Operating synchrotron light sources with a high gain free electron laser

Abstract

Since the 1980s synchrotron light sources have been considered as drivers of a high repetition rate (RR), high gain free electron laser (FEL) inserted in a by-pass line or in the ring itself. As of today, the high peak current required by the laser is not deemed to be compatible with the standard multi-bunch filling pattern of synchrotrons, and in particular with the operation of insertion device (ID) beamlines. We show that this problem can be overcome by virtue of magnetic bunch length compression in a ring section, and that, after lasing, the beam returns to equilibrium conditions without beam quality disruption. Bunch length compression brings a double advantage: the high peak current stimulates a high gain FEL emission, while the large energy spread makes the beam less sensitive to the FEL heating and to the microwave instability in the ring. The beam's large energy spread at the undulator is matched to the FEL energy bandwidth through a transverse gradient undulator. Feasibility of lasing at 25 nm is shown for the Elettra synchrotron light source at 1 GeV, and scaling to shorter wavelengths as a function of momentum compaction, beam energy and transverse emittance in higher energy, larger rings is discussed. For the Elettra case study, a low (100 Hz) and a high (463 kHz) FEL RR are considered, corresponding to an average FEL output power at the level of ∼1 W (∼10 13 photons per pulse) and ∼300 W (∼10 11 photons per pulse), respectively. We also find that, as a by-product of compression, the ∼5 W Renieri's limit on the average FEL power can be overcome. Our conclusion is that existing and planned synchrotron light sources may be made compatible with this new hybrid IDs-plus-FEL operational mode, with little impact on the standard beamlines functionality. Background and outlookFree electron lasers (FELs) in the ultra-violet (UV) and x-rays driven by high brightness radiofrequency linear accelerators (RF linacs) [1, 2] have become complementary to synchrotron light sources for the investigation of matter, from molecular to atomic scale. Despite their high peak brilliance and degree of coherence, high gain FELs are surpassed by synchrotrons in number of pulses per second and in number of beamlines that can be served simultaneously. A high repetition rate (RR) of the FEL is a prerequisite for multi-beamline operation; with present technology, it requires a superconducting linac [3-5] and a complex fan-like set of transfer lines [6].Since the 1980s synchrotron light sources have been considered for driving a high gain self-amplified spontaneous emission (SASE) FEL [7,8] in a by-pass line [9][10][11][12][13][14][15][16] or in the ring itself [17], in order to approach RRs of up to 100 kHz without a superconducting linac investment, and taking advantage of existing or planned synchrotron light sources. At present, synchrotron light sources target emittances compatible with lasing at (sub-)nm wavelength [18,19], whereas FELs require emittances at the diffraction limit, 4πε x  λ [20]. Transverse gradient u...

One way only to synchrotron light sources upgrade?

“…In one of the very first conceptual designs of a storage-ringdriven by-pass FEL (Kim et al, 1985), the ring was devoted to single-bunch operation at relatively high peak current, and therefore the FEL emission was not compatible with the standard multi-bunch synchrotron operation. In a recent proposal of ours (De Mitri & Cornacchia, 2015, and references therein), single-bunch peak currents comparable with those achieved in RF linacs can be obtained through magneticbunch length compression. In that proposal, beam gymnastics are carried out inside the storage ring, and therefore limited by the RF cavity impedance and the large momentum compaction implied by the compression scheme.…”

Features and futures of X-ray free-electron lasers

“…Although various schemes have been developed in recent decades, only few of them seem to be compatible with the preservation of high intensity. Although, the SASE principle has been considered for SRs 147 , 148 , 149 , the SASE mechanism typically requires a relatively long undulator ( 100 m) to amplify the initial emission from electron beam noise; thus, the capacity of SR-based SASE is undoubtedly limited. Furthermore, a beam energy spread of 1%, which significantly exceeds the typical FEL gain bandwidth of 0.1%, is expected in SR undulators.…”

Lattice design for angular dispersion enhanced microbunching in storage rings

“…The 𝛽 𝑦 in figure 5 is 200 meters at the entrance of the first dipole, it is difficult to accommodate such a large beta function in the storage ring which may shorten the beam lifetime and the linear optics will be very sensitive to the field errors. For this reason, we propose to implement the OISL scheme in a bypass section [24]- [26], as shown in figure 11. The electron beam normally circulates in the ring without passing through the OISL section and stays at the equilibrium status.…”