Development of focusing mirror for X-ray free electron laser
Hidekazu Mimura1, Hitoshi Ohmori2 and Kazuto Yamauchi3
1The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, JAPAN2RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, JAPAN
3Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, JAPAN

Abstract – We describe the fabrication of a long mirror for focusing hard X-rays to nanometer dimension. We established the figuringsystem with an accuracy of nanometer-level for X-ray mirror optics. Electrolytic in-process dressing grinding is used for first-stepfiguring and elastic emission machining is employed for final figuring and surface smoothing. The focusing mirror has an ellipticalcurved shape with a length of 400 mm and focal length of 550 mm. Figure accuracy with a peak-to-valley height of 2 nm is achieved.
A focusing test was performed at BL29XUL of SPring-8 and found the focused beam size to be approximately 75 nm at 15 keV, very
similar to the theoretical value. Qqｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐｐqqqqqqqqqqqqqqqqqqqqqqq
Key Words: XFEL, X-ray mirror, ELID grinding, EEM, On-machine measurement, MSI, RADSI
1. Introduction

requirement of a wide acceptance aperture, the employment of longer

Focusing X-rays to a small spot is a greatly desired technology for

mirrors is indispensable. In this report, we describe the design,increasing photon density at a sample position. Such extremely

fabrication and evaluation of a 400-mm-long mirror for hard X-ray

narrow beams would provide a high spatial resolution for X-ray

focusing. The focusing mirror has an elliptical curved shape with amicroscopy. Recently, nanofabrication technologies have advanced

length of 400 mm and focal length of 550 mm. Electrolytic in-process

rapidly and high performance focusing optical components for X-rays

dressing (ELID) grinding was used for first-step figuring. Elastic

are now readily available. Focusing optics can be divided into threeemission machining (EEM) was employed for the final figuring andclasses, (1) reflective, (2) refractive and (3) diffractive. Each withsurface smoothing.

advantages and drawbacks depend on the specific application, photonenergy, achievable spot size and aperture and robustness. Currently,

2. Optical systemthese optics can achieve focused beam sizes of less than 100 nm1-5).

We designed a two-dimensional focusing system with two longSince total reflection mirrors have high efficiency and no chromaticmirrors. The lengths of the vertical and horizontal focusing mirrors

aberration, mirrors are permanently installed in many beamlines inare 400 mm and 200 mm, respectively. The focal lengths are 550 mm

synchrotron facilities, typically set in Kirkpatrick–Baez (KB)

and 200 mm. The working distance is 100 mm. The incident X-raygeometry6). Two mirrors, with elliptical curve figures, are required forenergy and glancing angle are set at 15 keV and 1.4 mrad, respectively.

two-dimensional focusing. To obtain an ideal focused beam size andElliptical shapes are employed to realize a hard X-ray beam with afocusing efficiency, unprecedented figure accuracy and smoothness

size less than 100 nm at the 1-km-long beamline (BL29XUL) ofare demanded for the mirror surfaces. In the past decade, mirrorSPring-812,13). The figure profile and employed ellipse parameters arefabrication technology has greatly progressed and mirror surface

shown in Fig. 1 and Fig. 2, respectively. Figure 3 shows thefigure accuracy of 1-nm level can be achieved. The best spot sizes arewave-optically calculated intensity profile of a beam focused by the3)of the order of 25-nm at 15 keV . Hence, the performance is limited

ideally shaped ellipse of the 400-mm-long mirror, for an incident slit

not by mirror surface figure errors, but by the optical design, which iswidth of 100 μm. The FWHM and the interval between the first twodetermined by the geometrical optics and/or wave-optical theories.

minima are found to be 75 nm and 150 nm, respectively.

Hard X-ray free electron laser (XFEL) facilities are currently under40

available10). An XFEL produces high-intensity ultra-short X-ray

35

flashes with the properties of laser light. This excellent new light30

source will open up a range of new perspectives for the naturalsciences. Many researchers consider focusing mirror optics to be akey technology for the effective use of XFELs. In focusing devices,

Height (mm)construction in Germany, the US and Japan7)-9). Soft XFEL are already

2520
15

reflective optics are considered to be more promising, because mirrors

10

have overwhelming advantages with regard to X-ray radiation

5hardness and focusing efficiency3).

0To date, the lengths of the employed mirrors in reported studies areshorter than 100 mm. One reason for this short length is the difficultyin machining and measuring long mirror optics. Considering the riskof damage to a mirror surface under XFEL illumination11) and the0100

200Position(mm)

300

400

Figure 1 Figure profiles of focusing mirrors with lengths of 400 mm.
The optical system is diagramed in Fig. 2

MA(Vertical focusing)Slit(100μm×100μm)
From undulator

400 mmMB(Horizontal focusing)

50 mm 200 mm

MA：f=550mm

productivity18). The measuring force is so light that the probe does not

MB：f=200mm

scratch the mirror surface. The vertical measurement resolution is 1nm.

100 mmFiguring is performed by precisely controlling the z-position of the200 mm
1 km

550mm

grinding tool during the back and forth motion of the stage. Figure

Figure 2 Hard X-ray focusing system with long mirrors

accuracy of the order of 10 nm is possible using this machine. Figure

Intensity (arb.units)

4 shows a photograph of the X-ray mirror before and after machining,350

showing that a non-mirrored surface is turned into the mirrored

300surface.

250
200
150
100
500-500 -400 -300 -200 -100
0100
Position(nm)

200

300

400500Figure 3 Intensity profiles of focused beam obtained by wave-opticalsimulations
3. Elastic Emission Machining(a) Before Machining

In the past decade, accuracy of the order of nanometers has beenachieved for both flat and elliptical mirrors having lengths shorter
than 100 mm14), 15). The primary machining method for figuring
ultra-precise X-ray mirrors is EEM. EEM is a surface preparation
method utilizing the chemical reaction between the surfaces of fine
powder particles and the process surface. In EEM, silica powder
particles are typically employed as a reactive species. They areuniformly mixed with ultrapure water and are applied to the process
surface via a controlled flow. When the powder particle surface andprocess surface come into contact and then separate, there is asignificant probability that the atoms on the workpiece surface will
adhere and move onto the surface of the powder particles.
Observations of the surface structure after the EEM process using(b) After Machining

scanning tunneling microscopy (STM) clarified that the processed Si

Figure 4 Photograph of a mirror substrate before and after ELID

(001) surface is composed of almost three atomic layers. However,processing

there is an obstacle in fabricating a 400-mm-long mirror. Since EEM
can achieve removal rates of only about 10-15 m3/h, a preliminary

5. Interferometric metrology -figuring method is required to prepare the mirror shape with figure

The final surface figure testing is based on interferometric stitching

accuracy of the order of at least 100 nm (P-V).

technology. Surface profiles, measured by combining microstitchinginterferometry (MSI)19) and relative angle determinable stitching

4. Electrolytic In-process Dressing Grinding

interferometry (RADSI)20), are employed as profiles before and after

In ELID grinding, a mirror surface can be easily and rapidly

processing for surface figure correction with EEM. In MSI, by16),17). ELID involves dressing a metal bondstitching small area surface profiles having a spatial resolution of 0.03

diamond wheel by the electrolytic effect in the process of grinding. Amm, which are measured by a Michelson-type microscopic

large ultraprecision aspheric grinding system with ELID has beeninterferometer, acquisition of the surface profiles of the entire area ofdeveloped for the fabrication of large optical elements. The machinea mirror is possible with the same resolution. In RADSI, by stitchingtable size of this system is 1500 mm in length and 550 mm in width.

partially measured profiles of curved mirrors, using a stitching angle

Rectangular X-ray aspherical mirrors such as toroidal or cylindricalaccurately measured as the time profile data are acquired, ellipticallyshapes can be fabricated. The machine slide ways use double-V

curved profiles can be obtained. By mixing these two stitchingrollers for 3-axis dimension. An air hydrostatic bearing is installed forinterferome-try methods, surface figure measurement is possible withthe grinding wheel spindle. An ultraprecision contact-type form

measurement accuracy matching surface figure accuracy required formeasuring probe was equipped for on-machine measurements.

ideal performance. These surface testing systems were upgraded for

Alternation of the surface profiles after each process can be measured

the measurement of 400-mm-long X-ray mirrors21).

fabricated from a substrate

without separating the work from the stage table, which allows high6. Mirror fabrication

minimum beam size was attained. Other parameters, such as the yawIn this study, the 400-mm-long mirror was fabricated by combining

angle relative to the incident X-ray beam, were finely adjusted when

ELID grinding and EEM. The non-mirror surface was shaped directly

the mirror was inserted on the stage. Figure 7 shows the measured andinto an elliptical shape with figure accuracy of the order of 100 nm

ideal intensity profiles of the focused beam. The focused beam size(P-V) by using an NC ELID grinding system. The surface waswas 75 nm, as defined by the full width at half maximum of thesimultaneously smoothed to a mirrored surface. Final figuring andprofile. This size is almost equal to the ideal size undersurface smoothing were then performed by EEM. Figure 5 shows thediffraction-limited conditions at 15 keV.

figure error profiles after several EEM figuring processes. The surface
quality is gradually improved after each figuring. Finally, we achieved
figure accuracy of 2 nm (P-V).

Figure 5 Figure error profile after EEM figuring process
7. Evaluation

Figure 7 Intensity profile of the focused X-ray beam
8. Discussion

Figure 6 shows a photograph of the experimental set-up for aIn this study, a 400-mm-long mirror for hard X-ray focusing wasone-dimensional focusing test. The experiment was performed using

designed, fabricated and evaluated. The mirror will not only be usefulthe 1-km-long beamline of SPring-8. The distance between theat XFEL facilities, but may also be used at current third-generation

incident slit placed behind the monochromator and the mirror was 950

synchrotron radiation facilities, due to the fact that when using a longm. The average incident angle was 1.4 mrad. Monochromatic 15-keV

mirror, a long work distance can be selected with a small focused

X-rays were one-dimensionally focused. The mirror was supported bybeam size. This advantage promises breakthroughs in X-ray

only the three Bessel points on the rigidly structured table to minimize

microscopy. The various systems for analysis can be set aroundthe change in the overall length. The mirror shape determined using

samples.

X-rays was the same as that determined by RADSI profiling during

Prior to this study, there were two obstacles in producing long mirror

fabrications. The focused X-ray beam was probed by inserting a goldoptics. The first was the difficulty of measuring a curved surface

wire and analyzing the relationship between the vertical height andprofile. The second was the requirement for improved production

the intensity passing though the wire. The passing X-ray photons wereefficiency. In this report, we describe the effective solutions to thesecounted using an avalanche photodiode, because of its high linearity

obstacles. This technological development will make available highlyin photon counting.

accurate mirror optics of long length, which will be of great use tomany researchers.
Acknowledgement
This study was largely supported by the X-ray Free Electron Laser
Utilization Research Project and CREST program of the Ministry ofEducation, Culture, Sports, Science and Technology of Japan (MEXT).
This research was partially supported by a Grant-in-Aid for Specially
Promoted Research 18002009 and Global COE Research from MEXT.
The use of BL29XU at SPring-8 was supported by RIKEN.
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measuringprobeforon-machine