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Ultra short electron beam bunches from a laser plasma cathode
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 261 (2007) 5–8 www.elsevier.com/locate/nimb
Ultra short electron beam bunches from a laser plasma cathode Akira Maekawa a,*, Ryosuke Tsujii a, Kennichi Kinoshita a, Yamazaki Atsushi a, Kazuyuki Kobayashi a, Mitsuru Uesaka a, Yukio Shibata a, Yasuhiro Kondo a, Takeru Ohkubo b, Tomonao Hosokai c, Alexei Zhidkov d, Toshiharu Takahashi e b
a Nuclear Professional School, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188, Japan Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma, Japan c Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan d Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa, Japan e Kyoto University Research Reactor Institute, Asahiro-nishi2, Kumatori, Sennan, Osaka, Japan
Available online 14 April 2007
Abstract The fluctuation of the electron bunch duration due to energy spectrum instability in a laser plasma cathode has been examined. Previous experiments clearly proved that a laser plasma cathode can generate ultrashort electron bunches with a bunch duration of 130 fs (FWHM) and a geometrical emittance 0.07p mm mrad. The effect of temporal elongation of electron bunches due to their energy spread is estimated and the results are in good agreement with previous experiments. It is also clarified that the instability of the energy spectrum not only leads to a fluctuation of the bunch shape but also to a time-of-flight jitter, affecting possible future applications of a laser plasma cathode. 2007 Elsevier B.V. All rights reserved. PACS: 52.38.Kd; 41.60. m; 41.75.Ht Keywords: Laser plasma cathode; Femtosecond bunch duration; Transition radiation
1. Introduction A laser plasma cathode is one of the most promising approaches to generate ultrashort electron bunches. The bunch duration of the electron bunch at its origin is expected to be on the order of 10 fs, owing to the high frequency of plasma waves. Hence a laser plasma cathode has the great advantage of femtosecond time-resolved applications, such as an ultrafast beam based pump-and-probe analysis [1] and the generation of a femtosecond X-ray pulse by relativistic Thomson scattering. These future applications require the measurement and the evaluation
*
Corresponding author. Tel.: +81 29 287 8413; fax: +81 29 287 8488. E-mail address: [email protected] (A. Maekawa).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.04.081
of femtosecond electron bunches. Several approaches for the measurement have been proposed [2–4], e.g. the CTR (Coherent Transition Radiation) Michelson Interferometer, the FIR (Far-Infrared) polychromator, E/O (electrooptic) method and fluctuation method. At the Nuclear Professional School, University of Tokyo, we have performed bunch duration measurements for electrons emitted by a laser plasma cathode using CTR spectrum analysis, resulting in a bunch duration time of 130 fs (FWHM) [5,6]. In that experiment it was also shown that the electron bunch elongates temporally due to the energy spread of the electron bunch. Furthermore, the bunch elongation changes shot by shot because of a shot-to-shot instability of the energy spectrum of the electron bunch. In this paper, we estimate the fluctuation of the bunch elongation and of the time-of-flight and their influence on future applications of a laser plasma cathode.
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A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
2. Bunch duration measurement When a charged particle crosses the boundary between two media with different dielectric constants, transition radiation is emitted from the boundary [4]. The radiation from an electron bunch can be expressed by superposition of the radiation from each electron in the electron bunch. The total intensity of the radiation from an electron bunch is obtained by Itotal = NIe + N(N 1)f(k)Ie, where Ie is the intensity of the radiation from one electron, N the number of electrons in the electron bunch and f(k) the bunch form factor. When the wavelength of the radiation is longer than the bunch length, the radiation becomes coherent; the bunch form factor becomes unity and the total intensity of the radiation is proportional to the square of N. One can derive the longitudinal shape of the electron bunch by the spectrum analysis of the coherent radiation. In previous experiments [5,6], we have obtained the average spectrum of the CTR using a bolometer with several band-pass filters. The experimental setup is shown in Fig. 1. The Ti:Sapphire laser system based on the chirped pulse amplification (CPA) technique generates an ultrashort intense laser pulse with an energy up to 600 mJ and a pulse duration of 38 fs (FWHM). The laser pulse is focused by a f/3.5 off-axis parabolic mirror (OAP) into a helium gas jet. The gas pressure is operated at 35 atm and the corresponding electron density is 4 · 1019 cm 3. The focal spot size is 6 lm at 1/e2 in intensity and therefore the laser intensity is estimated to be 3.5 · 1019 W/cm2. A titanium foil with a thickness of 300 lm is placed 180 mm downstream from the gas jet. The transition radiation emitted from the Ti-foil is delivered into the bolometer with OAP. Fig. 2 shows the result of the spectrum analysis of the CTR and the average bunch duration is estimated to be 130 ± 30 fs (FWHM) at the Ti-foil (see details in [5]). 3. Bunch elongation effect The electron bunch elongates temporally because of the energy spread of the electron beam, while propagating the
Fig. 1. Experimental setup for the bunch duration measurement [6]. Transition radiation emitted from the 300 lm Ti-foil is delivered into the bolometer and the spectrum of the radiation is obtained.
Fig. 2. Spectra of the transition radiation measured by a bolometer (see also [5,6]). The circles show averaged signals of the transition radiation. The calculated spectra of the transition radiation for several bunch duration times are also inserted.
distance of 180 mm between the gas jet and the titanium foil. In a laser plasma cathode, although the generation of a quasi-monoenergetic electron beam has been reported recently from several laboratories [7,8], the energy spread and the reproducibility are still not enough compared to an RF linac. Fig. 3 shows the energy spectra of the electron bunches obtained in the previous experiments. A quasimonoenergetic electron beam with a mean energy of E 20 MeV and an energy spread of DE/E 0.2 is observed (Fig. 3(a)). However, a less monoenergetic electron beam and sometimes even an electron beam with Maxwell-like energy spectra are also generated (Fig. 3(b) and (c), respectively). This shot-to-shot instability of the energy spectrum leads to a fluctuation of the bunch elongation. To evaluate the bunch elongation effect, we first assume that the initial bunch shape at the plasma edge is a Gaussian distribution with the bunch duration of 30 fs (FWHM) based on our PIC simulation [9]. The energy spectrum of the electron beam is the one obtained in the previous experiments and the energy distribution in the electron bunch is assumed to be uniform. With these assumptions, the bunch shape at the Ti-foil is calculated as shown in Fig. 4. The bunch shape in Fig. 4(a)–(c) corresponds to the energy spectrum in Fig. 3(a)–(c), respectively. The quasi-monoenergetic electron bunches (Fig. 4(a) and (b)) elongate to 110– 250 fs (FWHM) and the electron bunch with a Maxwell distribution (Fig. 4(c)) elongates to more than 2 ps (FWHM) at the Ti-foil. In the previous experiments, the spectrum of the CTR is obtained at a wavelength range from 30 lm to 400 lm. Therefore it is supposed that we have measured the CTR only from the quasi-monoenergetic electron bunch in the previous experiments, since the electron bunch elongates too long to detect the CTR when the energy spectrum is a Maxwell distribution. Even in the case of quasi-monoenergetic electron bunches, the instability of the energy spectrum results in a fluctuation of the bunch shape at the Ti-foil and of the
A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
7
Fig. 3. Spatial distributions of the electrons deflected by the spectrometer (left) and the corresponding energy spectra (right).
Fig. 4. Estimated bunch shapes at the Ti-foil placed 180 mm downstream from the gas jet. Each bunch shape, (a)–(c), corresponds to the energy spectrum in Fig. 3(a)–(c), respectively. The horizontal axis indicates the electron bunch propagation time between the gas jet and the Ti-foil.
time-of-flight from the gas jet to the Ti-foil. It is clearly seen that the bunch duration of 130 fs obtained in the previous experiments is the average bunch duration of the fluctuating electron bunches. The difference of the time-of-flight is 105 fs between two monoenergetic electron bunches (Fig. 4(a) and (b)), while that is 1.12 ps between two beams of Fig. 4(a) and (c). These fluctuations will become an error factor, such as a time-of-flight jitter, in future applications of a laser plasma cathode. 4. TOF jitter in future applications Fig. 5 shows the schematic diagram of a future plan of a pulseradiolysis based on a laser plasma cathode. In this scheme, the timing between the electron bunch (pump pulse) and the laser pulse (probe pulse) is passively synchronized, because a single laser pulse is divided into two
Fig. 5. Schematic diagram of a future plan of a pulseradiolysis based on a laser plasma cathode. Single laser pulse is divided into two pulses by a beam splitter and generates both the electron bunch (pump pulse) and the laser pulse (probe pulse).
pulses by a beam splitter and generates both the electron bunch and the laser pulse. Hence a laser plasma cathode has intrinsically no jitter originating from the synchronization electric devices. By the estimation as described above, however, a laser plasma cathode has also a time-of-flight jitter due to the instability of the energy spectrum and this jitter, as well as the fluctuation of the bunch duration, will reduce its time-resolution. Nevertheless it is still sure that a laser plasma cathode is the promising approach for ultrafast time-resolved applications. It is because the fluctuation of the order of 100 fs due to the instability of energy spectrum is less than the synchronization jitter in a conventional linear accelerator. Actually in the S-band linear accelerator at University of Tokyo the jitter in short period is 340 fs (rms) [10]. For the more precise time-resolution, the distance between the beam source and the target is critical in practical applications. For example, the bunch duration and the difference of time-of-flight in Fig. 4 will decrease to 65–130 fs and to 40 fs, respectively when the source-sample distance is reduced from 180 mm to 50 mm.
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A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
5. Conclusion We have examined the temporal elongation of the electron bunch due to the energy spread. The quasi-monoenergetic electron bunch is estimated to extend to 110–250 fs (FWHM) while propagating the distance of 180 mm and this estimation is consistent with previous experimental results [5,6]. The shot-to-shot instability of the energy spectrum leads the fluctuation of the bunch shape and of the time-of-flight. As a result, it is shown that a laser plasma cathode has the time-of-flight jitter depending on the source-sample distance to be taken into account in future applications. As the next step, we will perform a single-shot measurement using a FIR polychromator and evaluate more precisely the bunch shape and its fluctuation. We
hope that such continuous bunch duration measurements are going to realize the ultrafast time-resolved analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M. Uesaka et al., Nucl. Instr. and Meth. B 241 (2005) 880. T. Watanabe et al., Nucl. Instr. and Meth. A 480 (2002) 315. Y. Shibata et al., Phys. Rev. E. 50 (1994) 1479. J. Tilborg et al., Phys. Rev. Lett. 96 (2006) 014801. T. Ohkubo et al., Phys. Rev. ST-AB 10 (2007) 031301. A. Maekawa, et al., in: Proceedings of EPAC2006, 2006, p. 2451. S. Mangles et al., Nature 431 (2004) 535. T. Hosokai et al., Phys. Rev. E. 73 (2006) 036407. A. Zhidkov et al., Phys. Plasmas. 11 (2004) 5379. A. Sakumi, et al., in: Proceedings of PAC2005, 2005, p. 3079.
Nuclear Instruments and Methods in Physics Research B 261 (2007) 5–8 www.elsevier.com/locate/nimb
Ultra short electron beam bunches from a laser plasma cathode Akira Maekawa a,*, Ryosuke Tsujii a, Kennichi Kinoshita a, Yamazaki Atsushi a, Kazuyuki Kobayashi a, Mitsuru Uesaka a, Yukio Shibata a, Yasuhiro Kondo a, Takeru Ohkubo b, Tomonao Hosokai c, Alexei Zhidkov d, Toshiharu Takahashi e b
a Nuclear Professional School, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188, Japan Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma, Japan c Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan d Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa, Japan e Kyoto University Research Reactor Institute, Asahiro-nishi2, Kumatori, Sennan, Osaka, Japan
Available online 14 April 2007
Abstract The fluctuation of the electron bunch duration due to energy spectrum instability in a laser plasma cathode has been examined. Previous experiments clearly proved that a laser plasma cathode can generate ultrashort electron bunches with a bunch duration of 130 fs (FWHM) and a geometrical emittance 0.07p mm mrad. The effect of temporal elongation of electron bunches due to their energy spread is estimated and the results are in good agreement with previous experiments. It is also clarified that the instability of the energy spectrum not only leads to a fluctuation of the bunch shape but also to a time-of-flight jitter, affecting possible future applications of a laser plasma cathode. 2007 Elsevier B.V. All rights reserved. PACS: 52.38.Kd; 41.60. m; 41.75.Ht Keywords: Laser plasma cathode; Femtosecond bunch duration; Transition radiation
1. Introduction A laser plasma cathode is one of the most promising approaches to generate ultrashort electron bunches. The bunch duration of the electron bunch at its origin is expected to be on the order of 10 fs, owing to the high frequency of plasma waves. Hence a laser plasma cathode has the great advantage of femtosecond time-resolved applications, such as an ultrafast beam based pump-and-probe analysis [1] and the generation of a femtosecond X-ray pulse by relativistic Thomson scattering. These future applications require the measurement and the evaluation
*
Corresponding author. Tel.: +81 29 287 8413; fax: +81 29 287 8488. E-mail address: [email protected] (A. Maekawa).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.04.081
of femtosecond electron bunches. Several approaches for the measurement have been proposed [2–4], e.g. the CTR (Coherent Transition Radiation) Michelson Interferometer, the FIR (Far-Infrared) polychromator, E/O (electrooptic) method and fluctuation method. At the Nuclear Professional School, University of Tokyo, we have performed bunch duration measurements for electrons emitted by a laser plasma cathode using CTR spectrum analysis, resulting in a bunch duration time of 130 fs (FWHM) [5,6]. In that experiment it was also shown that the electron bunch elongates temporally due to the energy spread of the electron bunch. Furthermore, the bunch elongation changes shot by shot because of a shot-to-shot instability of the energy spectrum of the electron bunch. In this paper, we estimate the fluctuation of the bunch elongation and of the time-of-flight and their influence on future applications of a laser plasma cathode.
6
A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
2. Bunch duration measurement When a charged particle crosses the boundary between two media with different dielectric constants, transition radiation is emitted from the boundary [4]. The radiation from an electron bunch can be expressed by superposition of the radiation from each electron in the electron bunch. The total intensity of the radiation from an electron bunch is obtained by Itotal = NIe + N(N 1)f(k)Ie, where Ie is the intensity of the radiation from one electron, N the number of electrons in the electron bunch and f(k) the bunch form factor. When the wavelength of the radiation is longer than the bunch length, the radiation becomes coherent; the bunch form factor becomes unity and the total intensity of the radiation is proportional to the square of N. One can derive the longitudinal shape of the electron bunch by the spectrum analysis of the coherent radiation. In previous experiments [5,6], we have obtained the average spectrum of the CTR using a bolometer with several band-pass filters. The experimental setup is shown in Fig. 1. The Ti:Sapphire laser system based on the chirped pulse amplification (CPA) technique generates an ultrashort intense laser pulse with an energy up to 600 mJ and a pulse duration of 38 fs (FWHM). The laser pulse is focused by a f/3.5 off-axis parabolic mirror (OAP) into a helium gas jet. The gas pressure is operated at 35 atm and the corresponding electron density is 4 · 1019 cm 3. The focal spot size is 6 lm at 1/e2 in intensity and therefore the laser intensity is estimated to be 3.5 · 1019 W/cm2. A titanium foil with a thickness of 300 lm is placed 180 mm downstream from the gas jet. The transition radiation emitted from the Ti-foil is delivered into the bolometer with OAP. Fig. 2 shows the result of the spectrum analysis of the CTR and the average bunch duration is estimated to be 130 ± 30 fs (FWHM) at the Ti-foil (see details in [5]). 3. Bunch elongation effect The electron bunch elongates temporally because of the energy spread of the electron beam, while propagating the
Fig. 1. Experimental setup for the bunch duration measurement [6]. Transition radiation emitted from the 300 lm Ti-foil is delivered into the bolometer and the spectrum of the radiation is obtained.
Fig. 2. Spectra of the transition radiation measured by a bolometer (see also [5,6]). The circles show averaged signals of the transition radiation. The calculated spectra of the transition radiation for several bunch duration times are also inserted.
distance of 180 mm between the gas jet and the titanium foil. In a laser plasma cathode, although the generation of a quasi-monoenergetic electron beam has been reported recently from several laboratories [7,8], the energy spread and the reproducibility are still not enough compared to an RF linac. Fig. 3 shows the energy spectra of the electron bunches obtained in the previous experiments. A quasimonoenergetic electron beam with a mean energy of E 20 MeV and an energy spread of DE/E 0.2 is observed (Fig. 3(a)). However, a less monoenergetic electron beam and sometimes even an electron beam with Maxwell-like energy spectra are also generated (Fig. 3(b) and (c), respectively). This shot-to-shot instability of the energy spectrum leads to a fluctuation of the bunch elongation. To evaluate the bunch elongation effect, we first assume that the initial bunch shape at the plasma edge is a Gaussian distribution with the bunch duration of 30 fs (FWHM) based on our PIC simulation [9]. The energy spectrum of the electron beam is the one obtained in the previous experiments and the energy distribution in the electron bunch is assumed to be uniform. With these assumptions, the bunch shape at the Ti-foil is calculated as shown in Fig. 4. The bunch shape in Fig. 4(a)–(c) corresponds to the energy spectrum in Fig. 3(a)–(c), respectively. The quasi-monoenergetic electron bunches (Fig. 4(a) and (b)) elongate to 110– 250 fs (FWHM) and the electron bunch with a Maxwell distribution (Fig. 4(c)) elongates to more than 2 ps (FWHM) at the Ti-foil. In the previous experiments, the spectrum of the CTR is obtained at a wavelength range from 30 lm to 400 lm. Therefore it is supposed that we have measured the CTR only from the quasi-monoenergetic electron bunch in the previous experiments, since the electron bunch elongates too long to detect the CTR when the energy spectrum is a Maxwell distribution. Even in the case of quasi-monoenergetic electron bunches, the instability of the energy spectrum results in a fluctuation of the bunch shape at the Ti-foil and of the
A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
7
Fig. 3. Spatial distributions of the electrons deflected by the spectrometer (left) and the corresponding energy spectra (right).
Fig. 4. Estimated bunch shapes at the Ti-foil placed 180 mm downstream from the gas jet. Each bunch shape, (a)–(c), corresponds to the energy spectrum in Fig. 3(a)–(c), respectively. The horizontal axis indicates the electron bunch propagation time between the gas jet and the Ti-foil.
time-of-flight from the gas jet to the Ti-foil. It is clearly seen that the bunch duration of 130 fs obtained in the previous experiments is the average bunch duration of the fluctuating electron bunches. The difference of the time-of-flight is 105 fs between two monoenergetic electron bunches (Fig. 4(a) and (b)), while that is 1.12 ps between two beams of Fig. 4(a) and (c). These fluctuations will become an error factor, such as a time-of-flight jitter, in future applications of a laser plasma cathode. 4. TOF jitter in future applications Fig. 5 shows the schematic diagram of a future plan of a pulseradiolysis based on a laser plasma cathode. In this scheme, the timing between the electron bunch (pump pulse) and the laser pulse (probe pulse) is passively synchronized, because a single laser pulse is divided into two
Fig. 5. Schematic diagram of a future plan of a pulseradiolysis based on a laser plasma cathode. Single laser pulse is divided into two pulses by a beam splitter and generates both the electron bunch (pump pulse) and the laser pulse (probe pulse).
pulses by a beam splitter and generates both the electron bunch and the laser pulse. Hence a laser plasma cathode has intrinsically no jitter originating from the synchronization electric devices. By the estimation as described above, however, a laser plasma cathode has also a time-of-flight jitter due to the instability of the energy spectrum and this jitter, as well as the fluctuation of the bunch duration, will reduce its time-resolution. Nevertheless it is still sure that a laser plasma cathode is the promising approach for ultrafast time-resolved applications. It is because the fluctuation of the order of 100 fs due to the instability of energy spectrum is less than the synchronization jitter in a conventional linear accelerator. Actually in the S-band linear accelerator at University of Tokyo the jitter in short period is 340 fs (rms) [10]. For the more precise time-resolution, the distance between the beam source and the target is critical in practical applications. For example, the bunch duration and the difference of time-of-flight in Fig. 4 will decrease to 65–130 fs and to 40 fs, respectively when the source-sample distance is reduced from 180 mm to 50 mm.
8
A. Maekawa et al. / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 5–8
5. Conclusion We have examined the temporal elongation of the electron bunch due to the energy spread. The quasi-monoenergetic electron bunch is estimated to extend to 110–250 fs (FWHM) while propagating the distance of 180 mm and this estimation is consistent with previous experimental results [5,6]. The shot-to-shot instability of the energy spectrum leads the fluctuation of the bunch shape and of the time-of-flight. As a result, it is shown that a laser plasma cathode has the time-of-flight jitter depending on the source-sample distance to be taken into account in future applications. As the next step, we will perform a single-shot measurement using a FIR polychromator and evaluate more precisely the bunch shape and its fluctuation. We
hope that such continuous bunch duration measurements are going to realize the ultrafast time-resolved analysis. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M. Uesaka et al., Nucl. Instr. and Meth. B 241 (2005) 880. T. Watanabe et al., Nucl. Instr. and Meth. A 480 (2002) 315. Y. Shibata et al., Phys. Rev. E. 50 (1994) 1479. J. Tilborg et al., Phys. Rev. Lett. 96 (2006) 014801. T. Ohkubo et al., Phys. Rev. ST-AB 10 (2007) 031301. A. Maekawa, et al., in: Proceedings of EPAC2006, 2006, p. 2451. S. Mangles et al., Nature 431 (2004) 535. T. Hosokai et al., Phys. Rev. E. 73 (2006) 036407. A. Zhidkov et al., Phys. Plasmas. 11 (2004) 5379. A. Sakumi, et al., in: Proceedings of PAC2005, 2005, p. 3079.