Abstract
We present an experimental and theoretical study of the effect of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and that the minimum micropulse length during the cycle can be significantly shorter than that which can be obtained without modulation. For example, when the desynchronism of our FEL is modulated at 40 kHz, the minimum measured micropulse length is 300 fs. Without modulation the minimum is about 700 fs. We show that when the desynchronism is modulated, the FEL can operate for part of the cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity losses. It is even possible for the FEL to operate periodically in the region of negative desynchronism where gain, as normally defined, does not exist.
Highlights
In a free-electron laser (FEL), a relativistic electron beam passes through the periodic transverse magnetic field of an undulator, transferring energy to a copropagating electromagnetic wave
We have studied the effect of desynchronism modulation on short pulse free-electron laser oscillators
Desynchronism modulation is achieved experimentally by modulating the energy of the electron beam that passes through a nonisochronous magnetic chicane in the FEL beam line
Summary
In a free-electron laser (FEL), a relativistic electron beam passes through the periodic transverse magnetic field of an undulator (or a wiggler), transferring energy to a copropagating electromagnetic wave. When the slippage distance is comparable with or larger than the electron pulse length (Lb), longitudinal overlap effects dominate the FEL dynamics. By varying the desynchronism between the periodic beam injection and the round-trip time of the radiation in the cavity, it is possible to control the overlap between the radiation and the electron pulses during many round-trips. A finite desynchronism dL is necessary to maintain synchronism between electron and optical pulses in the cavity because of the effect of laser lethargy [1], which causes the centroid of the optical pulse to travel slower than the speed of light in vacuum.
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