Synchrotron Optics
KB-SR1 (Kirkpatrick-Baez Focusing System)
The KB-SR1 Kirkpatrick-Baez focusing system provides grazing incidence reflecting mirrors with extreme surface precision to permit focusing x-rays to spot sizes on the micron spatial scale.
Because horizontal and vertical focusing are carried out by separate mirrors, the magnification for the horizontal and vertical can be different. This permits great flexibility when dealing with an asymmetric source, such as that from a synchrotron source.
The basic principle for focusing x-rays in orthogonal directions was originally suggested by Kirkpatrick and Baez in 1948¹. Since then, the technology has been developed by many others. Scientist at Lawrence Berkeley National Laboratory in particular, have spent over 20 years exploring and developing the technology².
The KB-SR1 is a complete, stand-alone, and turnkey Kirkpatrick-Baez (KB) mirror x-ray focusing system of proven design for convenient installation and operation on synchrotron beamlines. Sub-micrometer focusing is obtained using two platinum coated, 4" x 0.5" adjustable KB mirrors for vertical and horizontal focusing, each with a self-contained bending mechanism to set the desired elliptical surface figure. The mirrors are specifically shaped to achieve a good approximation of the ideal ellipse for a given image demagnification.
The KB-SR1 offers full remote control and readout of the absolute positions of motors for adjusting the translation of the system to the beam axis, angular alignment of the system to the input beam and focusing of the source to the desired image point by adjusting mirror curvature. A table indicating the allowed range of mirror curvature adjustment will be included for each KB-SR1. All adjustments can be made remotely through a hand-held control unit and the current absolute positions of eight control motors can be displayed separately using Linear Voltage Displacement Transducers (LVDT) absolute position encoders. The KB mirror system can be installed and aligned on a synchrotron beamline by the user using a procedures described in the KB-SR1's User's Manual. The clear aperture of the mirrors along the beam is approximately equal to the mirror length of 100 mm. The x-ray incidence angle is typically set to about 4 mrad and thus the KB mirrors intercept ~0.4 x 0.4 mm beam. At this incidence angle, the high-energy cutoff of the mirror reflectivity is about 17 keV. The user can vary both the intercepted beam size and the high-energy cutoff by selecting the incidence angle between 2-6 mrad. For example, selecting a larger incidence angle would increase the intersected beam but reduce the high-energy cutoff. The focal length (and working distance) can also be increased from its nominal design value for achieving optimal focal spot size, typically about 70 mm and user selectable at the time of order. These flexibilities are facilitated in the KB-SR1 by its use of the absolute encoders for all the adjustments. Users have been able to have the curvature of the mirrors pre-adjusted to suit their special beamline configuration and applications, e.g., several preferred incidence angles, by making arrangements to use the Long Trace Profiler at the Lawrence Berkeley National Laboratory. Some subsequent fine-tuning of curvature and tilt angle by the user may be required on initial installation.
The KB-SR1 is intended for extended use in the filtered, monochromatic beam from an insertion device or on a bending magnet beamline. Gradual deterioration of the x-ray reflectivity of the mirror reflection surface will take place due to radiation-induced damages, typically evidenced by visible discoloration of the mirror surface. The wide clear-aperture of the mirrors of about 10 millimeters allows the use of about 10 stripes of < 1-mm in width to extend the mirror's life. Eventually, deteriorated mirrors will have to be replaced, either on-site by the user or by Xradia at the factory. Care must be exercised to avoid exposure of the mirrors to a white, unfiltered beam from a synchrotron source that may physically damage the mirror or greatly shorten operating life. There is a significant lead time associated with the procurement of replacement mirrors, typically 10 to 12 weeks, so the purchase of a spare mirror is recommended.
The Xradia KB-SR1 focuses x-rays using specially prepared flat mirrors that are actively bent into the required shape—usually a plane ellipse or plane parabola. Additional tuning of the mirror's shape is achieved by varying the width, contributing to a perfect (< 1 µ rad) match between mirrors. Xradia takes great care in the mechanical design and choice of materials to making the entire assembly thermally insensitive.
¹ Formation of Optical Images by X-rays, Journal of the Optic Society of America, 38 (1948) 766 P. Kirkpatrick and A. V. Baez
² Theory and Practice of Elliptically Bent X-Ray Mirrors, Optical Engineering,
Malcolm Howells, S. Irick, A. MacDowell, H. A. Padmore, T. Renner, S. Y. Rah, R. SanlerT
KB-SR1(Kirkpatrick-Baez Focusing System) Specifications
Mirror Length |
101mm |
Energy Range |
Upper: ~ 20KeV, determined by grazing angle and mirror coating |
Grazing Angle |
3 to 8 mrad typical |
Environment |
Air or high vacuum |
Source to Mirror Distance |
2 to 40m |
Mirror to Focus Distance |
260mm and 120mm for the two mirrors |
| End of last mirror to focus distance (sample clearance) | 60mm |
Demagnifications |
10 to 300, depending on optics |