Effect of laser light polarization on generation of relativistic ion beams driven by an ultraintense laser

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1 Effect of laser light polarization on generation of relativistic ion beams driven by an ultraintense laser Jaroslaw Domanski, Jan Badziak, and Sławomir Jabłoński Citation: J. Appl. Phys. 113, (2013); doi: / View online: View Table of Contents: Published by the American Institute of Physics. Additional information on J. Appl. Phys. Journal Homepage: Journal Information: Top downloads: Information for Authors:

2 JOURNAL OF APPLIED PHYSICS 113, (2013) Effect of laser light polarization on generation of relativistic ion beams driven by an ultraintense laser Jaroslaw Domanski, 1,2 Jan Badziak, 2 and Sławomir Jabłonski 2 1 Warsaw University of Technology, Faculty of Physics, Koszykowa 75, Warsaw, Poland 2 Institute of Plasma Physics and Laser Microfusion, Euratom Association, Hery 23, Warsaw, Poland (Received 11 March 2013; accepted 16 April 2013; published online 7 May 2013) The effect of laser light polarization on properties of proton and carbon ion beams generated from a CH target irradiated by a 130 fs laser pulse of ultra-relativistic intensity ( W/cm 2 )is investigated using particle-in-cell simulations. It is shown that only circular light polarization ensures the production of quasi-monoenergetic relativistic beams of both protons and carbon ions from such a target while using the linear one results in the generation of quasi-monoenergetic protons accompanied with carbon ions of complex and broad energy spectrum. The influence of the target thickness and laser intensity on the ion energy spectrum and the laser-ions energy conversion efficiency is examined. VC 2013 AIP Publishing LLC.[ I. INTRODUCTION Laser-driven generation of high-energy ion beams has recently attracted considerable interest due to a variety of potential applications including proton radiography, inertial confinement fusion (ICF) fast ignition, nuclear physics, or hadron therapy (e.g., Refs. 1 3). Some of these applications (e.g., nuclear physics, hadron therapy) require light ions of sub-gev to multi-gev energies which are attainable only at very high laser intensities W/cm 2. Such laser intensities will soon become available with the Extreme Light Infrastructure (ELI) facilities which have just been started to be built in Europe. 4 At fixed target parameters (composition, thickness), the energy spectrum of ions driven by a short-pulse laser (the mean, E i, and the maximum, E m i, ion energy and the spectrum width de i ) and the laser-ions energy conversion efficiency, g, depend essentially on laser intensity and the laser light polarization which, in turn, determine a dominant mechanism of ion acceleration. At moderate laser intensities (say W/cm 2 ) and the linear polarization (LP), usually the Target Normal Sheath Acceleration (TNSA) mechanism 1,2,5 dominates the ion acceleration process. In TNSA, an intense short (1 ps) laser pulse interacting with the front surface of the target produce plasma and fast electrons. The electrons penetrate through the target and at the target rear surface they form a Debye sheath playing the role of virtual cathode. A high electric field generated by this cathode E c T h /(ek Dh ) efficiently ionizes the surface atoms and accelerates the produced ions (T h and k Dh are the temperature and the Debye length of fast (hot) electrons, respectively). Though maximum ion energies driven by TNSA can reach even hundreds of MeV, the ion energy spectrum is usually broad (de i /E i 1) and the laser-ions energy conversion efficiency is relatively low (g 0.1). 1,2,5,6 When the laser intensity exceeds W/cm 2, a significant role in fast ion generation from laser-produced plasma is played by Radiation Pressure Acceleration (RPA) 7 14 (also known as Skin-Layer Ponderomotive Acceleration SLPA 2,15 18 ). In the RPA scheme, the ponderomotive force (the radiation pressure) created by a laser pulse near the critical plasma surface in front of the target drives forward and compresses the target plasma in a piston-like manner (the hole boring stage). When the compressed plasma (ion) bunch reaches the target rear surface, it is detached from the target and accelerated further by the radiation pressure like a sail pushed by the wind (the light sail stage). As opposed to TNSA, where the accelerating field is determined by plasma properties, RPA employs basically an optical property of plasma near the critical surface (the ponderomotive force is determined by the gradient of laser energy density in plasma, e.g., Refs. 19 and 20), which only in secondary way depends on plasma parameters. The role of RPA in the ion acceleration process is the higher the higher is laser intensity, 7,21 and for ultrarelativistic intensities (>10 22 W/cm 2 ) RPA usually dominates other acceleration mechanism. 7,9,21 The RPA-driven ion beams have the energy spectrum much more narrow than those driven by TNSA and can be produced with the energetic efficiency of tens of percent. 7,9,11,14,21 A contribution of RPA to the ion acceleration process depends essentially on laser light polarization. Macchi et al. first demonstrated by particle-in-cell (PIC) simulations that using the circular polarization (CP) is beneficial for RPA and can lead to the dominance of RPA over TNSA even at moderate laser intensities (10 20 W/cm 2 ). 8 This conclusion has then been confirmed in numerous papers, 10,12,18,22,23 where the effect of light polarization (CP vs LP) on generation of non-relativistic proton beams from hydrogen plasma targets was investigated by PIC simulations. It was also shown that using CP at high laser intensities W/cm 2, quasimonoenergetic relativistic proton beams can be produced with a high energetic efficiency. 9,14,24,25 In this paper, we investigate the effect of light polarization (CP versus LP) on generation of relativistic protons and carbon ions from a CH target of realistic parameters (composition, density, thickness) by a subpicosecond laser pulse of ultra-relativistic intensity W/cm 2 predicted for the ELI laser facility. Using PIC simulations shows that only /2013/113(17)/173302/6/$ , VC 2013 AIP Publishing LLC

3 Domanski, Badziak, and Jabłonski J. Appl. Phys. 113, (2013) CP ensures the production of quasi-monoenergetic beams of both protons and carbon ions from such a target while using LP results in the generation of quasi-monoenergetic protons accompanied with carbon ions of a complex and broad energy spectrum. The influence of laser intensity and the target thickness on characteristics of the ion beams and the laser-ions energy conversion efficiency is examined. II. RESULTS AND DISCUSSION The numerical results presented in this paper were obtained using the one-dimensional (1D) relativistic PIC code, 26 which is a modified version of a well-known LPICþþ code. 27 The calculations were performed for a linearly or circularly polarized laser pulse of duration 130 fs and wavelength 800 nm. The laser pulse intensity was scanned in the range from W/cm 2 to W/cm 2. The shape of the laser pulse was described by a super-gaussian function. In the simulations, the laser pulse interacted with a polystyrene (CH) target of the thickness between 0.1 lm and 1 lm and densities of H þ and C þ6 equal to atoms/cm 3. In front of the target, a preplasma layer of the thickness 0.25 lm and the density shape described by an exponential function were situated. First, we will present results of calculations for fixed target thickness equal 1 lm. Below, there are presented plots showing the mean energy of carbon ions and protons as well as the energy conversion efficiency g and the standard deviation of ion energy de i =E i for both types of particles. The data obtained for different wave polarizations are compared assuming equal laser energy fluence for both polarizations. In addition, the mean energies of ions from the simulations are compared with the ones predicted from the Light Sail (LS) model. 13,21 In this model, the projectile is defined by the light reflection coefficient R and the areal mass density r ¼ q l, (q and l are the projectile mass density and thickness, respectively) and the projectile motion is described by the equation, 13,21 dðb cþ dt ¼ R 2 I L ðt z=cþ 1 b r c 1 þ b ; (1) where ¼ dz dt ; b ¼ =c; c ¼ð1 b2 Þ 1 2,IL is light intensity, and c is speed of light in vacuum. For R ¼ 1 (the projectile is assumed to be a perfectly reflected mirror), the final projectile velocity b f is determined by the expression, b f ¼ ð1 þ eþ2 1 ð1 þ eþ 2 þ 1 ; e ¼ 2 F L r c 2 ; (2) where F L ¼ Ð I L dt is the energy fluence of laser pulse. 13,21 The mean energies of carbon ions are similar for both types of polarizations, what we can see in Fig. 1(a), while the mean energies for protons are higher for LP, what is presented in Fig. 1(b). Maximum mean energies for both types of ions are achieved for the highest intensity of laser pulse (Fig. 1) and for the 1 lm target they are equal about 10 GeV for carbon ions for both polarizations and 1 GeV for protons at CP and 1.7 GeV for protons at LP. The light pressure produced on the target is proportional to the Pointing vector, therefore, for the circularly polarized FIG. 1. The mean energy of carbon ions (a) and protons (b) for the target thickness equal 1 lm and both types of polarization. The mean energy of carbon ions obtained from the LS model is added to the plot (a) for a comparison. wave the pressure is stable (there is no high-frequency oscillating component), while for the linearly polarized wave it varies between maximum value for the wave amplitude and 0 for the wave node. For this reason, we observe significant differences in ion acceleration for CP and LP. For CP (Fig. 2(a)), we can see a single high peak of the electric field E X (vector along the axis of propagation of the laser beam) created in the skin layer in front of the target, which is typical for the RPA mechanism of acceleration. 8,9,18 However, for LP (Fig. 2(b)) the electric field E X penetrates interior of the target forming many smaller peaks which is the result of the formation of the virtual cathodes in the plasma and appearance of the TNSA mechanism. For the circularly polarized wave, for which RPA dominates, due to the constant light pressure, velocities of carbon ions and protons are more or less equal to each other (Fig. 3(b)). As a result, energies of carbon ions are about twelve times higher than those of protons. However, for the linearly polarized wave, for which the contribution of TNSA is essential, protons are more efficiently accelerated by virtual cathode than carbon ions as they have more favourable ratio of the electric charge to the mass. Due to this fact, protons and carbon ions are separated in space (Fig. 3(a)). The protons achieve the higher velocities at the expense of carbon ions and, as a result, their energies are higher than in case of CP. A comparison of the mean energies of carbon ions obtained from the simulations and the LS model is presented in (Fig. 1(a)). A fairly good agreement between the simulations and the model can be seen in spite of the fact that the model does not take into account protons existing in the target. In Fig. 4, the energy conversion efficiencies for carbon

4 Domanski, Badziak, and Jabłonski J. Appl. Phys. 113, (2013) FIG. 2. The electric field vector (E X ) along the axis of propagation of the laser beam and the charge density in the early stage of simulation for the 1 lm target and the laser intensity equal to W/ cm 2 for both types of polarization: circular (a) and linear (b). ions and protons are presented. The conversion efficiency is calculated by dividing the sum of macro-particle energies by the energy of a laser pulse. The efficiency increases with an increase in laser intensity and achieves around 64% for carbon ions for both polarizations, while for protons it reaches 10.5% for LP and 6% for CP. Though the mean carbon ion energies are similar for CP and LP, the shape and the width of the carbon ion energy spectra are dramatically different for both the cases. It can be seen in Figs. 5 7 where the standard ion energy deviation de i =E i and the ion spectra shapes are presented. For CP, the carbon ion spectrum is quasi-monoenergetic and the ion energy dispersion is small: de i =E i For LP, the spectrum is complex and the carbon ion energies are dispersed in a broad energy range: de i =E i It is due to the fact that the motion of C ions, which are in a majority situated in the target interior (and not at the target rear surface) is determined in the LP case by the strong E X field oscillating inside the target (Fig. 2(b)), thus pushing the ions both in the forward and backward direction. In the case of protons, which are lighter and move faster, most of protons are situated near the target rear surface where the field E X of positive (þ) sign, produced by the virtual cathode, dominates (Fig. 2(b)). When the laser intensity is high enough (> W/cm 2 ), a majority of protons is accelerated by this field and, as a result, the produced proton beam has a narrow energy spectrum similar to that in the CP case (Fig. 5(b)). However, when the laser intensity is not sufficiently high the oscillating component of the E X field at the target rear is substantial, thus the proton energies are dispersed in a much broader energy range (Figs. 5(b) and 7). Now, we will present the results for the fixed laser intensity equal to W/cm 2 for CP and W/cm 2 for LP (the laser energy fluency for CP is equal to that for LP). Figs. 8, 10 and 11 show the mean energy, the energy conversion efficiency and the standard energy deviation for carbon ions and protons as a function of the target thickness. In Fig. 8(a), the mean energies of carbon ions predicted by the LS model are also added. For a thin target, we observe a significant difference between the ion beam parameters for two types of polarizations. For CP, the mean energy of both types of ions increases inversely proportionally to the thickness of the target and achieves around 72 GeV for carbon ions and 7.7 GeV for protons (Fig. 8). For LP, the mean energy reaches a maximum around the thickness L T ¼ 0.5 lm both for carbon ions and protons. For the thickness L T ¼ 0.1 lm, the mean energy of carbon ions drops to 650 MeV and for protons to 500 MeV. The energy spectrum for LP is much more dispersed than for CP. Due to this fact, in the acceleration process a matter forming the thinnest target becomes scattered over a large distance (Fig. 9). In such a case, the density of the matter forming the target becomes low, the target becomes transparent for the light wave and a significant part of the light energy is lost. As a result, the mean ion energies for the 0.1 um target are much smaller than that for the thicker one or for CP. In Fig. 10, the energy conversion efficiency for carbon ions and protons is presented. For LP, the efficiency increases with the target thickness and achieves around 60% for carbon ions and 10% for protons. The energy conversion efficiency for CP changes weakly with L T and reaches a value in the range from 60% to 75% for carbon ions, and around 6% for protons. The standard energy deviation for carbon ions and protons is presented in Fig. 11. For carbon ions, the energy FIG. 3. The charge density for 1 lm target and the laser intensity equal to W/cm 2 for both types of polarization: linear (a) and circular (b).

5 Domanski, Badziak, and Jabłonski J. Appl. Phys. 113, (2013) FIG. 6. The energy spectrum of carbon ions for the 1 lm target, laser intensity equal to W/cm 2 for CP and W/cm 2 for LP and both types of polarization. FIG. 4. The energy conversion efficiency for carbon ions (a) and proton (b) for the target thickness equal to 1 lm and both types of polarization. spectrums for CP are much more narrow than for LP. However, for protons, the width of energy spectrum for CP is comparable to that for LP and at L T ¼ 0.1lm, the energy deviation for both polarizations is large. FIG. 7. The energy spectrum of protons for the 1 lm target, laser intensity equal to W/cm 2 for CP and W/cm 2 for LP and both types of polarization. Fast protons and carbon ions of energies from hundreds MeV/amu to multi-gev/amu, which can be produced from lm thick CH target by the ELI laser of intensity FIG. 5. The standard ion energy deviation for carbon ions (a) and protons (b) for the target thickness equal to 1lm and both types of polarization. FIG. 8. The mean energy of carbon ions (a) and protons (b) for the laser intensity equal to W/cm 2 for CP and W/cm 2 for LP and both types of polarization. The mean energy of carbon ions obtained from the LS model is added to the plot (a) for a comparison.

6 Domanski, Badziak, and Jabłonski J. Appl. Phys. 113, (2013) FIG. 9. The charge density for the 0.1lm target and the laser intensity equal to W/cm 2 and linear polarization of the laser beam W/cm 2, have a potential to be used in nuclear physics or hadron therapy (after further narrowing the ion energy spectrum). A question arises whether the ELI laser would be capable of producing proton or carbon ion beams of ion energies E i and beam fluencies F b relevant to ion fast ignition of DT fusion, 1,2,28 i.e., of E i MeV/amu 28,29 and F b 1 GJ/cm Assuming that the laser pulse duration is still within the fs range, a prospect for reaching the required parameters for protons is rather negative. At laser intensities W/cm 2 and 1 lm CH target, the proton beam energy fluence reaches the required value 1 GJ/ cm 2, however, the mean proton energies are by about two orders of magnitude higher than those needed for optimum energy deposition to the compressed fuel (10 20 MeV 28,29 ). Slowing down the protons to the required energies by a significant decrease in laser intensity or/and an FIG. 11. The standard ion energy deviation for carbon ions (a) and protons (b) for the laser intensity equal to W/cm 2 for CP and W/cm 2 for LP and both types of polarization. increase in the target thickness will result, in turn, in a decrease in the energy fluence below the required value. The situation seems to be much better for carbon ions. Though for I L W/cm 2 and the 1 lm CH target, the mean carbon ion energies are much higher than the optimal ones (0.5 GeV 28,29 ), the carbon beam energy fluence (8 GJ/ cm 2 ) is higher than the threshold value (1 GJ/cm 2 ) as well. So, by careful optimizing the target thickness and the laser intensity (using a thicker, a few-lm target and decreasing intensity of the CP beam to a few W/cm 2 ), we can reach the beam parameters close to the required ones. Thus, although the ELI laser energy is by two orders of magnitude lower than needed for the full-scale fast ignition experiments, it can also be considered as a tool for research in inertial fusion. FIG. 10. The energy conversion efficiency for carbon ions (a) and protons (b) for the laser intensity equal to W/cm 2 for CP and W/cm 2 for LP and both types of polarization. III. CONCLUSIONS It is shown that for ultra-relativistic laser intensities predicted for the ELI laser an important factor determining quality of ion beams produced by the laser from realistic CH targets is the laser light polarization. Only CP ensures the production of quasi-monoenergetic relativistic beams of both protons and carbon ions from such a target while using LP results in the generation of quasi-monoenergetic protons accompanied with carbon ions of complex and broad energy spectrum. Moreover, CP enables us to employ thinner targets and, as a result, to achieve higher ion energies. Our results suggest that the ELI laser beam with CP would be capable of producing high-current quasi-monoenergetic proton and carbon ion beams of the mean ion energy reaching a few GeV/ nucleon. Such ion/proton beams have a potential to be

7 Domanski, Badziak, and Jabłonski J. Appl. Phys. 113, (2013) applied in various branches of research including nuclear physics, hadron therapy, or proton radiography of dense matter. 1 M. Borghesi, J. Fuchs, S. V. Bulanov, A. J. Mackinnon, P. K. Patel, and M. Roth, Fusion Sci. Technol. 49, 412 (2006) and references therein. 2 J. Badziak, Opto-Electron. Rev. 15, 1 (2007) and references therein. 3 K. W. D. Ledingham and W. Galster, New J. Phys. 12, (2010) and references therein. 4 The ELI parameters are available at 5 S. C. Wilks, A. B. Langdon, T. E. Cowan, M. Roth, M. Singh, S. Hatchett, M. H. Key, D. Pennington, A. MacKinnon, and R. A. Snavely, Phys. Plasmas 8, 542 (2001). 6 L. Robson, P. T. Simpson, R. J. Clarke, K. W. D. Ledingham, F. Lindau, O. Lundh, T. McCanny, P. Mora, D. Neely, C. G. Wahlstrom, M. Zepf, and P. McKenna, Nat. Phys. 3, 58 (2007). 7 T. Esirkepov, M. Borghesi, S. V. Bulanov, G. Mourou, and T. Tajima, Phys. Rev. Lett. 92, (2004). 8 A. Machi, F. Cattani, T. V. Liseykin, and F. Cornalti, Phys. Rev. Lett. 94, (2005). 9 X. Zhang, B. Shen, X. Li, Z. Jin, F. Wang, and M. Wen, Phys. Plasmas 14, (2007). 10 T. V. Liseykina and A. Macchi, Appl. Phys. Lett. 91, (2007). 11 A. P. L. Robinson, M. Zepf, S. Kar, R. G. Evans, and C. Bellei, New. J. Phys. 10, (2008). 12 T. V. Liseykina, M. Borghesi, A. Macchi, and S. Tuveri, Plasma Phys. Controlled Fusion 50, (2008). 13 A. Macchi, S. Veghini, and F. Pegoraro, Phys. Rev. Lett. 103, (2009). 14 B. Qiao, M. Zepf, M. Borghesi, and M. Geissler, Phys Rev. Lett. 102, (2009). 15 J. Badziak, H. Hora, E. Woryna, S. Jabłonski, L. Laska, P. Parys, K. Rohlena, and J. Wołowski, Phys. Lett. A 315, 452 (2003). 16 J. Badziak, S. Jabłonski, and S. Głowacz, Appl. Phys. Lett. 89, (2006). 17 J. Badziak, S. Jabłonski, P. Parys, M. Rosinski, J. Wołowski, A. Szydłowski, P. Antici, J. Fuchs, and A. Mancic, J. Appl. Phys. 104, (2008). 18 J. Badziak, G. Mishra, N. K. Gupta, and A. R. Holkundkar, Phys. Plasmas 18, (2011). 19 H. Hora, J. Badziak, M. N. Read, Y. T. Li, T. J. Liang, Y. Cang, H. Liu, Z. M. Sheng, J. Zhang, F. Osman, G. H. Miley, W. Zhang, X. He, H. Peng, S. Glowacz, S. Jablonski, J. Wolowski, Z. Skladanowski, K. Jungwirth, K. Rohlena, and J. Ullschmied, Phys. Plasmas 14, (2007). 20 H. Hora, Laser Part. Beams 30, 325 (2012). 21 J. Badziak and S. Jabłonski, Appl. Phys. Lett. 99, (2011). 22 O. Klimo, J. Psikal, J. Limpouch, and V. T. Tikhonchuk, Phys. Rev. ST Accel. Beams 11, (2008). 23 T. V. Liseykina, D. Prellino, F. Carnolti, and A. Macchi, IEEE Trans. Plasma Sci. 36, 1866 (2008). 24 K. H. Pae, I. W. Choi, and J. Lee, Laser Part. Beams 29, 11 (2011). 25 M. Grech, S. Skupin, A. Diaw, T. Schlegel, and V. T. Tikhonchuk, New. J. Phys. 13, (2011). 26 J. Badziak and S. Jabłonski, Phys. Plasmas 17, (2010). 27 R. Lichters, R. E. W. Pfund, and J. Meyer-Ter-Vehn, LPICþþ: A parallel one-dimensional relativistic electromagnetic particle-in-cell-code for simulating laser-plasma-interactions, Report MPQ 225, Max-Planck-Institut f ur Quantenoptik, Garching, Germany, 1997, the code is available at 28 J. C. Fernandez, J. J. Honrubia, B. J. Albright, K. A. Flippo, D. Cort Gautier, B. M. Hegelich, M. J. Schmitt, M. Temporal, and L. Yin, Nucl. Fusion 49, (2009). 29 J. Davis et al., Plasma Phys. Controlled Fusion 53, (2011). 30 N. Naumova, T. Schlegel, V. T. Tikhonchuk, C. Labaune, I. V. Sokolov, and G. Mourou, Phys. Rev. Lett. 102, (2009). 31 J. Badziak, S. Jabłonski, T. Pisarczyk, P. Rączka, E. Krousky, R. Liska, M. Kucharik, T. Chodukowski, Z. Kalinowska, P. Parys, M. Rosinski, S. Borodziuk, and J. Ullschmied, Phys. Plasmas 19, (2012). 32 P. Lalousis, H. Hora, S. Eliezer, J.-M. Martinez-Val, S. Moustaizis, G. H. Miley, and G. Mourou, Phys. Lett. A 377, 885 (2013).