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Clocking Ultra-fast Electron Bunches

Brookhaven researchers have developed a device that acts like a high-tech stopwatch for speedy packs of electrons just trillionths of a second long. This new diagnostic tool could aid in the development of x-ray free electron lasers (FEL), sources that produce pulses of light up to one billion times brighter and 1,000 times shorter than those produced at conventional storage ring light sources.

Like a high-speed strobe light, the ultra-short bursts of light produced from an FEL allow scientists to take stop-motion pictures of chemical reactions, biological processes, and various other atomic-scale events. FELs create this valuable x-ray light by shooting a series of ultra-short bunches of electrons through an array of specialized magnets. The shorter the electron bunches, the more powerful the resulting light.

In recent years, the bunch lengths of electrons in accelerators have decreased dramatically, producing extremely bright pulses of light that are on the same time scale as vibrations in molecules and the creation or breakage of chemical bonds. But in order to synchronize the time-dependent process being studied with the x-ray pulse, scientists need new beam diagnostics capable of measuring the length of picosecond-scale (one millionth of one millionth of a second) pulses.

At the NSLS Source Development Laboratory (SDL), Brookhaven researchers have demonstrated a high-resolution measurement system that’s nondestructive to the electron beam and can be easily transported among facilities. 

In the setup (first proposed by Brookhaven researchers in 2002), a beam of electron bunches is sent across the top edge of a thin, “electro-optical” crystal. Traveling at close to the speed of light, each electron bunch emits an electric field that travels downward through the crystal. At the same time, a single, line-focused laser pulse is sent straight through the broad side of the crystal, where sections of the laser line are encoded with spatial information from the electron bunches. A special camera at the end of the device collects these data.

Now, the goal is to measure electron pulses an order of magnitude smaller, in femtosecond duration, or quadrillionth of a second range.

X. Yang, T. Tsang, T. Rao, J.B. Murphy, Y. Shen, X.J. Wang, “Electron Bunch Length Monitors Using Spatially Encoded Electro-Optical Technique in an Orthogonal Configuration,” Appl. Phys. Lett., 95, 231106 (2009).

Top: Schematic layout of the electro-optical arrangement.

Middle: The electro-optical module with a YAG crystal for electron beam position monitor.

Bottom: Clockwise from front: Triveni Rao, Thomas Tsang, Xi Yang, Xijie Wang, Jim Murphy, and Yuzhen Shen

Figure 1 Figure 2