From the Director of LCLS: LCLS II and Beyond
When the Linac Coherent Light Source was planned and built, LCLS and SLAC management already envisioned how it could be expanded. Design sketches dating back to 2004 show the futuristic concept of multiple tunnels piercing through the hill at the end of the linac and ending up in additional underground experimental halls. The three-kilometer linac itself, the centerpiece of SLAC, also lends itself to LCLS expansion. It consists of three, one-kilometer sections that can each be fed by a beam injector. LCLS takes advantage of this modular linac design. The electron beam energy required for X-ray production in the Undulator Hall can be achieved by use of only the last third of the linac. The LCLS electron beam is therefore not produced at the beginning of the linac, but by an injector that is located in a vault at the two-kilometer point, leaving the first two thirds of the linac unused. While over the next five years the front part of the linac will be the home of the FACET project, we are already making plans to incorporate it afterwards into our envisioned LCLS expansion.
When we proposed LCLS II late last year and discussed it with our users, the early plans envisioned creating a second electron beam injector and using another kilometer of linac to produce a second, independent electron beam that feeds a second undulator X-ray source located next to the existing undulator. The concept avoids the construction of new tunnels and halls, and fully exploits the existing buildings. In addition to a second linac section, the original or baseline design adds a pair of undulators into the existing hall to create a second X-ray beam, and by clever manipulation of the two X-ray beams—one "soft" (low x-ray energy) and one "hard" (high x-ray energy)—allows simultaneous operation of at least five of the six experimental stations. The LCLS II project was envisioned to be completed by 2017.
On second thought, this summer, we realized shortcomings of the baseline design and began working on an alternative option with different opportunities and risks. We started from a vision that went beyond the 2017 time horizon of LCLS II. Based on the success of the FLASH facility in Hamburg and especially the early success of LCLS, the international landscape is rapidly changing. Many countries have now developed plans for building X-ray free electron lasers, and it is important for us to keep an eye on the competition. Since the planning horizon of most international facilities extends to about 2025, we started with the question where LCLS needs to be in 2025 to remain competitive.
Over the summer we developed a plan for "LCLS 2025" that will allow us to remain at the international forefront. Our plan constitutes a measured and financially prudent response to investments around the world and aims to have comparable capacity and capabilities as our competitors. For LCLS 2025 we envision use of the entire linac with three independent injectors for the three one-kilometer linac sections. The three linac beams then feed two LCLS branches, each consisting of an undulator tunnel and experimental halls. The new second branch would be located right next to the existing one. The two tunnels each contain two pairs of undulators, each pair being combined in a self-seeding arrangement to create well controlled, bright X-ray pulses. The four X-ray beams are then distributed to 10 experimental stations, three located in the present Near Experimental Hall or NEH, three in the present Far Experimental Hall, and four in a new experimental hall located partly above ground next to the existing NEH.
In light of LCLS 2025, we then asked whether the baseline LCLS II design was still an optimal strategy. Our brainstorming sessions came up with a new option which not only provides a good LCLS II solution by 2017 but also offers dramatic improvements in instrumentation and operation in the time period 2017–2025 toward LCLS 2025 . The key change from the baseline LCLS II option is to already incorporate construction of the second branch, consisting of a new tunnel and experimental hall. In contrast to the baseline or "one tunnel" option, which upgrades and extends instrumentation within the existing branch, the new "two-tunnel" option offers additional flexibility for instrumentation. Both options include a second injector and one-kilometer linac section, but in the two-tunnel option the second beam is guided into the new branch and feeds a pair of undulators in the second tunnel. Because of the additional tunnel cost, the pair of undulators is configured somewhat more simply to stay within the LCLS II budget scope. We eliminated the soft X-ray seeding capability and dramatically simplified polarization control. X-rays produced by the new undulators will serve the AMO and SXR soft X-ray stations, which are moved from the NEH into the new experimental hall. Each branch then contains space for an additional undulator pair and two more experimental stations.
At the end of the LCLS II project, the two tunnel option can therefore be extended to the LCLS 2025 configuration without major conventional construction by simply filling empty spaces. For example, while upgrading or extending instrumentation in one branch, we can still do experiments in the other. The flexibility of gradually adding and upgrading instrumentation therefore leads to a much higher integrated scientific productivity.
On September 15, the two LCLS II options were presented at an LCLS Roadmap Workshop to members of the LCLS Proposal Review Panel, the Scientific Advisory Committee, the Users Executive Committee and selected experts. All workshop participants favored the two tunnel option. We will present the two LCLS II options to the entire user community at the upcoming LCLS users meeting, and will also discuss them at the next LCLS Scientific Advisory Committee meeting in late October and the SLAC Science Policy Board meeting in early November. We expect to receive their endorsement for proceeding with the two tunnel concept in our next step, the conceptual design of LCLS II.