The focal length of an X-ray lens depends on the photon energy, the radius of curvature of the refracting surfaces, the lens material and the number of lens elements arranged in a row. Since the refractive index of the lens material of a CRL cannot be changed and the radius of curvature of the refracting surfaces of a manufactured X-ray lens cannot be changed, a variable focal length of an X-ray lens can be achieved by changing the number of lens elements in the X-ray beam. Adjustable focal length X-ray lenses are known as transfocators or zoom CRLs. Strictly speaking, transfocators are optics whose focal length can be changed in steps, and zoom lenses are optics whose focal length can be changed continuously. Refractive X-ray optics with an almost continuously variable focal length are referred to as zoom CRLs.
Transfocators
Most transfocators are systems in which pneumatic or other electromotive actuators move individual beryllium lens elements or groups (usually consisting of 1, 2, 4, 8,... lens elements) of such lens elements into or out of the X-ray beam. Since I have no rights to the pictures, here are some links to such devices: Transfokator, 2010; Microfocusing transfocator, 2012; F-Switch, 2016; Compact transfocator, 2019
The advantages are that the built-in beryllium lens elements can be cooled well due to their high thermal conductivity and can therefore be used in the white polychromatic X-ray beam. The disadvantage is that most of the devices are relatively large, measuring more than 1/2 m, and are therefore more difficult to integrate into experimental setups. In addition, the beryllium lens elements have only been manufactured with discrete radii of curvature of 50 µm, 100 µm, 200 µm, 300 µm, 500 µm, 1 mm, 1.5 mm,... This means that even with clever combinations of lens elements, not all desired focal lengths can be achieved.
LIGA-Zoom-CRLs
Since 2017, KIT/IMT has been developing zoom CRLs (Fig. 1) for monochromatic radiation based on CRLs manufactured using deep X-ray lithography, in which the lens elements are positioned on the ends of bending fingers made of silicon. By flipping eccentric levers, the approximately 30 bending fingers (each for the vertical and horizontal focus direction) can be bent and thus the lens elements on the respective bending finger can be removed from the X-ray beam. The radii of curvature of the optical surfaces of the lens elements along the CRL vary in the percentage range, allowing an almost continuous adjustment of the focal length by a clever combination of different lens elements. As the focal length can be adjusted independently in the vertical and horizontal directions, astigmatic optics can also be realized.
For example (depending on the CRL installed), a desired focal length of 300 mm can be kept constant to within 25 µm, even if the photon energy varies in the range 8-18 keV. Or a long focal length of, say, 5 m can be adjusted to within 2 mm for a photon energy of 12 keV with only a few lens elements in the beam.
The Zoom-CRL has a mass of about one kilogram and a volume of about one litre. It takes about 25 seconds to change the focal length. To do this, a stepper motor moves a gripper to the appropriate bending fingers and a second stepper motor moves a swivel arm with the gripper to move the appropriate eccentric lever. The focal length is maintained even in the event of a power failure. The desired focal length and photon energy are entered into a program that calculates which lens elements must be in the X-ray beam and sets this combination via a microcontroller on the LIGA-zoom-CRL.
Fig. 1: CAD view of a LIGA zoom CRL with lens elements 1, bending fingers 2, eccentric levers 3, stop rods 4, stepper motors 5 and 6, swing arm with toothed belt 7, limit switch 8, position sensor 9 and connection sockets 10 (left); photo of a LIGA zoom CRL (right)
A video of the zoom CRL can be downloaded here (5 MB). Figure 2 shows the complete system.
Fig. 2: View of the zoom CRL with control electronics, power supply and notebook with control software
Rotary zoom optics
Zoom X-ray optics with point focus could also be realised with so-called alligator lenses (Fig. 3). This would require two pairs of almost parallel, grooved substrates to be arranged in such a way that the wedge angle between the adjacent substrates could be controlled very precisely. This would require four rotary actuators with angular increments of around 100 µrad and would therefore be mechanically complex.
Fig. 3: Alligator zoom optics with line focus. To achieve a point focus, another lens of this type would have to be placed behind this lens, rotated 90° around the optical axis.
W. Jark has proposed a mechanically simpler variant with a prismatic rotating zoom optics with point focus. A microstructure as shown in Figure 4 focuses the incident X-ray beam. When rotated around the 45° axis shown in grey, the focal length increases with the angle of rotation. The advantages of such an optics are the very fast adjustment of the focal length and the simple mechanical design. The disadvantage is the reduced quality of focus due to the rounding of the prism edges in the beam.
Fig. 4: CAD drawing of a prismatic rotary zoom lens with point focus; the focal length is adjusted by rotation around the axis shown
Figure 5 shows the measured intensity at the focus (5 µm x 19 µm, spectral intensity enhancement SIE = 85) of a rotating prism zoom lens 580 mm behind the optics. The two orange squares at the top left and bottom right show the beam intensity without the optics.
Fig. 5: Intensity measured at the focus of a rotating prism zoom lens, corresponding to an image edge length of approximately 1 mm.