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UV pulse shaping for the photocathode RF gun
Nuclear Instruments and Methods in Physics Research A 637 (2011) S127–S129
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
UV pulse shaping for the photocathode RF gun Lixin Yan a,b,c,, Qiang Du a,b,c, Yingchao Du a,b,c, Jianfei Hua a,b,c, Wenhui Huang a,b,c, Chuanxiang Tang a,b,c a b c
Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084,China Key Laboratory of Particle and Radiation Imaging (Tsinghua University), Ministry of Education, Beijing 100084,China Key Laboratory of High Energy Radiation Imaging Fundamental Science for National Defense, Beijing 100084,China
a r t i c l e in fo
abstract
Available online 11 February 2010
Recently, manipulation with the drive laser plays a significant role in high brightness electron beam production by the photocathode RF gun. The article takes efforts on the temporal shaping of the driving laser for the photocathode RF gun. Method based on pulse stacking by birefringent crystal of a-BBO serials was tried to directly shape ultraviolet laser pulse. Using four pieces of a-BBO crystals to separate an input UV pulse with appropriate duration into 16 sub-pulses can form a ps-spaced pulse train suitable for coherent THz production. The group delay dispersion induced by the crystals was also carefully considered. To avoid beam deterioration by long path propagation, imaging relay of the shaped pulse was applied. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Photocathode RF gun Laser pulse shaping a-BBO crystal Group delay dispersion
1. Introduction High brightness electron beams based on photocathode RF gun are playing an increasingly significant role in many scientific fields such as XFEL, ILC, ultrafast electron diffraction, Thomson scattering based X-ray light sources and coherent THz production. Interest has been excited across the scientific community to do theoretical and experimental research work in this field. For the purpose to develop a multi-objective (X-ray, UED, THz, etc) scientific test facility, a photocathode RF gun based on copper has been set up at our laboratory. Experiments using the photoelectron beams to produce X-rays by Thomson scattering [1] and diffraction pattern by Mev ultrafast electron diffraction [2] have been performed. By virtue of two interesting features: the relativistic effect of single electron and the coherent effect of multiple electrons and relativistic ultrashort electron beams (o1 ps, tens of MeV) can produce high power coherent THz radiation. The THz modulated electron beams, or THz-repetition-rate ultrashort electron pulse trains exhibits further enhancement of the THz radiation [3,4]. How to achieve this kind of pre-bunched electron beams is the focus of many publications [5–7]. Due to the negligible emission time of the photocathode illuminated by the ultrashort drive laser pulses, the initial temporal distribution of the electron beam is the same as that of the laser pulse. Electron beams with arbitrary temporal profile may be achieved by laser pulse shaping. This article will focus on the laser pulse shaping method for UV pulse Corresponding author at: Accelerator laboratory, Department of engineering physics, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62795424. E-mail address: [email protected] (L. Yan).
train generation by the birefringent a-BBO crystal serials. THzrepetition-rate ultrashort electron pulse trains can not only be applied to radiate coherent THz radiation by mechanisms such as FEL, CSR, CTR, but also have important applications in timeresolved ultrafast electron diffraction, multi-pulse Thomson scattering and other areas. This shaping method of laser pulse by a-BBO crystals can also be applied to reduce the initial emittance of the electron beam from the photocathode RF gun.
2. Pulse shaping method In recent years, several kinds of shaping methods for ultrashort laser pulses have been developed for various applications, such as frequency domain shaping techniques (based on liquid crystals, acousto-optical crystals, or deformable mirrors, etc, represented by AOPDF [8]) for temporally flattop pulse production. These methods are mainly for infrared pulses and only suitable for low pulse energies. To produce modulated laser pulses, other methods have been proposed and tested, such as Neumann’s method by Fabry–Perot interferometer [5], pulse stacking by beamsplitter plates [9], or by polarizing beamsplitter cubes [10], or a-YVO4 crystals [11], or calcite crystals [12]. Several publications mentioned the possibilities of using a-BBO crystals, which can transmit UV light [10,13]. 2.1. Properties of a-BBO crystal The a-BBO crystal is a kind of negative, uniaxial crystal with a center of symmetry in its structure. It is not suitable for nonlinear optical processes, but its large birefringence over the broad
0168-9002/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.038
S128
L. Yan et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S127–S129
transparency range from 190 to 3500 nm makes it an excellent candidate material for linear optics [14]. The crystal is cut so that its extraordinary axis lies in its surface and perpendicular to the incident laser pulse. Its Sellmeier equations for ordinary (no) and extraordinary (ne) light are expressed as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:01878 2 0:01354 l no ðlÞ ¼ 2:7471þ 2 l 0:01822 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:01224 2 0:01516 l ne ðlÞ ¼ 2:3174þ 2 l 0:01667
Fig. 3. The input UV pulse can be split into 16 sub-pulses with alternatively varied polarization by a-BBO crystal serials.
where l is the wavelength in micrometer. For the UV laser to drive copper photocathode RF gun at central wavelength 266.7 nm (triple of laser at wavelength 800 nm), no(l0) =1.758, ne(l0) =1.612. The birefringence is 0.146. So the initially coincided such two types of polarized light centered at wavelength 266.7 nm will have relative retardation t (in ps) after propagation in the crystal for length l (in mm), which satisfies l = ct/(no ne), where c =2.99792458 10 1 m/ps is the velocity of light in vacuum, as shown in Fig. 1. To make the delay time between ordinary light and extraordinary light to be 1 ps, the thickness of the crystal should be 2.058 mm (about 2 mm). 2.2. Pulse stacking by a-BBO crystal As described above, if linear polarized sub-ps UV laser pulse centered at 266.7 nm is normally incident on the left surface of aBBO crystal, with the angle between the polarization direction and extraordinary axis to be 451, then a-BBO crystal of about 2 mm in thickness will split the input pulse into two perpendicular polarized sub-pulses spacing 1 ps in time. Accordingly, suppose there is four pieces of a-BBO crystals with thickness L0/2n (n= 0, 1, 2, 3) placed successfully as depicted in Fig. 2. The extraordinary axis of the odd crystals is parallel with the original input pulse polarization direction, while the extraordinary axis of the even crystals is tuned to be 451 with that of adjacent crystals. Without consideration of the dispersion difference between ordinary and extraordinary light by the crystals, the crystal serials will produce such pulse train including 16 alternatively varied polarization sub-pulses with equal spacing and amplitude, shown in Fig. 3. Using such a-BBO crystal serials with L0 = 16, 8, 4, 2 mm, and initial input pulse with 0.5 ps duration (FWHM), without consideration of dispersion induced by the crystals, one can
t=l(no-ne)/c t l
Fig. 4. THz-rep-rate pulse train formation with 0.5 ps initial pulse by (16, 8, 4, 2) mm crystal serials.
Table 1 Parameters of sub-pulses through a-BBO crystals. Sub-pulse index
GDD (104 fs2)
Broadening (%)
oooo oooe ooeo ooee oeoo oeoe oeeo oeee eooo eooe eoeo eoee eeoo eeoe eeeo eeee
1.62 1.58 1.54 1.51 1.47 1.43 1.39 1.36 1.32 1.28 1.24 1.21 1.17 1.13 1.09 1.06
1.60 1.53 1.45 1.39 1.32 1.25 1.19 1.13 1.06 1.01 0.95 0.89 0.84 0.78 0.73 0.68
obtain pulse train depicted in Fig. 4. The time interval of the adjacent sub-pulses is about 1 ps, thus the THz-rep-rate pulse train with equal amplitude is formed. Because of the alternatively varied polarization, interference does not play considerable role in the pulse train formation, as can also be seen in Fig. 4.
Fig. 1. The principle of birefringent crystal: ordinary light (o) will be left behind of extraordinary light after it pass through a-BBO crystal.
2.3. The group delay dispersion effect
Fig. 2. schematic of four pieces of a-BBO (with thickness L0/2n , n= 0, 1, 2, 3) crystals.
For the propagation of ultrashort laser pulse in optical materials, the group delay dispersion effect always has to be considered, which may considerably change the shape of the initial pulse. The total length of the a-BBO crystals to form above mentioned UV pulse train is about 3 cm. Each sub-pulse undergoes different optical path throughout the crystal serials due to its different combination of light types with the four crystals. For example, the first sub-pulse to go through the crystal serials can be denoted as ‘eeee’, for it to pass all the four pieces of crystals as extraordinary light. The second sub-pulse is then denoted as
L. Yan et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S127–S129
Iris f1
f2
f3
f4
f5
f6
f7
f8
f9
S129
f10
PC
Fig. 5. Schematic of imaging relay from the exit of a-BBO serials to the photocathode (PC).
‘eeeo’, for it to pass the first three crystals as extraordinary light and only for the fourth crystal as ordinary light. In this manner, the 16 sub-pulses can be denoted as oooo, oooe, ooeo, ooee, oeoo, oeoe, oeeo, oeee, eooo, eooe, eoeo, eoee, eeoo, eeoe, eeeo and eeee. So using the parameters of a-BBO crystals at wavelength 266.7 nm, the group delay dispersion of each sub-pulse can be calculated. The second derivatives of index for a-BBO crystals at wavelength 266.7 nm are: 00
b0 ¼ 0:5394 mm1 fs2 ;
00
be ¼ 0:3524 mm1 fs2
For initial transform limited input pulse with 0.5 ps pulse duration (FWHM), the GDD’s and pulse width broadening of each sub-pulse are listed in Table 1. From Table 1, we can see that no significant pulse broadening was induced by such crystal serials and no considerable differences between the sub-pulses, so the stacked pulse shape does not differ much with that of Fig. 4. But further calculations show that for initial pulse width less than 100 fs, the pulse broadening becomes inacceptable. For such cases, measures have to be taken to compensate the broadening effect induced by the inserted crystals.
3. Imaging relay To transport the UV pulse to the photocathode located about 20 m away from the exit of the THG (third harmonic generator) where fixed the a-BBO crystals, the imaging relay technique was applied to limit the diffraction effect by light propagation and the walk-off effect by the birefringent crystal serials. As Fig. 5 shows, five imaging relay units including 10 pieces of lens (f1 f10, whose focal lengths were 92, 92, 138, 138, 110, 69, 69, 46, 110,110 cm correspondingly) were appropriately arranged along the UV beamline. The spot size on the photocathode can be conveniently altered by changing the iris diameter.
4. Conclusions THz-rep-rate electron beam train which is suitable for coherent THz production can be realized by copper photocathode
RF gun illuminated by ps-spaced sub-ps UV pulse train. Pulse stacking method by birefringent crystal of a-BBO serials can be applied to directly shape ultraviolet laser pulse into such pulse train. Using four pieces of a-BBO crystals (with thickness 16, 8, 4, 2 mm in turn) to separate an input UV pulse with 0.5 ps of duration into 16 sub-pulses can form a ps-spaced pulse train with approximately equal amplitude. The group delay dispersion induced by the crystal serials does not play significant role in such cases. The temporal envelope of the shaped UV pulse can be measured single shot by deflecting cavity through electron beam after photocathode RF gun. The work is under way in our laboratory.
Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC) under Grant nos. 10975088, 10735050, 10704087 and 10805031 and by the National Basic Research Program of China (973 Program) under Grant no. 2007CB815102.
Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C.X. Tang, W.H. Huang, et al., Nucl. Instrum. Methods A 608 (2009) S70. R. K. Li, et al, Rev. Sci. Instrum. 80 (2009) 083303. A. Gover, Phys. Rev. ST. Accel. Beams 8 (2005) 03701. A. Gover, E. Dyunin, Y. Lurie, et al., Phys. Rev. ST Accel. Beams 8 (2005) 03702. J.G. Neumann, et al., Nucl. Instrum. Methods A 507 (2003) 498. M. Boscolo, M. Ferrario, I. Boscolo, et al., Nucl. Instrum. Methods A 577 (2007) 409. Y. Li, K.-J. Kim, Appl. Phys. Lett. 92 (2008) 014101. F. Verluise, V. Laude, Z. Cheng, et al., Opt. Lett. 25 (2000) 575. C.W. Siders, J.L.W. Siders, A.J. Taylor, et al., Appl. Opt. 37 (1998) 5302. H. Tomizawa, H. Dewa, H. Hanaki, et al., Quantum Electron. 37 (2007) 697. S. Zhou, D. Ouzounov, H. Li, et al., Appl. Opt. 46 (2007) 8488. B. Dromey, M. Zepf, M. Landreman, et al., Appl. Opt. 46 (2007) 5142. A.K. Sharma, T. Tsang, T. Rao, Phys. Rev. ST Accel. Beams 12 (2009) 033501. V.P. Solntsev, et al., J. Cryst. Growth 236 (2002) 290.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
UV pulse shaping for the photocathode RF gun Lixin Yan a,b,c,, Qiang Du a,b,c, Yingchao Du a,b,c, Jianfei Hua a,b,c, Wenhui Huang a,b,c, Chuanxiang Tang a,b,c a b c
Accelerator Laboratory, Department of Engineering Physics, Tsinghua University, Beijing 100084,China Key Laboratory of Particle and Radiation Imaging (Tsinghua University), Ministry of Education, Beijing 100084,China Key Laboratory of High Energy Radiation Imaging Fundamental Science for National Defense, Beijing 100084,China
a r t i c l e in fo
abstract
Available online 11 February 2010
Recently, manipulation with the drive laser plays a significant role in high brightness electron beam production by the photocathode RF gun. The article takes efforts on the temporal shaping of the driving laser for the photocathode RF gun. Method based on pulse stacking by birefringent crystal of a-BBO serials was tried to directly shape ultraviolet laser pulse. Using four pieces of a-BBO crystals to separate an input UV pulse with appropriate duration into 16 sub-pulses can form a ps-spaced pulse train suitable for coherent THz production. The group delay dispersion induced by the crystals was also carefully considered. To avoid beam deterioration by long path propagation, imaging relay of the shaped pulse was applied. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Photocathode RF gun Laser pulse shaping a-BBO crystal Group delay dispersion
1. Introduction High brightness electron beams based on photocathode RF gun are playing an increasingly significant role in many scientific fields such as XFEL, ILC, ultrafast electron diffraction, Thomson scattering based X-ray light sources and coherent THz production. Interest has been excited across the scientific community to do theoretical and experimental research work in this field. For the purpose to develop a multi-objective (X-ray, UED, THz, etc) scientific test facility, a photocathode RF gun based on copper has been set up at our laboratory. Experiments using the photoelectron beams to produce X-rays by Thomson scattering [1] and diffraction pattern by Mev ultrafast electron diffraction [2] have been performed. By virtue of two interesting features: the relativistic effect of single electron and the coherent effect of multiple electrons and relativistic ultrashort electron beams (o1 ps, tens of MeV) can produce high power coherent THz radiation. The THz modulated electron beams, or THz-repetition-rate ultrashort electron pulse trains exhibits further enhancement of the THz radiation [3,4]. How to achieve this kind of pre-bunched electron beams is the focus of many publications [5–7]. Due to the negligible emission time of the photocathode illuminated by the ultrashort drive laser pulses, the initial temporal distribution of the electron beam is the same as that of the laser pulse. Electron beams with arbitrary temporal profile may be achieved by laser pulse shaping. This article will focus on the laser pulse shaping method for UV pulse Corresponding author at: Accelerator laboratory, Department of engineering physics, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62795424. E-mail address: [email protected] (L. Yan).
train generation by the birefringent a-BBO crystal serials. THzrepetition-rate ultrashort electron pulse trains can not only be applied to radiate coherent THz radiation by mechanisms such as FEL, CSR, CTR, but also have important applications in timeresolved ultrafast electron diffraction, multi-pulse Thomson scattering and other areas. This shaping method of laser pulse by a-BBO crystals can also be applied to reduce the initial emittance of the electron beam from the photocathode RF gun.
2. Pulse shaping method In recent years, several kinds of shaping methods for ultrashort laser pulses have been developed for various applications, such as frequency domain shaping techniques (based on liquid crystals, acousto-optical crystals, or deformable mirrors, etc, represented by AOPDF [8]) for temporally flattop pulse production. These methods are mainly for infrared pulses and only suitable for low pulse energies. To produce modulated laser pulses, other methods have been proposed and tested, such as Neumann’s method by Fabry–Perot interferometer [5], pulse stacking by beamsplitter plates [9], or by polarizing beamsplitter cubes [10], or a-YVO4 crystals [11], or calcite crystals [12]. Several publications mentioned the possibilities of using a-BBO crystals, which can transmit UV light [10,13]. 2.1. Properties of a-BBO crystal The a-BBO crystal is a kind of negative, uniaxial crystal with a center of symmetry in its structure. It is not suitable for nonlinear optical processes, but its large birefringence over the broad
0168-9002/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.02.038
S128
L. Yan et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S127–S129
transparency range from 190 to 3500 nm makes it an excellent candidate material for linear optics [14]. The crystal is cut so that its extraordinary axis lies in its surface and perpendicular to the incident laser pulse. Its Sellmeier equations for ordinary (no) and extraordinary (ne) light are expressed as: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:01878 2 0:01354 l no ðlÞ ¼ 2:7471þ 2 l 0:01822 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:01224 2 0:01516 l ne ðlÞ ¼ 2:3174þ 2 l 0:01667
Fig. 3. The input UV pulse can be split into 16 sub-pulses with alternatively varied polarization by a-BBO crystal serials.
where l is the wavelength in micrometer. For the UV laser to drive copper photocathode RF gun at central wavelength 266.7 nm (triple of laser at wavelength 800 nm), no(l0) =1.758, ne(l0) =1.612. The birefringence is 0.146. So the initially coincided such two types of polarized light centered at wavelength 266.7 nm will have relative retardation t (in ps) after propagation in the crystal for length l (in mm), which satisfies l = ct/(no ne), where c =2.99792458 10 1 m/ps is the velocity of light in vacuum, as shown in Fig. 1. To make the delay time between ordinary light and extraordinary light to be 1 ps, the thickness of the crystal should be 2.058 mm (about 2 mm). 2.2. Pulse stacking by a-BBO crystal As described above, if linear polarized sub-ps UV laser pulse centered at 266.7 nm is normally incident on the left surface of aBBO crystal, with the angle between the polarization direction and extraordinary axis to be 451, then a-BBO crystal of about 2 mm in thickness will split the input pulse into two perpendicular polarized sub-pulses spacing 1 ps in time. Accordingly, suppose there is four pieces of a-BBO crystals with thickness L0/2n (n= 0, 1, 2, 3) placed successfully as depicted in Fig. 2. The extraordinary axis of the odd crystals is parallel with the original input pulse polarization direction, while the extraordinary axis of the even crystals is tuned to be 451 with that of adjacent crystals. Without consideration of the dispersion difference between ordinary and extraordinary light by the crystals, the crystal serials will produce such pulse train including 16 alternatively varied polarization sub-pulses with equal spacing and amplitude, shown in Fig. 3. Using such a-BBO crystal serials with L0 = 16, 8, 4, 2 mm, and initial input pulse with 0.5 ps duration (FWHM), without consideration of dispersion induced by the crystals, one can
t=l(no-ne)/c t l
Fig. 4. THz-rep-rate pulse train formation with 0.5 ps initial pulse by (16, 8, 4, 2) mm crystal serials.
Table 1 Parameters of sub-pulses through a-BBO crystals. Sub-pulse index
GDD (104 fs2)
Broadening (%)
oooo oooe ooeo ooee oeoo oeoe oeeo oeee eooo eooe eoeo eoee eeoo eeoe eeeo eeee
1.62 1.58 1.54 1.51 1.47 1.43 1.39 1.36 1.32 1.28 1.24 1.21 1.17 1.13 1.09 1.06
1.60 1.53 1.45 1.39 1.32 1.25 1.19 1.13 1.06 1.01 0.95 0.89 0.84 0.78 0.73 0.68
obtain pulse train depicted in Fig. 4. The time interval of the adjacent sub-pulses is about 1 ps, thus the THz-rep-rate pulse train with equal amplitude is formed. Because of the alternatively varied polarization, interference does not play considerable role in the pulse train formation, as can also be seen in Fig. 4.
Fig. 1. The principle of birefringent crystal: ordinary light (o) will be left behind of extraordinary light after it pass through a-BBO crystal.
2.3. The group delay dispersion effect
Fig. 2. schematic of four pieces of a-BBO (with thickness L0/2n , n= 0, 1, 2, 3) crystals.
For the propagation of ultrashort laser pulse in optical materials, the group delay dispersion effect always has to be considered, which may considerably change the shape of the initial pulse. The total length of the a-BBO crystals to form above mentioned UV pulse train is about 3 cm. Each sub-pulse undergoes different optical path throughout the crystal serials due to its different combination of light types with the four crystals. For example, the first sub-pulse to go through the crystal serials can be denoted as ‘eeee’, for it to pass all the four pieces of crystals as extraordinary light. The second sub-pulse is then denoted as
L. Yan et al. / Nuclear Instruments and Methods in Physics Research A 637 (2011) S127–S129
Iris f1
f2
f3
f4
f5
f6
f7
f8
f9
S129
f10
PC
Fig. 5. Schematic of imaging relay from the exit of a-BBO serials to the photocathode (PC).
‘eeeo’, for it to pass the first three crystals as extraordinary light and only for the fourth crystal as ordinary light. In this manner, the 16 sub-pulses can be denoted as oooo, oooe, ooeo, ooee, oeoo, oeoe, oeeo, oeee, eooo, eooe, eoeo, eoee, eeoo, eeoe, eeeo and eeee. So using the parameters of a-BBO crystals at wavelength 266.7 nm, the group delay dispersion of each sub-pulse can be calculated. The second derivatives of index for a-BBO crystals at wavelength 266.7 nm are: 00
b0 ¼ 0:5394 mm1 fs2 ;
00
be ¼ 0:3524 mm1 fs2
For initial transform limited input pulse with 0.5 ps pulse duration (FWHM), the GDD’s and pulse width broadening of each sub-pulse are listed in Table 1. From Table 1, we can see that no significant pulse broadening was induced by such crystal serials and no considerable differences between the sub-pulses, so the stacked pulse shape does not differ much with that of Fig. 4. But further calculations show that for initial pulse width less than 100 fs, the pulse broadening becomes inacceptable. For such cases, measures have to be taken to compensate the broadening effect induced by the inserted crystals.
3. Imaging relay To transport the UV pulse to the photocathode located about 20 m away from the exit of the THG (third harmonic generator) where fixed the a-BBO crystals, the imaging relay technique was applied to limit the diffraction effect by light propagation and the walk-off effect by the birefringent crystal serials. As Fig. 5 shows, five imaging relay units including 10 pieces of lens (f1 f10, whose focal lengths were 92, 92, 138, 138, 110, 69, 69, 46, 110,110 cm correspondingly) were appropriately arranged along the UV beamline. The spot size on the photocathode can be conveniently altered by changing the iris diameter.
4. Conclusions THz-rep-rate electron beam train which is suitable for coherent THz production can be realized by copper photocathode
RF gun illuminated by ps-spaced sub-ps UV pulse train. Pulse stacking method by birefringent crystal of a-BBO serials can be applied to directly shape ultraviolet laser pulse into such pulse train. Using four pieces of a-BBO crystals (with thickness 16, 8, 4, 2 mm in turn) to separate an input UV pulse with 0.5 ps of duration into 16 sub-pulses can form a ps-spaced pulse train with approximately equal amplitude. The group delay dispersion induced by the crystal serials does not play significant role in such cases. The temporal envelope of the shaped UV pulse can be measured single shot by deflecting cavity through electron beam after photocathode RF gun. The work is under way in our laboratory.
Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC) under Grant nos. 10975088, 10735050, 10704087 and 10805031 and by the National Basic Research Program of China (973 Program) under Grant no. 2007CB815102.
Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C.X. Tang, W.H. Huang, et al., Nucl. Instrum. Methods A 608 (2009) S70. R. K. Li, et al, Rev. Sci. Instrum. 80 (2009) 083303. A. Gover, Phys. Rev. ST. Accel. Beams 8 (2005) 03701. A. Gover, E. Dyunin, Y. Lurie, et al., Phys. Rev. ST Accel. Beams 8 (2005) 03702. J.G. Neumann, et al., Nucl. Instrum. Methods A 507 (2003) 498. M. Boscolo, M. Ferrario, I. Boscolo, et al., Nucl. Instrum. Methods A 577 (2007) 409. Y. Li, K.-J. Kim, Appl. Phys. Lett. 92 (2008) 014101. F. Verluise, V. Laude, Z. Cheng, et al., Opt. Lett. 25 (2000) 575. C.W. Siders, J.L.W. Siders, A.J. Taylor, et al., Appl. Opt. 37 (1998) 5302. H. Tomizawa, H. Dewa, H. Hanaki, et al., Quantum Electron. 37 (2007) 697. S. Zhou, D. Ouzounov, H. Li, et al., Appl. Opt. 46 (2007) 8488. B. Dromey, M. Zepf, M. Landreman, et al., Appl. Opt. 46 (2007) 5142. A.K. Sharma, T. Tsang, T. Rao, Phys. Rev. ST Accel. Beams 12 (2009) 033501. V.P. Solntsev, et al., J. Cryst. Growth 236 (2002) 290.