Top PDF Characterization of multiterawatt laser-solid interactions for proton acceleration

Characterization of multiterawatt laser-solid interactions for proton acceleration

Characterization of multiterawatt laser-solid interactions for proton acceleration

After the pulse compressor and diagnostic station three flat turning mirrors, A, B, and C in Fig. 4, were used to steer the 790 nm laser pulse within the target chamber towards the focusing optic. The first of these 共 A 兲 had a dielectric coating to facilitate transmission of a 532 nm beam used in a retro- focus arrangement described in Sec. III. The remaining two mirrors were gold coated. A f /3, 15.1 cm focal length off- axis, parabolic mirror was used to focus the beam onto tar- get. Due to the quantity of debris that can be generated in high-repetition rate lasersolid interactions it was necessary to shield the gold coating on this optic with a transparent plastic pellicle. The pellicle is 10 ␮ m thick and therefore has a low B integral and does not effect the overall focus quality. To determine the size and quality of the laser focal spot a microscope objective was used to image it onto a change- coupled device 共 CCD 兲 camera. The spatial resolution of the system has been calibrated to measure the focal spot size. The beam energy was attenuated to facilitate this measure- ment. Figure 5 shows a typical image, where best focus cor- responded to a FWHM spot size of 2.5 ␮ m, which is almost diffraction limited. 41% of the total energy on target was determined to be contained within FWHM of this spot. The maximum energy on target was 225 mJ, as determined with a calorimeter positioned in the beam path just in front of the focusing optic. Using these pulse parameters, the focused intensity within FWHM of the main focal spot is calculated to be 3.2 ⫻ 10 19 W cm ⫺ 2 .
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Characterization of proton and heavier ion acceleration in ultrahigh-intensity laser interactions with heated target foils

Characterization of proton and heavier ion acceleration in ultrahigh-intensity laser interactions with heated target foils

Proton acceleration from the front and rear surfaces of the Fe target foils was also diagnosed for the same heated and unheated shots by measuring 共 p , n 兲 reactions in 63 Cu. The proton energy spectra were deduced by convoluting the lit- erature cross section for the 63 Cu 共 p , n 兲 63 Zn reaction [ 15 ] and the SRIM-2003 calculated proton stopping ranges in Cu. The technique is described in detail elsewhere [7]. The deduced proton spectra are shown in Fig. 5, and have been corrected for the protons passing through the 1-mm-thick carbon sample ( Fig. 1 ) . The measurement was limited to protons with energy above ⬃ 13 MeV. As observed in previous ex- periments [7] the proton maximum cutoff energy is lower for the protons measured at the front side of the target. Signifi- cantly, the numbers of protons accelerated from both the front and rear surfaces of the heated target are reduced by about 2 orders of magnitude compared to the unheated target. The energy conversion efficiency to protons in this energy range is of the order of 2% for the unheated target, falling to ⬃ 0.01% for the heated target. The total energy conversion efficiency to protons ( over the full proton energy range ) from unheated targets under similar experimental conditions was measured to be about 7%.
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Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

Figure 3(a) shows the total normalised flux of x-ray emission and number of protons as a function of laser defocus, the number of protons falls with increasing defocus whilst the x-ray signal remains relatively constant. This indicates to a first order approximation that as the laser is defocused, and the on-target laser intensity reduced, there are a similar number of accelerated electrons travelling within the target (creating bremsstrahlung as they travel) yet the number of these electrons reaching the rear surface and contributing to the acceleration of protons has decreased. The measured x-ray signal is consistent with K-alpha measurements modelled by Reich et al for copper targets irradiated at similar laser intensities [5, 37]. If we consider each region independently via the penumbral technique outlined above—figure 3(b) we see an increase in x-ray flux from the central source for larger defocus and, more pertinently, the ratio between the central and substrate source shifts significantly in favour of the central source—figure 4(a).
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Injection and transport properties of fast electrons in ultraintense laser-solid interactions

Injection and transport properties of fast electrons in ultraintense laser-solid interactions

The injection and transport divergence properties of a high current beam of energetic electrons in metallic targets irradiated by ultraintense, picosecond laser pulses is investi- gated using simultaneous measurements of Ka fluorescence and proton acceleration, and a programme of 3-D hybrid-PIC simulations. The Ka fluorescence measurements, which are sensitive to the overall lateral extent of the electron beam, indicate that the effective transport half-angle is between 10 and 38 (17 and 31 if previous measurements with the same laser are included) as defined by the degree of uncertainty in the measurements (best fit 24 ), and this is supported by the simulation results. The simulations further reveal that the fast electron beam transport is strongly affected by self-generated magnetic fields, which in turn are sensitive to the average injection angle of the electrons at the front side of the target.
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Angle-dependent modulated spectral peaks of proton beams generated in ultrashort intense laser-solid interactions

Angle-dependent modulated spectral peaks of proton beams generated in ultrashort intense laser-solid interactions

Laser-driven ion beams hashave advantages of short pulse duration, high brightness and small source size. For their potential applications such as proton radiography 1 , proton-driven fast ignition 2 , tumour therapy 3 , proton-driven nuclear reactions 4 , etc., monoenergetic spectral distributions of protons are preferred. However, most of the experimentally generated proton beams present exponential-like proton energy spectra. To produce proton beams with modulated spectral distributions, several mechanisms, such as radiation pressure acceleration 5 , break-out afterburner 6 , and laser-driven shock acceleration 7 , have been proposed, and successfully demonstrated by numerical simulations and experiments 8,9 . However, to implement these mechanisms the drive laser pulses must have very high contrast ratio better than 10 -10 and high focused intensity higher than 10 21 W/cm 2 , typically. Such requirements are great challenges for the laser systems in commission.
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Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

Figure 3 ( a ) shows the total normalised fl ux of x-ray emission and number of protons as a function of laser defocus, the number of protons falls with increasing defocus whilst the x-ray signal remains relatively constant. This indicates to a fi rst order approximation that as the laser is defocused, and the on-target laser intensity reduced, there are a similar number of accelerated electrons travelling within the target ( creating bremsstrahlung as they travel ) yet the number of these electrons reaching the rear surface and contributing to the acceleration of protons has decreased. The measured x-ray signal is consistent with K-alpha measurements modelled by Reich et al for copper targets irradiated at similar laser intensities [ 5, 37 ] . If we consider each region independently via the penumbral technique outlined above —fi gure 3 ( b ) we see an increase in x-ray fl ux from the central source for larger defocus and, more pertinently, the ratio between the central and substrate source shifts signi fi cantly in favour of the central source —fi gure 4 ( a ) .
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Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

T he use of high-power (multi-terawatt to petawatt) laser pulses to drive collective electron motion in plasma has given rise to compact laser-based particle accelerators with potentially wide-ranging applications in science, industry and medicine 1–3 . Control over the collective motion of high-energy plasma electrons enables the resulting beam properties to be varied and is therefore fundamental to the development of these promising sources. A pertinent example is the laser-wakefield acceleration of electrons in low-density plasma by the formation and control of ionized channels 4 or ‘bubbles’ 5 created by electron expulsion and re-injection. The introduction of increasingly sophisticated techniques to control the bubble evolution and the electron dynamics within it have had a transformational effect on the electron beam energies, energy spread, current and beam stability achieved 6 . It has also led to new types of secondary radiation sources, such as betatron 7,8 , and new applications such as phase contrast imaging 9 . By contrast, control of charged particle motion in plasma, which is too dense for laser light to propagate (termed overdense) is significantly more difficult to achieve. In such plasma, the laser light penetrates only to the region of the critical density (where the plasma frequency is equal to the laser frequency), at which point the relativistic electrons produced escape the influence of the laser field. Yet, intense laser pulse interactions with overdense plasma, and in particular thin solid density foils, have, for more than a decade, been shown to be important for ion acceleration 2,3 , high flux bremsstralung production 10 , high harmonic generation 11,12 , terahertz emission 13,14 and potentially nonlinear, high-energy synchrotron emission 7,15 . Controlling the collective motion of high-energy plasma electrons in thin foils would enable new perspectives for developing and applying these unique particle and radiation sources.
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Fabrication and characterization of porous opaque PMMA foils to be laser irradiated producing ion acceleration

Fabrication and characterization of porous opaque PMMA foils to be laser irradiated producing ion acceleration

The laser irradiation of the clear PMMA is lower absorbed and a cold plasma is obtained accelerating a low proton yield at a kinetic energy of 600 keV (Fig. 7a). The laser irradiation of the opaque PMMA in which Au NPs are embedded produces hot plasma and a very good forward acceleration in TNSA regime, accelerating protons up to 1.9 MeV (Fig. 7b). It means that the high intensity laser produces high relativistic electron acceleration at the polymer surface, thanks to the presence of high Au nanoparticle concentration. Such electrons are transmitted to the back face of the thin film producing high positive charge concentration in the polymeric film. Coulomb explosion is generated in the solid and ions are emitted. The high electric field generated in the back face of the target (forward direction) drives the ion (protons, carbon and oxygen) acceleration. The high proton energy is due to the increment of the electric field driving the acceleration which depends on the temperature and the electron density of the produced plasma, according to literature [17].
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Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

T he use of high-power (multi-terawatt to petawatt) laser pulses to drive collective electron motion in plasma has given rise to compact laser-based particle accelerators with potentially wide-ranging applications in science, industry and medicine 1–3 . Control over the collective motion of high-energy plasma electrons enables the resulting beam properties to be varied and is therefore fundamental to the development of these promising sources. A pertinent example is the laser-wakefield acceleration of electrons in low-density plasma by the formation and control of ionized channels 4 or ‘bubbles’ 5 created by electron expulsion and re-injection. The introduction of increasingly sophisticated techniques to control the bubble evolution and the electron dynamics within it have had a transformational effect on the electron beam energies, energy spread, current and beam stability achieved 6 . It has also led to new types of secondary radiation sources, such as betatron 7,8 , and new applications such as phase contrast imaging 9 . By contrast, control of charged particle motion in plasma, which is too dense for laser light to propagate (termed overdense) is significantly more difficult to achieve. In such plasma, the laser light penetrates only to the region of the critical density (where the plasma frequency is equal to the laser frequency), at which point the relativistic electrons produced escape the influence of the laser field. Yet, intense laser pulse interactions with overdense plasma, and in particular thin solid density foils, have, for more than a decade, been shown to be important for ion acceleration 2,3 , high flux bremsstralung production 10 , high harmonic generation 11,12 , terahertz emission 13,14 and potentially nonlinear, high-energy synchrotron emission 7,15 . Controlling the collective motion of high-energy plasma electrons in thin foils would enable new perspectives for developing and applying these unique particle and radiation sources.
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Demonstration of coherent terahertz transition radiation from relativistic laser-solid interactions

Demonstration of coherent terahertz transition radiation from relativistic laser-solid interactions

Relativistic electron beams can also be generated in the interactions of intense laser pulses with low-density gas or high-density solid targets. For gas targets, electrons can be accelerated to a GeV energy level with wakefields [11,12]. With such an electron beam, Leemans et al. have observed a ~0.3 μJ THz pulse through transition radiation [13]. Strong THz radiation from laser-solid interactions has also been demonstrated [14,15,16,17]. Compared with the cases of gas targets, electron beams from solid targets have much higher charge, up to nC-μC. Recently THz radiation with energies of >700 μJ has been reported from the rear surface of a foil target [18,19]. Since the THz yield is found to be correlated with the square of the proton number, it is attributed to a transient dipole-like charge distribution structure formed by target normal sheath acceleration (TNSA), referred to hereafter as TNSA radiation [18]. On the other hand, in principle, THz radiation can be produced efficiently via the transition radiation process at the rear surface of a laser-irradiated thin solid foil [20], since both the short electron bunch duration and high bunch charge are ideal for this mechanism. Previous studies on the transition radiation from solid targets mainly concern in the optical region [21,22]. While the radiation in the THz regime has so far not yet been verified experimentally.
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Investigations of fast electron transport in intense laser-solid interactions

Investigations of fast electron transport in intense laser-solid interactions

Ion emission is one of the key secondary processes resulting from fast electron transport and is described in detail in Section 3.8. The acceleration is the result of intense electric fields setup by the fast electron sheath across the surfaces of the target. The electrostatic potential at the rear surface can exceed 10 12 V/m and can effectively accelerate ions to multi-MeV energies over a few microns. The acceleration process is known as Target Normal Sheath Acceleration (TNSA) [25]. The geometry of the acceleration mechanism results in ion beams emitted normal to the surface of the target. Key characteristics of these beams include a compact source size both spatially (tens of microns) and tem- porally (ps at source) and very low transverse emittance. Having the largest charge to mass ratio, protons are accelerated more effectively than other heavier ions. At present, the maximum energy of protons accelerated by this method is ≈ 60 MeV [26], although recent reports have been made of 67.5 MeV [27]. The total transfer of energy between the laser ion beam via the fast electrons can reach 10% [26].
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Diagnosis of Weibel instability evolution in the rear surface scale lengths of laser solid interactions via proton acceleration

Diagnosis of Weibel instability evolution in the rear surface scale lengths of laser solid interactions via proton acceleration

We note that ion acceleration from a non zero density scale length can be complex [24–26, 27], however, in general the time taken for the completion of the ion acceleration becomes longer with increasing density scale length. Here, the increased acceleration time is consistent with the breaking time given by Grismayer and Mora [24]. This forecasts that the acceleration from a 0.3 m m plasma scale length will require around 2.5 times longer than a zero density scale length plasma, which itself has a typical acceleration time empirically found to be around 1.3 times the laser pulse duration [28]. This corresponds to a global simulation time of around 1.9 ps in the 0.3 m m case, while the strong filamentary structure in the magnetic field is observed to have faded at an earlier time of around 1.0 ps, and so the lower energy protons are not affected by these structures as they are accelerated toward the end of this time window.
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Proton driven plasma wakefield acceleration in AWAKE

Proton driven plasma wakefield acceleration in AWAKE

The support of the Max Planck Society is gratefully acknowledged. This work was supported in parts by the Siberian Branch of the Russian Academy of Science (project No. 0305-2017- 0021), a Leverhulme Trust Research Project Grant RPG-2017-143 and by STFC (AWAKE- UK, Cockroft Institute core and UCL consolidated grants), United Kingdom; a Deutsche Forschungsgemeinschaft project grant PU 213-6/1 “Three-dimensional quasi-static simulations of beam self-modulation for plasma wakefield acceleration”; the National Research Foundation of Korea (Nos. NRF-2015R1D1A1A01061074 and NRF-2016R1A5A1013277); the Portuguese FCT— Foundation for Science and Technology, through grants CERN/FIS-TEC/0032/2017, PTDC- FIS-PLA-2940-2014, UID/FIS/50010/2013 and SFRH/IF/01635/2015; NSERC and CNRC for TRIUMF’s contribution; and the Research Council of Norway. M. Wing acknowledges the support of the Alexander von Humboldt Stiftung and DESY, Hamburg. The AWAKE collaboration acknowledge the SPS team for their excellent proton delivery.
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Heigoldt, Matthias
  

(2017):


	Temporal dynamics of the longitudinal bunch profile in a laser wakefield accelerator.


Dissertation, LMU München: Fakultät für Physik

Heigoldt, Matthias (2017): Temporal dynamics of the longitudinal bunch profile in a laser wakefield accelerator. Dissertation, LMU München: Fakultät für Physik

Diese Doktorarbeit beschäftigt sich mit der Messung des zeitlichen Profils von Elektronenpulsen aus lasergetriebenen Plasmabeschleunigern. Bei der sogenannten laser wakefield acceleration (LWFA) treibt ein hochintensiver Ultrakurzpulslaser eine Plasmawelle, die Beschleunigungsfel- der von mehreren hundert GV/m aufrechterhalten kann, und somit die von derzeitigen Radiofre- quenzbeschleunigern erreichbaren Felder um vier Größenordnungen übertrifft. Dies eröffnet die Möglichkeit die Größe und somit die Kosten derartiger Beschleuniger in Zukunft zu reduzieren. Da der zu beschleunigende Elektronenpuls notwendigerweise auf die im Bereich von µm liegen- de Größe der Plasmawelle limitiert ist, liefern LWFAs darüber hinaus ultrakurze und brillante Elektronenpakete, die geeignet sind kompakte Kurzpuls-Röntgenquellen zu realisieren, sei es mittels Thomson-Rückstreuung, Betatronstrahlung oder durch Freie-Elektronen-Laser (FELs). Insbesondere für letztere Anwendung ist das Profil des Elektronenpakets ausschlaggebend, da es den zur Verfügung stehenden Spitzenstrom bestimmt, der eine wichtige Ausgangsgröße für ein Undulatordesign darstellt, welches den Selbstverstärkungsprozess optimal unterstützt.
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Return current and proton emission from short pulse laser interactions with wire targets

Return current and proton emission from short pulse laser interactions with wire targets

The observations show that in all three target configurations—single wire, two adjacent wires and foil with wire behind—there is a significant amount of second har- monic emission. Figure 5 shows for the two wire configura- tion the emission is most intense from around the focal spot on the target wire and from a localized region at the same height in the nearby second wire 共 the laser is in the horizon- tal plane 兲 . In the foil/wire target configuration 共 Fig. 4 兲 the 2 ␻ emission is most intense from the back of the foil. We be- lieve that this emission is due to the coherent optical transi- tion radiation, which is generated when high energy elec- trons cross the interface between media with different dielectric properties. 22 At the focal spot on the target the electron beam is produced by the ponderomotive J Ã B force of the laser, 23 and at the secondary wire, the beam is caused by electron acceleration in the electric field between the tar- get and the nearby wire. The emission is coherent and is at 2 ␻ since the electrons are generated at twice the laser frequency. 24,25
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Acceleration of electrons in the plasma wakefield of a proton bunch

Acceleration of electrons in the plasma wakefield of a proton bunch

FIG. 1. The layout of the AWAKE experiment. The proton bunch and laser pulse propagate from left to right across the image, through a 10 m column of rubidium vapour. This laser pulse (green, bottom images) singly ionises the rubidium (Rb) to form a plasma (yellow) which then interacts with the proton bunch (red, bottom left image). This interaction modulates the long proton bunch into a series of microbunches (bottom right image) which drive a strong wakefield in the plasma. The self-modulation of the proton bunch is measured in imaging stations 1 and 2 and the optical and coherent transition radiation (OTR, CTR) diagnostics. The rubidium is supplied by two flasks (pink) at each end of the vapour source. The density is controlled by changing the temperature in these flasks and a gradient may be introduced by changing their relative temperature. Electrons (blue), generated using a radio frequency (RF) source, propagate a short distance behind the laser pulse and are injected into the wakefield by crossing at an angle. Some of these electrons are captured in the wakefield and accelerated to high energies. The accelerated electron bunches are focused and separated from the protons by the spectrometer’s quadrupoles and dipole magnet (grey, right). These electrons interact with a scintillating screen (top right image), allowing them to be imaged and their energy inferred from their position.
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Effects of laser prepulses on laser-induced proton generation

Effects of laser prepulses on laser-induced proton generation

From an experimental point of view, the obvious way to characterize preplasma is by performing interferometry. This was done, for example, in [15]. The experimental characterization of the preplasma, supported by computer hydrodynamics simulations, allows one to predict the evolution of the preplasma. However, at the same time, hydro simulations allow one to follow the evolution of the shock travelling in the target up to shock breakout (again such predictions can be corroborated by experimental measurements, as shown in figure 10). In turn this finally allows one to determine the minimum target thickness that is required to prevent shock breakout on the rear side. An example of how front and rear side predictions can be used together is shown in figure 10. The laser source used in the experiment was a CPA Ti:sapphire yielding 40 mJ on the target in 150 fs duration, resulting in a peak intensity on the target of 4 × 10 15 W cm −2 . We see that in order to obtain L /λ ≈ 4 (as predicted in [14])
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High efficiency proton beam generation through target thickness control in femtosecond laser-plasma interactions

High efficiency proton beam generation through target thickness control in femtosecond laser-plasma interactions

Many key experiments in the field of laser-accelerated ion beams have been performed on large scale Nd:Glass laser systems where high laser pulse energies (>100 J) coupled with sub-picosecond pulse lengths have produced focused intensities of 10 21 W cm 2 to accelerate the high- est energy protons (60 MeV). 7–9 However, recent years have seen the commissioning of an increasing number of high intensity Ti:sapphire based lasers which operate at pulse lengths typically around 50 fs but with lower pulse energies (0.5–10 J). Such systems have the advantage that they not only occupy a smaller spatial footprint but also operate at significantly higher repetition rates (typically 1 shot per mi- nute here) when compared to similar intensity Nd:Glass lasers (1 shot per hour). Many conceivable applications of laser-accelerated ion beams will not only require a low cost,
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Multi-pulse enhanced laser ion acceleration using plasma half cavity targets

Multi-pulse enhanced laser ion acceleration using plasma half cavity targets

The proton energy spectra obtained for four shots in the experimental campaign are presented in Fig. 2(a). These were obtained for constant laser parameters incident on a sin- gle planar foil target as a control measurement and three plasma half cavity targets with radii of 100 lm, 210 lm, and 260 lm, corresponding to ds of 670 fs, 1400 fs, and 1730 fs, respectively. The interaction of the laser pulse with the pla- nar foil is found to give the single temperature quasi Max- wellian proton spectra typically observed in laser plasma interactions in this intensity regime. On introduction of the two larger half cavity targets a significant increase in laser to proton energy conversion efficiency of 55% is observed in the lower half of the energy spectrum and generally a higher proton number is measured over the full energy spectrum for all half cavities. This is shown more clearly by plotting the enhancement factors (Fig. 2(b)), where the dose observed at a given proton energy is normalized to the dose obtained at the same energy for the planar foil interaction. In general, the enhancement factor is larger at higher proton energies suggesting a slight increase in proton temperature which
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Review of Energetic Particle Generation and Electromagnetic Radiation from Intense Laser-Plasma Interactions at the Institute of Physics, Chinese Academy of Sciences

Review of Energetic Particle Generation and Electromagnetic Radiation from Intense Laser-Plasma Interactions at the Institute of Physics, Chinese Academy of Sciences

In our experiments, we focus the pulse from the XL- II laser system using an f/3.5 off-axis parabolic mirror onto planar and wedge-shaped copper targets. The stan- dard planar target is a copper disk (thickness 65 µm, diam- eter 1000 µm), and the wedge-shaped target has the fol- lowing dimensions: tip angle 25 ◦ , height 150 µm. The angular distributions of the forward fast electrons behind the targets are measured by an array of imaging plate (IP) stacks. Figure 4 shows the measured angular distributions for the planar and wedge-shaped targets. Fast electrons (E > 600 keV) are emitted with a single smooth peak near the target normal for the planar target, and with double peaks on both sides of the isosceles surfaces for the wedge- shaped target. However, a large number of E > 120 keV electrons are emitted in the tip direction, resulting in a distribution without obvious concavity at the central part. This is much di ff erent than for the E > 600 keV electrons.
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