The fundamental difference between an ERL and a conventional storage-ring is that the electron beam power can be recycled at near-perfect efficiency to accelerate new electron bunches. With the repetition rate about 3–4 orders of magnitude higher than the normal conducting linac, the superconducting linac based FELs provide higher average brightness radiation pulses and support more users in more potential applications.Įnergy recovery linac (ERL) 12, 13, 14, 15 is another type of high energy accelerator which in principle combines the advantages of storage ring and linac to profit light sources. In order to improve the repetition rate, superconducting linac based free electron lasers, such as FLASH, European-XFEL, LCLS-II and SHINE 10, 11, are built or under-constructing worldwide. However, the low repetition rate (~ 100 Hz with copper linac) of FELs leads to an equal average brightness. The high-quality electron beam, generated by the linear accelerator, travels through a long undulator line and produces coherent radiation pulses with the peak brightness of 10 33 phs/s/mm 2/mrad 2/0.1%BW, about 9 orders of magnitude higher than that of the 3rd generation light sources. The DLSRs provide high average brightness of about 10 22 phs/s/mm 2/mrad 2/0.1%BW and high coherent fractions of about 0.1 at photon energy of 10 keV, which are enhanced by 3 and 1 orders of magnitude respectively, comparing with those of the 3rd generation light sources.įree electron lasers (FELs) 5, 6, 7, 8, 9, capable of providing high peak power, coherent radiation, are recognized as another revolutionary research tool for various fields. It is known that it helps to provide higher brightness and space coherence with a so-called multi-bend achromat (MBA) storage ring lattice design, that is to decrease the bending angle in each of the dipole bending magnets, allowing stronger focusing by multipole magnets between the bending magnets, instead of the double or triple bend achromat (DBA or TBA) lattice mostly employed in the 3rd generation light sources. In order to further improve the brilliance of the synchrotron radiation light source, the diffraction limited storage rings (DLSRs), recognized as one type of the 4th generation light sources, have been developed in the past decades 1, 2, 3, 4. In addition, the synchrotron radiation pulses are incoherent in both transverse and temporal, limiting the application on many frontier sciences such as high-resolution spectroscopy and imaging experiments. However, the durations of radiation pulses from storage rings are still too long to measure the atomic motion and structural dynamics on the fundamental time scale of a vibrational period (~ 100 fs). The average brilliance (B) of the 3rd generation light sources, defined as the photon flux (F) over the transverse photon beam size (∑ x∑ y) and divergence (∑’ x∑’ y) in 0.1% spectral bandwidth, \(\), is typically 10 19 phs/s/mm 2/mrad 2/0.1%BW. To date, the 3rd generation light sources, with the radiation mainly generated from the insertion devices, have witnessed an impressive development worldwide, instead of the 1st and the 2nd generation light sources with the radiation mainly emitted from the bending magnets. Over the past half-century, remarkable interests and demands of the synchrotron radiation users in the extreme ultraviolet (EUV) and x-ray regime lead to the continuing improvements of the synchrotron radiation facilities (SRs) in four generations, impacting on many disciplines such as physics, chemistry, biology and material science 1.
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