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Cu Kα pulse generation in an X-ray tube with a plasma cathode induced by a femtosecond laser pulse
ARTICLE IN PRESS Radiation Physics and Chemistry 78 (2009) 375–379
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Cu Ka pulse generation in an X-ray tube with a plasma cathode induced by a femtosecond laser pulse H. Yamada a,, H. Murakami a,b, Y. Shimada a a b
Advanced Photon Research Center, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan Research Unit for Quantum Beam Life Science Initiative, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan
a r t i c l e in fo
abstract
Article history: Received 11 December 2008 Accepted 24 March 2009
An X-ray tube has been developed with a femtosecond laser-produced plasma at a Cu cathode by the acceleration of the plasma electrons onto a Cu anode. The source provided a Cu Ka pulse with 109 photons/4p and ffi100 ns width. Spectral observations of visible emissions from the cathode have revealed that the excitation of the plasma occurs on the same time scale as the X-ray-pulse generation, indicating that the electrostatic shielding of the plasma gradually breaks under the applied electric field. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Pulsed X-ray source Cu Ka Laser-plasma cathode Plasma emission spectroscopy Electrostatic shielding effect
1. Introduction There has been much attention paid to hard-X-ray pulse generation using a laser and the application to the study of various dynamic phenomena. A plasma X-ray source generated by an intense femtosecond laser has shown excellent performance in time-resolved measurements of X-ray diffraction (Siders et al., 1999) and X-ray absorption fine structure studies (Lee et al., 2005; Chen et al., 2007). However, such an X-ray source requires a highpower, typically a terawatt laser system which requires much space and cost. A subpicosecond X-ray pulse generated with 30 fs and 75 mJ laser pulses shows a brilliance of 109 photons/4p/pulse in generation of Cu Ka (Guo et al., 1997). Some recent research has been focused on plasma X-ray sources with laser pulses of submillijoule energies (Kutzner et al., 2004; Serbanescu et al., 2007), although the available X-ray photon number is relatively small, e.g., 107 photons/4p/pulse of Cu Ka with 0.3 mJ laser pulses (Serbanescu et al., 2007). A laser-driven X-ray tube offers promise in terms of its compactness and cost performance. In the source described in the present work, a conventional X-ray tube and a commercial pulsed laser are combined. Employing the photoelectric effect, an electron bunch is induced by a laser pulse at the cathode and accelerated by a high-voltage between the electrodes, generating
Corresponding author. Present address: Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto, 619-0292, Japan. Tel.: +81774 75 2305; fax: +81774 75 2318. E-mail address: [email protected] (H. Yamada).
0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.03.069
an X-ray pulse at the anode. Tomov et al. have developed a laserdriven X-ray tube mainly with a 193 nm picosecond laser pulse, and observed the laser-induced deformation of a Au crystal by time-resolved X-ray diffraction measurements using 8 ps Cu Ka X-ray pulses (Tomov et al., 1995; Tomov and Rentzepis, 2004). Hinze et al. have investigated the dependence of the X-ray pulse duration on the applied voltage and the laser pulse energy, and have achieved 3 ps duration Cu Ka X-ray pulses in an X-ray tube driven with 780 nm femtosecond laser pulses (Hinze et al., 2004). The widespread application of the laser-driven X-ray tube, however, has been hampered primarily by its low brilliance (105–106 photons/4p/pulse) owing to the quantum efficiency of the photoelectric effect at the cathode. Egbert et al. have found that an X-ray tube with plasma cathode promises outstanding performance in terms of average X-ray power (Egbert et al., 2001, 2002). They developed an X-ray tube with a steel cathode and a W anode, and produced the plasma at the cathode as an electron source, by irradiating a 780 nm femtosecond laser pulse with a fluence above the ablation threshold of the metal. As a result, they obtained the X-ray dose rate ffi104 times higher than that with the photocathode utilizing the photoelectric effect. Although they have pointed out that the duration of the electron pulse produced by the plasma cathode can by no means be in the subpicosecond range, because the duration is determined by a much longer hydrodynamic plasma expansion time (Egbert et al., 2002), the temporal profiles of the X-ray radiation have not been measured in their study. An X-ray tube with an anode of 3d-transition-metal predominantly provides its characteristic Ka X-rays (Egbert and Chichkov, 2003). Such a quasi-monochromatic X-ray source is useful for
ARTICLE IN PRESS H. Yamada et al. / Radiation Physics and Chemistry 78 (2009) 375–379
radiographic measurements (Sato et al., 2006) and also to efficiently induce X-ray fluorescence of specific elements. With these motivations, we have developed a laser-driven X-ray tube with Cu electrodes as a source of Cu Ka X-ray pulses aimed at a time-resolved X-ray fluorescence spectroscopy of Co, Fe, or Mn compounds. In this paper, we report the absolute photon number, spectrum, and temporal profile of the X-ray pulse from this X-ray tube, in which the difference between a photocathode and the plasma cathode is clearly shown by employing an ultraviolet femtosecond laser pulse and varying the laser pulse energy. Further, we analyze the plasma generated at the cathode by visible emission spectroscopy, including measurements of the time-resolved emission.
2. Experimental A schematic of a laser-driven X-ray tube is depicted in Fig. 1. A Cu anode with a target angle of 301 was fixed at 10 mm from a Cu columnar cathode with a diameter of 5 mm. Electrons produced at the cathode were accelerated to the anode by an applied voltage up to 50 kV with a power supply of a commercial X-ray generator (50 W). The X-ray tube was connected to a turbomolecular pump producing a pressure in the tube of 104–105 Pa. A femtosecond laser pulse (p2 mJ, 100 fs, 800 nm, 10 Hz) from a Ti:sapphire laser system (Clark-MXR, CPA-10) was used to generate its third harmonic (267 nm). The ultraviolet (UV) pulse energy was varied in the range from 0.5 to 70 mJ. The UV femtosecond pulses were focused on the Cu cathode in the X-ray tube by a fused silica lens with 20 cm focal length. An X-ray spectrometer was composed of a 3.5 mm thick LiF(2 0 0) crystal (SAINT-GOBAIN) and an X-ray charge-coupled device (CCD) sensor cooled with a liquid nitrogen (E2V Technologies, CCD05-30). The X-ray radiation through a Be window of the tube was dispersed by the crystal with an incident angle of 50.11, and the (4 0 0) diffraction lines were simultaneously detected by the CCD with an accumulation time of 60 s. In the dispersive method where an entire spectrum of interest is recorded simultaneously, fluctuations in the incident X-ray intensity do not influence the quality of the spectrum (Tomov and Rentzepis, 2004). The energy resolution of 6 eV at full-width at halfmaximum (FWHM) was achieved by setting a Ta slit in front of the Be window of the X-ray tube. The X-ray intensity from the plasma-cathode tube was measured by an X-ray streak camera system (HAMAMATSU, C4575-01). The absolute photon number of the X-ray pulse was obtained by using the values of the conversion efficiency of the
Laser pulse
Window (quartz)
Lens
streak camera from X-ray photons at 8 keV into electrical signals on the product data sheet, the area of its Au photocathode irradiated by the X-rays, and the distance between the Au photocathode and the X-ray source. The relative intensities of X-ray pulses were measured by using a scintillation crystal, NaI(Tl) (25 mm diameter, 2 mm thick) with a fast time-response (0.8 ns) photomultiplier (HAMAMATSU, R7400-03) and evaluated by the total photocurrent per pulse. The detection solid angle was changed to prevent saturation of the photocurrent by adjusting the position of the scintillator and the width of a Ta slit in front of the scintillator. The pulsed signal from the photomultiplier was fed into a digital oscilloscope (Tektronix, TDS380) and averaged over 256 times. As for time-resolved measurements of X-ray pulses, a BaF2 crystal (25 mm diameter, 5 mm thick) with a 10 nm bandpass filter centered at 220 nm was used instead of NaI(Tl). The scintillation light of BaF2 crystals at 220 nm decays with a time constant of 600 ps. A resolution of the temporal characterization of X-ray pulses was about 2 ns (FWHM) in the measurement with the BaF2 scintillator, the photomultiplier, and the oscilloscope. Visible light emission from the laser focus spot on the cathode was measured by a spectrograph (Acton Research, SpectraPro300i) equipped with a CCD (Roper Scientific, LN/CCD-1340/400) with an accumulation time of 5 s through a glass view port of the X-ray tube. The time evolution of the line emission was monitored by a combination of a monochromator (JYHoriba, H10UV) with the photomultiplier and the oscilloscope.
3. Results and discussion 3.1. Enhancement of X-ray generation due to the plasma cathode Fig. 2 shows that the intensity of the produced X-ray pulse changes drastically around the laser fluence of 0.2 J/cm2. Below this threshold, the X-ray pulse intensity varies linearly with the laser pulse energy; this is due to single-photon photoelectric effect, because the UV light possesses the energy exceeding the
Acceleration potential:
109
50 kV 30 kV
X-ray intensity [photon/4π/pulse]
376
108
107
106
105
Electrodes (Cu) ≅ 50 kV
104
0.2 J/cm2
Electrons 1
Vacuum chamber Window (Be) X-ray pulse Fig. 1. A schematic of laser-driven X-ray tube.
10 Laser energy [μJ/pulse]
100
Fig. 2. Dependence of the X-ray pulse intensity on the laser pulse energy. The focus spot size on the cathode is ffi0.1 mm in diameter. The dashed line indicates the ablation threshold for Cu.
ARTICLE IN PRESS H. Yamada et al. / Radiation Physics and Chemistry 78 (2009) 375–379
3.2. Spectral and temporal profiles of the X-ray pulse As for the spectrum of the X-rays thus generated, it is found from Fig. 3 that the X-rays are predominantly Cu Ka in the plasma-cathode regime. Studies using laser-driven X-ray tubes have shown that this is also the case in the photocathode regime
Intensity [arb.unit]
1.0
CuKα1
0.5
CuKα2
↓ 30 kV 50 kV
0.0 7.9
8.0
8.1
8.2
X-ray energy [keV] Fig. 3. Spectra of the X-ray radiation from the laser-driven X-ray tube with 0.76 J/ cm2 laser fluence (plasma-cathode regime, 60 mJ/pulse, focus spot size of 0.1 mm) at the applied voltages of 30 and 50 kV.
0.14 J/cm2, 50 kV
1.0
Response function
0.5 X-ray intensity [arb.units]
work function of Cu. By defocusing the laser beam, we have confirmed that the threshold is not determined by the laser pulse energy, but by the laser fluence. The threshold value is in agreement with that observed in the femtosecond laser ablation of Cu (Nolte et al., 1997; Hashida et al., 2002). Since a plasma is also produced at the cathode by the irradiation of the laser with fluences above the ablation threshold and can also serve as an electron source in the X-ray tube, it is reasonable that the plasma is source of the remarkable X-ray pulse intensity enhancement compared with that due to the photoelectric effect. Additionally, the X-ray pulse intensity in the plasma-cathode regime is independent of the laser fluence. This indicates that the current reaches its maximum determined by the characteristics of the electric circuit of the system consisting of the X-ray tube and the DC power supply, e.g., the space–charge-limitation between the electrodes (Child, 1911). The brilliance of the Cu Ka X-ray (see Section 3.2) in the plasma-cathode regime is 109 photons/4p/pulse at the repetition rate of 10 Hz with a power supply of 50 W. We did not measure the X-ray source size and assumed a point source in the calculation of the brilliance from the measured photon number. Therefore, the time-averaged brilliance of 1010 photons/4p/s is the minimum value and increases with increasing the actual source size. For comparison, in conventional X-ray tubes with a Cu anode, the time-averaged brilliance of Cu Ka radiation is estimated to be ffi1013 photons/4p/s at 50 kV and 1 mA (Honkima¨ki et al., 1990). This is equivalent to only 106 photons/4p in the temporal width of 100 ns that is required and obtained by the X-ray pulses for the time-resolved studies of this report. The X-ray source size in the laser-driven X-ray tube will be significantly lager than the laser spot size (0.1 mm in this study), because the electron bunch expands during the propagation between the electrodes. Further, it will be larger in the plasmacathode regime than in the photocathode regime, because of the expansion of the plasma (see Section 3.3). Egbert et al. have measured the spot size of a laser-driven X-ray tube in the photocathode regime and have reported that the X-ray spot size is three times larger than the laser spot one under the interelectrode distance of 10 mm and at the applied voltage of 50 kV (Egbert and Chichkov, 2003).
377
0.0 -4
-2
0
2
4
6
Time [ns] 0.64 J/cm2
50, 40, 30 kV
1.0
0.5
0.0 0
200
400 600 Time [ns]
800
1000
Fig. 4. Temporal profiles of X-ray pulses measured by using a BaF2 scintillator, (a) in the photocathode regime, and (b) in the plasma-cathode regime. In the upper panel (a), the temporal profile of X-ray pulse is compared with that of the instrumental response function for the measurement of the X-ray pulses. In the lower panel (b), time zero indicates the irradiation of the laser pulse.
(Tomov et al., 1995; Tomov and Rentzepis, 2004; Egbert and Chichkov, 2003). As shown in Fig. 4(a) and (b), temporal profiles of the X-ray pulses distinctly differ between the photocathode and the plasma-cathode regimes. In the photocathode regime, the temporal profile of the X-ray pulse agrees with that of the instrumental response function. This is explained by the fact that the duration of the X-ray pulse in the photocathode regime is much shorter than the temporal resolution of the measurement. The temporal broadening of the X-ray pulse is determined by the inter-electrode distance, the applied voltage, and the space–charge effect in the photoelectron bunch emitted from the photocathode. Hinze et al. have reported that the X-ray pulse duration is in the range from 5 to 20 ps under conditions similar to the inter-electrode distance, applied voltages, and laser pulse energies in the present study (Hinze et al., 2004). In the plasma-cathode regime, the X-ray pulse duration is remarkably broadened: 80, 140, and 400 ns (FWHM) at the voltages of 50, 40, and 30 kV, respectively, in Fig. 4(b). It should be noted that it takes only 8 and 10 ps for an electron with zero initial kinetic energy to move between 10 mm separated electrodes by the electrostatic potentials of 50 and 30 kV, respectively. The X-ray pulse broadening in the plasma-cathode regime can be explained by the electrostatic shielding effect in the plasma. The laser-induced plasma on the Cu cathode surface includes charged particles, i.e. electrons and ions. These charged particles do not immediately feel the electrostatic field between the electrodes owing to the plasma shielding. Further, the electrostatic shielding of the plasma gradually breaks as the plasma expands in vacuum. Therefore, the electrons in the plasma are gradually drawn apart toward the anode by the applied voltage, and so the X-ray pulse grows and decays slowly compared with that in the photocathode regime. Hence, the X-ray pulse temporally broadens as seen in Fig. 4(b). The X-ray pulse is temporally broadened at the lower applied voltage, because the electron detachment from the plasma proceeds relatively slowly.
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1.0
+ (a) Cu (d) Cu
50 kV 30 kV 0 kV
30 kV
Intensity [arb.unit]
(b) ↓ 200 400 Time [ns]
0 0.5
600
Cu atom
↓
(c) ↓
Cu+ ion (a) → (d) ↓
0.0
490
495
500
505 510 Wavelength [nm]
515
520
Fig. 5. Plasma emission spectra from the laser-focused spot on the Cu cathode at 0.64 J/cm2 laser fluence and the applied voltages of 0, 30, and 50 kV. The atomic and ionic lines with a transition probability over 106 s1 are indicated by filled (Cu atom) and open (Cu+ ion) triangles. The inset shows the time evolution of the emission monitored at (a) 521.8 nm (atomic line) and (d) 498.6 nm (ionic line) at 30 kV with a spectral resolution of 1 nm.
3.3. Spectral and temporal analysis of the plasma-cathode emissions Fig. 5 shows spectra of visible light emission from the laserfocused spot on the cathode. Three atomic lines: (a) Cu (4p 2P3/2’4d 2 D5/2) at 521.8 nm, (b) Cu (4p 2P1/2’4d 2D3/2) at 515.3 nm, and (c) Cu (4s2 2D5/2’4p 2P3/2) at 510.6 nm are observed without applying voltage. If we assume local thermodynamic equilibrium and apply the Boltzmann-plot method using the relative intensities of the atomic lines (a)–(c) (Sabsabi and Cielo, 1995; Hafez et al., 2003), the temperature of Cu plasma is determined to be 9000 K at the applied voltage of 0 kV. This temperature is in agreement with values previously reported on Cu plasmas (ffi104 K) (Pietsch, 1996; Hafez et al., 2003), which confirms that the plasma generation occurs on the cathode surface, although no ionic copper line is observed in the wavelength range examined. The expansion velocity of the plasma during the adiabatic expansion in vacuum is given as v ¼ {2/(g1)} (gkT/m)1/2, with the ratio of specific heat capacity at constant pressure to that at constant volume g, plasma temperature T, Boltzmann constant k, and molecular weight m (Hafez et al., 2003). If the abovedetermined temperature of 9000 K is employed, the velocity is calculated to be 5000 m/s for Cu vapor where g ¼ 1.52 and m ¼ 63.5 g/mol. This velocity roughly agrees with that (5500 m/s) in the Cu plasma at ffi104 K which was observed to be constant in the first 200 ns by time-of-flight measurements (Pietsch, 1996). We assume that the plasma expands at 5000 m/s in the first few hundred nanoseconds owing to the electrostatic shielding under the applied electric field. Under this assumption, in the first 200 ns, the plasma expands to the anode by approximately 1 mm, which is only 10% of the distance between the electrodes. Therefore, after the expansion, electrons pulled out from the plasma attain the kinetic energy sufficient to produce the X-ray radiation by acceleration to the anode. As seen in Fig. 4(b), the X-ray intensity begins to rise within 100 ns irrespective of the value of the applied voltage. Hence, the detachment of the electrons from the plasma is considered to actually occur during the plasma expansion. The rise of the X-ray intensity is explained by the fact that the surface area of the plasma exposed to the applied electric field increases with the expansion (Korobkin et al., 1999). In the rest of this paper, we discuss the dependence of the plasma emission spectrum on the applied voltage in Fig. 5. Emission lines of Cu+ superimposed on a continuous emission
clearly emerge at the applied voltage of 30 kV. The continuous emission is due to free–bound and free–free transitions of the electrons. Both the line and continuous emissions are further intensified at 50 kV. All the upper levels of ionic lines in this spectral range (e.g., (d)) are located at over 22 eV above the level of the Cu ground state, while the upper levels of lines (a) to (c) observed without applying voltage are located at several eV above it. Therefore, it is considered that the electric field due to the applied voltage is required for the excitation onto the upper levels of the ionic lines. The temporal profiles of atomic emission (a) and ionic one (d) Cu+ (4d 3D3’4f 3F4) at 498.6 nm at the applied voltage of 30 kV are also shown in Fig. 5 (inset). Although the natural lifetimes of both the upper states are below 10 ns, the observed emissions continue for longer times. The temporal broadening is considered to be due to the collisions between the plasma particles. That is, the excitation of Cu and Cu+ into the upper levels induced by the collisions continues in a few hundred ns. Further, the ionic emission rises slowly in a delay time of ffi200 ns after the laser pulse irradiation, while the atomic emission does so immediately. The difference is also explained by the electrostatic shielding effect in the plasma. Pumping onto the upper state responsible for emission (d) requires collisions among particles in the plasma under the electric field, as described above, and so the emission does not occur until the electrostatic shielding breaks to some extent. On the other hand, the upper state for emission (a) is populated on the laser-plasma formation. Recently, Moorti et al. have demonstrated the generation of Ti Ka X-ray in a vacuum discharge triggered by a 5 mJ picosecond laser pulse (Moorti et al., 2005). In their apparatus, a relatively high current (ffi10 kA) leads the expanding plasma to pinching, and the intense electron beam bombards a Ti anode. As a result of the pinching dynamics, the temporal profile of X-ray pulse has mostly two or three sharp peaks with each of ffi30 ns duration, unlike one temporally broad peak in the present case. In our X-ray tube, the current would be too low to induce such a pinching effect (Krinberg and Paperny, 2002).
4. Conclusions We have developed an X-ray tube with Cu electrodes which generates electrons through the plasma formation at the cathode
ARTICLE IN PRESS H. Yamada et al. / Radiation Physics and Chemistry 78 (2009) 375–379
by the irradiation of an ultraviolet femtosecond laser pulse. The wavelength spectrum, photon number and temporal profile of the X-ray pulse have been measured. Further, we have investigated the plasma by emission spectroscopy including a timeresolved measurement. The photon number is markedly increased above the laser ablation threshold of Cu, and is ffi104 times as large as that due to the photoelectric effect at the cathode. The X-ray pulse is ascribed to Cu Ka, and its temporal width varies from 80 to 400 ns (FWHM) upon decreasing the applied voltage from 50 to 30 kV. The optical emission from the copper ions in the plasma and detachment of the electrons from the plasma occur on the same time scale which is of the order of 100 ns. This is interpreted in terms of the gradual breakdown of the electrostatic shielding under the applied electric field. From the viewpoint of the application of the X-ray source with the laserplasma cathode in the present study to time-resolved X-ray spectroscopy, one could use a commercially available kHz Ti:Sapphire laser as a pump source for the X-ray tube to enlarge the signal-to-noise ratio per unit time, because a 267 nm femtosecond pulse with intensity above the ablation threshold of Cu is still attained.
Acknowledgments We thank Doctors K. Yokoyama, A. Nishimura, and A. Yokoyama for their fruitful discussions. This study was partly supported by Grant-in-Aid for Scientific Research (19750015). References Chen, J., Zhang, H., Tomov, I.V., Ding, X., Rentzepis, P.M., 2007. Electron transfer and dissociation mechanism of ferrioxalate: a time resolved optical and EXAFS study. Chem. Phys. Lett. 437, 50–55. Child, C.D., 1911. Discharge from hot CaO. Phys. Rev. 32, 492–511. Egbert, A., Chichkov, B.N., Ostendorf, A., 2001. Ultrashort X-ray source driven by femtosecond laser pulses. Europhys. Lett. 56, 228–233. Egbert, A., Chichkov, B.N., Fallnich, C., Zimmermann, S., Reinhold, A., 2002. Femtosecond laser-driven X-ray tube. Opt. Eng. 41, 2658–2667. Egbert, A., Chichkov, B.N., 2003. High-repetition rate compact ultrashort X-ray source and applications. Proc. SPIE 4978, 92–100.
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Cu Ka pulse generation in an X-ray tube with a plasma cathode induced by a femtosecond laser pulse H. Yamada a,, H. Murakami a,b, Y. Shimada a a b
Advanced Photon Research Center, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan Research Unit for Quantum Beam Life Science Initiative, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan
a r t i c l e in fo
abstract
Article history: Received 11 December 2008 Accepted 24 March 2009
An X-ray tube has been developed with a femtosecond laser-produced plasma at a Cu cathode by the acceleration of the plasma electrons onto a Cu anode. The source provided a Cu Ka pulse with 109 photons/4p and ffi100 ns width. Spectral observations of visible emissions from the cathode have revealed that the excitation of the plasma occurs on the same time scale as the X-ray-pulse generation, indicating that the electrostatic shielding of the plasma gradually breaks under the applied electric field. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Pulsed X-ray source Cu Ka Laser-plasma cathode Plasma emission spectroscopy Electrostatic shielding effect
1. Introduction There has been much attention paid to hard-X-ray pulse generation using a laser and the application to the study of various dynamic phenomena. A plasma X-ray source generated by an intense femtosecond laser has shown excellent performance in time-resolved measurements of X-ray diffraction (Siders et al., 1999) and X-ray absorption fine structure studies (Lee et al., 2005; Chen et al., 2007). However, such an X-ray source requires a highpower, typically a terawatt laser system which requires much space and cost. A subpicosecond X-ray pulse generated with 30 fs and 75 mJ laser pulses shows a brilliance of 109 photons/4p/pulse in generation of Cu Ka (Guo et al., 1997). Some recent research has been focused on plasma X-ray sources with laser pulses of submillijoule energies (Kutzner et al., 2004; Serbanescu et al., 2007), although the available X-ray photon number is relatively small, e.g., 107 photons/4p/pulse of Cu Ka with 0.3 mJ laser pulses (Serbanescu et al., 2007). A laser-driven X-ray tube offers promise in terms of its compactness and cost performance. In the source described in the present work, a conventional X-ray tube and a commercial pulsed laser are combined. Employing the photoelectric effect, an electron bunch is induced by a laser pulse at the cathode and accelerated by a high-voltage between the electrodes, generating
Corresponding author. Present address: Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto, 619-0292, Japan. Tel.: +81774 75 2305; fax: +81774 75 2318. E-mail address: [email protected] (H. Yamada).
0969-806X/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2009.03.069
an X-ray pulse at the anode. Tomov et al. have developed a laserdriven X-ray tube mainly with a 193 nm picosecond laser pulse, and observed the laser-induced deformation of a Au crystal by time-resolved X-ray diffraction measurements using 8 ps Cu Ka X-ray pulses (Tomov et al., 1995; Tomov and Rentzepis, 2004). Hinze et al. have investigated the dependence of the X-ray pulse duration on the applied voltage and the laser pulse energy, and have achieved 3 ps duration Cu Ka X-ray pulses in an X-ray tube driven with 780 nm femtosecond laser pulses (Hinze et al., 2004). The widespread application of the laser-driven X-ray tube, however, has been hampered primarily by its low brilliance (105–106 photons/4p/pulse) owing to the quantum efficiency of the photoelectric effect at the cathode. Egbert et al. have found that an X-ray tube with plasma cathode promises outstanding performance in terms of average X-ray power (Egbert et al., 2001, 2002). They developed an X-ray tube with a steel cathode and a W anode, and produced the plasma at the cathode as an electron source, by irradiating a 780 nm femtosecond laser pulse with a fluence above the ablation threshold of the metal. As a result, they obtained the X-ray dose rate ffi104 times higher than that with the photocathode utilizing the photoelectric effect. Although they have pointed out that the duration of the electron pulse produced by the plasma cathode can by no means be in the subpicosecond range, because the duration is determined by a much longer hydrodynamic plasma expansion time (Egbert et al., 2002), the temporal profiles of the X-ray radiation have not been measured in their study. An X-ray tube with an anode of 3d-transition-metal predominantly provides its characteristic Ka X-rays (Egbert and Chichkov, 2003). Such a quasi-monochromatic X-ray source is useful for
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radiographic measurements (Sato et al., 2006) and also to efficiently induce X-ray fluorescence of specific elements. With these motivations, we have developed a laser-driven X-ray tube with Cu electrodes as a source of Cu Ka X-ray pulses aimed at a time-resolved X-ray fluorescence spectroscopy of Co, Fe, or Mn compounds. In this paper, we report the absolute photon number, spectrum, and temporal profile of the X-ray pulse from this X-ray tube, in which the difference between a photocathode and the plasma cathode is clearly shown by employing an ultraviolet femtosecond laser pulse and varying the laser pulse energy. Further, we analyze the plasma generated at the cathode by visible emission spectroscopy, including measurements of the time-resolved emission.
2. Experimental A schematic of a laser-driven X-ray tube is depicted in Fig. 1. A Cu anode with a target angle of 301 was fixed at 10 mm from a Cu columnar cathode with a diameter of 5 mm. Electrons produced at the cathode were accelerated to the anode by an applied voltage up to 50 kV with a power supply of a commercial X-ray generator (50 W). The X-ray tube was connected to a turbomolecular pump producing a pressure in the tube of 104–105 Pa. A femtosecond laser pulse (p2 mJ, 100 fs, 800 nm, 10 Hz) from a Ti:sapphire laser system (Clark-MXR, CPA-10) was used to generate its third harmonic (267 nm). The ultraviolet (UV) pulse energy was varied in the range from 0.5 to 70 mJ. The UV femtosecond pulses were focused on the Cu cathode in the X-ray tube by a fused silica lens with 20 cm focal length. An X-ray spectrometer was composed of a 3.5 mm thick LiF(2 0 0) crystal (SAINT-GOBAIN) and an X-ray charge-coupled device (CCD) sensor cooled with a liquid nitrogen (E2V Technologies, CCD05-30). The X-ray radiation through a Be window of the tube was dispersed by the crystal with an incident angle of 50.11, and the (4 0 0) diffraction lines were simultaneously detected by the CCD with an accumulation time of 60 s. In the dispersive method where an entire spectrum of interest is recorded simultaneously, fluctuations in the incident X-ray intensity do not influence the quality of the spectrum (Tomov and Rentzepis, 2004). The energy resolution of 6 eV at full-width at halfmaximum (FWHM) was achieved by setting a Ta slit in front of the Be window of the X-ray tube. The X-ray intensity from the plasma-cathode tube was measured by an X-ray streak camera system (HAMAMATSU, C4575-01). The absolute photon number of the X-ray pulse was obtained by using the values of the conversion efficiency of the
Laser pulse
Window (quartz)
Lens
streak camera from X-ray photons at 8 keV into electrical signals on the product data sheet, the area of its Au photocathode irradiated by the X-rays, and the distance between the Au photocathode and the X-ray source. The relative intensities of X-ray pulses were measured by using a scintillation crystal, NaI(Tl) (25 mm diameter, 2 mm thick) with a fast time-response (0.8 ns) photomultiplier (HAMAMATSU, R7400-03) and evaluated by the total photocurrent per pulse. The detection solid angle was changed to prevent saturation of the photocurrent by adjusting the position of the scintillator and the width of a Ta slit in front of the scintillator. The pulsed signal from the photomultiplier was fed into a digital oscilloscope (Tektronix, TDS380) and averaged over 256 times. As for time-resolved measurements of X-ray pulses, a BaF2 crystal (25 mm diameter, 5 mm thick) with a 10 nm bandpass filter centered at 220 nm was used instead of NaI(Tl). The scintillation light of BaF2 crystals at 220 nm decays with a time constant of 600 ps. A resolution of the temporal characterization of X-ray pulses was about 2 ns (FWHM) in the measurement with the BaF2 scintillator, the photomultiplier, and the oscilloscope. Visible light emission from the laser focus spot on the cathode was measured by a spectrograph (Acton Research, SpectraPro300i) equipped with a CCD (Roper Scientific, LN/CCD-1340/400) with an accumulation time of 5 s through a glass view port of the X-ray tube. The time evolution of the line emission was monitored by a combination of a monochromator (JYHoriba, H10UV) with the photomultiplier and the oscilloscope.
3. Results and discussion 3.1. Enhancement of X-ray generation due to the plasma cathode Fig. 2 shows that the intensity of the produced X-ray pulse changes drastically around the laser fluence of 0.2 J/cm2. Below this threshold, the X-ray pulse intensity varies linearly with the laser pulse energy; this is due to single-photon photoelectric effect, because the UV light possesses the energy exceeding the
Acceleration potential:
109
50 kV 30 kV
X-ray intensity [photon/4π/pulse]
376
108
107
106
105
Electrodes (Cu) ≅ 50 kV
104
0.2 J/cm2
Electrons 1
Vacuum chamber Window (Be) X-ray pulse Fig. 1. A schematic of laser-driven X-ray tube.
10 Laser energy [μJ/pulse]
100
Fig. 2. Dependence of the X-ray pulse intensity on the laser pulse energy. The focus spot size on the cathode is ffi0.1 mm in diameter. The dashed line indicates the ablation threshold for Cu.
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3.2. Spectral and temporal profiles of the X-ray pulse As for the spectrum of the X-rays thus generated, it is found from Fig. 3 that the X-rays are predominantly Cu Ka in the plasma-cathode regime. Studies using laser-driven X-ray tubes have shown that this is also the case in the photocathode regime
Intensity [arb.unit]
1.0
CuKα1
0.5
CuKα2
↓ 30 kV 50 kV
0.0 7.9
8.0
8.1
8.2
X-ray energy [keV] Fig. 3. Spectra of the X-ray radiation from the laser-driven X-ray tube with 0.76 J/ cm2 laser fluence (plasma-cathode regime, 60 mJ/pulse, focus spot size of 0.1 mm) at the applied voltages of 30 and 50 kV.
0.14 J/cm2, 50 kV
1.0
Response function
0.5 X-ray intensity [arb.units]
work function of Cu. By defocusing the laser beam, we have confirmed that the threshold is not determined by the laser pulse energy, but by the laser fluence. The threshold value is in agreement with that observed in the femtosecond laser ablation of Cu (Nolte et al., 1997; Hashida et al., 2002). Since a plasma is also produced at the cathode by the irradiation of the laser with fluences above the ablation threshold and can also serve as an electron source in the X-ray tube, it is reasonable that the plasma is source of the remarkable X-ray pulse intensity enhancement compared with that due to the photoelectric effect. Additionally, the X-ray pulse intensity in the plasma-cathode regime is independent of the laser fluence. This indicates that the current reaches its maximum determined by the characteristics of the electric circuit of the system consisting of the X-ray tube and the DC power supply, e.g., the space–charge-limitation between the electrodes (Child, 1911). The brilliance of the Cu Ka X-ray (see Section 3.2) in the plasma-cathode regime is 109 photons/4p/pulse at the repetition rate of 10 Hz with a power supply of 50 W. We did not measure the X-ray source size and assumed a point source in the calculation of the brilliance from the measured photon number. Therefore, the time-averaged brilliance of 1010 photons/4p/s is the minimum value and increases with increasing the actual source size. For comparison, in conventional X-ray tubes with a Cu anode, the time-averaged brilliance of Cu Ka radiation is estimated to be ffi1013 photons/4p/s at 50 kV and 1 mA (Honkima¨ki et al., 1990). This is equivalent to only 106 photons/4p in the temporal width of 100 ns that is required and obtained by the X-ray pulses for the time-resolved studies of this report. The X-ray source size in the laser-driven X-ray tube will be significantly lager than the laser spot size (0.1 mm in this study), because the electron bunch expands during the propagation between the electrodes. Further, it will be larger in the plasmacathode regime than in the photocathode regime, because of the expansion of the plasma (see Section 3.3). Egbert et al. have measured the spot size of a laser-driven X-ray tube in the photocathode regime and have reported that the X-ray spot size is three times larger than the laser spot one under the interelectrode distance of 10 mm and at the applied voltage of 50 kV (Egbert and Chichkov, 2003).
377
0.0 -4
-2
0
2
4
6
Time [ns] 0.64 J/cm2
50, 40, 30 kV
1.0
0.5
0.0 0
200
400 600 Time [ns]
800
1000
Fig. 4. Temporal profiles of X-ray pulses measured by using a BaF2 scintillator, (a) in the photocathode regime, and (b) in the plasma-cathode regime. In the upper panel (a), the temporal profile of X-ray pulse is compared with that of the instrumental response function for the measurement of the X-ray pulses. In the lower panel (b), time zero indicates the irradiation of the laser pulse.
(Tomov et al., 1995; Tomov and Rentzepis, 2004; Egbert and Chichkov, 2003). As shown in Fig. 4(a) and (b), temporal profiles of the X-ray pulses distinctly differ between the photocathode and the plasma-cathode regimes. In the photocathode regime, the temporal profile of the X-ray pulse agrees with that of the instrumental response function. This is explained by the fact that the duration of the X-ray pulse in the photocathode regime is much shorter than the temporal resolution of the measurement. The temporal broadening of the X-ray pulse is determined by the inter-electrode distance, the applied voltage, and the space–charge effect in the photoelectron bunch emitted from the photocathode. Hinze et al. have reported that the X-ray pulse duration is in the range from 5 to 20 ps under conditions similar to the inter-electrode distance, applied voltages, and laser pulse energies in the present study (Hinze et al., 2004). In the plasma-cathode regime, the X-ray pulse duration is remarkably broadened: 80, 140, and 400 ns (FWHM) at the voltages of 50, 40, and 30 kV, respectively, in Fig. 4(b). It should be noted that it takes only 8 and 10 ps for an electron with zero initial kinetic energy to move between 10 mm separated electrodes by the electrostatic potentials of 50 and 30 kV, respectively. The X-ray pulse broadening in the plasma-cathode regime can be explained by the electrostatic shielding effect in the plasma. The laser-induced plasma on the Cu cathode surface includes charged particles, i.e. electrons and ions. These charged particles do not immediately feel the electrostatic field between the electrodes owing to the plasma shielding. Further, the electrostatic shielding of the plasma gradually breaks as the plasma expands in vacuum. Therefore, the electrons in the plasma are gradually drawn apart toward the anode by the applied voltage, and so the X-ray pulse grows and decays slowly compared with that in the photocathode regime. Hence, the X-ray pulse temporally broadens as seen in Fig. 4(b). The X-ray pulse is temporally broadened at the lower applied voltage, because the electron detachment from the plasma proceeds relatively slowly.
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1.0
+ (a) Cu (d) Cu
50 kV 30 kV 0 kV
30 kV
Intensity [arb.unit]
(b) ↓ 200 400 Time [ns]
0 0.5
600
Cu atom
↓
(c) ↓
Cu+ ion (a) → (d) ↓
0.0
490
495
500
505 510 Wavelength [nm]
515
520
Fig. 5. Plasma emission spectra from the laser-focused spot on the Cu cathode at 0.64 J/cm2 laser fluence and the applied voltages of 0, 30, and 50 kV. The atomic and ionic lines with a transition probability over 106 s1 are indicated by filled (Cu atom) and open (Cu+ ion) triangles. The inset shows the time evolution of the emission monitored at (a) 521.8 nm (atomic line) and (d) 498.6 nm (ionic line) at 30 kV with a spectral resolution of 1 nm.
3.3. Spectral and temporal analysis of the plasma-cathode emissions Fig. 5 shows spectra of visible light emission from the laserfocused spot on the cathode. Three atomic lines: (a) Cu (4p 2P3/2’4d 2 D5/2) at 521.8 nm, (b) Cu (4p 2P1/2’4d 2D3/2) at 515.3 nm, and (c) Cu (4s2 2D5/2’4p 2P3/2) at 510.6 nm are observed without applying voltage. If we assume local thermodynamic equilibrium and apply the Boltzmann-plot method using the relative intensities of the atomic lines (a)–(c) (Sabsabi and Cielo, 1995; Hafez et al., 2003), the temperature of Cu plasma is determined to be 9000 K at the applied voltage of 0 kV. This temperature is in agreement with values previously reported on Cu plasmas (ffi104 K) (Pietsch, 1996; Hafez et al., 2003), which confirms that the plasma generation occurs on the cathode surface, although no ionic copper line is observed in the wavelength range examined. The expansion velocity of the plasma during the adiabatic expansion in vacuum is given as v ¼ {2/(g1)} (gkT/m)1/2, with the ratio of specific heat capacity at constant pressure to that at constant volume g, plasma temperature T, Boltzmann constant k, and molecular weight m (Hafez et al., 2003). If the abovedetermined temperature of 9000 K is employed, the velocity is calculated to be 5000 m/s for Cu vapor where g ¼ 1.52 and m ¼ 63.5 g/mol. This velocity roughly agrees with that (5500 m/s) in the Cu plasma at ffi104 K which was observed to be constant in the first 200 ns by time-of-flight measurements (Pietsch, 1996). We assume that the plasma expands at 5000 m/s in the first few hundred nanoseconds owing to the electrostatic shielding under the applied electric field. Under this assumption, in the first 200 ns, the plasma expands to the anode by approximately 1 mm, which is only 10% of the distance between the electrodes. Therefore, after the expansion, electrons pulled out from the plasma attain the kinetic energy sufficient to produce the X-ray radiation by acceleration to the anode. As seen in Fig. 4(b), the X-ray intensity begins to rise within 100 ns irrespective of the value of the applied voltage. Hence, the detachment of the electrons from the plasma is considered to actually occur during the plasma expansion. The rise of the X-ray intensity is explained by the fact that the surface area of the plasma exposed to the applied electric field increases with the expansion (Korobkin et al., 1999). In the rest of this paper, we discuss the dependence of the plasma emission spectrum on the applied voltage in Fig. 5. Emission lines of Cu+ superimposed on a continuous emission
clearly emerge at the applied voltage of 30 kV. The continuous emission is due to free–bound and free–free transitions of the electrons. Both the line and continuous emissions are further intensified at 50 kV. All the upper levels of ionic lines in this spectral range (e.g., (d)) are located at over 22 eV above the level of the Cu ground state, while the upper levels of lines (a) to (c) observed without applying voltage are located at several eV above it. Therefore, it is considered that the electric field due to the applied voltage is required for the excitation onto the upper levels of the ionic lines. The temporal profiles of atomic emission (a) and ionic one (d) Cu+ (4d 3D3’4f 3F4) at 498.6 nm at the applied voltage of 30 kV are also shown in Fig. 5 (inset). Although the natural lifetimes of both the upper states are below 10 ns, the observed emissions continue for longer times. The temporal broadening is considered to be due to the collisions between the plasma particles. That is, the excitation of Cu and Cu+ into the upper levels induced by the collisions continues in a few hundred ns. Further, the ionic emission rises slowly in a delay time of ffi200 ns after the laser pulse irradiation, while the atomic emission does so immediately. The difference is also explained by the electrostatic shielding effect in the plasma. Pumping onto the upper state responsible for emission (d) requires collisions among particles in the plasma under the electric field, as described above, and so the emission does not occur until the electrostatic shielding breaks to some extent. On the other hand, the upper state for emission (a) is populated on the laser-plasma formation. Recently, Moorti et al. have demonstrated the generation of Ti Ka X-ray in a vacuum discharge triggered by a 5 mJ picosecond laser pulse (Moorti et al., 2005). In their apparatus, a relatively high current (ffi10 kA) leads the expanding plasma to pinching, and the intense electron beam bombards a Ti anode. As a result of the pinching dynamics, the temporal profile of X-ray pulse has mostly two or three sharp peaks with each of ffi30 ns duration, unlike one temporally broad peak in the present case. In our X-ray tube, the current would be too low to induce such a pinching effect (Krinberg and Paperny, 2002).
4. Conclusions We have developed an X-ray tube with Cu electrodes which generates electrons through the plasma formation at the cathode
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by the irradiation of an ultraviolet femtosecond laser pulse. The wavelength spectrum, photon number and temporal profile of the X-ray pulse have been measured. Further, we have investigated the plasma by emission spectroscopy including a timeresolved measurement. The photon number is markedly increased above the laser ablation threshold of Cu, and is ffi104 times as large as that due to the photoelectric effect at the cathode. The X-ray pulse is ascribed to Cu Ka, and its temporal width varies from 80 to 400 ns (FWHM) upon decreasing the applied voltage from 50 to 30 kV. The optical emission from the copper ions in the plasma and detachment of the electrons from the plasma occur on the same time scale which is of the order of 100 ns. This is interpreted in terms of the gradual breakdown of the electrostatic shielding under the applied electric field. From the viewpoint of the application of the X-ray source with the laserplasma cathode in the present study to time-resolved X-ray spectroscopy, one could use a commercially available kHz Ti:Sapphire laser as a pump source for the X-ray tube to enlarge the signal-to-noise ratio per unit time, because a 267 nm femtosecond pulse with intensity above the ablation threshold of Cu is still attained.
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