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Article title: Measuring Extremely Bright Pulses of Light.

X-RAY free-electron lasers (XFELs) are tunable, high-power sources of short photon pulses. These new machines offer significant promise for scientific and medical breakthroughs by capturing the motion of molecules and even atoms. The intense pulses generated by XFELs are beginning to enhance time-resolved research in biology, chemistry, materials science, and physics. Designing diagnostic instruments for x rays is challenging, particularly for high-energy pulses, because the x-ray beam can damage the equipment.

In collaboration with SLAC National Accelerator Laboratory in Stanford, California, Lawrence Livermore scientists won an R&D 100 Award for developing a detector, called the XFEL energy monitor, that measures this pulse-by-pulse energy in real time without being damaged by the beam and with minimal effect on beam quality. The team probed nitrogen gas at x-ray energies up to 8 kiloelectronvolts, and the photoluminescence-based pulse-energy detector provided the required calibration information for the team to study the interaction. Nitrogen gas was chosen for the experiments because it is nonhazardous and its ultraviolet luminescence behavior is well understood from use in previous studies to detect and characterize cosmic rays.

Three XFEL energy monitors are currently in use at SLAC’s Linac Coherent Light Source facility. In operation since April 2009, this light source is the world’s first hard XFEL, capable of generating x rays with wavelengths comparable to atomic distances. It is also the first XFEL available for scientific research, providing users with virtually instantaneous pulse-energy data. During the next few years, XFEL facilities in Japan, Germany, Italy, Switzerland, and England are scheduled to come online.

Revealing the Unseen
X rays are useful for examining all kinds of matter, from DNA, bones, and lungs to the materials comprising stars. If x rays are sufficiently intense and produced in ultrashort pulses, they can reveal information about dynamic processes in many states of matter, such as solid, liquid crystal, and extremely dense plasma. These pulses are much like flashes from a high-speed strobe light, enabling scientists to take stop-motion pictures of atoms and molecules. XFEL pulses allow researchers to measure extremely fast physical events with atomic resolution. The technology can be applied to study regimes for the first time in a vast range of fields including ultrahigh-energy-density physics, structural biology, fundamental quantum electrodynamics, warm dense matter, and atomic physics.

An XFEL’s beam pulse lasts less than 100 femtoseconds (a femtosecond is one-quadrillionth of a second), and its beam wavelength measures just about 0.1 nanometers (about the diameter of the smallest atom). The laser’s peak brightness—up to a few gigawatts—is billions of times larger than that generated by synchrotrons, formerly the brightest light sources. “The revolutionary output characteristics of an XFEL propels us into a completely new regime of x-ray–matter interaction,” says Livermore physicist Stefan Hau-Riege, who headed the detector development team.

Photo of energy monitor development team.
Energy monitor development team: (from left) Donn McMahon, Mark McKernan, Richard Kemptner, Dmitri Ryutov, Richard Bionta, Daniel Behne, Keith Kishiyama, Stefan Hau-Riege, Vasco daCosta, Marty Roeben, and Robert Geer. (Not shown: Elden Ables, Stewart Shen [now retired], Alan Wootton [formerly of Livermore], Jacek Krzywinski [SLAC], and Marc Messerschmidt [SLAC]).

Instrument Is Not Intrusive
According to Hau-Riege, studying how matter interacts with x rays requires continuous, detailed characterization of the ultrahigh-intensity photon beam, with minimal intrusion. Knowledge of beam parameters such as photon flux (the number of photons arriving at a point) is essential because the parameters determine how the beam interacts with the experimental sample. However, ascertaining XFEL beam characteristics is particularly challenging because the beam can easily saturate or even destroy commonly used solid-state detectors. The XFEL energy monitor, which barely disturbs the intensity of the x-ray beam, preserves beam coherence and can be operated in a regime in which the attenuation is less than 1 percent.

The total pulse energy is inferred from ultraviolet radiation generated by the nitrogen gas contained in the vessel through which the beam travels. When the beam traverses the vessel, the nitrogen is excited and emits fluorescent radiation that is detected by the energy monitor’s photon-multiplier tubes. The nitrogen gas is continuously replenished and maintained at a constant pressure by a series of pumps. Hau-Riege says the interaction of hard x-ray photons with nitrogen is small, and if necessary, the nitrogen gas density can be decreased during operation.

Photos of an energy monitor installed at the Linac Coherent Light Source facility.
(left) At the Linac Coherent Light Source facility, new energy monitors installed on the x-ray free-electron laser nonintrusively measure photon beam energy. (right) The x-ray energy monitor is part of the photon diagnostic and conditioning suite.

A Bright Future
Although the energy monitor was developed primarily for characterizing ultrahigh-brightness x-ray pulses from hard XFEL facilities, it may also be used to characterize less bright x-ray sources. In fact, the team successfully tested a prototype of the device at the Stanford Synchrotron Radiation Light Source, which generates x-ray beams with intensities billions of times weaker than the Linac Coherent Light Source.

A research paper describing the XFEL energy monitor appeared in the July 23, 2010, edition of Physical Review Letters. The paper was co-authored by the Livermore researchers, together with colleagues from SLAC and the Center for Free-Electron Laser Science in Hamburg, Germany. The paper has helped to communicate to the physics community that the most energetic x-ray beams ever produced can be well characterized.

—Arnie Heller

Key Words: Linac Coherent Light Source, R&D 100 Award, SLAC National Accelerator Laboratory, x-ray free-electron laser (XFEL) energy monitor.

For further information contact Stefan Hau-Riege (925) 422-5892 (hauriege1@llnl.gov).


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