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Emission direction of fast electrons in high-intensity laser interactions with solids
High Energy Density Physics 1 (2005) 61e65 www.elsevier.com/locate/hedp
Emission direction of fast electrons in high-intensity laser interactions with solids J. Zhang*, Y.T. Li, Z.M. Sheng, Z.Y. Wei, Q.L. Dong, X. Lu Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, 8 ZhongGuanCun NanSanJie, HaiDian District, PO Box 603, Beijing 100080, China Received 2 August 2005 Available online 21 October 2005
Abstract Energetic fast electron beams can be generated in ultrashort and ultraintense lasereplasma interactions. In this paper the dependence of the emission direction of the fast electron beams on the experimental conditions of the laser and plasmas, such as intensity, polarization, incident angle, scale length of the preplasma, as well as the possible ways to control the emission direction of fast electrons are discussed. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Intense lasereplasma interaction; Emission direction of fast electron
1. Introduction Intensive studies on fast electron generation and transport in high-intensity laseresolid interactions have been carried out because of its significance for the fast ignition concept in the laser confined fusion and other applications. A collimated fast electron beam with a definite direction is crucial for these applications. Unfortunately, the physics processes related with the emission direction of the laser-produced fast electron beams are very complicated. The emission direction depends on a variety of parameters, such as laser intensity, laser polarization, laser incident angle, scale length of the preplasma and target materials. Under different conditions, fast electron emission has been observed in various directions, such as the forward direction, the normal direction of the target surface, the direction of the laser electric field, the specular direction of the incident laser. However, one can still obtain very well-collimated fast electron beams emitted in some particular directions with a careful control of the laser and the plasma conditions. In this paper we discuss the basic physics and the possible ways to control the emission direction of
* Corresponding author. Tel.: C86 10 82649356; fax: C86 10 82649531. E-mail address: [email protected] (J. Zhang). 1574-1818/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.hedp.2005.09.002
fast electrons generated in the interactions of a high-intensity laser pulse with a solid target. 2. Intrinsic directions of laser pulses and plasmas First let us consider the basic geometry of a laser beam and a solid target. The intrinsic directions of the laser beam and the intrinsic properties of the target will decide the initial directions of electrons. For the laser beam, there are three intrinsic directions which are propagation vector k, electric field E (polarization), and magnetic field B. For the target, the gradient of the electron density of the plasma generated from the target, Vne (target normal direction), the vector of the electron plasma wave induced in collective interactions, kp, are usually related with electron emission directions. These directions are often associated with the interaction conditions and sometimes are distorted during the interaction. Secondly, self-induced electrostatic fields and magnetic fields always co-exist with the plasma. These introduce new factors to change the emission directions. The initial directions of the accelerated fast electrons inside the plasma will be modified when the electrons pass through these fields. Therefore, one has to consider the effects of the spontaneous electromagnetic fields when studying fast electron emission. All of these factors make the emission directions of fast electrons very complicated, but also very
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informative to understand the physics process in the interaction. In the following sections, we shall present some of our observations in ultrashort intense laseresolid interactions. 3. Emission direction of fast electrons from ultrashort laser interactions with solid targets 3.1. Emission of fast electron beam in the intrinsic laser directions Fig. 1 shows the cutaway view of the angular distribution of the backward fast electrons generated in the interaction of an s-polarized femtosecond laser pulse with an aluminum plasma with a steep density profile at a laser intensity of 2 ! 1016 W/ cm2 [1]. The experiments were carried out with a Ti:sapphire laser at the Institute of Physics, Chinese Academy of Sciences. The laser delivered 5 mJ energy at 800 nm in 150 fs pulses. The diameter of the focal spot was less than 15 mm. The laser incident angle was 45 . The electrons were measured by DEF films. The sharp boundary of the exposure on the DEF films and the geometry of the target ensured that the electrons recorded by the film came from the front side of the Al target. The ejected fast electrons were found to be collimated along the laser electric field direction in a plane perpendicular to the incident plane. In the incident plane, no fast electrons were measured. It was also found that the fast electrons with higher energies have narrower angular divergence. This suggests that the ejected fast electrons were mainly accelerated by the electric field of the s-polarized laser pulses. This phenomenon seems to be similar to the results of laser accelerator injector based on laser ionization and the ponderomotive acceleration of electrons in gas, where electrons are accelerated in the polarization direction [2], but the heating mechanisms may be different. When the laser intensity is increased to be relativistic (the classical normalized momentum of electrons quivering in the laser electric field a0 O 1 or laser intensity of Il2 O 1.37 ! 1018 W/cm2), the laser magnetic field also becomes 90 150
important in the laser acceleration. The v ! B force perceived by electrons is comparable with that of the electric field. The electron motions will be modified greatly by the laser magnetic field, resulting in a longitudinal acceleration. More electrons will be accelerated along the laser propagation direction. Fig. 2 shows the angular distribution of the forward fast electrons generated in the interaction of a p-polarized femtosecond laser pulse with a 30 mm thick aluminum foil at a laser intensity of 2 ! 1018 W/cm2. Fast electrons with energies O0.6 MeV were measured by a stack of imaging plates. The experiments were carried out using the eXtreme Light II (XL-II) laser system at the Institute of Physics, Chinese Academy of Sciences. The laser system can produce a linearly polarized pulse with an energy up to 0.6 J in a duration of 30 fs at a wavelength of 800 nm. The laser incident angle was 45 . The diameter of the focal spot is 15 mm. One can see that a well-collimated fast electron beam is generated along the propagation direction of the laser pulse within G5 . When an ultrashort laser beam is incident onto a solid target obliquely, a portion of the laser beam is reflected in the specular direction near the critical density surface of the plasma. Theoretical simulations predicted that the reflected laser field can also accelerate electrons [3]. Fig. 3 shows such an angular distribution, where fast electrons are ejected in the specular direction of the laser beam. This measurement was also performed with the XL-II laser system. The laser beam was incident on an aluminum target at 45 and the laser intensity was 2 ! 1017 W/ cm2. Similar electron beam was also observed by other groups previously [4]. 3.2. Emission of fast electron beam in the target normal direction The geometry of the solid target will affect the spatial shape of the plasma and the critical density surface. When a p-polarized intense laser obliquely irradiates a plasma with a very steep density boundary, vacuum heating or resonance absorption can produce fast electrons in the target normal direction [5]. Fig. 3 shows a fast electron beam ejected almost in the front
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Fig. 2. The angular distribution of forward fast electrons with energies over 600 keV for p-polarized laser at an intensity 2 ! 1018 W/cm2.
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target normal direction besides the one in the laser specular direction. 4. Dependence of the emission directions of fast electrons on laser and target parameters and possible control of the emission directions We have shown some examples of the intrinsic emission directions of fast electrons so far. However, in a specific experiment, the fast electron emission depends on the laser intensity, polarization, incident angle, and the status of the preplasma generated by the prepulses before the arrival of the main laser pulse. Careful studies on these dependences are the first step to a possible control of emission directions of fast electrons, which is a pending problem so far. The physics is not fully understood. However, we would like to present several examples to shed some lights on this aspect. If one only considers the laser pulse, the key parameter is the normalized quivering momentum of electrons in the laser electric field a0 Z eE/meu0c Z 8.5 ! 10ÿ10Il1/2, where I is the laser intensity in W/cm2, and l is the laser wavelength in mm, e is the electron charge, E is the magnitude of the laser field, me is the electron mass, u0 is the laser angular frequency, and c is the light velocity. For a laser intensity of Il2 1.37 ! 1018 W/cm2 (a0 1), say, w1016 W/cm2, the laser electric field is the predominant factor. For the s-polarized laser field, electrons are mainly dominated by transverse acceleration (see Fig. 1), while for the p-polarized laser field, the processes such as resonance absorption and vacuum heating will accelerate electrons in the target normal direction (see Fig. 3). Almost all of the outgoing fast electrons were emitted in the normal direction. The emission direction of the fast electrons obeys the momentum conservation [3]. However, when the laser intensity is increased to be relativistic, Il2 O 1.37 ! 1018 W/cm2 (a0 O 1), the amplitude of the magnetic field of the laser beam is comparable to that of the laser electric field, the electrons tend to be accelerated in the longitudinal direction by J ! B heating (see Fig. 2). These forward
fast electrons are of significance in the fast ignition concept [6]. If laser peak amplitude a0 w 1, the situation will be much more complicated, since the transverse and longitudinal forces are comparable and will compete each other. Fig. 4 shows the angular distribution of the forward fast electrons behind the target illuminated by a p-polarized laser beam with an incidence angle of 45 for laser intensities from the sub-relativistic to the relativistic [7]. Here 0 and 45 correspond to the laser propagation vector and the target normal, respectively. In the experiment a 0.6e1 ps, 1.053 mm linearly polarized laser pulse with an energy up to 10 J, output by the GEKKO Module II (GM-II) laser facility at the Institute of Laser Engineering, Osaka University, was focused onto a 5 mm thick aluminum foil target at an incident angle of 45 . The diameter of the focus was about 30 mm. The laser intensity on targets was adjustable in the range of (0.2e4) ! 1017 W/cm2. A prepulse with an energy about 3 ! 10ÿ3 of the main pulse, originating from the regenerative amplifier, starts from 700 ps ahead of the main pulse peak. Therefore, the main laser pulse interacted with a preplasma. The forward hot electrons were measured behind the target by LiF (Mg, Cu, P) thermoluminescence dosimeters (TLD). One can see the peak of the forward fast electron beam produced by the p-polarized laser beam moves to the laser propagation direction from the target normal direction as the laser intensity is increased to be relativistic. In the experiments, s-polarized laser pulses were also used to study the effects of polarization on fast electrons. We have found that the effects of laser polarization on fast electron emission become weaker at a relativistic intensity than that at a sub-relativistic intensity. This is mainly caused by the following two reasons. One is that the forward acceleration due to laser magnetic field is dominated at the regime of a0 O 1. This acceleration is less dependent on laser polarization. The other is that the critical surface of plasma becomes curved due to the high laser pressure. This leads to the formation of a p-polarized component of the laser electric field. 1500 2.8x1018 W/cm2 8.6x1017 W/cm2 1000
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Fig. 5. Angular distributions of the fast electrons with energies greater than 300 keV in the laser incident plane for the p-polarized laser pulse at an intensity of 1e2 ! 1018 W/cm2 for three different incidence angles of (a) 22.5 , (b) 45 , and (c) 70 .
A preplasma is usually formed before the main laser pulse arrives at the target surface because of the practical limit of the laser technology, i.e., the existence of the amplified spontaneous emission (ASE), especially for a relativistically intense lasereplasma interaction. When a preplasma is present in advance of the peak of the main laser pulse, the interaction will depend, to a large extent, on the properties of the preformed plasma, particularly its scale length [8]. For example, for a preplasma with a short-scale length, the vacuum heating or the resonance absorption may dominate the absorption process, while for the case of large-scale length, the stochastic heating [9] and various parametric instabilities such as the stimulated Raman scattering, two-plasma decay may also accelerate electrons [10]. In this case, electrons are accelerated directly by the laser fields or by the excited electron plasma waves. Therefore, control of the plasma density scale length may lead to desired fast electron emissions. The spontaneous electrostatic fields and magnetic fields in the plasma will modify the original directions of fast electrons. This indicates another possible way to control the fast electron emission. After the fast electrons are generated in the laser focal region, part of them will eject backward into vacuum from the underdense plasma in front of the target. The others move forward into overdense region and propagate inside the cold target region. About 20e40% laser energy can be converted to fast electrons at intensities w1019 W/cm2. This forms a highly directional fast electron beam with a current as high as multi-Mega (even Giga)-Ampere [11]. This huge current will induce strong electrostatic fields and magnetic fields inside the plasma or target bulk. The bulk fields, in turn, will affect the fast electron transport. Studies have shown that the forward fast electron beam will be collimated by the static magnetic fields [12]. The fast electron beam may be also broken up into filaments due to the two-stream instability and Weibel instability [13]. The spontaneous electrostatic fields and magnetic fields near the front target surface can also be used to control the directions of fast electrons. Our recent experiments have demonstrated a fast electron jet emitted along the target surface when the electron density gradient is steep. It can be explained by the confinement of quasistatic magnetic fields and electrostatic sheath fields. The experiments were also carried out using the XL-II laser system. The angular distributions of fast electrons were measured by a stack array of imaging plates (IP). Fig. 5(a)e(c) shows the angular distributions of fast electrons in the polarization plane for laser incident angles of 22.5 , 45 , 70 , respectively, at a laser intensity of 1e2 ! 1018 W/cm2. The most striking aspect of our measurements is the presence of fast electrons ejected along the front target surface, marked as ‘‘surface electrons’’ in the figures. In particular, the number of the surface fast electrons increases with the laser incident angle. The emission peak along the target surface reaches w5 times higher than that close to the target normal for the case of incident angle 70 . Moreover, in this case the surface electron jet is also well-collimated with a small cone angle less than 15 (the full width at half maximum). To understand the characteristics of the surface fast electrons, a 2D3V fully
J. Zhang et al. / High Energy Density Physics 1 (2005) 61e65
scattering, stochastic heating, two-plasma decay, and other mechanisms. However, in a specific experiment, such processes have to be considered (see the review paper in Ref. [15]). Control of the emission direction of the fast electrons can be achieved by careful control of the laser and plasma parameters.
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relativistic PIC code has been used to simulate the generation of the surface electromagnetic fields in the interactions. In the 2D PIC simulations a p-polarized laser pulse with an irradiance of 2 ! 1018 W/cm2 is incident at 70 onto an 8nc, 4l thick plasma slab with a sharp boundary. The diameter of the laser focus is 10l. The profiles of the electrostatic field (Ex), and the magnetic field (Bz) at 250 fs are shown in Fig. 6. The fields are normalized by the incident laser amplitude. Part of the fast electrons generated in the interaction region will be reflected back to the vacuum by the surface magnetic field Bz. However, the negative sheath field Ex, whose peak position slightly shifts to the vacuum relative to that of the Bz, will push them back to target again. Therefore the directions of those fast electrons will be deviated from their initial directions and tend to travel along the target surface. Our results have shown that the surface electron jet is wellcollimated, highly directional, and stable. It can be used as an electron injector and as a laser-based electron gun with energies up to MeV. Recently Kodama reported a demonstration of guiding a high-density MeV electron beam using a fine carbon wire coupled to a hollow-cone target [14]. The electron beams emitted along the fiber-like plasma are also due to the confinement self-induced electric and magnetic field. 5. Summary In this paper we focus our discussions on the basic physics that governs the electron emission, instead of the different generation mechanisms, such as the vacuum heating, ponderomotive acceleration, resonance absorption, stimulated Raman
The authors are indebted to Prof. R. Kodama and colleagues for the Gekko joint experiments. This work was jointly supported by the NSFC (Grant No. 10374115, 60321003, 10335020, 10425416 and 10390161), the National High-Tech ICF program, the NKBRSF (Grant No. G1999075206), the JSPS-CAS Core University Program on Plasma Physics and Nuclear Fusion, and National Key Laboratory of High Temperature and High Density Plasmas.
References [1] L.M. Chen, J. Zhang, Y.T. Li, H. Teng, T.J. Liang, Z.M. Sheng, Q.L. Dong, L.Z. Zhao, Z.Y. Wei, X.W. Tang, Phys. Rev. Lett. 87 (2001) 225001. [2] C.I. Moore, et al., Phys. Rev. Lett. 82 (1999) 1688. [3] Z.-M. Sheng, et al., Phys. Rev. Lett. 85 (2000) 5340; Y. Sentoku, et al., Phys. Plasmas 6 (1999) 2855. [4] S. Bastiani, et al., Phys. Rev. E 56 (1997) 7179. [5] Y.T. Li, J. Zhang, L.M. Chen, Y.F. Mu, T.J. Liang, Z.Y. Wei, Q.L. Dong, Z.L. Chen, H. Teng, S.T. Chun-Yu, W.M. Jiang, Z.J. Zheng, X.W. Tang, Phys. Rev. E 64 (2001) 046407. [6] M. Tabak, et al., Phys. Plasmas 1 (1994) 1626. [7] Y.T. Li, J. Zhang, Z.M. Sheng, J. Zheng, Z.L. Chen, R. Kodama, T. Matsuoka, M. Tampo, K.A. Tanaka, T. Tsutsumi, T. Yabuuchi, Phys. Rev. E 69 (2004) 036405. [8] M.I.K. Santala, M. Zepf, I. Watts, F.N. Beg, E. Clark, M. Tatarakis, K. Krushelnick, A.E. Dangor, T. McCanny, I. Spencer, R.P. Singhal, K.W.D. Ledingham, S.C. Wilks, A.C. Machacek, J.S. Wark, R. Allott, R.J. Clarke, P.A. Norreys, Phys. Rev. Lett. 84 (2000) 1459. [9] Z.-M. Sheng, K. Mima, Y. Sentoku, M.S. Jovanovic´, T. Taguchi, J. Zhang, J. Meyer-ter-Vehn, Phys. Rev. Lett. 88 (2002) 055004. [10] C. Rousseaux, M. Rabec le Gloahec, S.D. Baton, F. Amiranoff, J. Fuchs, L. Gremillet, J.C. Adam, A. He´ron, P. Mora, Phys. Plasmas 9 (2002) 4261. [11] M.H. Key, et al., Phys. Plasma 5 (1998) 1966; R. Kodama, et al., Phys. Plasmas 8 (2001) 2268. [12] A.R. Bell, et al., Phys. Rev. Lett. 91 (2003) 035003; M. Tatarakis, et al., Phys. Rev. Lett. 81 (1998) 999; Borghesi, et al., Phys. Rev. Lett. 83 (1999) 4309; L. Gremillet, et al., Phys. Rev. Lett. 83 (1999) 5015; H. Teng, J. Zhang, Z.L. Chen, Y.T. Li, K. Li, X.Y. Peng, J.X. Ma, Phys. Rev. E 67 (2003) 026408. [13] M.S. Wei, et al., Phys. Rev. E 70 (2004) 056412 and references therein. [14] R. Kodama, et al., Nature 423 (2004) 1005. [15] F. Amiranoff, Meas. Sci. Technol. 12 (2001) 1795e1800.
Emission direction of fast electrons in high-intensity laser interactions with solids J. Zhang*, Y.T. Li, Z.M. Sheng, Z.Y. Wei, Q.L. Dong, X. Lu Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, 8 ZhongGuanCun NanSanJie, HaiDian District, PO Box 603, Beijing 100080, China Received 2 August 2005 Available online 21 October 2005
Abstract Energetic fast electron beams can be generated in ultrashort and ultraintense lasereplasma interactions. In this paper the dependence of the emission direction of the fast electron beams on the experimental conditions of the laser and plasmas, such as intensity, polarization, incident angle, scale length of the preplasma, as well as the possible ways to control the emission direction of fast electrons are discussed. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Intense lasereplasma interaction; Emission direction of fast electron
1. Introduction Intensive studies on fast electron generation and transport in high-intensity laseresolid interactions have been carried out because of its significance for the fast ignition concept in the laser confined fusion and other applications. A collimated fast electron beam with a definite direction is crucial for these applications. Unfortunately, the physics processes related with the emission direction of the laser-produced fast electron beams are very complicated. The emission direction depends on a variety of parameters, such as laser intensity, laser polarization, laser incident angle, scale length of the preplasma and target materials. Under different conditions, fast electron emission has been observed in various directions, such as the forward direction, the normal direction of the target surface, the direction of the laser electric field, the specular direction of the incident laser. However, one can still obtain very well-collimated fast electron beams emitted in some particular directions with a careful control of the laser and the plasma conditions. In this paper we discuss the basic physics and the possible ways to control the emission direction of
* Corresponding author. Tel.: C86 10 82649356; fax: C86 10 82649531. E-mail address: [email protected] (J. Zhang). 1574-1818/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.hedp.2005.09.002
fast electrons generated in the interactions of a high-intensity laser pulse with a solid target. 2. Intrinsic directions of laser pulses and plasmas First let us consider the basic geometry of a laser beam and a solid target. The intrinsic directions of the laser beam and the intrinsic properties of the target will decide the initial directions of electrons. For the laser beam, there are three intrinsic directions which are propagation vector k, electric field E (polarization), and magnetic field B. For the target, the gradient of the electron density of the plasma generated from the target, Vne (target normal direction), the vector of the electron plasma wave induced in collective interactions, kp, are usually related with electron emission directions. These directions are often associated with the interaction conditions and sometimes are distorted during the interaction. Secondly, self-induced electrostatic fields and magnetic fields always co-exist with the plasma. These introduce new factors to change the emission directions. The initial directions of the accelerated fast electrons inside the plasma will be modified when the electrons pass through these fields. Therefore, one has to consider the effects of the spontaneous electromagnetic fields when studying fast electron emission. All of these factors make the emission directions of fast electrons very complicated, but also very
J. Zhang et al. / High Energy Density Physics 1 (2005) 61e65
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informative to understand the physics process in the interaction. In the following sections, we shall present some of our observations in ultrashort intense laseresolid interactions. 3. Emission direction of fast electrons from ultrashort laser interactions with solid targets 3.1. Emission of fast electron beam in the intrinsic laser directions Fig. 1 shows the cutaway view of the angular distribution of the backward fast electrons generated in the interaction of an s-polarized femtosecond laser pulse with an aluminum plasma with a steep density profile at a laser intensity of 2 ! 1016 W/ cm2 [1]. The experiments were carried out with a Ti:sapphire laser at the Institute of Physics, Chinese Academy of Sciences. The laser delivered 5 mJ energy at 800 nm in 150 fs pulses. The diameter of the focal spot was less than 15 mm. The laser incident angle was 45 . The electrons were measured by DEF films. The sharp boundary of the exposure on the DEF films and the geometry of the target ensured that the electrons recorded by the film came from the front side of the Al target. The ejected fast electrons were found to be collimated along the laser electric field direction in a plane perpendicular to the incident plane. In the incident plane, no fast electrons were measured. It was also found that the fast electrons with higher energies have narrower angular divergence. This suggests that the ejected fast electrons were mainly accelerated by the electric field of the s-polarized laser pulses. This phenomenon seems to be similar to the results of laser accelerator injector based on laser ionization and the ponderomotive acceleration of electrons in gas, where electrons are accelerated in the polarization direction [2], but the heating mechanisms may be different. When the laser intensity is increased to be relativistic (the classical normalized momentum of electrons quivering in the laser electric field a0 O 1 or laser intensity of Il2 O 1.37 ! 1018 W/cm2), the laser magnetic field also becomes 90 150
important in the laser acceleration. The v ! B force perceived by electrons is comparable with that of the electric field. The electron motions will be modified greatly by the laser magnetic field, resulting in a longitudinal acceleration. More electrons will be accelerated along the laser propagation direction. Fig. 2 shows the angular distribution of the forward fast electrons generated in the interaction of a p-polarized femtosecond laser pulse with a 30 mm thick aluminum foil at a laser intensity of 2 ! 1018 W/cm2. Fast electrons with energies O0.6 MeV were measured by a stack of imaging plates. The experiments were carried out using the eXtreme Light II (XL-II) laser system at the Institute of Physics, Chinese Academy of Sciences. The laser system can produce a linearly polarized pulse with an energy up to 0.6 J in a duration of 30 fs at a wavelength of 800 nm. The laser incident angle was 45 . The diameter of the focal spot is 15 mm. One can see that a well-collimated fast electron beam is generated along the propagation direction of the laser pulse within G5 . When an ultrashort laser beam is incident onto a solid target obliquely, a portion of the laser beam is reflected in the specular direction near the critical density surface of the plasma. Theoretical simulations predicted that the reflected laser field can also accelerate electrons [3]. Fig. 3 shows such an angular distribution, where fast electrons are ejected in the specular direction of the laser beam. This measurement was also performed with the XL-II laser system. The laser beam was incident on an aluminum target at 45 and the laser intensity was 2 ! 1017 W/ cm2. Similar electron beam was also observed by other groups previously [4]. 3.2. Emission of fast electron beam in the target normal direction The geometry of the solid target will affect the spatial shape of the plasma and the critical density surface. When a p-polarized intense laser obliquely irradiates a plasma with a very steep density boundary, vacuum heating or resonance absorption can produce fast electrons in the target normal direction [5]. Fig. 3 shows a fast electron beam ejected almost in the front
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Fig. 2. The angular distribution of forward fast electrons with energies over 600 keV for p-polarized laser at an intensity 2 ! 1018 W/cm2.
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Fig. 3. The angular distribution of ejected fast electrons with energies over 300 keV in the incident plane for p-polarized laser at an intensity 2 ! 1017 W/cm2.
target normal direction besides the one in the laser specular direction. 4. Dependence of the emission directions of fast electrons on laser and target parameters and possible control of the emission directions We have shown some examples of the intrinsic emission directions of fast electrons so far. However, in a specific experiment, the fast electron emission depends on the laser intensity, polarization, incident angle, and the status of the preplasma generated by the prepulses before the arrival of the main laser pulse. Careful studies on these dependences are the first step to a possible control of emission directions of fast electrons, which is a pending problem so far. The physics is not fully understood. However, we would like to present several examples to shed some lights on this aspect. If one only considers the laser pulse, the key parameter is the normalized quivering momentum of electrons in the laser electric field a0 Z eE/meu0c Z 8.5 ! 10ÿ10Il1/2, where I is the laser intensity in W/cm2, and l is the laser wavelength in mm, e is the electron charge, E is the magnitude of the laser field, me is the electron mass, u0 is the laser angular frequency, and c is the light velocity. For a laser intensity of Il2 1.37 ! 1018 W/cm2 (a0 1), say, w1016 W/cm2, the laser electric field is the predominant factor. For the s-polarized laser field, electrons are mainly dominated by transverse acceleration (see Fig. 1), while for the p-polarized laser field, the processes such as resonance absorption and vacuum heating will accelerate electrons in the target normal direction (see Fig. 3). Almost all of the outgoing fast electrons were emitted in the normal direction. The emission direction of the fast electrons obeys the momentum conservation [3]. However, when the laser intensity is increased to be relativistic, Il2 O 1.37 ! 1018 W/cm2 (a0 O 1), the amplitude of the magnetic field of the laser beam is comparable to that of the laser electric field, the electrons tend to be accelerated in the longitudinal direction by J ! B heating (see Fig. 2). These forward
fast electrons are of significance in the fast ignition concept [6]. If laser peak amplitude a0 w 1, the situation will be much more complicated, since the transverse and longitudinal forces are comparable and will compete each other. Fig. 4 shows the angular distribution of the forward fast electrons behind the target illuminated by a p-polarized laser beam with an incidence angle of 45 for laser intensities from the sub-relativistic to the relativistic [7]. Here 0 and 45 correspond to the laser propagation vector and the target normal, respectively. In the experiment a 0.6e1 ps, 1.053 mm linearly polarized laser pulse with an energy up to 10 J, output by the GEKKO Module II (GM-II) laser facility at the Institute of Laser Engineering, Osaka University, was focused onto a 5 mm thick aluminum foil target at an incident angle of 45 . The diameter of the focus was about 30 mm. The laser intensity on targets was adjustable in the range of (0.2e4) ! 1017 W/cm2. A prepulse with an energy about 3 ! 10ÿ3 of the main pulse, originating from the regenerative amplifier, starts from 700 ps ahead of the main pulse peak. Therefore, the main laser pulse interacted with a preplasma. The forward hot electrons were measured behind the target by LiF (Mg, Cu, P) thermoluminescence dosimeters (TLD). One can see the peak of the forward fast electron beam produced by the p-polarized laser beam moves to the laser propagation direction from the target normal direction as the laser intensity is increased to be relativistic. In the experiments, s-polarized laser pulses were also used to study the effects of polarization on fast electrons. We have found that the effects of laser polarization on fast electron emission become weaker at a relativistic intensity than that at a sub-relativistic intensity. This is mainly caused by the following two reasons. One is that the forward acceleration due to laser magnetic field is dominated at the regime of a0 O 1. This acceleration is less dependent on laser polarization. The other is that the critical surface of plasma becomes curved due to the high laser pressure. This leads to the formation of a p-polarized component of the laser electric field. 1500 2.8x1018 W/cm2 8.6x1017 W/cm2 1000
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A preplasma is usually formed before the main laser pulse arrives at the target surface because of the practical limit of the laser technology, i.e., the existence of the amplified spontaneous emission (ASE), especially for a relativistically intense lasereplasma interaction. When a preplasma is present in advance of the peak of the main laser pulse, the interaction will depend, to a large extent, on the properties of the preformed plasma, particularly its scale length [8]. For example, for a preplasma with a short-scale length, the vacuum heating or the resonance absorption may dominate the absorption process, while for the case of large-scale length, the stochastic heating [9] and various parametric instabilities such as the stimulated Raman scattering, two-plasma decay may also accelerate electrons [10]. In this case, electrons are accelerated directly by the laser fields or by the excited electron plasma waves. Therefore, control of the plasma density scale length may lead to desired fast electron emissions. The spontaneous electrostatic fields and magnetic fields in the plasma will modify the original directions of fast electrons. This indicates another possible way to control the fast electron emission. After the fast electrons are generated in the laser focal region, part of them will eject backward into vacuum from the underdense plasma in front of the target. The others move forward into overdense region and propagate inside the cold target region. About 20e40% laser energy can be converted to fast electrons at intensities w1019 W/cm2. This forms a highly directional fast electron beam with a current as high as multi-Mega (even Giga)-Ampere [11]. This huge current will induce strong electrostatic fields and magnetic fields inside the plasma or target bulk. The bulk fields, in turn, will affect the fast electron transport. Studies have shown that the forward fast electron beam will be collimated by the static magnetic fields [12]. The fast electron beam may be also broken up into filaments due to the two-stream instability and Weibel instability [13]. The spontaneous electrostatic fields and magnetic fields near the front target surface can also be used to control the directions of fast electrons. Our recent experiments have demonstrated a fast electron jet emitted along the target surface when the electron density gradient is steep. It can be explained by the confinement of quasistatic magnetic fields and electrostatic sheath fields. The experiments were also carried out using the XL-II laser system. The angular distributions of fast electrons were measured by a stack array of imaging plates (IP). Fig. 5(a)e(c) shows the angular distributions of fast electrons in the polarization plane for laser incident angles of 22.5 , 45 , 70 , respectively, at a laser intensity of 1e2 ! 1018 W/cm2. The most striking aspect of our measurements is the presence of fast electrons ejected along the front target surface, marked as ‘‘surface electrons’’ in the figures. In particular, the number of the surface fast electrons increases with the laser incident angle. The emission peak along the target surface reaches w5 times higher than that close to the target normal for the case of incident angle 70 . Moreover, in this case the surface electron jet is also well-collimated with a small cone angle less than 15 (the full width at half maximum). To understand the characteristics of the surface fast electrons, a 2D3V fully
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scattering, stochastic heating, two-plasma decay, and other mechanisms. However, in a specific experiment, such processes have to be considered (see the review paper in Ref. [15]). Control of the emission direction of the fast electrons can be achieved by careful control of the laser and plasma parameters.
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relativistic PIC code has been used to simulate the generation of the surface electromagnetic fields in the interactions. In the 2D PIC simulations a p-polarized laser pulse with an irradiance of 2 ! 1018 W/cm2 is incident at 70 onto an 8nc, 4l thick plasma slab with a sharp boundary. The diameter of the laser focus is 10l. The profiles of the electrostatic field (Ex), and the magnetic field (Bz) at 250 fs are shown in Fig. 6. The fields are normalized by the incident laser amplitude. Part of the fast electrons generated in the interaction region will be reflected back to the vacuum by the surface magnetic field Bz. However, the negative sheath field Ex, whose peak position slightly shifts to the vacuum relative to that of the Bz, will push them back to target again. Therefore the directions of those fast electrons will be deviated from their initial directions and tend to travel along the target surface. Our results have shown that the surface electron jet is wellcollimated, highly directional, and stable. It can be used as an electron injector and as a laser-based electron gun with energies up to MeV. Recently Kodama reported a demonstration of guiding a high-density MeV electron beam using a fine carbon wire coupled to a hollow-cone target [14]. The electron beams emitted along the fiber-like plasma are also due to the confinement self-induced electric and magnetic field. 5. Summary In this paper we focus our discussions on the basic physics that governs the electron emission, instead of the different generation mechanisms, such as the vacuum heating, ponderomotive acceleration, resonance absorption, stimulated Raman
The authors are indebted to Prof. R. Kodama and colleagues for the Gekko joint experiments. This work was jointly supported by the NSFC (Grant No. 10374115, 60321003, 10335020, 10425416 and 10390161), the National High-Tech ICF program, the NKBRSF (Grant No. G1999075206), the JSPS-CAS Core University Program on Plasma Physics and Nuclear Fusion, and National Key Laboratory of High Temperature and High Density Plasmas.
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