# The Linac Coherent Light Source: Recent Developments and Future Plans
## 3 LCLS-II Science Opportunities  
The representative recent highlights illustrated above are just a glimpse of the remarkable scientific impact achieved by LCLS and related facilities that comprise the first generation of X-ray free-electron lasers. Similar facilities are just beginning operation or are under constructions around the world, including PAL-FEL in the Republic of Korea and Swiss-FEL in Switzerland [26]. However, despite the enormous peak brightness, the average X-ray brightness from this initial generation of XFEL facilities is quite modest, owing to the low repetition rate associated with pulsed-RF accelerator technology. This restricts their impact in many important areas of science that require both high average brightness and ultrafast time resolution.  
A new generation of XFELs is now under development that will overcome this limitation by exploiting superconducting RF accelerator technology (SCRF) to provide ultrafast X-ray pulses at  
Figure 7: Fresh-slice multi-pulse scheme. The electron bunch propagates off-axis in the dechirper to create a strong transverse head–tail kick. The subsequent oscillating orbit (in combination with fixed-magnet corrections) is exploited so that the tail of the bunch (orange) lases in the first undulator section (at energy E\({}_{1}\)) and the head of the bunch (blue) lases in the second undulator section (at energy E\({}_{2}\)). The current LCLS layout allows for up to three pulses with controlled photon energies and pulse delays. Reprinted with permission from [25]. Copyright Nature Publishing Group, 2016.  
high repetition rate. This development is driven by important new science opportunities that have been identified and advanced over the past decade through scientific workshops around the world. The European-XFEL in Germany is the first of this new generation, spanning the hard and soft X-ray ranges and delivering 10 Hz bursts of 2700 pulses at \(\sim\)4.5 MHz, representing an average repetition rate of up to 27 kHz. Most recently, a series of science workshops held at SLAC National Accelerator Laboratory in 2015 focused on the new science opportunities [27] that will be enabled by the LCLS upgrade project (LCLS-II), based on a novel continuous wave SCRF (CW-SCRF) accelerator technology. As shown in Figure 8, LCLS-II will provide ultrafast X-rays in the 0.25 keV-5 keV range at repetition rates up to 1 MHz with two independent XFELs based on adjustable-gap undulators: soft X-ray undulator (SXU) spanning the range from 0.25 to 1.6 keV, and the hard X-ray undulator (HXU) spanning the range from 1 keV to 5 keV [4].  
Here we highlight some of the important new science opportunities enabled by such a facility in the areas of (1) fundamental charge and energy flow in molecular complexes; (2) photo-catalysis and coordination chemistry; (3) quantum materials; and (4) coherent imaging at the nanoscale. These examples represent just a few of the many science opportunities where a high repetition rate is particularly enabling, and is not intended to be comprehensive of the broad range of science that is driving the development of such facilities.