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Near-infrared storage ring free electron laser experiments at AIST
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Infrared Physics & Technology 51 (2008) 375–377 www.elsevier.com/locate/infrared
Near-infrared storage ring free electron laser experiments at AIST N. Sei *, K. Yamada, H. Ogawa, M. Yasumoto Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Available online 15 December 2007
Abstract The development of free electron lasers (FELs) with a compact storage ring NIJI-IV in the near- and middle-infrared regions has been advanced at the National Institute of Advanced Industrial Science and Technology. The optical klystron ETLOK-III was installed in one of the long straight sections of the NIJI-IV, and spontaneous emission spectra were observed in the visible and near-infrared regions. Optical cavity chambers for infrared FELs were installed this February, and it was confirmed that the vibration amplitude of the optical cavity chambers was below 0.5 lm in an optical beam axis. FEL experiments in the near-infrared region will be performed this winter. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Free electron laser; Storage ring NIJI-IV; Infrared region; Optical klystron ETLOK-III; Optical cavity; Spontaneous emission
1. Introduction The oscillations of the storage ring free electron lasers (SRFELs) and their applications have been developed with a compact storage ring NIJI-IV at the National Institute of Advanced Industrial Science and Technology (AIST). After the first lasing at 595 nm in 1992 [1], improvements of the NIJI-IV electron beam have been carried out for shortening the FEL wavelength. The wavelength of 212 nm, at which an FEL oscillation was achieved with the NIJI-IV in 1998, was the shortest wavelength of FELs [2]. The wavelength of the NIJI-IV FEL was reduced to 198 nm in 2003, and the NIJI-IV FEL reached the VUV region. Experiments with photoelectron emission microscopy have also been developed using the NIJI-IV FELs as intense light source in the DUV and VUV regions. We achieved to observe catalytic CO oxidation on Palladium surfaces with video-rate time resolution [3]. Moreover, we planned to develop FEL oscillations in the infrared region using another straight section in the NIJI-IV [4]. Although many infrared FEL facilities based on linear accelerators are operating, no SRFEL has been achieved in the wide *
Corresponding author. Tel.: +81 29 861 5680; fax: +81 29 861 5683. E-mail address: [email protected] (N. Sei).
1350-4495/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2007.12.005
infrared region. In general, an SRFEL has the advantage of a narrow spectrum line width and stability of the lasing wavelength compare with a linear accelerator FEL. It can be used as well as synchrotron radiation passed a monochromator. The target output power and the wavelength region are 1 mW and 0.5–10 lm, respectively. FEL experiments in the near-infrared region will be performed this winter. In this paper, we provide an outline of the nearinfrared FEL experiments with the NIJI-IV. 2. Storage ring NIJI-IV The storage ring NIJI-IV was constructed in 1991 focusing on FEL oscillations [1]. Although it is a compact storage ring with a circumference of 29.6 m, it has two 7.25 m straight sections. The signal frequency for an RF cavity of the NIJI-IV is 162.2 MHz, and the harmonic number is 16. However, FEL experiments are performed in a singlebunch mode to avoid bunch lengthening due to coupledbunch instability. The electron-beam energy is set from 310 to 340 MeV in the injection and FEL experiments. The natural emittance and energy spread at the electronbeam energy of 340 MeV are 6.7 10 8 m rad and 2.6 10 4, respectively [5]. Although a microwave instability appeared at the electron-beam current above 15 mA,
376
N. Sei et al. / Infrared Physics & Technology 51 (2008) 375–377
the peak-electron density in a bunch is over 1.4 1017 m 3. High FEL gain can be achieved with the NIJI-IV. 3. Optical klystron ETLOK-III The insertion device for the infrared FEL oscillations is the optical klystron ETLOK-III, which is an improvement on a 3 m planar undulator [4]. It has two 1.4 m undulator sections and a 72 cm dispersive section. The gap in the undulator section gu can be changed between 36 and 150 mm, and the number of periods in one undulator section is seven. The maximum K value is estimated to be 10.4 from the observed spontaneous emission spectra. Although the gap in the dispersive section gd can be changed between 42 and 188 mm, it must satisfy a condition of gd 6 gu + 38. The wavelength of the fundamental harmonics emitted from the ETLOK-III ranges from the visible region to about 10 lm at the electron-beam energy of 340 MeV. We measured the spectra of the spontaneous emission using a photodiode array attached to a monochromator in the visible and near-infrared regions. Fig. 1 shows the spectra of the fundamental and the third harmonics at a wavelength of around 475 nm. The energy spread evalu-
a
1.0
Intensity [a.u.]
0.8
Single-bunch operation E = 340 [MeV] I = 8.7 [mA]
0.6
0.4
0.2
0.0 455
460
465
470
475
480
485
490
495
500
505
Wavelength [nm]
b
4. Optical cavity In February, new mirror chambers were set at both ends of an optical cavity for the infrared FEL. The cavity length was 14.8 m, i.e., half the circumference of the NIJI-IV. We adopted granite with a weight of 1.3 ton as a stand for the mirror chamber to suppress low-frequency vibrations. Moreover, a vibration dumper, which was able to intercept low-frequency vibrations from the NIJI-IV, was installed between the bending magnet chamber and the mirror chamber. Fig. 2 shows the vibration spectra at the upstream side before and after the installation of the mirror chamber. As shown in Fig. 2, it was confirmed that the stand and the vibration dumper decreased the vibration amplitude on the mirror chamber by about 80%. The vibration amplitude on an optical beam axis was within 0.5 lm, which was the upper limit for maintaining a continuous wave FEL in the NIJI-IV [6]. A mirror holder installed in the mirror chamber was controlled using stepping motors for three-axis positioning and two-tilt angles perpendicular to the optical beam axis. The nominal resolutions for the position on the axis and the tilt angles were 0.1 lm and 4 lrad, respectively. The mirror holder contained two remotely inter-changeable 0
1.0 Single-bunch operation E = 340 [MeV] I = 1.0 [mA]
-20
Magnitude [dB gal]
0.8
Intensity [a.u.]
ated from these spectra was about 2.7 10 4 for the fundamental harmonics and about 3.1 10 4 for the third harmonics, respectively. The value estimated from the spectrum of the fundamental harmonics was in good agreement with the design value, but the value estimated from the spectrum of the third harmonics was slightly more than the design value. In general, further precise adjustment of the electron-beam orbit is necessary for the higher harmonics. The overestimation of the energy spread was caused by the misalignment of the electron-beam orbit in the ETLOK-III and/or by a kick force in the dispersive section. However, the deep modulation of the spectra of the spontaneous emission would guarantee a high FEL gain in the visible and near-infrared regions.
0.6
0.4
0.2
0.0 455
-40
-60
-80
460
465
470
475
480
485
490
495
500
Wavelength [nm]
Fig. 1. Spontaneous emission spectra of (a) the fundamental harmonics and (b) the third harmonics. The undulator gap and the dispersive gap are 140 and 170 mm in (a). In (b), the undulator gap and the dispersive gap are 100 and 130 mm.
-100 1
10
100
Z-axis Frequency [Hz]
Fig. 2. Observed vibration spectra. The dotted line is a spectrum measured on the floor before the installation of the mirror chamber, and the solid line represents a spectrum measured on the stand after the installation.
N. Sei et al. / Infrared Physics & Technology 51 (2008) 375–377
mirrors. The effective diameter of the cavity mirror was 46 mm, therefore a diffraction loss at the cavity mirrors would be negligible at wavelengths below 10 lm. We obtained high-reflection mirrors whose loss was below 0.1% at a wavelength of around 850 nm. The radius of the cavity-mirror curvature was 10 m, which is not optimized for the FEL gain [4]. The maximum FEL gain with the cavity mirrors was estimated to be about 1.3%. When the radius of the mirror curvature was 8 m, the FEL gain increased up to 2.0%. However, the initial cavity loss of the mirrors was much smaller than 1.3%, and therefore, SRFEL oscillation in the near-infrared region will be realized. 5. Conclusions A study to develop SRFELs in the infrared region has been conducted at AIST. The optical klystron ETLOKIII was installed in one of the long straight sections of the NIJI-IV. The spontaneous emission spectra emitted from the ETLOK-III were observed below the near-infrared region. The modulation of the spontaneous emission spectrum was so deep that a high FEL gain could be obtained with the ETLOK-III. Stable mirror chambers were set at both ends of the optical cavity for the infrared
377
FEL. The stand and the vibration dumper of the mirror chamber could decrease the vibration amplitude on the mirror chamber by about 80%. The amplitude of the vibrations at the cavity mirror could be controlled within 0.5 lm on the optical beam axis. High-reflection mirrors whose loss is less than 0.1% at a wavelength of around 850 nm were obtained. The maximum FEL gain with the mirrors is estimated to be about 1.3%. The FEL experiments in the near-infrared region will be performed this winter, and SRFEL oscillations will be achieved. Acknowledgement This work was supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] T. Yamazaki et al., Nucl. Inst. and Meth. A 331 (1993) 27. [2] K. Yamada et al., Nucl. Inst. and Meth A 429 (1999) 159. [3] H. Ogawa, et al., in: Proceedings of 28th Free Electron Lasers Conference, Berlin, 2006, p. 375. [4] N. Sei et al., Jpn. J. Appl. Phys. 41 (2002) 1595. [5] N. Sei et al., Jpn. J. Appl. Phys. 42 (2003) 5848. [6] N. Sei et al., Jpn. J. Appl. Phys. 46 (2007) 3644.
Infrared Physics & Technology 51 (2008) 375–377 www.elsevier.com/locate/infrared
Near-infrared storage ring free electron laser experiments at AIST N. Sei *, K. Yamada, H. Ogawa, M. Yasumoto Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Available online 15 December 2007
Abstract The development of free electron lasers (FELs) with a compact storage ring NIJI-IV in the near- and middle-infrared regions has been advanced at the National Institute of Advanced Industrial Science and Technology. The optical klystron ETLOK-III was installed in one of the long straight sections of the NIJI-IV, and spontaneous emission spectra were observed in the visible and near-infrared regions. Optical cavity chambers for infrared FELs were installed this February, and it was confirmed that the vibration amplitude of the optical cavity chambers was below 0.5 lm in an optical beam axis. FEL experiments in the near-infrared region will be performed this winter. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Free electron laser; Storage ring NIJI-IV; Infrared region; Optical klystron ETLOK-III; Optical cavity; Spontaneous emission
1. Introduction The oscillations of the storage ring free electron lasers (SRFELs) and their applications have been developed with a compact storage ring NIJI-IV at the National Institute of Advanced Industrial Science and Technology (AIST). After the first lasing at 595 nm in 1992 [1], improvements of the NIJI-IV electron beam have been carried out for shortening the FEL wavelength. The wavelength of 212 nm, at which an FEL oscillation was achieved with the NIJI-IV in 1998, was the shortest wavelength of FELs [2]. The wavelength of the NIJI-IV FEL was reduced to 198 nm in 2003, and the NIJI-IV FEL reached the VUV region. Experiments with photoelectron emission microscopy have also been developed using the NIJI-IV FELs as intense light source in the DUV and VUV regions. We achieved to observe catalytic CO oxidation on Palladium surfaces with video-rate time resolution [3]. Moreover, we planned to develop FEL oscillations in the infrared region using another straight section in the NIJI-IV [4]. Although many infrared FEL facilities based on linear accelerators are operating, no SRFEL has been achieved in the wide *
Corresponding author. Tel.: +81 29 861 5680; fax: +81 29 861 5683. E-mail address: [email protected] (N. Sei).
1350-4495/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2007.12.005
infrared region. In general, an SRFEL has the advantage of a narrow spectrum line width and stability of the lasing wavelength compare with a linear accelerator FEL. It can be used as well as synchrotron radiation passed a monochromator. The target output power and the wavelength region are 1 mW and 0.5–10 lm, respectively. FEL experiments in the near-infrared region will be performed this winter. In this paper, we provide an outline of the nearinfrared FEL experiments with the NIJI-IV. 2. Storage ring NIJI-IV The storage ring NIJI-IV was constructed in 1991 focusing on FEL oscillations [1]. Although it is a compact storage ring with a circumference of 29.6 m, it has two 7.25 m straight sections. The signal frequency for an RF cavity of the NIJI-IV is 162.2 MHz, and the harmonic number is 16. However, FEL experiments are performed in a singlebunch mode to avoid bunch lengthening due to coupledbunch instability. The electron-beam energy is set from 310 to 340 MeV in the injection and FEL experiments. The natural emittance and energy spread at the electronbeam energy of 340 MeV are 6.7 10 8 m rad and 2.6 10 4, respectively [5]. Although a microwave instability appeared at the electron-beam current above 15 mA,
376
N. Sei et al. / Infrared Physics & Technology 51 (2008) 375–377
the peak-electron density in a bunch is over 1.4 1017 m 3. High FEL gain can be achieved with the NIJI-IV. 3. Optical klystron ETLOK-III The insertion device for the infrared FEL oscillations is the optical klystron ETLOK-III, which is an improvement on a 3 m planar undulator [4]. It has two 1.4 m undulator sections and a 72 cm dispersive section. The gap in the undulator section gu can be changed between 36 and 150 mm, and the number of periods in one undulator section is seven. The maximum K value is estimated to be 10.4 from the observed spontaneous emission spectra. Although the gap in the dispersive section gd can be changed between 42 and 188 mm, it must satisfy a condition of gd 6 gu + 38. The wavelength of the fundamental harmonics emitted from the ETLOK-III ranges from the visible region to about 10 lm at the electron-beam energy of 340 MeV. We measured the spectra of the spontaneous emission using a photodiode array attached to a monochromator in the visible and near-infrared regions. Fig. 1 shows the spectra of the fundamental and the third harmonics at a wavelength of around 475 nm. The energy spread evalu-
a
1.0
Intensity [a.u.]
0.8
Single-bunch operation E = 340 [MeV] I = 8.7 [mA]
0.6
0.4
0.2
0.0 455
460
465
470
475
480
485
490
495
500
505
Wavelength [nm]
b
4. Optical cavity In February, new mirror chambers were set at both ends of an optical cavity for the infrared FEL. The cavity length was 14.8 m, i.e., half the circumference of the NIJI-IV. We adopted granite with a weight of 1.3 ton as a stand for the mirror chamber to suppress low-frequency vibrations. Moreover, a vibration dumper, which was able to intercept low-frequency vibrations from the NIJI-IV, was installed between the bending magnet chamber and the mirror chamber. Fig. 2 shows the vibration spectra at the upstream side before and after the installation of the mirror chamber. As shown in Fig. 2, it was confirmed that the stand and the vibration dumper decreased the vibration amplitude on the mirror chamber by about 80%. The vibration amplitude on an optical beam axis was within 0.5 lm, which was the upper limit for maintaining a continuous wave FEL in the NIJI-IV [6]. A mirror holder installed in the mirror chamber was controlled using stepping motors for three-axis positioning and two-tilt angles perpendicular to the optical beam axis. The nominal resolutions for the position on the axis and the tilt angles were 0.1 lm and 4 lrad, respectively. The mirror holder contained two remotely inter-changeable 0
1.0 Single-bunch operation E = 340 [MeV] I = 1.0 [mA]
-20
Magnitude [dB gal]
0.8
Intensity [a.u.]
ated from these spectra was about 2.7 10 4 for the fundamental harmonics and about 3.1 10 4 for the third harmonics, respectively. The value estimated from the spectrum of the fundamental harmonics was in good agreement with the design value, but the value estimated from the spectrum of the third harmonics was slightly more than the design value. In general, further precise adjustment of the electron-beam orbit is necessary for the higher harmonics. The overestimation of the energy spread was caused by the misalignment of the electron-beam orbit in the ETLOK-III and/or by a kick force in the dispersive section. However, the deep modulation of the spectra of the spontaneous emission would guarantee a high FEL gain in the visible and near-infrared regions.
0.6
0.4
0.2
0.0 455
-40
-60
-80
460
465
470
475
480
485
490
495
500
Wavelength [nm]
Fig. 1. Spontaneous emission spectra of (a) the fundamental harmonics and (b) the third harmonics. The undulator gap and the dispersive gap are 140 and 170 mm in (a). In (b), the undulator gap and the dispersive gap are 100 and 130 mm.
-100 1
10
100
Z-axis Frequency [Hz]
Fig. 2. Observed vibration spectra. The dotted line is a spectrum measured on the floor before the installation of the mirror chamber, and the solid line represents a spectrum measured on the stand after the installation.
N. Sei et al. / Infrared Physics & Technology 51 (2008) 375–377
mirrors. The effective diameter of the cavity mirror was 46 mm, therefore a diffraction loss at the cavity mirrors would be negligible at wavelengths below 10 lm. We obtained high-reflection mirrors whose loss was below 0.1% at a wavelength of around 850 nm. The radius of the cavity-mirror curvature was 10 m, which is not optimized for the FEL gain [4]. The maximum FEL gain with the cavity mirrors was estimated to be about 1.3%. When the radius of the mirror curvature was 8 m, the FEL gain increased up to 2.0%. However, the initial cavity loss of the mirrors was much smaller than 1.3%, and therefore, SRFEL oscillation in the near-infrared region will be realized. 5. Conclusions A study to develop SRFELs in the infrared region has been conducted at AIST. The optical klystron ETLOKIII was installed in one of the long straight sections of the NIJI-IV. The spontaneous emission spectra emitted from the ETLOK-III were observed below the near-infrared region. The modulation of the spontaneous emission spectrum was so deep that a high FEL gain could be obtained with the ETLOK-III. Stable mirror chambers were set at both ends of the optical cavity for the infrared
377
FEL. The stand and the vibration dumper of the mirror chamber could decrease the vibration amplitude on the mirror chamber by about 80%. The amplitude of the vibrations at the cavity mirror could be controlled within 0.5 lm on the optical beam axis. High-reflection mirrors whose loss is less than 0.1% at a wavelength of around 850 nm were obtained. The maximum FEL gain with the mirrors is estimated to be about 1.3%. The FEL experiments in the near-infrared region will be performed this winter, and SRFEL oscillations will be achieved. Acknowledgement This work was supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] T. Yamazaki et al., Nucl. Inst. and Meth. A 331 (1993) 27. [2] K. Yamada et al., Nucl. Inst. and Meth A 429 (1999) 159. [3] H. Ogawa, et al., in: Proceedings of 28th Free Electron Lasers Conference, Berlin, 2006, p. 375. [4] N. Sei et al., Jpn. J. Appl. Phys. 41 (2002) 1595. [5] N. Sei et al., Jpn. J. Appl. Phys. 42 (2003) 5848. [6] N. Sei et al., Jpn. J. Appl. Phys. 46 (2007) 3644.