A proposed new facility, called LUX, will be able to combine accelerator and laser systems to study ultrafast dynamics across a wide range of sciences.
Ultrafast X-rays have been identified in numerous workshops and reports around the world as a key area that is ripe for new scientific investigations – with femtosecond pulses allowing the detailed study of atomic motion during physical, chemical and biological reactions. Ultrafast lasers covering most of the visible, infrared and ultraviolet regions of the spectrum already provide the capability to measure bond breaking in chemical reactions with both excellent timing resolution and very short pulses. Thus, experimenters have used lasers to tremendous advantage in thousands of investigations of time dynamics, many of which are absolutely critical to research in solid-state physics, semiconductors, photochemistry and photobiology. However, until now, ultrafast time-domain studies in the X-ray region have been almost completely lacking, even though they are needed to refine the picture of dynamics at the timescales of atomic vibration periods – about 100 fs or less, and even the possibility of resolving electron dynamics with sub-femtosecond resolution.
Through the use of synchrotron radiation, and by the novel conversion of intense laser pulses into soft and hard X-rays, scientists have recently been able to perform some innovative experiments for the first time, such as Bragg diffraction studies of phase transitions and even attosecond electron redistribution in Auger electron processes. However, the laser-based X-ray fluxes are low, the signal levels weak and the experiments are challenging to accomplish.
LUX – a Linac-based ultrafast X-ray/laser facility – is a concept that is designed to produce ultrashort X-ray pulses in a highly refined manner for experiments across all areas of the physical, chemical and biological sciences. The facility will provide an increase of X-ray flux by several orders of magnitude, and would be accessible to a large number of users. Ultrafast lasers would be available for “pump-probe” experiments at femtosecond resolutions, where a pulse from a laser excites or “pumps” the system under study, while the X-ray pulse is used to probe the system configuration as a snapshot in time after the pump pulse. Figure 1 shows a schematic of this concept.
While the approximately 40 available light sources in the world are largely limited to static spectroscopies, microscopies and structures, LUX will be the first to be designed from the start as a user facility for femtosecond X-ray dynamics, with precise timing as an integral requirement. It will offer high repetition rates, tunability and multiple laser sources for excitation and probe experiments, with pulses 1000 times shorter than typical third-generation light sources.
Although pump-probe experiments represent some of the most important techniques, involving a femtosecond laser as a pump and the ultrafast linac-based X-ray source as the probe, the facility will also be designed to accommodate multidimensional coherent laser spectroscopies, such as three-laser pump beams and an X-ray probe, as well as two X-ray wavelengths for double-resonance X-ray pump and probe spectroscopies. Most of these novel forms of spectroscopies with X-rays have not even been delineated yet.
The LUX proposal is based on a recirculating electron linac, which provides a compact and cost-effective configuration for the production of intense ultrafast extreme ultraviolet (EUV) and X-ray pulses, with tight synchronization to sample excitation lasers. The provision of a broad photon spectrum covering the whole range from EUV to hard X-ray wavelengths allows for both spectroscopy and diffraction studies, probing nuclear positions as well as electronic, chemical or structural properties. The design specification of a 10 kHz pulse repetition rate is matched to pump-probe experiments and allows rapid data acquisition and sample relaxation or replacement.
The facility is designed to produce ultrafast EUV and soft X-rays by a harmonic-cascade free-electron laser (FEL) technique, while hard X-rays are produced by a novel manipulation of the electron bunches followed by compression of the photon beam. The FEL process is initiated by a “seed” laser, which allows tunability of both wavelength and pulse duration from hundreds to tens of femtoseconds. Hard X-ray pulses are produced in superconducting insertion devices – undulators produce narrow-band peaks with harmonics out to 10 keV and higher, and wigglers produce broadband pulses extending to even shorter wavelengths.
The major components and systems of LUX involve existing accelerator technologies: radiofrequency (RF) photo-injector guns, superconducting linear accelerators, magnet lattices in the arcs and straight sections, transversely deflecting cavities, harmonic generation in FELs, narrow-gap short-period undulators, X-ray manipulation in optical beamlines, and a variety of short-pulse laser systems. Figure 2 shows the layout of the machine. In LUX, high-quality (low emittance, high charge) electron bunches produced in an RF photocathode are accelerated to approximately 100 MeV in an injector linac before being turned into the main linac. The main linac accelerates the electron bunches by about 700 MeV on each pass, resulting in a final energy of approximately 3 GeV after four passes. After acceleration to 3 GeV the electron bunches pass through insertion devices to produce radiation, supplied to multiple beamlines.
The beam-quality requirements of the RF photocathode gun have already been demonstrated, with a normalized emittance of approximately 3 mm mrad at 1 nC charge. The flexibility of the LUX lattice design allows the control and preservation of the transverse and longitudinal emittances of the electron beam, minimizing the influence of collective effects and allowing the manipulation of the picosecond electron bunches to produce femtosecond X-ray pulses.
To produce ultrafast hard X-rays, at the exit of the final arc the electron bunches receive a time-correlated vertical kick in a dipole-mode RF cavity – the head is kicked up and the tail is kicked down, while the centroid is unperturbed. The electrons then radiate X-rays in the downstream chain of undulators and wiggler magnets, imprinting this head-tail correlation in the geometrical distribution of the X-ray pulse. The correlated X-ray pulse is then compressed using asymmetrically cut crystal optics in order to achieve the ultrashort X-ray pulse length.
In addition, high-flux, short-pulse photons will be produced over an energy range of tens of electron volts to a thousand electron volts using a laser-seeded harmonic-cascade FEL. The high-brightness electron beam is extracted from the recirculating linac and passed through an undulator, where a co-propagating seed laser results in a modulation of the charge distribution over a short length of the bunch. This modulation enhances radiation in a following undulator at shorter wavelengths that are harmonically related to the seed. The process is repeated by modulating a fresh portion of the beam, this time with the harmonic radiation produced in the previous undulator.
Sophisticated laser systems will be an integral part of the LUX facility, providing experimental excitation pulses and stable timing signals, as well as the electron source through the photocathode laser. Each endstation will have its own dedicated laser system with optical filtering and diagnostics, all contained within a stable and controlled environment. Multiple tunable lasers covering infrared to ultraviolet wavelengths with a range of pulse durations are required for experiment initiation, together with sophisticated temporal and spatial filtering to optimize the performance for specific experimental applications.
The synchronization and timing of the ultrashort X-ray pulses with respect to the experimental excitation pulse is critical in studies of ultrafast dynamics. For LUX, the techniques of optically seeded systems and bunch manipulation prove insensitive to the usual timing jitter that arises from electron acceleration in RF systems. A laser master oscillator provides stable optical pulses, and optical distribution systems transport these pulses to each beamline, with feedback based on interferometric measurements to stabilize the path lengths. The conversion to microwave signals by photodiodes allows the generation of the RF signals for the accelerator, and for phase-locking of endstation lasers. The lasers may also be optically seeded directly from the master oscillator.
The LUX project is currently in a pre-conceptual design phase, and the facility design is being optimized in order to meet the demands of the growing number of scientific applications. Combining state-of-the-art accelerator and laser systems to produce a unique X-ray facility for the study of ultrafast dynamics presents some exciting challenges and the prospect of a bountiful future in new areas of science.