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Proton emission from resonant laser absorption and self-focusing effects from hydrogenated structures
Applied Surface Science 272 (2013) 50–54
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Proton emission from resonant laser absorption and self-focusing effects from hydrogenated structures M. Cutroneo a,∗ , L. Torrisi a , D. Margarone b , A. Picciotto c a b c
Dip.di Fisica, Università di Messina, V. F. Stagno D’Alcontres 31, 98166 S. Agata (ME), Italy Institute of Physics, ASCR-PALS, Na Slovance 2, 18221 Prague 8, Czech Republic Fondazione Bruno Kessler – IRST, Trento, Italy
a r t i c l e
i n f o
Article history: Available online 6 April 2012 Keywords: Resonant absorption Self-focusing Thomson parabola spectrometer IC SiC IEA
a b s t r a c t Effects of resonant absorption and self-focusing are investigated by using fast and intense laser pulses. The ion emission and acceleration in the non-equilibrium laser-generated plasma are investigated at low and high intensities, from 1010 up to about 1016 W/cm2 . The properties of plasma are strongly dependent on the time and space, laser intensity and wavelength. A special interest concerns the energetic and intense proton generation for the multiplicity use that proton beams have in different scientific fields (Nuclear Physics, Astrophysics, Bio-Medicine, Microelecronics, etc.). Investigations have been performed at INFN-LNS of Catania and at PALS Laboratory of Prague, by using thick and thin targets and different technique of ion analysis. The mechanisms of resonant absorption of the laser light, produced in special targets containing nanostructures with dimensions comparable with the laser wavelength, enhances the proton energy. The mechanisms of self-focusing, obtained by changing the laser focal distance from the target surface, increase the local intensity and consequently the high directional ion acceleration. Real-time ion detections were performed through Thomson parabola spectrometer (TPS), ion collectors (IC), SiC detectors and ion energy analyzer (IEA) employed in time-of-flight configuration (TOF). The energy and the amount of ions increase significantly when the two non-linear phenomena occurs, as will be described. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The interaction of short laser pulses with solid targets has become an important field of study because of many applications, such as the fast ignition scheme of inertia confinement fusion, the plasma-based particle accelerator, coherent x/␥-ray sources, etc. For most of these applications, the nature of the absorption process must be determined. The density scale length of the plasmas generated from the target surfaces can be estimated as: L = cs p
(1)
where cs is the ion sound speed and p is the laser pulse duration [1]. For high intensities (>1016 W/cm2 ) and very short pulses (<1 ps) the scale length is too short to generate sufficient absorption effects and resonance absorption at the critical surface is suggested to be one of the major absorption mechanisms. Some experiments show
∗ Corresponding author. E-mail address: [email protected] (M. Cutroneo). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.129
that it plays an important role even for plasmas with a scale length considerably shorter than the laser wavelength 0 . However many theoretical works on resonance absorption are only valid for the case in which L > 0 [2]. At higher laser intensity, say L < vos /ω0 , the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking. Thus, the electron plasma wave is hard to develop and vacuum heating tends to be dominant. Here, vos = eE0 /me ω0 is the quiver velocity of the electron, E0 is the electric field normal to the interface, and ω0 is the laser frequency. A simple model is used to calculate the energy absorption efficiency when a laser of short pulse length impinges on a dielectric slab that is doped with an impurity with a resonant line at the laser frequency. It is found that the energy absorption efficiency is maximized for a certain degree of doping concentration (at a given pulse length) and also for a certain pulse length (at a given doping concentration). Dimensionless parameters are constructed, allowing calculations with one set of parameters be used to infer the results expected for other sets of parameters.
M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
Absorption processes are generally dependent on the density scale length. For shorter scale length or higher laser intensity, vacuum heating tends to be dominant. Literature reports that the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking. This can lead to heating of the plasma at the expanse of the wave energy [3]. Interaction of the laser radiation above some threshold intensity with a plasma of defined properties may significantly increase the charge state and energy of the produced ions, due to a peculiar effect occurring in the plasma, which focalizes further the laser pulse (self-focusing effect) acting so as a small vapor lens placed in front of the target surface. Advances in laser technology have recently enabled the observation of self-focusing in the interaction of intense laser pulses with plasmas. Self-focusing in plasma can occur through thermal, relativistic, and ponderomotive effects [4]. Thermal self-focusing is due to collisional heating of plasma exposed to electromagnetic radiation: the rise in temperature induces a hydrodynamic expansion, which leads to an increase of the refraction index and further heating. Relativistic selffocusing is caused by the mass increase of electrons traveling at speed approaching the speed of light, which modifies the plasma refractive index, depending on the electromagnetic and plasma frequencies. Ponderomotive self-focusing is caused by the ponderomotive force, which pushes electrons away from the region where the laser beam is more intense, therefore increasing the refractive index and inducing a focusing effect. Both effects of resonant absorption and self-focusing were investigated in order to produce high yield of energetic proton emission from laser irradiated targets, as will be presented and discussed.
2. Experimental set-up The main experiments have been performed by using the Nd:Yag laser of INFN-LNS of Catania and the Iodine Asterix laser of PALS Laboratory of Prague. The first can be employed at 1064 nm, 532 nm and 355 nm, 9 ns pulse duration, 1 J maximum pulse energy, with intensities between 108 and 1011 W/cm2 . The second can be employed at 1315 nm (1ω) and 438 nm (3ω), 300 ps pulse duration, 800 J maximum pulse energy, with intensities between 1013 and 1016 W/cm2 . At higher intensities the data were collected from literature and compared with our measurements in order to evaluate the generalized law of I2 scale factor. In order to generate protons, the irradiated targets were thick and thin hydrogenated films. Many of these were polyethylene based (CH2 -mnomer) with inclusions of nanostructures such as carbon-nanotubes (CNT) of different length and oxides (such as Fe2 O3 ), other consisted of hydrogenated Si, thin films of mylar covered by Au or Al films, hydrates and metals. Generally thick films (1 mm thickness) were used at LNS for irradiation at low laser intensities to generate backward directed plasmas, while thin films (of the order of 1 m in thickness) were employed at high laser intensity at PALS in order to generate forward directed plasmas. Time-of-flight (TOF) measurements have been obtained with ion collectors (IC), semiconductor detectors based on SiC, and electrostatic deflector ion energy analyzer (IEA) that permits to measure the average ion energy, the ion energy and the charge state distributions, respectively. Details on IC, SiC and IEA detector are given in the literature [5,6]. The ion plasma temperature, Ti , was measured though the Coulomb–Boltzmann shifted (CBS) fit of the experimental ion energy distributions [7]; the electronic plasma temperature, ne , was measured through the evaluation of the atoms removed from the
51
Fig. 1. Experimental set-up scheme at PALS Laboratory of Prague.
laser crater and volume of the visible plasma observed by a fast CCD camera. A Thomson parabola spectrometer (TPS) was also employed at PALS in forward direction along the normal to the target surface in order to measure the energy, charge states and ion species of ejected particles from plasma, thanks to their magnetic and electric deflection that use a magnetic field, of the order of 0.1 T, and an electric field, of about 3 kV/cm, to deflect the ions toward a multichannel-plate (MCP) detector placed orthogonally to the incident high collimated ion beams. A streak camera was employed at PALS to measure the laser focal position (FP) distance with respect to the sample surface. Negative distances mean a focus in front of the surface while positive distances mean a focus inside the target. Silicon photodiodes of different types were tested for the possibility of measurement of high-intensity X-ray pulses in the energetic range 0.7–23 keV. The main problems encountered were non-linearity and overloading of the detectors, especially for detectors assigned for soft radiation. These problems were overcome by operating the photodiodes in the integrating mode, which accomplishes a much greater dynamic range. Photodiodes BPYP03 were used for the measurement of the soft component (0.7–1.5 keV), while FLM photodiodes for the harder component (4–23 keV), both realized at the Institute of Electron Technology, Warsaw [8]. The photodiodes were placed at the distances from 100 to 200 cm from the target and at 30◦ detection angle from the normal to the target surface in backward direction. Different filters (Be, Al) of various thicknesses changed the range of detector sensitivity. Fig. 1 shows a schematic experimental set-up employed at PALS Laboratory of Prague. 3. Results At low intensities, of the order of 1010 W/cm2 , with 3–9 ns pulse duration and 1064 nm wavelength, a typical spectrum of ions
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M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
Fig. 2. Typical IC spectra obtained at low intensity at LNS of Catania relative to polyethylene irradiation (a) and relative CBS deconvolution process (c). Typical resonant absorption process at low intensity obtained by irradiating Fe2 O3 (b) and CNT (d) nanostructures, 0.1% in concentration, embedded in polyethylene.
emitted from polyethylene, detected by IC shows a large peak due to the carbon charge states and a little peak due to proton detection, as reported in Fig. 2a. In this case the TOF distance is 60 cm, thus the corresponding proton peak energy is about 75 eV. The proton and carbon peak can be interpreted as convolution of CBS distribution, according to the literature [8]. This approach permits to evaluate the plasma temperature and the acceleration voltage, that correspond to 10 eV and 67 V, respectively, as reported in Fig. 2c. The four charge states for carbon was evaluated though IEA spectrometer. The insertion of the nanostructures in the polyethylene change strongly the result of the ion emission at low laser intensity, as reported in Fig. 2b and d showing that the proton/carbon ratio increases from 0.05 in pure polyethylene up to 0.67 and 1.5 for 0.1% concentration of Fe2 O3 and carbon nanotubes. In these last cases the TOF length was 150 cm thus the corresponding maximum proton energy, calculated at the FWHM of the proton peak, is about 120 eV in the two cases. Thus the insertion of absorbent nanostructures, with length of the order of 1 m, i.e. comparable with the laser wavelength, produces effects of resonant absorption that can be responsible of a strong increment of the proton yield emission and of the proton kinetic energy. This result confirms that also the plasma temperature and acceleration voltage increase due to the resonant absorption effect. At high intensity, of the order of 1016 W/cm2 , with 300 ps pulse duration and 1315 nm wavelength, a typical spectrum of ions emitted from amorphous surface layers of hydrogenated silicon (Si:H) is reported in Fig. 3 for a thick sample producing plasma ions in backward direction (a) and for a thin target, 17 m in thickness, producing hot plasma in forward direction (b). In the first case the proton peak at 70 ns corresponds to a kinetic energy of 1 MeV while in the second case the position at 40 ns, well measurable without
the photopeak using a 8 m aluminum absorber placed in front of the IC detector, corresponds to a kinetic energy of 4.5 MeV. In this last case the short target length, producing electron emission in forward direction and enhances the effect of proton drive acceleration due to a sort of absorption resonance effect increasing significantly the proton kinetic energy. The appearance of new peaks in the IC signal clearly indicates a change of the main ion production (acceleration) mechanism. In addition to the ambipolar acceleration of ions non linear forces, including ponderomotive relativistic and self-focusing, which lead to very high laser intensity in a self-focused channel may become the main reason for the presence of high kinetic energy and high charged ions. The complexity of the laser interaction mechanisms with solid targets is due to the non-linearity of the processes occurring in the pre-plasma and of the plasma non linear optical properties which are dependent on the laser intensity and that occurs above a threshold of about 1014 W/cm2 . Self-focusing increases the intensity of the part of laser beam on the target due to the higher focusing which may reduce the spot of two order of magnitude increasing the intensities above 1018 W/cm2 . The X-ray radiation fluxes from Au-produced plasmas at comparable experimental conditions are presented in the following. Records of hard X-ray emission (HX), produced at 500 J laser pulse at 1ω normal irradiation and detected by FLM photodiodes, filtered by 1200 m Be filters, are shown in Fig. 4. The maximum of hard X-ray component (5–20 keV) was recorded at FP = −200 m, i.e. focusing the laser 200 m in front of the target surface. This operation produces a self-focusing due to the prepulse irradiation of the surface with consequent increase of the laser intensity, plasma temperature and hard X-ray production. Records of soft X-ray emission (SX), produced in the same experimental conditions, are also reported in the same plot and demonstrating that at the FP = −200 m
M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
53
ions with higher charge states, connected with the presence of fast electrons, and generated by resonant absorption mechanisms, create a maximum yield at FP = −200 m. The inverse bremsstrahlung prevails for both the lateral maxima, which I2 , 1015 W/cm2 , while the maximum yield is related to the resonant absorption process. The initial laser beam diameter of 29 cm, focused by an aspheric lens it reaches a minimum diameter value of 70 m at FP = 0. When the focal position is FP = −200 m and the laser energy is very low, the self-focusing cannot happen because the conditions are below the threshold value but increasing the laser energy above the threshold of the order of 1014 W/cm2 [9], it occurs because the high light refraction effect produces a further laser beam focalization, due to the dense plasma volume in front of the target, which converges the beam so as a focusing lens. Thus special effects occur, as evident by the higher charge state distributions observed through IEA spectrometer, higher plasma temperature effects and higher proton energy and yield emission. 4. Discussion and conclusions
Fig. 3. Typical SiC detector spectrum, in backward direction, relative to the ion emission from high intensity laser irradiation of thick hydrogenated silicon (a) and IC detector spectrum, in forward direction, relative to the ion emission from a thin film of the same material irradiated in the same conditions (b).
conditions their yield decreases, confirming that an increment of X-ray energy occurs during the self-focusing effect. The ion yield versus focus position plot of Fig. 3 indicates that for low charge states ions are due to ionization by thermal electrons generated by inverse bremsstrahlung mechanism. Such a result was ascribed to the volume effect of produced plasma due to the interaction of continuously decreasing diameter of the laser beam with respect to the target surface that, in the case of self-focusing mechanisms, is found to a forward negative focus positions. In contrast,
The existence of an optimum laser focus position for generation of the fastest ions with the highest charge states in front of the target surface is consistent with our old observations [10]. The course of dependencies and similar values of the highest zmax indicate a threshold for the appearance of relativistic self-focusing of laser beam and a principal limitation of the maximum attainable laser intensity. At PALS differences for 1ω and 3ω could be ascribed to a different absorption of laser radiation, in accordance with the scaling relation I2 . The front part of the 300 ps laser pulse interacts with the target and creates an expanding plasma plume. Considering for simplicity, the expansion velocity v = 106 m/s, the plasma plume attains the distance of 100 m within the first 100 ps. For the laser beam diameter of 70 m, the self-focusing length should be about 100–200 m, at least. For FP = 0, the more the plasma plume expands, the longer the interaction length, but the lower the laser intensity with which the front of the plasma interacts. In contrast, starting from FP = −400 m, the plasma plume faces during the expansion, increasing laser intensity only. The following conclusions can be made: - Nano and micrometric structures, such as carbon nanotubes, polymeric chains and molecular groups with dimensions comparable with the laser wavelength may induce resonant absorption effects increasing the plasma temperature and the acceleration ion drive mechanisms. - Self-focusing processes influence significantly the generation of ions with the highest charge states, using high power iodine laser with the pulse length of 300 ps and an optimal FP distance can be found to enhance this effect of intensity increase due to the focal spot decreasing. Acknowledgments This work was supported by INFN 5th National Committee of PLATONE and LIANA Projects. References
Fig. 4. Typical X-ray yield versus focal position with respect to the target surface from PALS experiment obtained by irradiating a thin Au film.
[1] H. Cai, W. Yu, S. Zhu, C. Zheng, L. Cao, B. Li, Z.Y. Chen, A. Bogerts, Physics of Plasmas 13 (2006) 094504. [2] W.L. Kruer, Physics of Laser Plasma Interactions, Addison-Wesley, New York, 1988. [3] S.C. Wilks, W.L. Kruer, IEEE Journal of Quantum Electronics 33 (1997) 1954. [4] L. Torrisi, D. Margarone, L. Laska, J. Krasa, A. Velyhan, M. Pfeifer, J. Ullschmied, L. Ryc, Laser and Particle Beams 26 (2008) 379–387.
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[5] E. Woryna, P. Parys, J. Wolowski, W. Mroz, Laser and Particle Beams 14 (1996) 293. [6] L. Torrisi, G. Foti, L. Giuffrida, D. Puglisi, J. Wolowski, J. Badziak, P. Parys, M. Rosinski, D. Margarone, J. Krasa, A. Velyhan, J. Ullschmied, Journal of Applied Physics 105 (2009) 123304. [7] L. Torrisi, S. Gammino, L. Andó, L. Laska, J. Krasa, K. Rohlena, J. Ullschmied, J. Wolowski, J. Badziak, P. Parys, Journal of Applied Physics 99 (2006) 083301. [8] L. Laska, L. Ryc, J. Badziak, F.P. Boody, S. Gammino, K. Jungwirth, J. Krasa, E. Krousky, A. Mezzasalma, P. Parys, M. Pfeifer, K. Rohlena, L. Torrisi, J. Ullschmied,
J. Wolowski, Radiation Effects and Defects in Solids 160 (10–12) (2005) 557–566. [9] L. Laska, K. Jungwirth, J. Krasa, E. Krousky, M. Pfeifer, K. Rohlena, J. Ullschmied, J. Badziak, P. Parys, J. Wolowski, S. Gammino, L. Torrisi, F.P. Boody, Laser and Particle Beams 24 (1) (2006) 175–179. [10] L. Laska, K. Jungwirth, J. Krasa, M. Pfeifer, K. Rohlena, J. Ullschmied, J. Badziak, P. Parys, L. Ryc, J. Wolowski, S. Gammino, L. Torrisi, F.P. Boody, Czechoslovak Journal of Physics 55 (6) (2005) 691–699.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Proton emission from resonant laser absorption and self-focusing effects from hydrogenated structures M. Cutroneo a,∗ , L. Torrisi a , D. Margarone b , A. Picciotto c a b c
Dip.di Fisica, Università di Messina, V. F. Stagno D’Alcontres 31, 98166 S. Agata (ME), Italy Institute of Physics, ASCR-PALS, Na Slovance 2, 18221 Prague 8, Czech Republic Fondazione Bruno Kessler – IRST, Trento, Italy
a r t i c l e
i n f o
Article history: Available online 6 April 2012 Keywords: Resonant absorption Self-focusing Thomson parabola spectrometer IC SiC IEA
a b s t r a c t Effects of resonant absorption and self-focusing are investigated by using fast and intense laser pulses. The ion emission and acceleration in the non-equilibrium laser-generated plasma are investigated at low and high intensities, from 1010 up to about 1016 W/cm2 . The properties of plasma are strongly dependent on the time and space, laser intensity and wavelength. A special interest concerns the energetic and intense proton generation for the multiplicity use that proton beams have in different scientific fields (Nuclear Physics, Astrophysics, Bio-Medicine, Microelecronics, etc.). Investigations have been performed at INFN-LNS of Catania and at PALS Laboratory of Prague, by using thick and thin targets and different technique of ion analysis. The mechanisms of resonant absorption of the laser light, produced in special targets containing nanostructures with dimensions comparable with the laser wavelength, enhances the proton energy. The mechanisms of self-focusing, obtained by changing the laser focal distance from the target surface, increase the local intensity and consequently the high directional ion acceleration. Real-time ion detections were performed through Thomson parabola spectrometer (TPS), ion collectors (IC), SiC detectors and ion energy analyzer (IEA) employed in time-of-flight configuration (TOF). The energy and the amount of ions increase significantly when the two non-linear phenomena occurs, as will be described. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The interaction of short laser pulses with solid targets has become an important field of study because of many applications, such as the fast ignition scheme of inertia confinement fusion, the plasma-based particle accelerator, coherent x/␥-ray sources, etc. For most of these applications, the nature of the absorption process must be determined. The density scale length of the plasmas generated from the target surfaces can be estimated as: L = cs p
(1)
where cs is the ion sound speed and p is the laser pulse duration [1]. For high intensities (>1016 W/cm2 ) and very short pulses (<1 ps) the scale length is too short to generate sufficient absorption effects and resonance absorption at the critical surface is suggested to be one of the major absorption mechanisms. Some experiments show
∗ Corresponding author. E-mail address: [email protected] (M. Cutroneo). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.129
that it plays an important role even for plasmas with a scale length considerably shorter than the laser wavelength 0 . However many theoretical works on resonance absorption are only valid for the case in which L > 0 [2]. At higher laser intensity, say L < vos /ω0 , the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking. Thus, the electron plasma wave is hard to develop and vacuum heating tends to be dominant. Here, vos = eE0 /me ω0 is the quiver velocity of the electron, E0 is the electric field normal to the interface, and ω0 is the laser frequency. A simple model is used to calculate the energy absorption efficiency when a laser of short pulse length impinges on a dielectric slab that is doped with an impurity with a resonant line at the laser frequency. It is found that the energy absorption efficiency is maximized for a certain degree of doping concentration (at a given pulse length) and also for a certain pulse length (at a given doping concentration). Dimensionless parameters are constructed, allowing calculations with one set of parameters be used to infer the results expected for other sets of parameters.
M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
Absorption processes are generally dependent on the density scale length. For shorter scale length or higher laser intensity, vacuum heating tends to be dominant. Literature reports that the electrons being pulled out and then returned to the plasma at the interface layer by the wave field can lead to a phenomenon like wave breaking. This can lead to heating of the plasma at the expanse of the wave energy [3]. Interaction of the laser radiation above some threshold intensity with a plasma of defined properties may significantly increase the charge state and energy of the produced ions, due to a peculiar effect occurring in the plasma, which focalizes further the laser pulse (self-focusing effect) acting so as a small vapor lens placed in front of the target surface. Advances in laser technology have recently enabled the observation of self-focusing in the interaction of intense laser pulses with plasmas. Self-focusing in plasma can occur through thermal, relativistic, and ponderomotive effects [4]. Thermal self-focusing is due to collisional heating of plasma exposed to electromagnetic radiation: the rise in temperature induces a hydrodynamic expansion, which leads to an increase of the refraction index and further heating. Relativistic selffocusing is caused by the mass increase of electrons traveling at speed approaching the speed of light, which modifies the plasma refractive index, depending on the electromagnetic and plasma frequencies. Ponderomotive self-focusing is caused by the ponderomotive force, which pushes electrons away from the region where the laser beam is more intense, therefore increasing the refractive index and inducing a focusing effect. Both effects of resonant absorption and self-focusing were investigated in order to produce high yield of energetic proton emission from laser irradiated targets, as will be presented and discussed.
2. Experimental set-up The main experiments have been performed by using the Nd:Yag laser of INFN-LNS of Catania and the Iodine Asterix laser of PALS Laboratory of Prague. The first can be employed at 1064 nm, 532 nm and 355 nm, 9 ns pulse duration, 1 J maximum pulse energy, with intensities between 108 and 1011 W/cm2 . The second can be employed at 1315 nm (1ω) and 438 nm (3ω), 300 ps pulse duration, 800 J maximum pulse energy, with intensities between 1013 and 1016 W/cm2 . At higher intensities the data were collected from literature and compared with our measurements in order to evaluate the generalized law of I2 scale factor. In order to generate protons, the irradiated targets were thick and thin hydrogenated films. Many of these were polyethylene based (CH2 -mnomer) with inclusions of nanostructures such as carbon-nanotubes (CNT) of different length and oxides (such as Fe2 O3 ), other consisted of hydrogenated Si, thin films of mylar covered by Au or Al films, hydrates and metals. Generally thick films (1 mm thickness) were used at LNS for irradiation at low laser intensities to generate backward directed plasmas, while thin films (of the order of 1 m in thickness) were employed at high laser intensity at PALS in order to generate forward directed plasmas. Time-of-flight (TOF) measurements have been obtained with ion collectors (IC), semiconductor detectors based on SiC, and electrostatic deflector ion energy analyzer (IEA) that permits to measure the average ion energy, the ion energy and the charge state distributions, respectively. Details on IC, SiC and IEA detector are given in the literature [5,6]. The ion plasma temperature, Ti , was measured though the Coulomb–Boltzmann shifted (CBS) fit of the experimental ion energy distributions [7]; the electronic plasma temperature, ne , was measured through the evaluation of the atoms removed from the
51
Fig. 1. Experimental set-up scheme at PALS Laboratory of Prague.
laser crater and volume of the visible plasma observed by a fast CCD camera. A Thomson parabola spectrometer (TPS) was also employed at PALS in forward direction along the normal to the target surface in order to measure the energy, charge states and ion species of ejected particles from plasma, thanks to their magnetic and electric deflection that use a magnetic field, of the order of 0.1 T, and an electric field, of about 3 kV/cm, to deflect the ions toward a multichannel-plate (MCP) detector placed orthogonally to the incident high collimated ion beams. A streak camera was employed at PALS to measure the laser focal position (FP) distance with respect to the sample surface. Negative distances mean a focus in front of the surface while positive distances mean a focus inside the target. Silicon photodiodes of different types were tested for the possibility of measurement of high-intensity X-ray pulses in the energetic range 0.7–23 keV. The main problems encountered were non-linearity and overloading of the detectors, especially for detectors assigned for soft radiation. These problems were overcome by operating the photodiodes in the integrating mode, which accomplishes a much greater dynamic range. Photodiodes BPYP03 were used for the measurement of the soft component (0.7–1.5 keV), while FLM photodiodes for the harder component (4–23 keV), both realized at the Institute of Electron Technology, Warsaw [8]. The photodiodes were placed at the distances from 100 to 200 cm from the target and at 30◦ detection angle from the normal to the target surface in backward direction. Different filters (Be, Al) of various thicknesses changed the range of detector sensitivity. Fig. 1 shows a schematic experimental set-up employed at PALS Laboratory of Prague. 3. Results At low intensities, of the order of 1010 W/cm2 , with 3–9 ns pulse duration and 1064 nm wavelength, a typical spectrum of ions
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M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
Fig. 2. Typical IC spectra obtained at low intensity at LNS of Catania relative to polyethylene irradiation (a) and relative CBS deconvolution process (c). Typical resonant absorption process at low intensity obtained by irradiating Fe2 O3 (b) and CNT (d) nanostructures, 0.1% in concentration, embedded in polyethylene.
emitted from polyethylene, detected by IC shows a large peak due to the carbon charge states and a little peak due to proton detection, as reported in Fig. 2a. In this case the TOF distance is 60 cm, thus the corresponding proton peak energy is about 75 eV. The proton and carbon peak can be interpreted as convolution of CBS distribution, according to the literature [8]. This approach permits to evaluate the plasma temperature and the acceleration voltage, that correspond to 10 eV and 67 V, respectively, as reported in Fig. 2c. The four charge states for carbon was evaluated though IEA spectrometer. The insertion of the nanostructures in the polyethylene change strongly the result of the ion emission at low laser intensity, as reported in Fig. 2b and d showing that the proton/carbon ratio increases from 0.05 in pure polyethylene up to 0.67 and 1.5 for 0.1% concentration of Fe2 O3 and carbon nanotubes. In these last cases the TOF length was 150 cm thus the corresponding maximum proton energy, calculated at the FWHM of the proton peak, is about 120 eV in the two cases. Thus the insertion of absorbent nanostructures, with length of the order of 1 m, i.e. comparable with the laser wavelength, produces effects of resonant absorption that can be responsible of a strong increment of the proton yield emission and of the proton kinetic energy. This result confirms that also the plasma temperature and acceleration voltage increase due to the resonant absorption effect. At high intensity, of the order of 1016 W/cm2 , with 300 ps pulse duration and 1315 nm wavelength, a typical spectrum of ions emitted from amorphous surface layers of hydrogenated silicon (Si:H) is reported in Fig. 3 for a thick sample producing plasma ions in backward direction (a) and for a thin target, 17 m in thickness, producing hot plasma in forward direction (b). In the first case the proton peak at 70 ns corresponds to a kinetic energy of 1 MeV while in the second case the position at 40 ns, well measurable without
the photopeak using a 8 m aluminum absorber placed in front of the IC detector, corresponds to a kinetic energy of 4.5 MeV. In this last case the short target length, producing electron emission in forward direction and enhances the effect of proton drive acceleration due to a sort of absorption resonance effect increasing significantly the proton kinetic energy. The appearance of new peaks in the IC signal clearly indicates a change of the main ion production (acceleration) mechanism. In addition to the ambipolar acceleration of ions non linear forces, including ponderomotive relativistic and self-focusing, which lead to very high laser intensity in a self-focused channel may become the main reason for the presence of high kinetic energy and high charged ions. The complexity of the laser interaction mechanisms with solid targets is due to the non-linearity of the processes occurring in the pre-plasma and of the plasma non linear optical properties which are dependent on the laser intensity and that occurs above a threshold of about 1014 W/cm2 . Self-focusing increases the intensity of the part of laser beam on the target due to the higher focusing which may reduce the spot of two order of magnitude increasing the intensities above 1018 W/cm2 . The X-ray radiation fluxes from Au-produced plasmas at comparable experimental conditions are presented in the following. Records of hard X-ray emission (HX), produced at 500 J laser pulse at 1ω normal irradiation and detected by FLM photodiodes, filtered by 1200 m Be filters, are shown in Fig. 4. The maximum of hard X-ray component (5–20 keV) was recorded at FP = −200 m, i.e. focusing the laser 200 m in front of the target surface. This operation produces a self-focusing due to the prepulse irradiation of the surface with consequent increase of the laser intensity, plasma temperature and hard X-ray production. Records of soft X-ray emission (SX), produced in the same experimental conditions, are also reported in the same plot and demonstrating that at the FP = −200 m
M. Cutroneo et al. / Applied Surface Science 272 (2013) 50–54
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ions with higher charge states, connected with the presence of fast electrons, and generated by resonant absorption mechanisms, create a maximum yield at FP = −200 m. The inverse bremsstrahlung prevails for both the lateral maxima, which I2 , 1015 W/cm2 , while the maximum yield is related to the resonant absorption process. The initial laser beam diameter of 29 cm, focused by an aspheric lens it reaches a minimum diameter value of 70 m at FP = 0. When the focal position is FP = −200 m and the laser energy is very low, the self-focusing cannot happen because the conditions are below the threshold value but increasing the laser energy above the threshold of the order of 1014 W/cm2 [9], it occurs because the high light refraction effect produces a further laser beam focalization, due to the dense plasma volume in front of the target, which converges the beam so as a focusing lens. Thus special effects occur, as evident by the higher charge state distributions observed through IEA spectrometer, higher plasma temperature effects and higher proton energy and yield emission. 4. Discussion and conclusions
Fig. 3. Typical SiC detector spectrum, in backward direction, relative to the ion emission from high intensity laser irradiation of thick hydrogenated silicon (a) and IC detector spectrum, in forward direction, relative to the ion emission from a thin film of the same material irradiated in the same conditions (b).
conditions their yield decreases, confirming that an increment of X-ray energy occurs during the self-focusing effect. The ion yield versus focus position plot of Fig. 3 indicates that for low charge states ions are due to ionization by thermal electrons generated by inverse bremsstrahlung mechanism. Such a result was ascribed to the volume effect of produced plasma due to the interaction of continuously decreasing diameter of the laser beam with respect to the target surface that, in the case of self-focusing mechanisms, is found to a forward negative focus positions. In contrast,
The existence of an optimum laser focus position for generation of the fastest ions with the highest charge states in front of the target surface is consistent with our old observations [10]. The course of dependencies and similar values of the highest zmax indicate a threshold for the appearance of relativistic self-focusing of laser beam and a principal limitation of the maximum attainable laser intensity. At PALS differences for 1ω and 3ω could be ascribed to a different absorption of laser radiation, in accordance with the scaling relation I2 . The front part of the 300 ps laser pulse interacts with the target and creates an expanding plasma plume. Considering for simplicity, the expansion velocity v = 106 m/s, the plasma plume attains the distance of 100 m within the first 100 ps. For the laser beam diameter of 70 m, the self-focusing length should be about 100–200 m, at least. For FP = 0, the more the plasma plume expands, the longer the interaction length, but the lower the laser intensity with which the front of the plasma interacts. In contrast, starting from FP = −400 m, the plasma plume faces during the expansion, increasing laser intensity only. The following conclusions can be made: - Nano and micrometric structures, such as carbon nanotubes, polymeric chains and molecular groups with dimensions comparable with the laser wavelength may induce resonant absorption effects increasing the plasma temperature and the acceleration ion drive mechanisms. - Self-focusing processes influence significantly the generation of ions with the highest charge states, using high power iodine laser with the pulse length of 300 ps and an optimal FP distance can be found to enhance this effect of intensity increase due to the focal spot decreasing. Acknowledgments This work was supported by INFN 5th National Committee of PLATONE and LIANA Projects. References
Fig. 4. Typical X-ray yield versus focal position with respect to the target surface from PALS experiment obtained by irradiating a thin Au film.
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