Experimental observation of limit-cycle oscillations in a short-pulse free-electron laser

Abstract

The first experimental observation of limit-cycle power oscillations in a short-pulse free-electron laser is presented. These are due to a nonlinear modulation of the optical micropulse shape and phase by the electrons, which leads to the formation of a train of subpulses. Experimentally, the oscillations have been found to depend on the slippage distance and on the desynchronism between optical pulses and electron bunches, comparing well with theoretical predictions.PACS numbers: 41.60.CrThe free-electron laser (FEL) is a novel type of laser opening up many new applications, due to its high power, short pulse length, and wavelength tunability [1]. The FEL uses a beam of relativistic electrons injected into a periodic magnetic structure, the so-called undulator. The alternating magnetic field forces the electrons to move along sine-like trajectories and, consequently, to emit radiation. Usually this radiation, which is known as spontaneous emission, is captured in an optical cavity and amplified on successive passes through the undulator in the presence of newly injected electrons. The electrons are bent into a beam dump after they have traversed the undulator.A radio-frequency (rf) accelerator provides the electron beam in most FEL oscillators operating in the infrared and visible spectral range. This type of accelerator has the property that electrons emerge as short bunches, which are separated by fixed periods. The light wave in the FEL cavity mimics this time structure, i.e., consists of a train of short micropulses. Careful synchronization of the circulating micropulses and the electron bunches is required to achieve laser oscillation. This is done by matching the length of the optical cavity to the electron bunch repetition frequency.Electrons slip back relative to an optical micropulse on their mutual travel through the undulator, because (i) their velocity is somewhat smaller than the velocity of light and (ii) they move along an undulating path. The FEL resonance condition implies that the slippage is one radiation wavelength X per undulator period traversed and, therefore, the bunch slips back by a distance NX in an undulator consisting of TV periods [2]. In view of the fact that the amplification grows toward the downstream end of the undulator, where the electrons have shifted back relative to the front of the optical micropulse, the optical pulse peaks up at its rear end. This has the consequence that the optical group velocity is somewhat smaller than its vacuum value. Hence, in a perfectly synchronized resonator, the micropulse would continue to narrow and retard on subsequent passes through the undulator. The reduced overlap with the gain medium leads to a reduction of the gain per pass, which impedes the growth of the laser power. This effect is known as "laser lethargy" [2]. In order to restore the gain, it is necessary to slightly reduce the cavity length from the value giving perfect synchronism, so that the peak of the optical pulse is advanced relative to the peak of the electron bunch at t...

Femtosecond single-shot timing and direct observation of subpulse formation in an infrared free-electron laser

“…Our experimental observations are in agreement with previous investigations of the optical pulse dynamics in other IR free-electron laser oscillators [5,7], however, not seen in such clarity as here due to the delay-and real-timeresolved cross-correlation measurements. The subpulse creation and power oscillations arise without external perturbation, caused by a nonlinear interaction of the short electron bunches and optical pulses.…”

“…Furthermore, pronounced nonlinear processes like subpulse formation and limit-cycle oscillations (LCOs) of the power have been predicted for short electron bunches within an oscillator-type FEL [20,21]. Our experimental cross-correlation measurements provide both delay-time as well as real-time resolution and thus give insight into these processes in yet unprecedented contrast and detail, as opposed to previous one-dimensional studies [5,7]. The results are complemented by 4D simulations of the FEL process, which confirm the pulse shape evolution and guide the understanding of the FEL oscillator physics.…”

“…Here, significant short pulse effects in the FEL interaction can occur on the sub-ps scale [5,7]. Due to the large slippage length as compared to the electron pulse duration, the lasing dynamics are quite sensitive to the temporal overlap of the electron and optical pulses.…”

“…Infrared FEL oscillators, on the other hand, are usually operated using picosecond-long electron pulses [5,6]. Here, significant short pulse effects in the FEL interaction can occur on the sub-ps scale [5,7].…”

Modulated desynchronism in short pulse free-electron laser oscillators

“…The optimum frequency for obtaining short pulse with high power is therefore around 40 kHz, a result which agrees with our experimental observations. It is interesting to note that, if our system is simulated with a 0 0.016 (a reduction of about a factor of 2 from our actual system), our FEL enters the well-known limit cycle [3,7] regime and oscillates at about 40 kHz (see Fig. 9).…”