Superradiant Cascade in a Seeded Free-Electron Laser

Abstract

We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation ( rad ¼ 200 nm) at the second harmonic of a short, intense seed laser pulse ( seed ¼ 400 nm) was generated by the cascaded FEL scheme at the transition between the modulator and radiator undulator sections. The superradiance of the ultrashort pulse is confirmed by detailed measurements of the resulting spectral structure, the intensity level of the produced harmonics, and the trend of the energy growth along the undulator. These results are compared to numerical particle simulations using the FEL code GENESIS 1.3 and show a satisfactory agreement. Fourth generation light sources open the way towards the exploration of molecular and atomic phenomena, yielding unprecedented benefits to a wide range of scientific disciplines [1][2][3][4][5][6]. Free-electron lasers (FELs), operating in self-amplified spontaneous emission (SASE) mode, have demonstrated the capability of reaching the subnanometer wavelength range at the femtosecond time scale [7,8]. The multielectron dynamics in atoms and molecules involved in chemical transformation processes evolves on a femtoto attosecond time domain. Thus, ultrashort pulses in the XUV spectral range are the potential tool to control and map the collective electronic and nuclei rearrangements. In a SASE FEL the generation of radiation is due to the passage of a relativistic electron beam through the periodic magnetic field of an undulator with maximum field amplitude B u and period u , inducing emission at a resonant2 (linear undulator) and its higher order harmonics, where is the Lorentz factor of the electrons and K ¼ eB u u =ð2m e cÞ the deflection parameter of the undulator. At the onset of saturation, the electrons emit coherently [9] over a characteristic frequency bandwidth Á!=! $ , where is the Pierce parameter [10,11]. Considering that typical values of the Pierce parameter range from 10 À3 to 10 À4 , the associated FWHM pulse length at the Fourier limit is ¼ 2 ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 lnð2Þ p L c =c, where L c ¼ rad =4 is known as cooperation length and, at rad ¼ 1 nm, assumes values in the range c $ 0:6-6 fs. When the electron bunch is longer than L c , several independent processes of SASE amplification can occur, leading to a pulse structure far from the Fourier limit and composed of a number of independent spikes [12]. A selective amplification of only one of these spikes has been demonstrated by shaping the electron beam phase space, in order to enable the field growth only in a limited portion of the bunch [13,14]. A single mode may also be generated by seeding the FEL amplifier with an external source. Seeding with a pulse shorter than c results in a spike of rms duration given by the cooperation length, at the onset of saturation when P sat % P beam (P beam is the electron beam power). Pulses shorter than the cooperation length may still be obtained at intensities higher than the saturation level, when the FEL emits in the superra...

Observation of Time-Domain Modulation of Free-Electron-Laser Pulses by Multipeaked Electron-Energy Spectrum

“…The amplification of one single SASE spike has been demonstrated by compressing the electron beam close or below the FEL coherence length [15,16], by using a chirped bunch energy combined with a matched undulator taper [17][18][19], or by spoiling the whole electron beam except a limited fraction [20,21], a technique that has also been implemented to produce double pulse two-color radiation for pump and probe experiments [22]. Short single or multiple pulses have also been produced in seeded or cascaded FELs [23][24][25][26][27], with increased coherence and shot to shot stability. More sophisticated seeding concepts [28,29] lead to the possibility of producing single-or two-color subfemtosecond pulses in the soft x-ray's range [30,31], but with similar limitation in terms of gain bandwidth.…”

On the Importance of Electron Beam Brightness in High Gain Free Electron Lasers

“…The FEL pulse duration is typically specified by users in facilities relying on SASE, whilst its lower-limit is set by the duration of the external seed laser, such as in High Gain Harmonic Generation (HGHG) schemes [10,15,16,[33][34][35][36][37]. In both cases, the electron bunch duration at the undulator should be longer than the required FEL pulse duration because of effects such as FEL slippage in SASE FELs, arrival time jitter, and multi-stage cascade in HGHG FELs.…”

Longitudinal space charge assisted echo seeding of a free-electron laser with laser-spoiler noise suppression

“…When the temporal coherence of FEL light is determined by the shot noise of an electron beam, as in self-amplified spontaneous emission (SASE), it is poor [13][14][15], but if it is determined by an external seed laser, the FEL light takes on the excellent temporal coherence properties of the external laser in the region that has been seeded. However, if the seed pulse is short while the electron bunch is long, the noisy SASE background signal can overwhelm the seeded radiation, wiping out the benefits of the seed.To reduce this noisy background, the FEL radiator is typically made short enough that the unseeded portion of the bunch does not reach saturation while the seeded portion does, but this can be limited in the case of short seeds with low seed power [1,[5][6][7]. Alternatively, the electron bunch could be made short relative to the seed, but this puts challenging constraints on the synchronization between the femtosecond-scale seed and the electron bunch.…”

“…Short-wavelength, high-brightness light sources, like free-electron lasers (FELs) driven by particle accelerators, are in demand for experiments studying ultrafast processes in matter. The FEL community has pursued methods to improve the temporal coherence properties of the light [1][2][3][4][5][6][7] and to generate shorter, tunable FEL pulses [5][6][7][8][9][10][11][12]. When the temporal coherence of FEL light is determined by the shot noise of an electron beam, as in self-amplified spontaneous emission (SASE), it is poor [13][14][15], but if it is determined by an external seed laser, the FEL light takes on the excellent temporal coherence properties of the external laser in the region that has been seeded.…”