JAERI-FEL has the maximum power at the perfect synchronism between the optical pulse and the electron hunch, while usual EEL oscillators require cavity-length shortening to compensate laser lethargy. Numerical analyses show that the lasing observed at JAERI-FEL is a new type of FEL lasing quasi-stationary superradiance, which gives extraction efficiency higher than the theoretical prediction previously reported. Our analyses also indicate that long stable macro-pulses generated by superconducting linac are indispensable to establish this new type of lasing.
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