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NEW APPARATUS & UPGRADES

Development of a Differential Pumping System ofSoft X-Ray Beamline for Windowless Experiments

under Normal Atmospheric ConditionsIn synchrotron radiation facilities, thin metal and

polymer films are usually used as a vacuum windowto separate the ultra-high vacuum of the storagering or the optics chamber from the high-pressureenvironment of the experimental apparatus. Aberyllium (Be) window is often used as a standardvacuum window owing to its high optical transmissionof the X-ray beam. However, Be windows absorblow energy photons, and cannot be used for a softX-ray beamline. Instead, the standard procedure in asoft X-ray beamline is to install all the equipmentnecessary for experiments inside the high-vacuumchamber. This method can have drawbacks as well.On one hand, the high-vacuum chamber producesan ideally clean environment. On the other hand,technologically significant phenomena take placeunder higher-pressure regions (1~ 760 mbar). Recently,the demands to study samples under a high-pressureenvironment have been increasing in the soft X-rayregion. For example, several groups have attemptedto obtain photoemission data at pressures of up to100 mbar by separating the sample region from thebeamline with a vacuum window [1]. The vacuumwindows used in the soft X-ray regions need to beultra-thin to transmit the photons to the sample region.However, thin films are fragile and have difficultywithstanding a high pressure difference. Furthermore,organic contamination of the vacuum window reducesthe photon intensity in the absorption edges of the lightelements. In order to overcome these complications,the development of a windowless method must beinvestigated. A windowless connection between a

high-vacuum beamline and a high-pressure samplechamber would provide an opportunity that opens newscientific possibilities in soft X-ray sciences.

One approach to overcoming the window problemis to use a differential pump [2]. In a differentialpump, the conductance between two vacuumchambers is limited by an open aperture or pipes. Itallows photons of all energy ranges to pass freelyalong a l ine of the l ight path between the twodifferent pressure regions. A challenge of the presentinvestigation is to create a windowless connectionbetween an experimental chamber filled by 1 atmhelium (He) and the high-vacuum beamline. Sincesoft X-rays below 1.5 keV are fairly absorbed inatmospheric air, a He path was used instead of anair path. While He gas has a higher transmissionthan air in the soft X-ray region owing to its smalleratomic number, it is more difficult to evacuate Heusing vacuum pumps. In this study, a differentialpumping system was developed for use withwindowless soft X-ray experiments under normalatmospheric conditions. The performance of thedifferential pump was evaluated by connecting it toboth atmospheric air and helium environments. Thetransmission of soft X-rays was measured under thecondition of atmospheric helium to demonstratethe performance of the apparatus.

Figure 1 shows a photograph of the presentdifferential pumping system. The details of the systemwere described in a recent report [3]. The apparatusconsists of an aperture-based four-stage differentialpump, which was designed on the basis of a simple

SR P4 P3 P2 P1 P0

A1A2A3A3

500 mm

A4

Fig. 1. Photograph of the fabricated differential pumping system. Ax (x = 0 - 4) is the location of eachconductance-limiting aperture, and Px (x = 0 - 4) is the pressure at each differential pumping stage.Inset is a photograph of the aperture installed between the third and fourth stages at BL27SU.

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model calculation. The design has relativelyshort conductance-limiting components that alloweasy installation and alignment of the system on thesynchrotron beamline. The total length of the systemis about 500 mm, which is significantly shorter thanthose of other similar systems previously reported [2].

Figure 2 shows the pressure distribution curvesobtained from the simulation and the testmeasurement of the system. The pressure P0 in thesample region is 1×105 Pa (= 1 atm). When P0 isthe atmospheric air, the pressure at the uppermoststage (P4) is 8.5×10-5 Pa. The present apparatusachieved differential pressures of about 9 orders ofmagnitude less than the atmospheric pressure of air.The next challenge was to connect the experimentalchamber filled with 1 atm of helium gas to the high-vacuum beamline in the windowless system. Thepressure of the P4 section increased by a factor ofabout 40, compared with the P4 value for air. Thepressure deteriorated for mainly two reasons [3]. Thefirst reason is that a gas mass increases the gas flow,as was the case with helium. The second reason isthat the turbomolecular pumps had lower pumpingspeeds with helium than with air, and the lowerspeeds reduced the pumpsʼ performance. Althoughthe performance of the system degraded by about oneorder of magnitude when using helium, the observedP4 value (3.3×10-3 Pa) is sufficient to connect thissystem to the beamline. The present system providesexcellent isolation between the atmospheric pressureof the sample environment and the high-vacuum ofthe beamline across the short distance.

Figure 3 shows the transmission curves of softX-rays under atmospheric helium. The experimentallydetermined transmission curves are nearly identicalto the simulated ones. The transmission at 550 eV(the oxygen K-edge region) measured at 10 mmdownstream of the first aperture is about 85%. Whenthe photon flight path in the atmospheric pressureregion is expanded to 50 mm, more than 70% of theincident photons are transmitted to the detector.Although the transmission is decreased in the lowphoton energy region, about 60% of the incidentphotons are transmitted to the detector at 350 eV.The fabricated differential pump was a success:low energy soft X-ray photons passed through theatmospheric pressure environment.

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105

102

101

100

10-1

10-2

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10-4

10-5

103

104

Measurement (Air)Measurement (He)Simulation (Air)Simulation (He)

Differential Pumping Stage

Pre

ssur

e (P

a)

Fig. 2. Performance measurement of the presentdifferential pumping system. The pressuredistribution curves were measured for theatmospheric pressures of air (solid circles) andhelium (solid triangles). The calculated pressurefor the optimized design is also indicated for air(empty circles) and helium (empty triangles).

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10 mm measurement50 mm measurement10 mm calculated50 mm calculated

Photon Energy(eV)

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issi

on

Fig. 3. Experimental and calculated transmissioncurves of soft X-ray photons obtained under helium-path conditions. The experimental transmissioncurves were measured at the helium-path lengths of10 mm (solid circles) and 50 mm (empty circles).The solid (10 mm) and dashed (50 mm) linesindicate the simulated transmission curves of thefabricated differential pumping system.

Yusuke Tamenori

SPring-8 /JASRI

E-mail: tamenori@spring8.or.jp

References

[1] D.F. Ogletree et al.: Rev. Sci. Instrum. 73 (2002) 3872.[2] T.Gog et al.: J. Synchrotron Rad. 14 (2007) 339.[3] Y. Tamenori: J. Synchrotron Rad. 17 (2010) 243.