Optimization of echo-enabled harmonic generation toward coherent EUV and soft X-ray free-electron laser at NSLS-II

Abstract

Prebunching via echo-enabled harmonic generation (EEHG) is an efficient way to reduce the radiator length and improve the longitudinal coherence as well as output stability in storage-ring-based free-electron lasers. We propose a conceptual design, which uses two straight sections to seed coherent extreme-ultraviolet (EUV) and soft X-ray emission with nearly MHz repetition rate. To take the large energy spread (10−3) of a storage ring into account and utilize the existing bending magnets between the two straight sections as the first chicane, we implement a special modeling tool, named EEHG optimizer. This tool has been successfully applied to maximize the prebunching with a reasonably low energy modulation, thereby generating intense coherent X-ray pulses within a short undulator length (a few meters) limited by the available space of a storage ring. Numerical simulations confirm that the optimized EEHG parameters can be directly applied to generate a 10 MW scale peak power with fully coherent ultrafast EUV to soft X-ray pulses based on the NSLS-II parameters. This method can be easily extended to other types of diffraction-limited storage rings.

Twin-pulse seeding enables pump-probe capabilities in the EUV to soft X-ray spectrum at synchrotron light sources

“…Seeding coherent EUV to soft X-ray emission can be accomplished using a compact EEHG design via two SSs of a SR 1 , 2 . This design utilizes the BM section between these two SSs as the first chicane.…”

“…This design utilizes the BM section between these two SSs as the first chicane. However, one of the key EEHG parameters, the momentum compaction of chicane 1, is completely determined by the SR lattice while the energy modulation of stage 1 is chosen to have an optimal and reasonably low value (typically ) 1 . For each harmonic, the final energy spread only varies as a function of the momentum compaction of the SR lattice, as shown in Fig.…”

“…Compared to linac sources, the 4th generation SLSs have several key constraints: (1) long bunch length (at least a few picoseconds); (2) large energy spread; (3) low peak current. Because of these limitations, some form of external seeding via a laser system is required to generate shorter radiation pulses 1 , 2 . When conventional high-gain harmonic generation (HGHG) 3 – 6 is utilized, a great deal of seed laser power is required to generate coherent radiation (CR) at wavelengths significantly below what a conventional laser technology can deliver due to the large energy spread.…”

“…The initial stage requires a significant momentum compaction; however, it can be set to a specific value still having a reasonable range of tunability. Especially, this scheme is well suited to SR-based FELs, as we have shown that the two stages of EEHG can be placed in two straight sections (SSs) separated by one or more bending magnet (BM) sections, which provide the required momentum compaction 1 , 2 . Thus, EEHG produces short CR pulses 8 – 16 at high repetition rates, is fully compatible with SLSs (no need of any lattice change), as well as allows the accessibility to much higher harmonics toward the soft X-ray spectrum.…”

“…Building on our recent development of utilizing two straight sections (SSs) of a SR to seed coherent emission in the EUV to soft X-ray range 1 , 2 , we present a modified EEHG, twin-pulse seeding, where multiple laser pulses are used for the first stage of seeding within a single modulator to generate distinct regions of modulation, and the final stage of seeding for each region can be performed in a different SS to allow a broad range of independent tunability. Twin-pulse seeding is fully compatible with the large energy spread of a SR as well as the small momentum compaction per BM section intrinsically associated with a 4th generation SLS, as the desired momentum compaction can be accumulated over multiple BM sections.…”