As a transformative tool for science, the Next Generation Light Source (NGLS) will comprise an array of x-ray lasers providing temporally and spatially coherent pulses with unprecedented average brightness extending to photon energies beyond 1 keV. Individual lasers and beamlines will be optimized for specific applications requiring, for example, high repetition rates, time resolution to the few-femtosecond regime (and potentially shorter), high spectral resolution, tunability, and polarization control. This powerful combination of capabilities will enable cinematic imaging of dynamics, determination of the structure of heterogeneous systems, and development of novel nonlinear x‑ray spectroscopies. These unique resources will lead to a new understanding of how electronic and nuclear motions in molecules and solids are coupled and how functional systems perform and evolve in situ.
NGLS will dramatically impact a wide range of energy applications—from natural and artificial photosynthesis to catalysts, batteries, superconductors, carbon sequestration, and biofuels. Solving the complex long-term energy challenges facing both the nation and world is the subject of a wide-ranging set of reports from the DOE Office of Basic Energy Sciences (science.energy.gov/bes/news-and-resources/reports/). These reports highlight the urgent need for deeper understanding of the basic science underpinning energy technologies. The NGLS—with its combination of high average power, ultrashort pulses, and coherence—is a revolutionary observational tool that will bridge the critical gaps in our understanding.
The current design concept for NGLS is a coherent, x-ray free-electron laser (FEL) array powered by a superconducting accelerator capable of delivering electron bunches to a suite of up to 10 independently configured FEL beamlines. Each beamline, operating simultaneously at a nominal repetition rate of 100 KHz, will be optimized for specific science needs. With multiple beam capabilities in each FEL beamline, up to 20 primary x-ray experimental end stations can operate in parallel, serving up to some 2,000 users per year.
Figure 1 shows the major components of NGLS: the injector, laser heater, continuous-wave (CW) superconducting linac sections, linearizer and bunch compression systems, beam distribution, array of independent FELs, and x-ray beamlines.
Figure 1. Schematic layout of the main components of NGLS (not to scale).
The NGLS approach combines significant recent advances in high-brightness
photocathode beam generation, acceleration, and transport with state-of-the-art
superconducting radio frequency (SCRF) technology, as well as revolutionary
concepts for laser-seeded FEL operation and undulator designs. The uniform
pulse spacing at a high repetition rate will provide unprecedented capabilities
at startup, accommodating more diverse and challenging experiments than those
enabled by current or planned sources. This feature also offers the potential
to exploit conceptual and technical advances in areas such as seed lasers,
superconducting undulators, x-ray optics, and FEL oscillators. The distributed
multibeam approach allows for enhanced capacity by expanding the number of end
stations. There also is tremendous opportunity to exploit advances in machine
control and operation to provide greater flexibility and new capabilities for
generating x-ray pulses tailored to specific science needs.
Figure 2. Comparison of coherent pulse characteristics of existing and planned light sources.
As illustrated in Figure 2, no other technology can provide the average power, precision, and simultaneous utilization of multiple beams by many researchers. First-generation x-ray FELs with low repetition rates provide orders-of-magnitude improvement over existing sources, primarily in peak brightness and temporal resolution. However, peak brightness is not a substitute for average brightness, particularly when probing valence electron structure, chemical bonding, electron correlation, and charge dynamics in condensed matter systems. A next-generation light source is needed that provides (1) high average brightness along with ultrafast temporal resolution down to the femtosecond regime; (2) polarization control; and (3) tunability through the important absorption edges of carbon, oxygen, nitrogen and the L-edges of first-row transition metals. High repetition rates are essential for capturing events that are rare or have low scattering rates and for probing the structures of heterogeneous ensembles in situ. Such rates also are needed to maintain the required average brightness for experiments in which the peak brightness is necessarily restricted to avoid significant perturbation of the sample.
These scientific requirements can be uniquely met by an array of x-ray lasers with high repetition rates, as envisioned for NGLS. As a unique tool for exploring the structure and dynamics of matter at fundamental scales of length, time, momentum, and energy, NGLS will provide many benefits and advantages over existing and planned light sources. The facility will feature the following capabilities:
The NGLS provides a unique combination of ultrafast capability, high longitudinal coherence, high average power, multiuser operability, and flexibility in time structure (both repetition rate and pulse duration). Moreover, the facility’s upgrade potential could provide both additional capacity (up to seven more FELs) and new capabilities—including, for example, higher repetition rates and resolving power; higher average and peak x-ray power; longer pulses; shorter- and longer-wavelength FELs; additional synchronization and shaping possibilities; and x-ray pulse feedback control. The following sections describe NGLS baseline parameters for one preliminary design point while acknowledging the significant flexibility around these point parameters. The design options discussed here are meant to illustrate the exciting scientific opportunities presented by the convergence of rapidly advancing FEL technology; superconducting accelerator technology; and sophisticated micromanipulation of high-energy, high-brightness electron beams. A detailed optimization of cost, performance, and risk has yet to be performed but will build on the baseline pre-conceptual design outlined here.
Figure 3. Three nominal initial FEL beamline types and performance features.
Flux may be controlled from ~108 to ~1012 photons per pulse in the fundamental, depending on the desired wavelength, pulse duration, and repetition rate; harmonics will be available at reduced intensity. Laser seeding will be implemented to produce pulses with duration as short as ~1 fs and even sub-femtosecond pulses with reduction in flux, with (1) temporal coherence approaching fundamental transform limits, (2) the possibility of some control over chirp or longitudinal pulse-shape, and (3) synchronization of the x-ray pulses and timing with respect to end station lasers with ≤10 fs precision. One of the FELs will be capable of producing “two-color” x-ray pulses; another will provide better energy resolution with longer pulses and high temporal coherence. The third FEL will be a self-seeded SASE device capable of operating at the full repetition rate of the linac, thereby providing very high average power x-ray beams. At about constant average electron beam power, NGLS can operate at a higher pulse repetition rate using bunches of lower charge and shorter duration but higher brightness. Table 1 summarizes estimates for major FEL performance parameters for the nominal initial configurations. The machine and FEL designs will be developed to address the requirements required for each FEL beamline. Figure 3 shows the major performance characteristics of the three nominal initial FEL configurations for the NGLS. Figure 4 shows the estimated flux per pulse for a SASE FEL.
Figure 4. Photon flux and tuning range for two different undulator designs: in green a superconducting device with period 19.4 mm, magnetic gap 7.5 mm, undulator K-parameter 4.3 (at 0.27 keV) and 1.6 (at 1.2 keV), in red a permanent magnet device with period 29.2 mm, magnetic gap 7.5 mm, undulator K-parameter 3.4 (at 0.27 keV) and 1.0 (at 1.2 keV). Electron beam parameters are 0.6 µm emittance, 600 A peak current, 250 fs pulse duration, and 100 keV rms slice energy spread. The solid lines show the fundamental, while the dashed and dotted curves show the 3rd harmonic and 5th harmonics, respectively. The fundamental provides > 1012 photons/pulse below 0.9 keV and ≥5x1011 photons/pulse at 1.2 keV. In addition, the 3rd harmonic provides greater than 109 photons/pulse at 3 keV, and >108 photons/pulse at 6 keV (superconducting), while the 5th harmonic provides 108 photons/pulse at 6 keV (superconducting). Values are for SASE FEL output and might be increased with self-seeding and a tapered undulator, however the calculations do not include errors, which will reduce the output. Superconducting undulators are under development at LBNL in an R&D project.
Table 1. Nominal performance parameter ranges for three initial FEL configurations.