Optical path and upstream optics¶

The beamline splits naturally into upstream optics (everything before the spherical experimental chamber) and in-chamber sample and detector hardware.

1. EPU source¶

Soft X-rays are produced by an elliptically polarizing undulator (EPU) on the storage ring. The undulator gap and polarization setting determine flux, harmonic content, and polarization state (linear vs circular vs intermediate) as a function of energy. Ring beam current is read as an analog channel for long-term normalization and drift awareness.

2. Horizontal exit slit¶

After the undulator, a horizontal exit slit selects the useful portion of the fan of radiation before dispersion.

3. M101 monochromator¶

Beamline energy is set by the M101 monochromator, a grating instrument. Different gratings cover different energy ranges (for example lower vs higher photon energies). Monochromator motors include grating selection, vessel position, feedback, and small deflections used for alignment and stability.

4. M103 mirror and focusing¶

Downstream of the monochromator, M103 (and related mirror optics) focus and steer the beam toward the experiment. Mirror motors adjust yaw and bend to shape the focal spot at the sample.

5. Higher-order suppressor (HOS)¶

A four-bounce mirror assembly removes higher harmonic content that would otherwise contaminate low-energy scattering or spectroscopy. Its incidence angle is adjustable over a limited range, or the assembly can be withdrawn from the beam when unfiltered spectrum is required (with the trade-off that harmonics are no longer suppressed).

6. Gold mesh and flux monitoring¶

A gold mesh can be inserted upstream of the HOS for photoelectric current-based flux measurement. A second mesh assembly downstream of the HOS but before the first JJ slits provides incident flux suitable for normalization when the upstream spectrum is not appropriate (for example certain low-energy scattering conditions). The corresponding analog channel is commonly used as an Izero reference for absorbed intensity.

7. JJ scatter slits (three stations)¶

JJ scatter slits define beam size and position at three distances:

Each station has four motors: vertical and horizontal translation (centering) and aperture (size).

8. Shutter and chamber entry¶

Before and inside the chamber, shutters (often piezo or air-assisted) gate X-ray exposure. Digital outputs open and close the beam; digital inputs report dump status and shutter-related interlocks. In-chamber lighting is separate from the X-ray shutter and is used for alignment cameras.

Summary diagram (conceptual)¶

Ring -> EPU -> exit slit -> M101 (energy) -> mirrors (e.g. M103) -> HOS
  -> [optional mesh] -> Upstream JJ -> Middle JJ -> shutter path
  -> In-chamber JJ -> sample -> detector arm (photodiode / CMOS area detector)

Next: Sample and detector geometry for coordinate axes and alignment roles.