Coherent Harmonics in the Super-radiant Regime from an Fel*

Early work that quantified the ratio of coherent harmonic power to the fundamental power as a function of cavity length found that the ratio peaked for a large cavity length detuning. This is consistent with an overbunching model of coherent harmonic production. Recent measurements on the IR Demo FEL at Jefferson Lab have shown that the ratio is also high very near the synchronous point. In this regime the laser is operating a super-radiant mode, similar to what might be present in a SASE device. The recent data is compared to predictions of harmonic production in both long pulse oscillators and SASE FELs.