Ultrafast Lasers at the Linac Coherent Light Source
The Linac Coherent Light Source at SLAC is the world’s first hard X-ray free-electron laser, or FEL, and one of the most complex light sources ever developed. Its ultrashort pulses of X-ray laser light, a billion times brighter than any light source before it, are uniquely capable of probing the detailed structure and dynamics of atoms, molecules, and materials. But this brilliant beam is not the only laser at work in the LCLS. Other ultrafast lasers kick off the process that generates the X-ray laser beam and play an essential role in experiments at the LCLS.
The process of generating the X-ray laser beam starts in the photoinjector, which is stationed partway down the two-mile-long SLAC linear accelerator. Here scientists hit the copper surface of a photocathode with a conventional, ultrafast laser to generate bursts, or bunches, of electrons, which accelerate to high energies as they travel down the linac. At the end the electrons wiggle through a series of undulator magnets. This causes them to emit radiation in the form of X-ray light. And this light, in turn, changes the distribution of the electrons in a way that synchronizes their light emissions, thus producing “coherent” X-ray laser radiation.
Timing is critical from the start. The bunches of electrons must leave the copper surface of the photocathode at exactly the right times so they can synchronize properly with the rest of the electron accelerator. The ultrafast pulses from the conventional laser system in the photoinjector – which are just a few picoseconds, or trillionths of a second, long – allow this tight level of control; and by changing the characteristics of the laser pulses, scientists can optimize the sizes and timing of the electron bunches to maximize the intensity of the resulting X-ray laser beam.
During FEL operation the photoinjector laser is operated 24/7 for weeks at a time, and must produce pulses of exceptionally stable size, timing and energy. The LCLS laser department has dedicated scientists and engineers on call around the clock to ensure optimal operation of this vital part of the LCLS.
Lasers in Experiments
To date, about half of the experiments at LCLS use ultrafast optical lasers along with the X-ray laser to probe the structure and dynamics of atomic and molecular systems. Many of these experiments use a “pump-probe” configuration in which the experimental sample is pumped with the optical laser and probed with the FEL X-rays, or vice versa.
The complete list of experiments is exceptionally diverse. For example, the X-ray laser may manipulate or probe the inner electrons of atoms and molecules, then scientists use the ultrafast optical laser to watch how the atom or molecule re-arranges itself in real time. In biochemistry, an ultrafast optical pulse may stimulate structural changes in a biological molecule, followed by an X-ray pulse that looks for those changes with X-ray diffraction, a technique originally intended for exploring the unfolding of proteins. In materials science, a sample is hit with an ultrafast optical pulse to compress its lattice-like molecular structure, while the X-ray pulse observes the compression and subsequent relaxation in real time, again relying on X-ray diffraction. In other experiments, an ultrafast optical pulse changes the sample’s magnetic properties, which are then probed with X-rays from the FEL.
In the LCLS’s Near Experimental Hall, the ultrafast lasers are located in a laboratory above the experimental hutches, where they are continuously operated, monitored, and optimized. There are three complete laser systems, each producing pulses of several millijoules of energy at a wavelength of 800 nanometers. Each pulse lasts 35-150 femtoseconds, or millionths of a billionth of a second. There is one complete laser system for each hutch, with the ability to send the output of any laser into any hutch as a backup in case a laser goes down during an experiment. The outputs of each laser are sent through vacuum relay tubes into laser-safe enclosures in the experimental hutches, where the beams are manipulated to meet the needs of each individual experiment. Almost every experiment has different needs, and there are two new experiments every week, so the laser group maintains a full-time laser engineer or scientist dedicated to each hutch to ensure that all of the diverse laser needs are met for every experiment. There is currently is not enough space in the Far Experimental Hall for a dedicated laser lab, and the laser systems will be located in the experimental hutches.
Synchronizing the FEL with an External Ultrafast Laser
In experiments that combine optical lasers and the FEL, the quality of the data often depends critically on knowing the precise timing, within a few femtoseconds, of the X-ray pulses relative to the optical laser pulses. But this is not easy to achieve.
The “master clock” of the SLAC linac operates at a radio frequency (RF) of 476 megahertz. The lasers are synchronized to this RF clock, but only within about 100 femtoseconds. Unfortunately, this is quite a lot of jitter compared to the duration of the laser pulses and X-ray pulses, and this makes it difficult for scientists to understand very fast changes in the systems under study.
Efforts are under way to improve the locking of the optical lasers to the RF, but these improvements are not expected to decrease the timing jitter enough to meet the needs of future LCLS experiments. Therefore additional efforts have begun to develop techniques to measure the relative time of arrival of the FEL X-ray pulses and optical laser pulses within just a few femtoseconds. These techniques will not eliminate the jitter between the optical lasers and the FEL, but, if successful, they will tell experimenters the precise timing between the two sources, enabling much better understanding of the experimental data.
Ultrafast laser systems are key components in both enabling superior FEL operation and control, and vastly enhancing the experimental capabilities of this unique light source. SLAC has invested heavily in ultrafast laser systems and skilled personnel to ensure that the LCLS maintains and enhances its expertise in this critical technology.