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Observation of hard X-ray pulses with a highly sensitive streak camera
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
Observation of hard X-ray pulses with a highly sensitive streak camera T. Hara*, Y. Tanaka, H. Kitamura, T. Ishikawa Institute of Physical and Chemical Research, Harima Institute, SPring-8/RIKEN, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan
Abstract We have developed a highly sensitive X-ray streak camera system, which synchronously operates with the RF signal of the SPring-8 storage ring. The streak camera was installed at an undulator beamline of SPring-8, and the beam loading effect for various electron bunch structures (filling pattern) has been observed. The camera has also been operated as a timing monitor for a synchronization system of synchrotron radiation and a Ti:sapphire laser. The highly sensitive X-ray streak camera system can be used not only as an accelerator diagnosis, but also as a fast temporal detector for beamline applications, such as the observation of fast temporal transitions of diffraction images and relaxation process. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 07.85.Fv; 07.68.+m; 79.60. i Keywords: X-ray streak camera; X-ray detector; Streak camera; CsI
1. Introduction Streak cameras are one of the most powerful tools for the observation of fast temporal phenomena and they have been widely used for visible and soft X-ray light detection. In third generation synchrotron radiation (SR) sources, the most of beamlines are designed and optimized for soft X-ray or X-ray utilization, thus installation of visible streak cameras requires a specially designed light port or a beamline for electron bunch observation. On the other hand, X-ray streak cameras are compatible with existing beamlines *Corresponding author. Tel.: +81-791-58-2809; fax: +81791-58-2810. E-mail address: [email protected] (T. Hara).
and they can be set up in an experimental hutch after a monochromator. In addition, the X-ray streak cameras can be used not only as an electron beam diagnostic but also as a detector for beamline experiments. One defect of the X-ray streak cameras is relatively low sensitivity of photocathode compared with that of visible streak cameras. An Au cathode is widely used in X-ray region for its material stability, but accumulative measurements are necessary due to its low quantum efficiency and it may lead to large temporal resolution. One way to improve the temporal resolution is to develop a jitter free accumulative system [1], and the other way is to improve cathode sensitivity. We have modified a photocathode disc of a commercially available camera (Hamamatsu Photonics, C5680-06) and
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 6 1 1 - 8
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T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
installed a CsI photocathode (100 nm thickness). As a result, 20–100 times higher sensitivity and better time response were obtained compared with a conventional Au cathode (30 nm thickness) in the energy range of 8–46 keV [1,2]. Using this highly sensitive X-ray streak camera, we observed bunch loading effect in SPring-8.
2. Instrumentation The X-ray streak camera is installed at an undulator beamline (BL29XU) of SPring-8 and Fig. 1 shows a schematic view of the camera setup. BL29XU is equipped with a double crystal monochromator, so monochromatized SR photons are observed by the streak camera [3]. The camera has two pairs of deflection electrodes. Fast deflection electrodes implement a ps order vertical scan at 84.76 MHz, and slow deflection electrodes displace vertical ps images horizontally on a phospher screen. This double scan makes a ms or ms order record of fast repetitive signals possible. The ring RF frequency is 508.58 MHz, so one of the six bunches are detected on the camera when all RF buckets are filled with electron bunches (fully filled mode). The camera scan trigger is generated from 508.58 MHz RF signal of SPring-8 after the frequency being down converted to 84.76 MHz by a 16 counter. Due to low photocathode quantum efficiency to X-ray,
measurements are made in an accumulative manner, so pulse to pulse jitter on the scan trigger affects the camera temporal resolution. In order to reduce jitter on the scan trigger, we first operate a synchronized Ti::sapphire mode locked laser [4] with a 16 counter output, and we detect the laser pulses with a fast PIN detector and use it as the camera scan trigger. In this configuration, the laser oscillator works as a frequency filter on the 84.76 MHz RF signal and the jitter reduces from 16 to 5 ps [2]. The intrinsic temporal resolution of the camera is about 4 ps (FWHM) measured by a single shot image of a 2 ps laser pulse (266 nm), and the overall temporal resolution for accumulative measurements is estimated to be about 6 ps. A 50 mm Ta slit is inserted in front of the photocathode and it limits vertical photon beam size to avoid degradation of the temporal resolution of the camera. The whole camera is tilted by 58 from the SR beam axis to prevent the transmitted X-ray directly hitting multi-channel plates (MCP).
3. Observation of X-ray SR pulses Fig. 2 is a comparison of the observed SR pulses at 46.5 keV using Au and CsI photocathodes. Both plots are accumulations of 8 105 SR pulses, and 20 times higher MCP gain is applied for Au. The
Fig. 1. Instruments setup at BL29XU of SPring-8.
T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
Fig. 2. SR pulses observed using CsI (solid line) and Au (dotted line) cathodes. Figure shows 8 105 accumulative shots. The Au plot is measured with a 20 times higher MCP gain.
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pulse profile obtained using the CsI cathode is close to Gaussian, whereas that of Au is statistically less accurate. This is due to the small number of secondary electron emission from the Au cathode compared with the CsI [1]. Fig. 3 shows the beam loading effect measured by the double scanning mode of the streak camera. Full scale of the horizontal axis corresponds to one round trip of the SPring-8 storage ring and the vertical position of the image shows longitudinal displacement of the bunches. When an electron bunch passes through the ring RF cavity, the field induced by the bunch reduces effective acceleration field of the cavity. This will be compensated by RF power supplied by klystrons, but it takes time because of a high Q value of the cavity. As a result, following electron bunches gain less energy and
Fig. 3. Longitudinal bunch positions when the ring is operated in (a) 24/29 filled mode and (b) 1/12 filled+10 equally spaced bunches mode.
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are shifted longitudinally in order to sit on the RF phase where the bunch can gain energy equivalent to radiation loss. The deformation of the RF field is significant when beam current per bunch is large. In Fig. 3 (a), the ring is filled with 2026 bunches (24/29 RF buckets are filled) with 0.05 mA/bunch. The RF cavity regains acceleration voltage during a train of empty buckets (5/29), so the longitudinal bunch positions at the top and tail of 2026 bunch train are shifted by about 20 ps. This is more significant in the case of 203 bunch train (1/12 filled)+10 bunches (Fig. 3 (b)). In this case, the current per bunch is ten times larger, and the positions at the top and tail of 203 bunch train shift by about 90 ps. Insertion devices also change the longitudinal bunch position due to the variation of radiation loss caused by gap movement. At SPring-8, we observed that the bunch moves 42 ps when 14 insertion devices close the gap [4]. In the beamline experiment, we have used the X-ray streak camera to observe time delay on the
wave front of SR pulses caused by asymmetric crystal reflection [5]. Acknowledgements The authors would like to acknowledge H.Yamazaki, K.Tamasaku and M.Yabashi for their assistance in this work.
References [1] K. Scheidt, G. Naylor, Proceedings of the 4th European Workshop on Diagnostics and Beam Instrumentation for Particle Accelerators, Chester, UK, 16–18 May 1999, p.51. [2] T. Hara, Y. Tanaka, H. Kitamura, T. Ishikawa, Rev. Sci. Instrum. 71 (2000) 3624. [3] K. Tamasaku, Y. Tanaka, M. Yabashi, H. Yamazaki, N. Kawamura, M. Suzuki, T. Ishikawa, Nucl. Instr. and Meth. A 467–468 (2001), in this proceedings. [4] Y. Tanaka, T. Hara, H. Kitamura, T. Ishikawa, Rev. Sci. Instrum. 71 (2000) 1268. [5] Y. Tanaka et al., to be submitted.
Observation of hard X-ray pulses with a highly sensitive streak camera T. Hara*, Y. Tanaka, H. Kitamura, T. Ishikawa Institute of Physical and Chemical Research, Harima Institute, SPring-8/RIKEN, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan
Abstract We have developed a highly sensitive X-ray streak camera system, which synchronously operates with the RF signal of the SPring-8 storage ring. The streak camera was installed at an undulator beamline of SPring-8, and the beam loading effect for various electron bunch structures (filling pattern) has been observed. The camera has also been operated as a timing monitor for a synchronization system of synchrotron radiation and a Ti:sapphire laser. The highly sensitive X-ray streak camera system can be used not only as an accelerator diagnosis, but also as a fast temporal detector for beamline applications, such as the observation of fast temporal transitions of diffraction images and relaxation process. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 07.85.Fv; 07.68.+m; 79.60. i Keywords: X-ray streak camera; X-ray detector; Streak camera; CsI
1. Introduction Streak cameras are one of the most powerful tools for the observation of fast temporal phenomena and they have been widely used for visible and soft X-ray light detection. In third generation synchrotron radiation (SR) sources, the most of beamlines are designed and optimized for soft X-ray or X-ray utilization, thus installation of visible streak cameras requires a specially designed light port or a beamline for electron bunch observation. On the other hand, X-ray streak cameras are compatible with existing beamlines *Corresponding author. Tel.: +81-791-58-2809; fax: +81791-58-2810. E-mail address: [email protected] (T. Hara).
and they can be set up in an experimental hutch after a monochromator. In addition, the X-ray streak cameras can be used not only as an electron beam diagnostic but also as a detector for beamline experiments. One defect of the X-ray streak cameras is relatively low sensitivity of photocathode compared with that of visible streak cameras. An Au cathode is widely used in X-ray region for its material stability, but accumulative measurements are necessary due to its low quantum efficiency and it may lead to large temporal resolution. One way to improve the temporal resolution is to develop a jitter free accumulative system [1], and the other way is to improve cathode sensitivity. We have modified a photocathode disc of a commercially available camera (Hamamatsu Photonics, C5680-06) and
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 6 1 1 - 8
1126
T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
installed a CsI photocathode (100 nm thickness). As a result, 20–100 times higher sensitivity and better time response were obtained compared with a conventional Au cathode (30 nm thickness) in the energy range of 8–46 keV [1,2]. Using this highly sensitive X-ray streak camera, we observed bunch loading effect in SPring-8.
2. Instrumentation The X-ray streak camera is installed at an undulator beamline (BL29XU) of SPring-8 and Fig. 1 shows a schematic view of the camera setup. BL29XU is equipped with a double crystal monochromator, so monochromatized SR photons are observed by the streak camera [3]. The camera has two pairs of deflection electrodes. Fast deflection electrodes implement a ps order vertical scan at 84.76 MHz, and slow deflection electrodes displace vertical ps images horizontally on a phospher screen. This double scan makes a ms or ms order record of fast repetitive signals possible. The ring RF frequency is 508.58 MHz, so one of the six bunches are detected on the camera when all RF buckets are filled with electron bunches (fully filled mode). The camera scan trigger is generated from 508.58 MHz RF signal of SPring-8 after the frequency being down converted to 84.76 MHz by a 16 counter. Due to low photocathode quantum efficiency to X-ray,
measurements are made in an accumulative manner, so pulse to pulse jitter on the scan trigger affects the camera temporal resolution. In order to reduce jitter on the scan trigger, we first operate a synchronized Ti::sapphire mode locked laser [4] with a 16 counter output, and we detect the laser pulses with a fast PIN detector and use it as the camera scan trigger. In this configuration, the laser oscillator works as a frequency filter on the 84.76 MHz RF signal and the jitter reduces from 16 to 5 ps [2]. The intrinsic temporal resolution of the camera is about 4 ps (FWHM) measured by a single shot image of a 2 ps laser pulse (266 nm), and the overall temporal resolution for accumulative measurements is estimated to be about 6 ps. A 50 mm Ta slit is inserted in front of the photocathode and it limits vertical photon beam size to avoid degradation of the temporal resolution of the camera. The whole camera is tilted by 58 from the SR beam axis to prevent the transmitted X-ray directly hitting multi-channel plates (MCP).
3. Observation of X-ray SR pulses Fig. 2 is a comparison of the observed SR pulses at 46.5 keV using Au and CsI photocathodes. Both plots are accumulations of 8 105 SR pulses, and 20 times higher MCP gain is applied for Au. The
Fig. 1. Instruments setup at BL29XU of SPring-8.
T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
Fig. 2. SR pulses observed using CsI (solid line) and Au (dotted line) cathodes. Figure shows 8 105 accumulative shots. The Au plot is measured with a 20 times higher MCP gain.
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pulse profile obtained using the CsI cathode is close to Gaussian, whereas that of Au is statistically less accurate. This is due to the small number of secondary electron emission from the Au cathode compared with the CsI [1]. Fig. 3 shows the beam loading effect measured by the double scanning mode of the streak camera. Full scale of the horizontal axis corresponds to one round trip of the SPring-8 storage ring and the vertical position of the image shows longitudinal displacement of the bunches. When an electron bunch passes through the ring RF cavity, the field induced by the bunch reduces effective acceleration field of the cavity. This will be compensated by RF power supplied by klystrons, but it takes time because of a high Q value of the cavity. As a result, following electron bunches gain less energy and
Fig. 3. Longitudinal bunch positions when the ring is operated in (a) 24/29 filled mode and (b) 1/12 filled+10 equally spaced bunches mode.
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T. Hara et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 1125–1128
are shifted longitudinally in order to sit on the RF phase where the bunch can gain energy equivalent to radiation loss. The deformation of the RF field is significant when beam current per bunch is large. In Fig. 3 (a), the ring is filled with 2026 bunches (24/29 RF buckets are filled) with 0.05 mA/bunch. The RF cavity regains acceleration voltage during a train of empty buckets (5/29), so the longitudinal bunch positions at the top and tail of 2026 bunch train are shifted by about 20 ps. This is more significant in the case of 203 bunch train (1/12 filled)+10 bunches (Fig. 3 (b)). In this case, the current per bunch is ten times larger, and the positions at the top and tail of 203 bunch train shift by about 90 ps. Insertion devices also change the longitudinal bunch position due to the variation of radiation loss caused by gap movement. At SPring-8, we observed that the bunch moves 42 ps when 14 insertion devices close the gap [4]. In the beamline experiment, we have used the X-ray streak camera to observe time delay on the
wave front of SR pulses caused by asymmetric crystal reflection [5]. Acknowledgements The authors would like to acknowledge H.Yamazaki, K.Tamasaku and M.Yabashi for their assistance in this work.
References [1] K. Scheidt, G. Naylor, Proceedings of the 4th European Workshop on Diagnostics and Beam Instrumentation for Particle Accelerators, Chester, UK, 16–18 May 1999, p.51. [2] T. Hara, Y. Tanaka, H. Kitamura, T. Ishikawa, Rev. Sci. Instrum. 71 (2000) 3624. [3] K. Tamasaku, Y. Tanaka, M. Yabashi, H. Yamazaki, N. Kawamura, M. Suzuki, T. Ishikawa, Nucl. Instr. and Meth. A 467–468 (2001), in this proceedings. [4] Y. Tanaka, T. Hara, H. Kitamura, T. Ishikawa, Rev. Sci. Instrum. 71 (2000) 1268. [5] Y. Tanaka et al., to be submitted.