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Hard X-ray beamline of 7.5 T superconducting wiggler at Pohang Light Source
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
Hard X-ray beamline of 7.5 T superconducting wiggler at Pohang Light Source Y.U. Sohn*, J.-Y. Choi, S.N. Kim, K.R. Kim, J.S. Bak, Y.M. Koo Pohang Accelerator Laboratory, POSTECH, Engineering Department, San 31 Hyoja-dong, Kyungbuk, Pohang, South Korea
Abstract A superconducting wiggler of 7.5 T magnetic field will be installed for researches using hard X-ray photons up to 100 keV at Pohang Light Source. It consists of three poles: a central main pole and two side poles on either side. Three beamlines will be constructed in a total radiation fan of 17 mrad: the spectroscopy beamline with 2 mrad, the scattering/ medical application beamline with 5 mrad in common. The spectroscopy beamline is now under construction and the other beamlines will be constructed in the next stage. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconducting wiggler; Hard X-ray; Spectroscopy; Beamline
1. Introduction The 7.5 T superconducting wiggler (SCW), the first insertion device in the hard X-ray range at Pohang Light Source (PLS), will increase the photon critical energy from 5.5 to 31 keV, and provide the experimental opportunities in the energy range from 30 up to 100 keV at PLS. Its main scientific objectives are spectroscopy, scattering, and medical applications; the main experiments at the spectroscopy beamline will be EXAFS for high-Z elements, the experiments at the scattering beamline include high-pressure diffraction, residual stress measurements among
*Corresponding author. Tel.: +82-562-279-1815; fax: +82562-279-1899. E-mail address: [email protected] (Y.U. Sohn).
other studies, and angiography, tomography, and hard X-ray imaging will be performed at the medical beamline. The available horizontal radiation fan of 17 mrad is divided into two branch beamlines; 2 mrad for the spectroscopy beamline, 5 mrad for scattering/medical application beamline in common use, leaving 10 mrad as the gap between the two branches. The scattering and medical beamlines will be constructed in tandem and will be operated in time-sharing mode. The medical beamline will be extended to a dedicated building outside about 60 m away from the source. Taking into account space constraint in the experimental hall the spectroscopy beamline will be placed near the source, and its experimental hutch will be situated at about 24 m from the source. At present, the spectroscopy beamline is under construction. This paper describes the SCW source and the design of the spectroscopy beamline.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 4 6 9 - 7
708
Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
2. Superconducting wiggler source The SCW has three superconducting dipoles: one central pole and two side poles [1]. The cryostat of the wiggler, which consists mainly of an inner helium vessel and an outer container and a helium supply tank, has a 106(H) mm 26(V) mm warm bore at its center for the electron beam path. The magnet is energized by two current sources to control the magnetic field integral along the beam path to zero. In addition to the primary radiation source from the central pole, the SCW has the secondary one from the side poles. The secondary source may be harmful from the experimental point of view and we designed a magnet scheme to prevent this secondary-source radiation from entering the beamlines; corrector magnets with weaker magnetic fields will be installed on either side of SCW and deflect the electron beam before it enters SCW so that the emission axes of radiation from the side poles are deviated off from the beamline. Although the radiation from the correctors is forwarded to the beamline instead, it can be filtered out more easily by the thick beryllium windows in the front end since its critical photon energy is much lower than that of the side poles. Fig. 1 shows the orbit deviation along the electron beam axis and the electron trajectory in phase space for this scheme. The secondary-source removal scheme and geometric constraint of the vacuum chamber in the storage ring limit the available horizontal beam fan to about 17 mrad. The brightness spectrum of SCW is shown in Fig. 2 together with that of the bending magnet. The power emitted per unit horizontal angle into the available beam fan is about 73 W/mrad at 2.5 GeV and 150 mA. The power estimation shows that 8.6 kW must be absorbed at non-optical components around the source like the vacuum chamber.
3. Spectroscopy beamline The major design goals of the spectroscopy beamline are as follows; operation energy from 10 keV to 100 keV, energy resolution less than
Fig. 1. (a) Orbit deviation of an electron in SCW. (b) Trajectory of an electron in phase space.
Fig. 2. Brightness of SCW radiation.
2 10 4, photon flux more than 108 ph/s at the sample surface of the horizontal spot size of 10 mm(H) and appropriate vertical size satisfying
Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
709
Fig. 3. Energy resolution of the spectroscopy beamline when using (a) Si(3 1 1) crystal and (b) Si(5 1 1) crystal for the monochromator.
the required energy resolution. Focusing optics is not applied for the present. The spectroscopy beamline consists of a monochromator, a higherharmonics rejection mirror and slits among others. 3.1. Monochromator and harmonics rejection mirror The monochromator uses double crystals with a fixed-exit mechanism. Two kinds of crystals will be used to cover the whole energy range: Si(3 1 1) crystals from 9 to 72 keV, and Si(5 1 1) crystals from 14 to 113 keV. The Bragg angle ranges from 38 to 258. The Si(5 1 1) crystal will be used mainly above 70 keV. We consider adopting the so-called adjustable inclined geometry mechanism to select these two reflecting planes from a pair of crystals without breaking vacuum instead of two crystals [2]. The broad brightness spectrum of the SCW source requires inevitably the rejection of the higher-harmonics components for EXAFS experiments. The third harmonics content in this energy range are considerable; 42% at 10 keV, 22% at 20 keV, and 12% at 30 keV, respectively, calculated from the brightness spectrum and rocking curve width. The harmonics above three also contribute considerably at the fundamental energies lower than 30 keV. The total reflection mirror will be installed downstream of the monochromator to reject the higher-harmonics components. The coating material is rhodium, and the mirror will be used at incident angles higher than
1.6 mrad (critical angle of 40 keV) to reject the higher harmonics of fundamental energies below 30 keV. The incident angle can be adjusted to reduce the harmonics content to much less than 1% while keeping the reflectivity of fundamental component photons higher than 90%. The incident beam height is confined within 1 mm to avoid an excessive length of the mirror, and in this case the maximum footprint length of the incident beam is 625 mm. Taking into account possible alignment errors, the actual mirror size will be 875 mm(L) 50 mm(W).
3.2. Energy resolution and photon flux The most important parameters regarding the EXAFS are energy resolution and photon flux. The calculated energy resolutions in this beamline are shown in Fig. 3. Setting the slit width to 0.2 mm, we can obtain energy resolutions lower than 2 10 4 below 60 keV with Si(3 1 1) crystals, and the same energy resolution up to 100 keV with Si(5 1 1) crystals. The calculated photon flux per horizontal divergence is shown in Fig. 4. In this calculation, the vertical slit width is varied with photon energy in order to have an energy resolution of 2 10 4 and 1 10 4, respectively. For the energy resolution of 2 10 4 in Fig. 4(a), the slit width is kept constant below 16 keV to accept only vertical divergence of 45.6 mrad, which is limited by the above-mentioned mirror length, although wider
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Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
Fig. 4. Photon flux on the sample irradiated area when using (a) Si (3 1 1) crystal and (b) Si (5 1 1) crystal. Solid line corresponds to the case of an energy resolution of 2 10 4, and the dotted line 1 10 4.
slit widths are possible below this energy for the required energy resolution. If we set the horizontal beam size at the sample position to 10 mm (0.416 mrad) with a slit, we can obtain photon fluxes of more than 9 108 ph/s on with Si(3 1 1) crystals, and 1.6 108 ph/s with Si(5 1 1) crystals, respectively, with an energy resolution of 2 10 4. The photon flux is enough for EXAFS experiments.
Acknowledgements This work was supported by POSCO and the Ministry of Science and Technology, Korea. We are grateful to the Dr. Mezentsev and the staff members at Budker Institute of Nuclear Physics at Novosibirsk, Russia for their helpful discussion on the SCW beamline, especially the SCW source.
4. Conclusion References We designed the spectroscopy beamline of SCW and we expect that the flux and energy resolution will be satisfied for EXAFS experiments. The construction of the spectroscopy beamline will be finished by the end of year 2002. The scattering/ medical beamline will be constructed in next stage according to the long-term plan of PAL.
[1] Y.U. Sohn, D.E. Kim, H.S. Suh, Y.D. Yoon, Y. M. Koo, Proc. ICSRS & AFSR 95, Kyungju, Korea, October 25–27, 1995; Mezentsev et al., 7.5 T Superconducting Wiggler Technical Report, Novosibirsk, Russia, 1995. [2] T. Uruga, H. Kimura, Y. Kohmura, M. Kuroda, H. Nagasawa, K. Ohtomo, H. Yamaoka, T. Ishikawa, T. Ueki, Rev. Sci. Instr. 66 (1995) 2254.
Hard X-ray beamline of 7.5 T superconducting wiggler at Pohang Light Source Y.U. Sohn*, J.-Y. Choi, S.N. Kim, K.R. Kim, J.S. Bak, Y.M. Koo Pohang Accelerator Laboratory, POSTECH, Engineering Department, San 31 Hyoja-dong, Kyungbuk, Pohang, South Korea
Abstract A superconducting wiggler of 7.5 T magnetic field will be installed for researches using hard X-ray photons up to 100 keV at Pohang Light Source. It consists of three poles: a central main pole and two side poles on either side. Three beamlines will be constructed in a total radiation fan of 17 mrad: the spectroscopy beamline with 2 mrad, the scattering/ medical application beamline with 5 mrad in common. The spectroscopy beamline is now under construction and the other beamlines will be constructed in the next stage. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconducting wiggler; Hard X-ray; Spectroscopy; Beamline
1. Introduction The 7.5 T superconducting wiggler (SCW), the first insertion device in the hard X-ray range at Pohang Light Source (PLS), will increase the photon critical energy from 5.5 to 31 keV, and provide the experimental opportunities in the energy range from 30 up to 100 keV at PLS. Its main scientific objectives are spectroscopy, scattering, and medical applications; the main experiments at the spectroscopy beamline will be EXAFS for high-Z elements, the experiments at the scattering beamline include high-pressure diffraction, residual stress measurements among
*Corresponding author. Tel.: +82-562-279-1815; fax: +82562-279-1899. E-mail address: [email protected] (Y.U. Sohn).
other studies, and angiography, tomography, and hard X-ray imaging will be performed at the medical beamline. The available horizontal radiation fan of 17 mrad is divided into two branch beamlines; 2 mrad for the spectroscopy beamline, 5 mrad for scattering/medical application beamline in common use, leaving 10 mrad as the gap between the two branches. The scattering and medical beamlines will be constructed in tandem and will be operated in time-sharing mode. The medical beamline will be extended to a dedicated building outside about 60 m away from the source. Taking into account space constraint in the experimental hall the spectroscopy beamline will be placed near the source, and its experimental hutch will be situated at about 24 m from the source. At present, the spectroscopy beamline is under construction. This paper describes the SCW source and the design of the spectroscopy beamline.
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 4 6 9 - 7
708
Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
2. Superconducting wiggler source The SCW has three superconducting dipoles: one central pole and two side poles [1]. The cryostat of the wiggler, which consists mainly of an inner helium vessel and an outer container and a helium supply tank, has a 106(H) mm 26(V) mm warm bore at its center for the electron beam path. The magnet is energized by two current sources to control the magnetic field integral along the beam path to zero. In addition to the primary radiation source from the central pole, the SCW has the secondary one from the side poles. The secondary source may be harmful from the experimental point of view and we designed a magnet scheme to prevent this secondary-source radiation from entering the beamlines; corrector magnets with weaker magnetic fields will be installed on either side of SCW and deflect the electron beam before it enters SCW so that the emission axes of radiation from the side poles are deviated off from the beamline. Although the radiation from the correctors is forwarded to the beamline instead, it can be filtered out more easily by the thick beryllium windows in the front end since its critical photon energy is much lower than that of the side poles. Fig. 1 shows the orbit deviation along the electron beam axis and the electron trajectory in phase space for this scheme. The secondary-source removal scheme and geometric constraint of the vacuum chamber in the storage ring limit the available horizontal beam fan to about 17 mrad. The brightness spectrum of SCW is shown in Fig. 2 together with that of the bending magnet. The power emitted per unit horizontal angle into the available beam fan is about 73 W/mrad at 2.5 GeV and 150 mA. The power estimation shows that 8.6 kW must be absorbed at non-optical components around the source like the vacuum chamber.
3. Spectroscopy beamline The major design goals of the spectroscopy beamline are as follows; operation energy from 10 keV to 100 keV, energy resolution less than
Fig. 1. (a) Orbit deviation of an electron in SCW. (b) Trajectory of an electron in phase space.
Fig. 2. Brightness of SCW radiation.
2 10 4, photon flux more than 108 ph/s at the sample surface of the horizontal spot size of 10 mm(H) and appropriate vertical size satisfying
Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
709
Fig. 3. Energy resolution of the spectroscopy beamline when using (a) Si(3 1 1) crystal and (b) Si(5 1 1) crystal for the monochromator.
the required energy resolution. Focusing optics is not applied for the present. The spectroscopy beamline consists of a monochromator, a higherharmonics rejection mirror and slits among others. 3.1. Monochromator and harmonics rejection mirror The monochromator uses double crystals with a fixed-exit mechanism. Two kinds of crystals will be used to cover the whole energy range: Si(3 1 1) crystals from 9 to 72 keV, and Si(5 1 1) crystals from 14 to 113 keV. The Bragg angle ranges from 38 to 258. The Si(5 1 1) crystal will be used mainly above 70 keV. We consider adopting the so-called adjustable inclined geometry mechanism to select these two reflecting planes from a pair of crystals without breaking vacuum instead of two crystals [2]. The broad brightness spectrum of the SCW source requires inevitably the rejection of the higher-harmonics components for EXAFS experiments. The third harmonics content in this energy range are considerable; 42% at 10 keV, 22% at 20 keV, and 12% at 30 keV, respectively, calculated from the brightness spectrum and rocking curve width. The harmonics above three also contribute considerably at the fundamental energies lower than 30 keV. The total reflection mirror will be installed downstream of the monochromator to reject the higher-harmonics components. The coating material is rhodium, and the mirror will be used at incident angles higher than
1.6 mrad (critical angle of 40 keV) to reject the higher harmonics of fundamental energies below 30 keV. The incident angle can be adjusted to reduce the harmonics content to much less than 1% while keeping the reflectivity of fundamental component photons higher than 90%. The incident beam height is confined within 1 mm to avoid an excessive length of the mirror, and in this case the maximum footprint length of the incident beam is 625 mm. Taking into account possible alignment errors, the actual mirror size will be 875 mm(L) 50 mm(W).
3.2. Energy resolution and photon flux The most important parameters regarding the EXAFS are energy resolution and photon flux. The calculated energy resolutions in this beamline are shown in Fig. 3. Setting the slit width to 0.2 mm, we can obtain energy resolutions lower than 2 10 4 below 60 keV with Si(3 1 1) crystals, and the same energy resolution up to 100 keV with Si(5 1 1) crystals. The calculated photon flux per horizontal divergence is shown in Fig. 4. In this calculation, the vertical slit width is varied with photon energy in order to have an energy resolution of 2 10 4 and 1 10 4, respectively. For the energy resolution of 2 10 4 in Fig. 4(a), the slit width is kept constant below 16 keV to accept only vertical divergence of 45.6 mrad, which is limited by the above-mentioned mirror length, although wider
710
Y.U. Sohn et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 707–710
Fig. 4. Photon flux on the sample irradiated area when using (a) Si (3 1 1) crystal and (b) Si (5 1 1) crystal. Solid line corresponds to the case of an energy resolution of 2 10 4, and the dotted line 1 10 4.
slit widths are possible below this energy for the required energy resolution. If we set the horizontal beam size at the sample position to 10 mm (0.416 mrad) with a slit, we can obtain photon fluxes of more than 9 108 ph/s on with Si(3 1 1) crystals, and 1.6 108 ph/s with Si(5 1 1) crystals, respectively, with an energy resolution of 2 10 4. The photon flux is enough for EXAFS experiments.
Acknowledgements This work was supported by POSCO and the Ministry of Science and Technology, Korea. We are grateful to the Dr. Mezentsev and the staff members at Budker Institute of Nuclear Physics at Novosibirsk, Russia for their helpful discussion on the SCW beamline, especially the SCW source.
4. Conclusion References We designed the spectroscopy beamline of SCW and we expect that the flux and energy resolution will be satisfied for EXAFS experiments. The construction of the spectroscopy beamline will be finished by the end of year 2002. The scattering/ medical beamline will be constructed in next stage according to the long-term plan of PAL.
[1] Y.U. Sohn, D.E. Kim, H.S. Suh, Y.D. Yoon, Y. M. Koo, Proc. ICSRS & AFSR 95, Kyungju, Korea, October 25–27, 1995; Mezentsev et al., 7.5 T Superconducting Wiggler Technical Report, Novosibirsk, Russia, 1995. [2] T. Uruga, H. Kimura, Y. Kohmura, M. Kuroda, H. Nagasawa, K. Ohtomo, H. Yamaoka, T. Ishikawa, T. Ueki, Rev. Sci. Instr. 66 (1995) 2254.