XFEL: Building the perfect mirror

Scientists at the European XFEL facility will need to ensure that its 27 000 X-ray flashes per second reach their intended targets without distortions. Since X-rays are a form of light, scientists can use mirrors to change their course as needed. But it takes a lot more than a basic looking glass to reflect the world’s most intense X-ray flashes.

Instead, teams at European XFEL have developed a unique mirror that can maintain the X-rays’ laser qualities. A prototype of this mirror was recently produced, and its properties are being measured and tested at Helmholtz-Zentrum Berlin, Germany. At approximately one metre long—the length needed to reflect the X-rays at the required extremely shallow angles—the scale of the mirror might not seem so special. But for its entire length, the mirror’s surface cannot deviate more than a few nanometres from perfect flatness. That’s the equivalent of a road only being able to go up or down by the width of a human hair for 28 kilometres.

“It is the first time we are realizing a long mirror with such a high quality,” says European XFEL scientist Maurizio Vannoni. Other mirrors used in science, such as those in telescopes, are much bigger in size, but they are not required to have anything close to this margin of error.

To reflect X-rays, which have wavelengths 1000 times smaller than those of visible light, very high-quality mirrors are required. And since at the European XFEL these X-rays have a laser-like quality, the mirrors will need to be better than the best quality available today.

That’s because within each flash, the X-ray laser waves are running exactly parallel with one another and form what is known as a wavefront . After reflection off the mirror, the waves need to remain in this parallel alignment. Any bend in the mirror will have the same effect as a wavy mirror in a carnival funhouse—it will reflect in a distorted manner.

“When you use a bad mirror, you destroy the wavefront quality,” Vannoni says. “These mirrors need to reflect perfectly—a real challenge.”

Left: The prototype was constructed in collaboration with three companies: Thales SESO in France, Bruker in the USA, and Cinel in Italy. Right: A drawing showing how the piezo-electric elements will allow for adjustment of the mirror.

Experiments at the European XFEL will rely on X-ray laser pulses coherently arriving at the scientific instruments, so all mirrors have to be as flat as possible.

To build the mirror, Vannoni and his colleagues have been developing advanced engineering methods to craft and polish it. The mirror is made from high-density silicon coated with a thin layer of a hardy chemical called boron carbide, which will help the silicon survive the X-ray flashes’ intensity. The team can precisely manipulate the mirror’s shape—and therefore its flatness—by using piezo-electric elements along its length. These piezos apply a voltage that can stretch or contract the mirror’s surface.

“The result depends on how good the starting surface is,” Vannoni says. “Ninety percent of the quality should be in the mirror, and the additional 10 percent you can get with the piezos.”

As a next step of this development, Vannoni and his colleagues are implementing a cooling system for the mirror so deformations of its surface caused by any absorption of X-rays can be avoided. The production of ten such mirrors is beginning for the European XFEL’s three starting beamlines. The first mirrors are scheduled to be installed in about one year.

“Very early on, we realized this would be a difficult task, but now we have a prototype and the test results at hand—and these confirm our modelling,” says European XFEL Scientific Director Thomas Tschentscher. “I’m excited to see the first fully set up system.”