- Email: [email protected]
Observation of THz coherent transition radiation from single-bunch beam at KURRI-LINAC as an intense pulsed light source
Available online at www.sciencedirect.com
Infrared Physics & Technology 51 (2008) 363–366 www.elsevier.com/locate/infrared
Observation of THz coherent transition radiation from single-bunch beam at KURRI-LINAC as an intense pulsed light source Toshiharu Takahashi a,*, Kiyoshi Takami b a
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan b Nippon Advanced Technology Co., Ltd., Kumatori, Osaka 590-0494, Japan Available online 23 December 2007
Abstract A single-bunch beam has been generated using a developed high-speed avalanche-type pulser in KURRI-LINAC in order to lift restrictions of the spectral resolution in the spectroscopic study and the delay time in the time-resolved measurement. Both of the rise and fall times of the developed pulser are 110 ps. The observation of CTR has confirmed the single-bunch beam. The degree of impurity of single bunch has been estimated to be 1.5% by the analysis of the interferogram. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Coherent radiation; THz spectroscopy; Single bunch; Beam diagostics
1. Introduction In the electron linear accelerator at Research Reactor Institute in Kyoto University (KURRI-LINAC), properties of several types of coherent radiation (synchrotron radiation [1], transition radiation [2], Cherenkov radiation [3], diffraction radiation, and Smith–Purcell radiation [4], pre-bunched FEL [5]) in the THz-wave and the millimeter-wave regions have been experimentally investigated since 1991. The beamline for the millimeter-wave spectroscopy has been constructed, in which coherent transition radiation (CTR) has been used as a light source [6] and the spectroscopic research for gas [6] and solid materials [7] have been demonstrated. There are three advantages of the linac-based coherent radiation: first, the high-peakpower coherent radiation is available because of a high amount of charge (several nC) in a bunch, second, various types of coherent radiation are available, and third, the interaction between the electron beam and the medium is available. Therefore, the linac-based coherent radiation is especially useful as a pico-second pulsed light source for *
Corresponding author. Tel.: +81 724 51 2409; fax: +81 724 51 2602. E-mail address: [email protected] (T. Takahashi).
1350-4495/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2007.12.029
the time-resolved spectroscopy and the pulseradiolysis study. The electron beam of KURRI-LINAC has a multibunch structure. Since the accelerating frequency is 1.3 GHz (L-band), the interval between pulses in the CTR pulse train is 770 ps (23 cm). The spectrum from successive bunches is constituted of the higher harmonics of 1.3 GHz [6]. As a result, the CTR can be treated as a light source with a continuous spectrum only when the spectral resolution is lower than 1/23 cm = 0.0434 cm1 and the optical delay in the time-resolved measurement is restricted within 770 ps. A single-bunch beam is necessary in order to lift these restrictions. In this paper we describe a new injection system of electrons for generating a single-bunch beam by a high-speed avalanche-type grid pulser and observation of CTR from a single-bunch beam. 2. High-speed avalanche-type pulser Though a sub-harmonic buncher system is usually used for generation of a single-bunch beam, we have developed a high-speed avalanche-type pulser to drive an electron gun due to a little space and small cost. The schematic diagram
364
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
is shown in Fig. 1. The pulser consists of 20 transistors on a strip line of 50 X, because the waveform of a multi-stage pulser has a sharp edge. The trigger signal to the pulser has been synchronized with 1.3-GHz radio frequency to stabilize the amount of electric charge in a bunch. The pulser has three advantages as follows. First, we could construct it at small cost because general-purpose transistors FMMT439 (Zetex) are used as avalanche transistors. The break-down voltage was actually measured and transistors of 291 V were selected. Second, the end of the strip line on the opposite side of the output is terminated by a 50-X resistor as shown in Fig. 1. The disorder of waveforms can be minimized though the output amplitude becomes half. Third, the pulse width is determined by the length of two coaxial-cables for clipping. Hence, we can generate not a single-bunch beam but two or three bunches. The high-voltage pulses by this pulser are shown in Fig. 2. The length of clipping cables and the pulse width are 2.5 cm and 250 ps in Fig. 2a, and 10 cm and 1 ns in Fig. 2b, respectively. The waveforms were obtained by an oscilloscope CSA803 (Tektronix) with a sampling head SD-26 through three 20-dB attenuators. The amplitude of 250-ps pulse is 750 V and that of 1-ns one is 940 V. Both of the rise and fall times are 110 ps.
3. Experimental procedures of CTR measurement The pulser was installed in the electron injector of KURRI-LINAC, in which the thermal-cathode type electron gun YU-156 (EIMAC) was used, to generate a single-bunch beam. In order to confirm a single-bunch beam, CTR was observed at the coherent radiation beamline. The schematic layout of the beamline is shown in Fig. 3. The superposition of the forward CTR from a Ti window and the backward CTR from an Al-foil was used as a light source. The CTR was guided to the experimental room by a parallel beam of 150 mm in diameter and detected by a liquid-helium-cooled Si bolometer (Infrared Laboratory) with the long-wavelength-pass filter of 35 cm1 through a home-made Martin–Puplett type interferometer. The interferometer was used in step-scan operation and the maximum optical path difference was 600 mm which exceeded two times of the interval between bunches. When the waveform of CTR was observed, a millimeterwave diode detector DXP-10 (Millitec) was used though its spectral bandwidth was narrow (75–110 GHz). The output signal was visualized by an oscilloscope DSA70404 (Tektronix). Ti window
Al foil
electron beam
from L-band Linac M2
Tirgger Input
+1650V
Accelerator Room R:50
50
M1
strip line Shield Wall
Experimental Room
M4
Output Interferometer
M3
-H.V. (variable)
Coaxial-cable clipping
Fig. 1. Schematic diagram of the developed avalanche-type pulser. It consists of 20 transistors in series on a strip line of 50 X.
Fig. 3. The schematic layout of coherent radiation beamline. Keys are: (M1, M4) flat mirrors; (M2) a spherical mirror of f = 1500 mm; and (M3) a spherical mirror of f = 750 mm. Both of a Ti-window and an Al-foil are the sources of CTR.
Fig. 2. Waveforms of high-voltage pulses from the developed pulser. The pulse width and amplitude are 250 ps and 750 V in (a) and 1 ns and 940 V in (b).
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
The waveform of CTR detected by a millimeter-wave diode detector with the 250-ps pulser is shown in Fig. 4. The horizontal scale is 400 ps/div and the width of the signal is about 130 ps (FWHM). This value represents not the actual bunch length (5 ps) but the response of the detector. The neighbor bunches are not seen at intervals of 770 ps from the main bunch. Therefore, the single-bunch beam has been successfully generated by the developed high-speed pulser. Wave packets of CTR from successive bunches interfere each other and cross-correlation interferograms are observed in addition to the autocorrelation interferogram under the multi-bunch operation [6]. The interferogram of CTR with the 250-ps pulser was shown in Fig. 5 detected by a Si bolometer through an interferometer. The interference structure around the optical path difference of 0 mm represents the autocorrelation interferogram. The crosscorrelation interferogram with small amplitude is seen around 240 mm though the theoretical interval between bunches are 230 mm. The ratio of amplitude of the crosscorrelation interferogram to that of the autocorrelation one was 1.5% in Fig. 5. By means of calculation of the superposition of the electric field emitted from electrons in two bunches, the autocorrelation interferogram IA and the cross-correlation one I12 of CTR from two bunches are written by Z 1 I A ðdÞ ¼ P 0 ðrÞf ðrÞðn21 þ n22 Þð1 cosð2pdrÞÞdr ð1Þ Z0 1 I 12 ðdÞ ¼ P 0 ðrÞf ðrÞðn21 þ n22 n1 n2 cosð2pdrÞÞdr ð2Þ 0
where d is the optical path difference, r the wavenumber, n1 the number of electrons in the first bunch, n2 that in the second one, and P0(r) the power spectrum from an electron.
The distributions of electrons in the first and second bunches are assumed to be similar, that is the same bunch form factor f1(r) = f2(r) = f(r). The ratio of amplitude of the cross-correlation interferogram in Eq. (2) to that of the autocorrelation one in Eq. (1) is given by R ¼ n1 n2 =ðn21 þ n22 Þ:
ð3Þ
When the second bunch is much smaller than the first one (n1 n2), the ratio R is reduced to n2/n1, namely the ratio of the number of electrons. Therefore, the degree of impurity of single bunch can be estimated to 1.5% from the interferogram in Fig. 5. In order to confirm the analysis by the interferogram we demonstrated the measurement of CTR with the 1-ns pulser. The waveform of CTR detected by a diode detector is shown in Fig. 6. Two CTR pulses with same amplitude are 0.2
OUTPUT VOLTAGE (V)
4. Results and discussion
365
0.15
0.1
0.05
0 -10
-5
0
5
10 230 235
240
245
250
OPTICAL PATH DIFFERENCE (mm)
Fig. 5. Observed interferograms of CTR detected by a Si bolometer with the 250-ps pulser around the optical path difference of 0 and 240 mm.
Fig. 4. The waveform of CTR detected by a millimeter-wave diode detector with the 250-ps pulser.
366
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
Fig. 6. The waveform of CTR detected by a millimeter-wave diode detector with the 1-ns pulser.
OUTPUT VOLTAGE (V)
0.1
0.05
0 -10
-5
0
5
10 230
235
240 245
465 470
475
480
OPTICAL PATH DIFFERENCE (mm)
Fig. 7. Observed interferograms of CTR detected by a Si bolometer with the 1-ns pulser around the optical path difference of 0, 240, and 470 mm.
observed. The interferogram of CTR with the 1-ns pulser was shown in Fig. 7. The cross-correlation interferogram was observed around the optical path difference of 235 mm, which amplitude was half of the autocorrelation interferogram. The ratio calculated from Eq. (3) is 1/2 for n1 = n2 and is equivalent with the experimental result. The second cross-correlation interferogram around 470 mm was not observed in Fig. 7.
Acknowledgements
5. Summary
References
We developed the high-speed avalanche-type pulser with 110-ps rise and fall times in order to generate a singlebunch beam. After installing the pulser to the electron injector of KURRI-LINAC, the autocorrelation and cross-correlation interferograms of CTR were measured. The degree of impurity of the single bunch was estimated to be 1.5% by the analysis of the interferograms.
[1] [2] [3] [4] [5] [6] [7]
One of the authors (T.T.) would like to thank Dr. Y. Shibata for helpful discussion about the analysis of interferogram. This work was partially supported by the comprehensive support program for the promotion of accelerator science and technology from the High Energy Accelerator Research Organization (KEK).
Y. Shibata et al., Phys. Rev. A 44 (1991) R3449. Y. Shibata et al., Phys. Rev. A 45 (1992) R8340. T. Takahashi et al., Phys. Rev. E 50 (1994) 4041. Y. Shibata et al., Phys. Rev. E 57 (1998) 1061. Y. Shibata et al., NIM 528 (2004) 162. T. Takahshi et al., Rev. Sci. Instrum. 69 (1998) 3770. Y.H. Matsuda et al., Physica B 346–347 (2004) 519–523.
Infrared Physics & Technology 51 (2008) 363–366 www.elsevier.com/locate/infrared
Observation of THz coherent transition radiation from single-bunch beam at KURRI-LINAC as an intense pulsed light source Toshiharu Takahashi a,*, Kiyoshi Takami b a
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan b Nippon Advanced Technology Co., Ltd., Kumatori, Osaka 590-0494, Japan Available online 23 December 2007
Abstract A single-bunch beam has been generated using a developed high-speed avalanche-type pulser in KURRI-LINAC in order to lift restrictions of the spectral resolution in the spectroscopic study and the delay time in the time-resolved measurement. Both of the rise and fall times of the developed pulser are 110 ps. The observation of CTR has confirmed the single-bunch beam. The degree of impurity of single bunch has been estimated to be 1.5% by the analysis of the interferogram. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Coherent radiation; THz spectroscopy; Single bunch; Beam diagostics
1. Introduction In the electron linear accelerator at Research Reactor Institute in Kyoto University (KURRI-LINAC), properties of several types of coherent radiation (synchrotron radiation [1], transition radiation [2], Cherenkov radiation [3], diffraction radiation, and Smith–Purcell radiation [4], pre-bunched FEL [5]) in the THz-wave and the millimeter-wave regions have been experimentally investigated since 1991. The beamline for the millimeter-wave spectroscopy has been constructed, in which coherent transition radiation (CTR) has been used as a light source [6] and the spectroscopic research for gas [6] and solid materials [7] have been demonstrated. There are three advantages of the linac-based coherent radiation: first, the high-peakpower coherent radiation is available because of a high amount of charge (several nC) in a bunch, second, various types of coherent radiation are available, and third, the interaction between the electron beam and the medium is available. Therefore, the linac-based coherent radiation is especially useful as a pico-second pulsed light source for *
Corresponding author. Tel.: +81 724 51 2409; fax: +81 724 51 2602. E-mail address: [email protected] (T. Takahashi).
1350-4495/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2007.12.029
the time-resolved spectroscopy and the pulseradiolysis study. The electron beam of KURRI-LINAC has a multibunch structure. Since the accelerating frequency is 1.3 GHz (L-band), the interval between pulses in the CTR pulse train is 770 ps (23 cm). The spectrum from successive bunches is constituted of the higher harmonics of 1.3 GHz [6]. As a result, the CTR can be treated as a light source with a continuous spectrum only when the spectral resolution is lower than 1/23 cm = 0.0434 cm1 and the optical delay in the time-resolved measurement is restricted within 770 ps. A single-bunch beam is necessary in order to lift these restrictions. In this paper we describe a new injection system of electrons for generating a single-bunch beam by a high-speed avalanche-type grid pulser and observation of CTR from a single-bunch beam. 2. High-speed avalanche-type pulser Though a sub-harmonic buncher system is usually used for generation of a single-bunch beam, we have developed a high-speed avalanche-type pulser to drive an electron gun due to a little space and small cost. The schematic diagram
364
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
is shown in Fig. 1. The pulser consists of 20 transistors on a strip line of 50 X, because the waveform of a multi-stage pulser has a sharp edge. The trigger signal to the pulser has been synchronized with 1.3-GHz radio frequency to stabilize the amount of electric charge in a bunch. The pulser has three advantages as follows. First, we could construct it at small cost because general-purpose transistors FMMT439 (Zetex) are used as avalanche transistors. The break-down voltage was actually measured and transistors of 291 V were selected. Second, the end of the strip line on the opposite side of the output is terminated by a 50-X resistor as shown in Fig. 1. The disorder of waveforms can be minimized though the output amplitude becomes half. Third, the pulse width is determined by the length of two coaxial-cables for clipping. Hence, we can generate not a single-bunch beam but two or three bunches. The high-voltage pulses by this pulser are shown in Fig. 2. The length of clipping cables and the pulse width are 2.5 cm and 250 ps in Fig. 2a, and 10 cm and 1 ns in Fig. 2b, respectively. The waveforms were obtained by an oscilloscope CSA803 (Tektronix) with a sampling head SD-26 through three 20-dB attenuators. The amplitude of 250-ps pulse is 750 V and that of 1-ns one is 940 V. Both of the rise and fall times are 110 ps.
3. Experimental procedures of CTR measurement The pulser was installed in the electron injector of KURRI-LINAC, in which the thermal-cathode type electron gun YU-156 (EIMAC) was used, to generate a single-bunch beam. In order to confirm a single-bunch beam, CTR was observed at the coherent radiation beamline. The schematic layout of the beamline is shown in Fig. 3. The superposition of the forward CTR from a Ti window and the backward CTR from an Al-foil was used as a light source. The CTR was guided to the experimental room by a parallel beam of 150 mm in diameter and detected by a liquid-helium-cooled Si bolometer (Infrared Laboratory) with the long-wavelength-pass filter of 35 cm1 through a home-made Martin–Puplett type interferometer. The interferometer was used in step-scan operation and the maximum optical path difference was 600 mm which exceeded two times of the interval between bunches. When the waveform of CTR was observed, a millimeterwave diode detector DXP-10 (Millitec) was used though its spectral bandwidth was narrow (75–110 GHz). The output signal was visualized by an oscilloscope DSA70404 (Tektronix). Ti window
Al foil
electron beam
from L-band Linac M2
Tirgger Input
+1650V
Accelerator Room R:50
50
M1
strip line Shield Wall
Experimental Room
M4
Output Interferometer
M3
-H.V. (variable)
Coaxial-cable clipping
Fig. 1. Schematic diagram of the developed avalanche-type pulser. It consists of 20 transistors in series on a strip line of 50 X.
Fig. 3. The schematic layout of coherent radiation beamline. Keys are: (M1, M4) flat mirrors; (M2) a spherical mirror of f = 1500 mm; and (M3) a spherical mirror of f = 750 mm. Both of a Ti-window and an Al-foil are the sources of CTR.
Fig. 2. Waveforms of high-voltage pulses from the developed pulser. The pulse width and amplitude are 250 ps and 750 V in (a) and 1 ns and 940 V in (b).
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
The waveform of CTR detected by a millimeter-wave diode detector with the 250-ps pulser is shown in Fig. 4. The horizontal scale is 400 ps/div and the width of the signal is about 130 ps (FWHM). This value represents not the actual bunch length (5 ps) but the response of the detector. The neighbor bunches are not seen at intervals of 770 ps from the main bunch. Therefore, the single-bunch beam has been successfully generated by the developed high-speed pulser. Wave packets of CTR from successive bunches interfere each other and cross-correlation interferograms are observed in addition to the autocorrelation interferogram under the multi-bunch operation [6]. The interferogram of CTR with the 250-ps pulser was shown in Fig. 5 detected by a Si bolometer through an interferometer. The interference structure around the optical path difference of 0 mm represents the autocorrelation interferogram. The crosscorrelation interferogram with small amplitude is seen around 240 mm though the theoretical interval between bunches are 230 mm. The ratio of amplitude of the crosscorrelation interferogram to that of the autocorrelation one was 1.5% in Fig. 5. By means of calculation of the superposition of the electric field emitted from electrons in two bunches, the autocorrelation interferogram IA and the cross-correlation one I12 of CTR from two bunches are written by Z 1 I A ðdÞ ¼ P 0 ðrÞf ðrÞðn21 þ n22 Þð1 cosð2pdrÞÞdr ð1Þ Z0 1 I 12 ðdÞ ¼ P 0 ðrÞf ðrÞðn21 þ n22 n1 n2 cosð2pdrÞÞdr ð2Þ 0
where d is the optical path difference, r the wavenumber, n1 the number of electrons in the first bunch, n2 that in the second one, and P0(r) the power spectrum from an electron.
The distributions of electrons in the first and second bunches are assumed to be similar, that is the same bunch form factor f1(r) = f2(r) = f(r). The ratio of amplitude of the cross-correlation interferogram in Eq. (2) to that of the autocorrelation one in Eq. (1) is given by R ¼ n1 n2 =ðn21 þ n22 Þ:
ð3Þ
When the second bunch is much smaller than the first one (n1 n2), the ratio R is reduced to n2/n1, namely the ratio of the number of electrons. Therefore, the degree of impurity of single bunch can be estimated to 1.5% from the interferogram in Fig. 5. In order to confirm the analysis by the interferogram we demonstrated the measurement of CTR with the 1-ns pulser. The waveform of CTR detected by a diode detector is shown in Fig. 6. Two CTR pulses with same amplitude are 0.2
OUTPUT VOLTAGE (V)
4. Results and discussion
365
0.15
0.1
0.05
0 -10
-5
0
5
10 230 235
240
245
250
OPTICAL PATH DIFFERENCE (mm)
Fig. 5. Observed interferograms of CTR detected by a Si bolometer with the 250-ps pulser around the optical path difference of 0 and 240 mm.
Fig. 4. The waveform of CTR detected by a millimeter-wave diode detector with the 250-ps pulser.
366
T. Takahashi, K. Takami / Infrared Physics & Technology 51 (2008) 363–366
Fig. 6. The waveform of CTR detected by a millimeter-wave diode detector with the 1-ns pulser.
OUTPUT VOLTAGE (V)
0.1
0.05
0 -10
-5
0
5
10 230
235
240 245
465 470
475
480
OPTICAL PATH DIFFERENCE (mm)
Fig. 7. Observed interferograms of CTR detected by a Si bolometer with the 1-ns pulser around the optical path difference of 0, 240, and 470 mm.
observed. The interferogram of CTR with the 1-ns pulser was shown in Fig. 7. The cross-correlation interferogram was observed around the optical path difference of 235 mm, which amplitude was half of the autocorrelation interferogram. The ratio calculated from Eq. (3) is 1/2 for n1 = n2 and is equivalent with the experimental result. The second cross-correlation interferogram around 470 mm was not observed in Fig. 7.
Acknowledgements
5. Summary
References
We developed the high-speed avalanche-type pulser with 110-ps rise and fall times in order to generate a singlebunch beam. After installing the pulser to the electron injector of KURRI-LINAC, the autocorrelation and cross-correlation interferograms of CTR were measured. The degree of impurity of the single bunch was estimated to be 1.5% by the analysis of the interferograms.
[1] [2] [3] [4] [5] [6] [7]
One of the authors (T.T.) would like to thank Dr. Y. Shibata for helpful discussion about the analysis of interferogram. This work was partially supported by the comprehensive support program for the promotion of accelerator science and technology from the High Energy Accelerator Research Organization (KEK).
Y. Shibata et al., Phys. Rev. A 44 (1991) R3449. Y. Shibata et al., Phys. Rev. A 45 (1992) R8340. T. Takahashi et al., Phys. Rev. E 50 (1994) 4041. Y. Shibata et al., Phys. Rev. E 57 (1998) 1061. Y. Shibata et al., NIM 528 (2004) 162. T. Takahshi et al., Rev. Sci. Instrum. 69 (1998) 3770. Y.H. Matsuda et al., Physica B 346–347 (2004) 519–523.