Observation of MeV multicharged ions and hot electrons accelerated by a 65-fs laser pulse

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1 Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) Observation of MeV multicharged ions and hot electrons accelerated by a 65-fs laser pulse T. Auguste a, A.Ya. Faenov b, I. Fukumoto c, S. Hulin a, A.I. Magunov b, P. Monot a, P. D Oliveira a, T.A. Pikuz b, A. Sasaki c, B.Yu. Sharkov d, I.Yu. Skobelev b;, T. Tajima c, A.G. Zhidkov c a Commissariat a l Energie Atomique, Centre d Etudes de Saclay, DRECAM, Service de Photons Atomes et Molecules, Bât.522, Gif-sur-Yvette, France b Multicharged Ion Spectra Data Center of VNIIFTRI, Mendeleevo , Russia c Advance Photon Research Center, JAERI, 25-1 Mii-minami-cho, Neyagawa-shi, Osaka 572, Japan d Institute of Experimental and Theoretical Physics, Moscow, Russia Abstract The generation of fast particles (both ions and electrons) in a femtosecond laser-produced plasma has been studied experimentally and the possibility of applying particle-in-cell (PIC) codes to the description of the experimental results has been investigated. The energy distribution function of fast ions has been measured directly by means of X-ray spectroscopy. It is shown that this diagnostic can also be used as an indirect method to measure fast electrons inside the plasma. The hot electron distribution in the 0:1 2 MeV energy range was measured directly using a standard electron magnetic spectrometer. The comparison of the experimental results with the numerical simulations show: (1) the electromagnetic PIC-FC code can be successfully applied for simulating plasmas created by the interaction of femtosecond-laser pulses with solid targets and (2) the existence of a transient population of hot electrons with a non-maxwellian distribution typical of interaction experiments with short-pulse lasers.? 2001 Elsevier Science Ltd. All rights reserved. Keywords: X-ray spectroscopy; Laser-produced plasma; Fast ions; Hot electrons 1. Introduction Emission of energetic particles from short-pulse-laser produced plasmas has been the matter of intensive studies for the last few years [1 16]. With decreasing laser pulse duration and Corresponding author. Tel.: address: skobelev@orc.ru (I.Yu. Skobelev) /01/$ - see front matter? 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 148 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) increasing intensity, hot electrons, which govern ion acceleration and X-ray production [11 15], constitute a transient, anisotropic distribution. Therefore, the study of the energy distribution of hot electrons becomes important in understanding the basic mechanisms underlying the interaction of short pulse lasers with matter. Usually, the hot electron distribution is characterized by a Maxwell-like distribution with an eective temperature T h having a known dependence with the laser intensity [17]. With very short laser pulses ( 100 fs), the hot electrons can constitute a transient population with an energy cut-o, because in this case the time of total randomization can be longer than the laser pulse duration. This transient distribution can aect ion acceleration, X-ray and -ray emission, and the emittance of energetic particles. Thus, the characterization of hot electrons and energetic ions through measurements of their energy distribution, for example, and the comparison with numerical calculations are very important not only to clarify the basic mechanisms of the interaction but also to determine future prospects for the applications of femtosecond laser-produced plasmas, and particularly for the development of table-top low-cost sources of high-energy particles. In the present paper, we use X-ray spectroscopy to study fast particles created by the interaction of high-intensity femtosecond laser pulses with solid targets. The main purposes of the present work are: (1) to study experimentally the generation of fast particles (both ions and electrons) in femtosecond laser-produced plasmas and (2) to investigate the possibility of applying particle-in-cell (PIC) codes to the description of the experimental results. Dierent experimental methods have been used here. Generally speaking, the methods to measure the energy distribution of fast particles can be divided into direct and indirect methods. The direct methods are based on the direct measurement of the particle energy, while the indirect ones are based on the observation of events resulting from the interaction of fast particles with matter. Mass-spectrometry, for example, is the most commonly used direct method to measure the energy of both heavy and light particles. It should be noted that direct methods provide the most reliable information. However, only particles that escape from the plasma can be collected and therefore no information about the acceleration process can be inferred. Thus, if we are interested in the investigation of fast particles inside the plasma, we must use other experimental techniques. For such investigations, X-ray spectroscopy provides the primary method. The spectroscopy can be either a direct or an indirect method, because one can either observe photons emitted directly by fast particles or photons radiated after the excitation of high-energy transitions by fast particles. For example, in the case of heavy particles, i.e., ions, the wavelength of spectral lines radiated by moving ions will be shifted due to Doppler-eect and the number of shifted photons will be proportional to the number of ions with a given velocity. This means that the measurement of spectral line proles can give the ion velocity distribution. It is obvious that this method has one serious limitation, namely, it permits study of only very fast ions, because the central part of spectral line proles, as a rule, is formed by other broadening mechanisms, such as Stark-eect, self-absorption, etc. Direct X-ray spectroscopic methods, generally speaking, can be also used to study fast electrons. In this case, bremsstrahlung emission in the hard X-ray range would be observed. Unfortunately, in the current case the spectrometers do not operate in the correct spectral range. We therefore used indirect spectroscopic methods that are not very sensitive to the electron velocity distribution and allow only qualitative estimates of fast electron energy to be

3 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) obtained. For quantitative measurements of the hot electrons external to the plasma we use a standard magnetic electron spectrometer. 2. Direct spectroscopic observation of high-energy ions The direct observation of high-energy ions by means of X-ray spectroscopy has been performed for a uorine plasma created by a femtosecond laser. The experiments are performed with the 10-TW laser (UHI10), which was designed to generate 65-fs pulses at a 10-Hz repetition rate, employing the standard CPA technique. Ti-Sapphire rods are used as the lasing medium and the operating wavelength of the system is 790 nm. In order to produce such high power ultrashort pulses with good contrast, one rst uses an aberration free Oner pulse stretcher to expand the low energy ultrashort pulse to 300 ps [18]. The pulse is then amplied in four stages to an energy of 1:8 J. Next, pulse compression is performed in a vacuum chamber connected directly to the experimental chamber. The contrast, measured with a high dynamic range cross-correlator is about 10 5 at 1 ps, and about 10 7 at several ns. The total energy in the 65 fs laser pulse is about 800 mj. The 80 mm diameter laser beam is focused with an f=2:35 o-axis parabolic mirror onto teon, aluminium, calcium and copper targets, which were placed at a 22:5 angle from the direction of the incident laser beam. The focal spot radius measured in vacuum is about m; giving a laser intensity on the target in the range of (2 4) W=cm 2 range. Spatially resolved X-ray spectra were obtained using the FSSR-2D, focusing spectrometers with spatial resolution [19 21]. Two large aperture (15 mm 50 mm) spherically bent mica crystals with a 150-mm radius of curvature were placed 25 and 29 cm from the plasma. These allowed measurement of helium- and hydrogen-like uorine emission, for which the spectral resolving power was limited to due to the limitation of the rocking curve of the crystal for these long wavelengths. In the experiments with aluminum, calcium and copper, the spectral resolution was improved to 5000 and The rst spectrometer was mounted so that the angle between the direction of observation and the target surface was smaller than 10. The second spectrometer was placed perpendicularly to the target surface. Spectra were recorded on RAR 2492 lms. The lm holder was protected by two layers of polypropylene lters coated with thin layers of aluminum on both sides. The results of a time-integrated measurement of the Ly line of uorine are shown in Fig. 1. We see that the spectrum recorded face-on to the target surface (Fig. 1a) exhibits a strong asymmetrical feature in contrast to the spectrum recorded in the direction parallel to the target surface. We attribute the blue feature in the spectrum of Fig. 2a to the Doppler-shift of the emission due to highly energetic emitting ions coming from the target. These spectra allow estimates of both the absolute value of the energetic ion velocities but also their direction of motion. It follows from Fig. 1a that ion velocities in the direction perpendicular to the target surface reach cm=s. The spectrum presented in Fig. 1b allows an estimate of the component of ion velocity in the direction parallel to the target surface. Because the broadening of the central part of the line prole is given not only by the ion motion, only an upper limit of the velocity can be obtained. In this case, the transverse velocity component was smaller than cm=s. These estimations show that ions with energy up to 2 MeV created

4 150 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) Fig. 1. Time-integrated spectral intensity of the 2p 1s line of H-like F IX parallel (a) and perpendicularly (b) to the plasma expansion. Fig. 2. Typical He-like F VIII ion energy distribution inferred from spectroscopic measurements of the He line prole: (1) q las =1: W=cm 2 ; (2) q las =2: W=cm 2, (3) q las = W=cm 2. inside the plasma move from the target surface in a well collimated beam with a divergence angle smaller than 10. It should be emphasized that all lines observed in these experiments exhibit the same dependence with the observation angle [11]. It proves that the dierence in spectra observed along and perpendicularly to the target surface is caused by Doppler eect, because only relative Doppler shifts are the same for all spectral transitions. We note that the observed line proles are sensitive to the laser intensity q las : for example, for increased laser intensity the prole becomes broader and, consequently, faster ions are produced.

5 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) The blue wing in the line proles can be used to infer the energy distribution of fast ions. In Fig. 2 are shown results obtained for He-like uorine ions at three dierent laser intensities. As it is seen from Fig. 2, when q las is (1 4) W=cm 2 uorine ions with kinetic energy of several MeVs are produced. 3. Direct electron spectrometer observation of fast electrons As mentioned above crystal X-ray spectrometers, as a rule, are not suitable to observe bremsstrahlung radiation caused by very fast electrons. Therefore, a standard electron spectrometer has been used to measure hot electrons external to the plasma. The electron spectrometer is composed of two parallel rectangular magnetic deectors shielded by a metal enclosure. Electrons entering through a 1 mm aperture follow a hemispheric trajectory before being detected by a 40-mm diameter MCP chevron assembly coupled to a phosphor screen. The rear side of the screen is then imaged onto a CCD camera with a 45 incidence mirror. The electron kinetic energy is related to the position D by T (ev) = A( 1+BD 2 1) 2 ; (1) where D is the position of the signal at the output of the spectrometer, while A and B are related to the geometry of the instrument. To limit the pressure for MCP operation, the instrument is set inside a 460-mm diameter vacuum chamber. The 0:18 T magnetic eld allows one to record an electron spectra from 100 kev to 2:2 MeV in a single experiment. The distance between the solid target and spectrometer is 1 m. Since the distance from the source to the spectrometer is much larger than the trajectory inside the spectrometer, the energy resolution is limited by the size of the spectrometer aperture image in the energy dispersion plane rather than by the angular admittance. The uncertainty T of the electron kinetic energy is thus given by T =D f(t ev ); (2) where D is the position uncertainty at the spectrometer output (1 mm) corresponding to 10% in our experimental conditions and f(t ev ) is a function of the temperature. This operation of the spectrometer was veried on the Van de Graa source of the Laboratoire des Solides Irradies (Ecole Polytechnique, France) for energies between 600 kev and 2:2 MeV. It was found that the width of the energy distribution of the source remains below 15 kev in this range, the dispersion corresponds to that predicted by Eq. (1), and the MCP response is found to be at in this range. 4. PIC simulations of the acceleration process To investigate the process of randomization of hot electrons and ion acceleration by a short pulse laser, we use a method based on the electromagnetic PIC-FP code to explore the dynamics of a solid density plasma irradiated by a femtosecond p-polarized, obliquely incident laser pulse. The method employs the Langevin equation to account for elastic collisions and a non-lte average ion model for plasma ionization including the ionization due to the laser eld (OFI) in

6 152 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) Fig. 3. Comparison of calculated hot electron distribution with the experimental data in the MeV range: Thin lines experimental distribution, (1) q las =1: W=cm 2, (2 q las =2: W=cm 2, (3) q las = W=cm 2 ; Thick line calculated distribution for q las = W=cm 2. Fig. 4. Comparison of the measured He F VIII line prole with the PIC-simulation. addition to particle ionization. The method conforms to a direct solution of the Fokker Planck equation. The details of the method can be found in [22,23]. To solve the Maxwell equations, we use the two wave approximation [22,23]. We include ionization processes in the PIC simulation by allowing change to the computational particle (CP) charges. A change in the charge of CPs representing plasma electrons is calculated by the standard electron balance equation in a kinetics cell, which includes many PIC cells. The OFI is included as a process of successive ionization whose probability depends on the electric eld strength, E; and ionization potential of the average ion, I Z, in a kinetics cell. The average charge of every CP representing ions is used in the equation of motion. To determine the initial conditions for the PIC simulation, we apply the hydrodynamics code HYADES [24,16]. We calculate the target density, temperature, and ion charge after the laser prepulse for a contrast ratio of 1 : 10 7 and a 4 ns duration. The relativistic electromagnetic PIC code with the square current and charge weighting is used to calculate the interaction of an intense obliquely incident p-polarized pulse laser with an overdense plasma. Collisions are computed as an eective force after calculation of the velocity and position of the CP. The calculation with movable ions is carried out for a uorine-like solid target. The laser intensity q las = (1 4) W=cm 2,at = 800 nm; (the maximum ee 0 =m!c is equal to 0.9) is chosen to be constant during the laser pulse and the pulse duration is 65 fs. The time step is set to 0:03=! 0 pl, where!0 pl is the initial plasma frequency. The number of CPs is N =10 5 per 1 m of the plasma. Examples of PIC-simulations are shown in Fig. 3. The calculated time-integrated electron energy distribution in the 0:1 2 MeV range is given in Fig. 3 (thick line) for laser intensity

7 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) q las = W=cm 2, together with the three measured distributions (thin lines). This distribution exhibits groups of hot electrons with dierent temperature. The calculation is performed including the solid part of the target that leads to the lower temperature in the rst group reecting the energy loss in the solid. One can see two dierent groups of hot electrons. The rst has a temperature ranging between 100 and 200 kev in the energy range up to 0:8 Mev; and the other has a temperature of about 500 kev in the energy range from 0.8 to 1:4 Mev. Due to the short laser pulse duration, a full randomization of hot electron energy does not occur, and the electron distribution is non-maxwellian. Further, in Fig. 3 we show a comparison of the calculated energy distribution of fast electron with experimental results. One can see a reasonable agreement between theoretical and experimental data in the region below 1:5 MeV. The PIC code allows the simulation of the blue wing of the spectral line proles resulting from the motion of emitting ions with MeV energy. In Fig. 4, one can see that a very good agreement is found between the experimental line shape of the He F VIII transition and the one calculated using the energetic ion velocity distribution provided by the code. Thus, our PIC simulations give a reasonable agreement with experimental results for both fast electrons and energetic ions. 5. Indirect observation of fast electrons by means of X-ray spectroscopy X-ray spectroscopy can also be used as an indirect method to measure the energy of fast electrons in situ. It is well known [2 5,7 10,16] that short-pulse-laser-produced plasma is an ecient source of K emission. Because only highly energetic electrons can excite such inner shell atomic transitions, the observation of K radiation an indirect analysis of the fast electron source. It should be noted that the use of sensitive spectrometers with good spatial resolution provides the way to study multidimensional eects such as, e.g., the fountain eect [25]. To observe the spatial distribution of K emission from femtosecond-laser-produced plasma three spectrometers have been used simultaneously in the present work [26]. Each spectrometer measured the spectra with one-dimensional resolution. The two spectrographs had spatial resolution along the target surface, while the third spectrograph was mounted at 10 to the target surface. The third spectrometer measured spectra with 20 m spatial resolution along the laser axis due to the focusing properties of spherically bent crystals. The experiments have been carried out with Al, Ca and Cu targets. In Fig. 5, we present the results obtained for Al targets, where we observed the K line of AlI and the resonance line of He-like Al simultaneously. One can see from this gure that the K line is emitted from a larger spatial region than the He line, which indicates that there is a spatial region where the plasma temperature is low fast electrons exists. We have also seen that the size of this region is about 1 mm and depends weakly laser intensity. Of course, it is not necessary to have very high energy electrons to excite K line of AlI; however, experiments with Ca and Cu targets give similar results with K emission coming from a larger region than the He-like emission. It should be noted that the energy of the K transition of Cu is about 8 kev, and energetic electrons are actually required to excite such a high-energy transition. We have performed calculations of the K emission eciency and of the number of emitted photons for dierent materials by using a Monte-Carlo simulation code. This code of the TIGER

8 154 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) Fig. 5. Emission spectra of aluminum plasma recorded with spatial resolution along the target surface. R and I are the resonance and intercombination lines of Al XII, correspondingly. Fig. 6. Eciency of backward K emission per electron from Al, Ca, and Cu slabs depending on the energy of the electron (a) and number of backward emitted K photons per sr and 1 J of laser energy for dierent temperatures of hot electrons (b). series [27,28] was developed for the solution of coupled electron=photon transport problems. It was used here to simulate the interaction of hot electrons with the solid target. The code uses an elaborate ionization=relaxation model including K; L1; L2; L3; M, and N shells to treat the transport of photons with energy in the 1 10 kev range. In the present work, we used

9 T. Auguste et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 71 (2001) the ACCEPTP code, which is valid for problems with axial symmetry, without magnetic and electric elds. Results of the calculation are presented in Fig. 6. It follows from the calculations that the detection of K emission indirectly conrms the generation of fast electrons in femtosecond-laserproduced plasmas. Note that the optimum hot electron temperature for K generation in Cu is about 100 kev. As it follows from our PIC-simulations [11], this value of the temperature is realized when the laser intensity is about W=cm 2. This means that the laser intensity achieved in our experiment was optimum for the generation of K radiation for medium Z elements. 6. Conclusion In the present work, the generation of fast particles (both ions and electrons) in a femtosecond laser-produced plasma have been studied experimentally and the possibility of using PIC codes for the description of the experimental results has been investigated. The distribution function of ions with energy up to several MeVs has been measured directly by means of X-ray spectroscopy. It was shown that this diagnostic can also be used as an indirect method to measure fast electrons in situ. The comparison of the experimental and the numerical results has shown that the electromagnetic PIC-FC code can be successfully applied for simulating plasmas created by the interaction of high-intensity short-pulse lasers with solid targets. Particularly, computations indicate the existence of a transient population of hot electrons with a non-maxwellian distribution, resulting from an incomplete randomization of the suprathermal electron energy during the laser pulse. Acknowledgements This work was partially supported by INTAS under Research Grant No References [1] Kruer WL, Wilks SC. Plasma Phys Controlled Fusion 1992;34:2061. [2] Cobble JA et al. Appl Phys 1991;69:3369. [3] Rousse A et al. Phys Rev E 1994;50:2200. [4] Feurer T et al. Phys Rev E 1997;56:4608. [5] Kmetec JD et al. Phys Rev Lett 1992;68:1527. [6] Beg FN et al. Phys Plasmas 1997;4:447. [7] Yu J, Jiang Z, Kieer J-C. Phys Plasmas 1999;6:1318. [8] Guo T et al. Proc SPIE 1997;84:3157. [9] Yoshida M et al. Appl Phys Lett 1998;73:2393. [10] Eder DC, Pretzler G, Fill E, Eidmann K, Saemann A. Appl Phys B 2000;70:211. [11] Zhidkov A et al. Phys Rev E 1999;60:3273. [12] Zhidkov A, Sasaki A, Tajima T. Phys Rev E 2000;61:R2224. [13] Zhidkov A, Sasaki A, Tajima T. Rev Sci Instrum 2000;71:931. [14] Hatchet SP et al. Phys Plasmas 2000;7:2069.

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