Getting synchrotron light onto a sample

An electron takes many small turns

Fast electrons in a storage ring radiate as magnetic fields bend their motion. A row of alternating magnetic poles makes an electron wiggle gently along a straight section. This device is an undulator. Each oscillation contributes radiation.

At suitable photon energies and observation directions, the contributions reinforce one another. Peaks appear in the energy distribution, including higher harmonics. Changing the magnetic field moves the peaks and helps make useful X-ray energies available.

Sources: [1][2]

A slit selects directions and changes the spectrum

Close the gap between absorbing blades and some light strikes the material. Far from the source, a smaller opening accepts a narrower range of directions. Undulator energy and emission angle are related, so this also changes the transmitted energy distribution.

Flux counts photons per second through a stated window. Spectral brightness additionally describes how concentrated the light is in source area, direction and energy range. Coherence concerns relationships between light-field oscillations; it requires appropriate field or interference information.

Sources: [3]

Two crystals select a narrow energy range

Atoms in a crystal form regularly spaced planes. X-rays scattered from successive planes travel slightly different distances. When those differences match the wavelength, reflection is enhanced: Bragg reflection. Rotating the crystal changes the selected wavelength and photon energy.

A double-crystal monochromator reflects the beam twice. The first crystal selects energy; the second turns the beam back toward its original direction. Maintaining a fixed exit height while changing energy also requires moving the second crystal. Tilting it alone can reduce throughput and steer the surviving light away from the sample.

Sources: [4][5]

Mirrors bring the accepted light to a focus

Useful hard-X-ray reflection usually requires a shallow grazing angle along a surface. A curved mirror redirects the accepted light toward a focus. A Kirkpatrick–Baez, or KB, system uses two mirrors to focus in horizontal and vertical planes.

Near focus, light occupies a small region. Move the sample along the outgoing path and its spot can broaden. Finite mirrors intercept only part of the beam, and their coatings reflect only a fraction of what arrives. Spot size, energy range and delivered flux must be considered together.

Sources: [6][7]

Measure what the sample changes

A transmission measurement brackets the sample exposure with open-beam measurements. Agreement between the two open readings supports the comparison. Independent references must also calibrate detector efficiency, electronic response and exposure time.

A thicker sample generally leaves fewer photons in the original beam. For a known material and photon energy, that change can support a thickness estimate. Removal includes both absorption and scattering into other directions. Determining deposited heat requires further information.

Sources: [8]

Each experiment needs a suitable beamline

Energy range, spot size and sample environment vary between experiments. At HEPS, the call issued on 12 October 2026 set a 30 November deadline and expected experiments from February 2027. These were the announced plans, checked on 12 November; individual beamline opening notes determine the conditions offered to users.

Sources: [9]