Atmospheric, Non-Contact and High Speed Electro Chemical Machining Processes for X-Ray Optics
Authors: Radhakrishnan Rajeswaran , Xu Jing , Lucatero Savidra , Hall Timothy D , Taylor E. J. 
Published: February 2020

High resolution X-ray focusing optics is critical component in many spatially resolved techniques at many synchrotron facilities, such as micro X-ray fluorescence, micro-diffraction, and micro-spectroscopy. In order for these optics to achieve modern capability goals, they must embody various key performance attributes such as a minimum in spatial resolution, a maximum in focusing efficiency, a practical working distance (no more than a few meters), and ideally robust achromaticity (negligible photon energy dependence). The state of the art in X-ray optics encompasses, among others, diffractive zone plate optics, Kirkpatrick-Baez (K-B) mirrors, and compound refractive lenses (CRLs). Among these, only K-B mirrors provide highly efficient collection of X-ray photons and essentially perfect achromaticity, but require elliptical mirrors of exceptionally precise figure and placement, as well as sub-nanometer smoothness1.

In attempting to identify potential advances in X-ray optics beyond this state of the art, one promising solution is the design of single-bounce, axially symmetric mirror optics with parabolic inner profiles capable of focusing x-rays simultaneously in two orthogonal directions, which may overcome the long focal length limitation of KB mirrors. Manufacture of such precise optics as these novel one-bounce mirrors or the traditional K-B mirrors requires highly advanced surface finishing techniques. Silicon is mostly used in this application due to its low CTE, low thermal deformation, low z, high reflectivity, and low radiation adsorption. The conventional manufacturing approaches used to prepare aspherical, low roughness, and high-z coated silicon mirrors, included Ion Beam Figuring, Elastic Emission Machining (EEM), Plasma Machining, Lapping/Polishing2,3. Of these techniques the Plasma and Ion Beam approaches require vacuum atmosphere for processing limiting the size of the components that can be produced. Meanwhile, EEM can achieve better final finishes (up to 1 A) under atmospheric, non-contact conditions, however it is several orders of magnitude slower (0.1 to 0.001 µm/min) than these approaches. Furthermore, contact machining techniques like Lapping/Polishing tend to lead to surface defects that must be removed prior to mirror usage. Therefore, the design of low cost/high speed machining and polishing systems that can form high resolution x-ray focusing optics over large mirror areas is critical for the formation of next generation of x-ray optics.

This paper will discuss ongoing efforts by Faraday to design an atmospheric, non-contact, low cost/high speed electrochemical machining process to produce next-generation ellipsoidal x-ray optics via a scalable pulse-reverse electrochemical manufacturing approach. Unlike conventional electrochemical surface finishing processes, the pulse-reverse process does not require low conductivity/high viscosity electrolytes or the addition of hazardous chemical species (like HF) to remove the oxide film associated with electropolishing of passive and strongly passive materials like Si. This paper will focus on the potential of pulse/pulse reverse electrofinishing process to produce optics such as Kirkpatrick-Baez mirrors and compound refractive lenses for X-ray optic technologies being used at synchrotron facilities. In particular, this talk will discuss fundamental processing techniques and the performance observed in ongoing studies. Initial trials of atmospheric, non-contact process demonstrated a material removal rate of 40 µm/h which is 100 times faster than currently used EEM technique.

Acknowledgement

 This material is based upon ‎work supported by the Department of Energy under ‎Award No‎. ‎DE‎-‎ SC0015201.

References

 S. Matsuyama., et.al., “Textbook for hard X-ray focusing with Kirkpatrick-Baez optics.” Online resource: http://cheiron2009.spring8.or.jp/images/PDF/Practicals?16_29XLU.pdf. Accessed 15 Oct 2015.

H. Takino., et.al., “ Ultraprecision Machining of Optical Surfaces.” Nikon Corporation.

Y. Mori., et.al., “Development of plasma chemical vaporization machining and elastic emission machining systems for coherent X-ray optics.” SPIE 4501, 2001.