Ti:Sapphire laser pulses at intensities of 3 Ã 1016 W/cm2 indicate the presence of hot electron jets with electron energies of 80 to over 250 keV and cone angles ...
J. Phys. IV France 133 (2006) 271–275 C EDP Sciences, Les Ulis DOI: 10.1051/jp4:2006133054
Hot electron jets from femtosecond heated plasmas at intensities of 1016 --1017 W/cm2 R. Fedosejevs1,2 , C. Serbanescu1 , F. Dorchies2 , C. Fourment2 , C. Bonté2 , F. Blasco2 , J. Santos2 , S. Petit2 , D. Romanov1 , W. Rozmus1 , V.Yu Bychenkov1,3 , C.E. Capjack1 and V. Tikhonchuk2 1
University of Alberta, Edmonton, AB T6G 2V4, Canada Centre Lasers Intenses et Applications (CELIA), Université Bordeaux I, France 3 P.N. Lebedev Physics Institute, Russian Academy of Sciences, Moscow 2
Abstract. Solid target experiments carried out on copper targets with both p- and s-polarized 130 fs Ti:Sapphire laser pulses at intensities of 3 × 1016 W/cm2 indicate the presence of hot electron jets with electron energies of 80 to over 250 keV and cone angles of the order of 10 degrees at the higher energies with directions dependent on the incident polarization. For comparison, studies on an Argon cluster jet targets with 50 fs duration Ti:Sapphire laser pulses at vacuum intensity of 1017 W/cm2 indicated no detectable hot electron jet emission with electron energies above 50 keV. A 2D PIC simulation of the p-polarized solid target interaction predicts features similar to those observed experimentally.
1. INTRODUCTION Previous reports [1, 2, 3] have indicated that high-energy electrons (50 to 250 keV) can be emitted in collimated jets from the interaction of femtosecond laser pulses with solid targets at intensities of 1016 – 1017W/cm2 . Such emission from solid targets has been observed into beams with cone angles on the order of tens of degrees or less depending on energy and target conditions. The electron jets occur in the plane of polarization of the incident laser beam and changes from s-polarized to p-polarized incidence. For solid target interactions mechanisms such as resonance absorption [4, 5], vacuum heating [6, 7], and two-plasmon decay [5, 8, 9] can lead to the acceleration of such electrons. Recently, the generation of high-energy electrons along the electric field direction via knock-on Coulomb collisions of the oscillating electrons in the incident laser field has been proposed [10] in a process similar to that leading to enhanced inverse Bremsstrahlung absorption for high field intensities [11].
2. EXPERIMENT 2.1 Solid Target Experiments The solid target experiments were carried out at the University of Alberta and the experimental layout for the solid target experiment shown in Fig.1(a) is similar to that in [12]. Ti:sapphire laser pulses of ∼ 250 J energy, 130 fs (FWHM) duration at 800 nm were employed. The laser beam was focused in vacuum by a 10X microscope objective onto a rotating polished solid copper disk target. A fresh target spot was used for every laser shot. The prepulse (8.5 ns prior) contrast ratio was on the order of 5 × 10−4 . The measured interaction spot size is 2.5 m (FWHM) resulting in a peak intensity of 3.6 × 1016 W/cm2 when using the 10X microscope objective for 250 J pulses. The electrons were measured both with aluminum filtered PIN diode detectors and with aluminum filtered DEF film. The angle of incidence was 30◦ and the polarization was rotated from s- to p-polarized incidence on target.
Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2006133054
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Figure 1. Experimental Setups: (a) solid target experiments and (b) cluster jet experiments.
2.2 Cluster Jet Experiments The cluster jet experiments were carried out at CELIA and the experimental layout is shown in Fig.1(b). Ti:sapphire laser pulses with ∼3 mJ energy, 50 fs (FWHM) duration at 800 nm were focused in vacuum by a 20 cm focal length simple lens onto a 4 mm diameter cluster jet target of pulsed Ar at a backing pressure of 40 bar. The jet has been characterized [13] with an average cluster size of 27.5 nm, a condensation fraction of 25% and an average atomic density of 8 × 1018 cm3 at this pressure. The prepulse (4 ns prior) contrast ratio was on the order of 10−5 , which was low enough that it played no role in the interaction process. The measured 1/e intensity diameter of the laser beam was 9.0 m resulting in a peak intensity of 9.5 × 1016 W/cm2 for a pulse length of 50 fs. Electron generation was measured by an aluminum filtered PIN diode and by an aluminum filtered Fluorescent screen (Kodak LANEX Medium Screen) imaged onto a cooled CCD camera (Andor DV-440). The response of the camera-CCD camera detector system was calculated based on the known response of the fluorescer screen, the details of the imaging system and the manufacturers calibration for the CCD camera system giving an estimated calibration accuracy of the order of a factor of two. 3. RESULTS AND DISCUSSION 3.1 Solid Target Results We have measured the 8 keV K x-ray emission from the solid target [12] indicating a significant fraction of electrons with energies of the order of 10 keV which do not escape the target region as expected from resonance absorption for our given intensities [4]. However, from the PIN diode signals and exposures on DEF film it was found that a directed flux of much hotter electrons was generated in certain directions with energy fluences of the order of 100-1000 times higher than the isotropic 8 keV x-ray emission. DEF has an emulsion on both sides of a plastic substrate and thus signals are obtained of the electron flux entering the first surface and exiting the back surface. The energy threshold for electrons to reach the first emulsion surface through the 33 m aluminum filter used was approximately 80 keV while the energy required to reach the backside emulsion is approximately 250 keV [14]. The resultant images in Fig.2 show a narrow cone of high energy electrons greater than 250 keV in the middle of a broader cone of electrons with energies greater than 80 keV. For p-polarization, a single electron jet is observed in the plane of incidence at an angle between the target normal and the specular direction. For s-polarized radiation, the electron jet is observed in the lateral directions in the plane of polarization at an angle of ∼21◦ back towards the incident direction from the E-field vector. The emission cone angle of the radiation
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Figure 2. Electron jet images on the front and back sides of DEF film for >80 and >250 kev energies ranges respectively: (a) p-polarized 30◦ incidence and (b) s-polarized 30◦ incidence.
>250 keV is of the order of 10◦ in both cases. The total energy in the >250 keV electron jet is estimated, based on the known x-ray energy response of the DEF film, to be ∼10−4 of the incident laser energy or 1.5 × 107 electrons per steradian. For the electrons with energy >80 keV the film is overexposed and one can only say that the energy flux appears to be more than an order of magnitude more than the electrons above 250 keV. One can also see distorted shadows of the edge of the target and microscope objective in the s-polarized image indicated deflection of the lower energy electrons from ambient electric and magnetic fields as they propagate to the film. These results are consistent with some of those previously reported in the literature [2, 3] in the presence of a small preplasma as is expected here. We also used the 250 keV electrons in an initial radiography imaging experiments showing a resolution of ∼100 microns. The origin of the electrons in this case is not clear at present. Given that there is a prepulse leading to a small preplasma in the solid target experiments [12], instabilities related to underdense plasma gradients may contribute. In part the electron jets may be related to the two-plasmon instability where hot electron temperatures of 30-60 keV have been reported with observable tails of up to 250 keV energy [8, 9]. In these cases the high-energy electrons were peaked at an angle of 45 degrees to the incident k-vector in the plane of polarization. In the present case the steep density gradient and ambient electric and magnetic fields may contribute to some deflection of any such electrons. 3.2 Cluster Jet Results The 12.5 m Al filtered PIN diode signal was measured with and without magnetic shielding for different backing pressures as shown in Fig.3(a). The difference in the two signals corresponds to the flux of electrons with energy greater than 47 keV. There is no observable difference within the 10% error bars of the measurement indicating that the predominant signal is due to x-rays with energies of ≥ 3 keV [15]. The aluminum filter was located 11 mm on front of the 5.6 mm diameter detector face so that the energy dependent large angle scattering of electrons must be taken into account in calculating the effective response of the detector. Using a limit of 10% of the measured signal, the maximum electron flux that could be present is estimated to be 1.7 × 106 , 6 × 104 and 7 × 103 srad−1 for electron energies of 50, 100 and 250 keV respectively. The fluorescent screen image, filtered with 13 and 26 m Al filters corresponding to electron cutoff energies of 57 and 76 keV (taking into account the plastic overcoat on the LANEX screen), for the case of 40,000 shots and a backing pressure of 40 bar is shown in Fig.3(b). Based on the calculated calibration factors for the imaging system, the brightness of the circular image of the aperture observed is within a factor of two of that estimated for the observed x-ray flux with energies of ≥3 keV [15].
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Figure 3. Cluster jet measurements (a) Signal from 12.5 m Al filtered PIN diode versus argon backing pressure and (b) LANEX screen image with split 26 um (left) and 13 um (right) Al filter.
There is no observable jet like structure within the 14.5◦ observation cone angle and the minor nonuniformities observed correspond to alignment marks on the fluorescent screen and additional tape on the filter (right hand side). However, assuming that the full signal observed were due to electrons would lead to a maximum observable fluences of approximately 6 × 104 , 3 × 104 and 1.2 × 104 srad−1 for 50, 100 and 250 keV electrons respectively. Previously electrons with energies up to a maximum of 6 keV (well below the observation range here) have been reported for cluster jet targets in cones around the incident electric field vector at intensities of the order of 1016 Wcm−2 [16, 17]. If one compares to the expected signals for the generation of hot electrons from knock on Coulomb collisions [10], for an intensity of 1 × 1017 Wcm−2 one would expect a hot electron plateau which extends to greater than 50 keV. However, only a small fraction of electrons with an angle of propagation within a cone angle of 9.5 mrad [11] will participate and only a small fraction have an impact parameter that would lead to an energy enhancement collision. The number of electrons produced with energies in the range of 40 to 50 keV is estimated to be significantly less than one per steradian per laser shot. The observation threshold of 6 × 104 srad−1 is well above this value and thus we would not be able to detect such a low fluence. 4. 2D PIC CODE MODELING FOR P-POLARIZED INCIDENCE ON SOLID TARGETS A 2D PIC simulation [18] has been run for the case of a 133 fs, 2x1016 Wcm−2 800 nm pulse incident at 45◦ on a slab target with a density scale length of 800 nm. As shown in Fig.4, near the peak of the laser pulse, an electron jet with energies of over 250 keV (>1.5) is observed in a direction between the specular and normal directions in agreement with the experimental observations. A jet is also observed at the rear side of the thin slab target which is not observed in the present experiment since a solid target was employed. In the simulation, a hot electron temperature of 69 keV is observed and the fraction of incident energy in the electrons above 100 keV is 0.04%. Thus about 0.01% of the energy would be above 250 keV in approximate agreement with the experiments. Further simulations are required to clarify the dependence on polarization and density scale length and 3D simulations are required for the case of S-polarization. Such simulations should help to elucidate the physical processes that lead to the generation of these hot electron jets. 5. CONCLUSIONS For solid target interactions electron, jets have been observed with electron energies of over 250 keV with ∼10−4 of the incident laser energy. The orientation of the jets changes from s- to p-polarization remaining in the plane of polarization. The beam is sufficiently intense that it can be used for electron radiography
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Figure 4. 2D PIC simulation of P-polarized solid target interaction (a) simulation geometry, (b) definition of angular geometry, (c) and (d) electron energy distribution versus angle at t = 67.9 fs and t = 74.7 fs relative to the peak of the laser pulse at 66.7 fs.
imaging experiments. In contrast, cluster jet targets do not show signs of such electron jets even up to intensities of 1 × 1017 Wcm−2 . Thus, it might be expected that cluster jet sources of multi-keV radiation should remain relatively free of contamination from very hard x-rays as compared to solid targets at similar fluences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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