Since summer 2024, the NanoMAX imaging endstation (EH1) uses a fixed KB-mirror system to create a nano-focused beam. The two mirrors with matching numerical aperture create a symmetric diffraction limited focus in the shape of two crossed one-dimensional sinc functions.
Beam size and intensity
The coherent flux / beam intensity and focal spot size at the NanoMAX imaging endstation (EH1) depend on the photon energy of the X-ray beam. The highest flux of about 8.e10 photons per second, can be achieved at around 7 to 8 keV. Towards both lower and higher photon energies from this optimum, the provided flux is reduced. Down to 5 keV and up to 13 keV, the flux is above or equal to 1.e.10 photons per second. Beyond 15 keV, the mirrors do not work in the total reflection regime anymore and the beam intensity is strongly reduced.
Depending on the method and sample it is possible, that not all of the available photons can be used, as the resulting signal strength could blind and damage the detectors. In such cases the primary beams intensity could be reduced by either inserting absorbers upstream of the mirrors or by closing the secondary source aperture beyond the setting needed for a coherent illumination of the mirrors. The later ensures a consistent beam quality.
With the mirrors design parameters (below) the following numbers for the diffraction limited focal spot size, depth of field and secondary source aperture size settings for a coherent illumination could be calculated:
| photon energy [keV] | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
|---|---|---|---|---|---|---|---|---|---|---|
| focal spot size [nm] | 78.7 | 67.5 | 59.0 | 52.5 | 47.2 | 42.9 | 39.4 | 36.3 | 33.7 | 31.5 |
| depth of field [μm] | 120.0 | 102.8 | 90.0 | 80.0 | 72.0 | 65.4 | 60.0 | 55.4 | 51.4 | 48.0 |
Hardware
The vertically focusing mirror (VFM) and horizontally focusing mirror are both made from single crystal silicon in shape of a elliptical cylinder. The design parameters of the two mirrors are listed in the following table:
| VFM | HFM | |
|---|---|---|
| Mirror material | Single crystal silicon | Single crystal silicon |
| Substrate shape | Elliptical cylinder | Elliptical cylinder |
| Reflection direction | Downward | Rightward |
| distance from source | 34.300 m | 34.420 m |
| distance to focus | 0.200 m | 0.080 m |
| distance between mirrors | 0.005 m | |
| free distance after HFM | 0.045 m | |
| substrate length | 0.160 m | 0.070 m |
| active mirror length | 0.150 m | 0.060 m |
| incidence angle | 3.5 mrad | 3.5 mrad |
From the design parameters above, the following parameters can be calculated:
| VFM | HFM | |
|---|---|---|
| distance from source to mirror center | 34.300 m | 34.420 m |
| focal length | 0.200 m | 0.080 m |
| acceptance aperture | 525 μm | 210 μm |
| acceptance angle | 15.3 μrad | 6.1 μrad |
| numerical aperture (NA) | 1.312 mrad | 1.312 mrad |
Options
Just upstream of the KB-mirror pair the endstation features four independently movable slit blades. Generally these are used to limit the acceptance aperture for the primary beam to the polished and thus reflective surface of the mirrors. If the science case requires a slightly larger probing beam on the sample, and simply placing the sample outside the focus is not an option, then it is possible to reduce the KB-mirrors acceptance aperture. This reduces the divergence of the beam and increases the focal spot size.
Increasing the beam size by a factor of X, in both horizontal and vertical direction, will reduce the flux by a factor of X² for usage with ptychography, as the flux density within the central beam pupil stays constant and the detector can not be over exposed.