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

Abstract

We experimentally demonstrate a single-shot arrival time monitor for short picosecond infrared freeelectron laser (IR FEL) pulses based on balanced optical cross-correlation with a synchronized fs table-top laser. Employing this timing tool at the Fritz Haber Institute IR FEL, we observe a shot-to-shot timing jitter of only 100 fs and minute-scale timing drifts of a few picoseconds, the latter being strictly correlated with the electron beam energy of the accelerator. We acquire sum-frequency cross-correlation data with micropulse resolution, providing full access to the IR FEL pulse shape evolution within the macropulse. These measurements provide unprecedented insights into the occurrence of limit-cycle oscillations of the FEL intensity as a consequence of subpulse formation. Our experimental results are complemented by fourdimensional simulations of the nonlinear pulse dynamics in a low-gain FEL oscillator based on MaxwellLorentz theory.

Three-Dimensional, Time-Dependent Analysis of High- and Low-Q Free-Electron Laser Oscillators

“…An analytic theory for this regime predicts a scaling of the optical pulse energy E L with the bunch charge q as E L ∝ q 3/2 and a scaling of the temporal width τ L of the optical pulse as τ L ∝ q −1/2 [138], which has been experimentally confirmed [139], although higher extraction efficiencies have also been observed for a perfectly synchronized cavity length, where the analytic theory breaks down [92,132]. Furthermore, short pulse FEL oscillators operating in the large-slippage regime also show a stable oscillation in the macropulse power, known as limit-cycle oscillations, for certain cavity lengths detuned from perfect synchronization, which has been observed both experimentally [102,137,[143][144][145][146] and in numerical simulations [136,138,[147][148][149].…”

Three-dimensional, time-dependent analysis of high- and low-Q free-electron laser oscillators

“…An analytic theory for this regime predicts a scaling of the optical pulse energy 𝐸 𝐿 with the bunch charge 𝑞 as 𝐸 𝐿 ∝ 𝑞 3/2 and a scaling of the temporal width 𝜏 𝐿 of the optical pulse as 𝜏 𝐿 ∝ 𝑞 −1/2 [129] which has been experimentally confirmed [130], although higher extraction efficiencies have also been observed for a perfectly synchronized cavity length where the analytic theory brakes down [91,124]. Furthermore, short pulse FEL oscillators operating in the large-slippage regime also show a stable oscillation in the macropulse power, known as limit-cycle oscillations, for certain cavity lengths detuned from perfect synchronization, which has been observed both experimentally [98,128,[134][135][136][137] and in numeric simulations [127,129,[138][139][140].…”

Advanced Scheme to Generate MHz, Fully Coherent FEL Pulses at nm Wavelength

“…As a final remark, it is important to point out that the cavity detuning results in a change in the temporal and spectral distribution of the stored FEL pulse. This has been extensively discussed in FEL oscillators in the past [54][55][56]. The consequences on the properties of the output FEL should be carefully considered before applying this method for power gain control.…”