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Sökning: WFRF:(Persson Anders) > Wahlström Claes Göran

  • Resultat 41-50 av 59
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41.
  • Lopez, Rodrigo, et al. (författare)
  • Amplitude and phase control of attosecond light pulses
  • 2005
  • Ingår i: Physical Review Letters. - 1079-7114. ; 94:3
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the generation, compression, and delivery on target of ultrashort extreme-ultraviolet light pulses using external amplitude and phase control. Broadband harmonic radiation is first generated by focusing an infrared laser with a carefully chosen intensity into a gas cell containing argon atoms. The emitted light then goes through a hard aperture and a thin aluminum filter that selects a 30-eV bandwidth around a 30-eV photon energy and synchronizes all of the components, thereby enabling the formation of a train of almost Fourier-transform-limited single-cycle 170 attosecond pulses. Our experiment demonstrates a practical method for synthesizing and controlling attosecond waveforms.
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42.
  • Lundh, Olle, et al. (författare)
  • Active steering of laser-accelerated ion beams
  • 2008
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951. ; 92:1
  • Tidskriftsartikel (refereegranskat)abstract
    • A technique for optical control of the spatial distribution of laser-accelerated ion beams is presented. An ultra-short laser pulse, tightly focused to relativistic intensities on a thin foil target, drives a beam of MeV ions. An auxiliary, nanosecond laser pulse drives a shock and locally deforms the initially flat target prior to the main pulse interaction. By changing the properties of the shock-driving laser pulse, the normal direction of the ion emitting surface is locally manipulated and the emission direction is thereby controlled. In the future, this method could be used to achieve dynamic control of the ion beam divergence.
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43.
  • Lundh, Olle, et al. (författare)
  • Influence of shock waves on laser-driven proton acceleration
  • 2007
  • Ingår i: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 76:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence of shock waves, driven by amplified spontaneous emission (ASE), on laser-accelerated proton beams is investigated. A local deformation, produced by a cold shock wave launched by the ablation pressure of the ASE pedestal, can under oblique laser irradiation significantly direct the proton beam toward the laser axis. This can be understood in the frame of target normal sheath acceleration as proton emission from an area of the target where the local target normal is shifted toward the laser axis. Hydrodynamic simulations and experimental data show that there exists a window in laser and target parameter space where the target can be significantly deformed and yet facilitate efficient proton acceleration. The dependence of the magnitude of the deflection on target material, foil thickness, and ASE pedestal intensity and duration is experimentally investigated. The deflection angle is found to increase with increasing ASE intensity and duration and decrease with increasing target thickness. In a comparison between aluminum and copper target foils, aluminum is found to yield a larger proton beam deflection. An analytic model is successfully used to predict the proton emission direction.
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44.
  • Mangles, SPD, et al. (författare)
  • Effect of contrast ratio on electron beam stability in laser wakefield acceleration experiments
  • 2006
  • Ingår i: Plasma Physics and Controlled Fusion. - 0741-3335. ; 48:12B, s. 83-90
  • Tidskriftsartikel (refereegranskat)abstract
    • Laser wakefield accelerators offer the possibility of compact electron acceleration. However one of the key outstanding issues with the results reported to date is the electron beam stability. Experiments on two laser systems reveal that the contrast ratio between the ASE pedestal and main pulse is an important factor in determining the quality of the electron beam. With a high contrast ratio (10^8) the electron beam profile is a well collimated single beam having a low pointing instability (<10 mrad rms). With a lower contrast (10^6) the beam profile contains multiple beamlets which exhibit a large pointing instability (~50 mrad rms). Ahigh contrast ratio not only improves the beam pointing stability (~6 mrad) but also stabilizes the electron beam energy reproducibility (5%).
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45.
  • Mangles, SPD, et al. (författare)
  • Laser-wakefield acceleration of monoenergetic electron beams in the first plasma-wave period
  • 2006
  • Ingår i: Physical Review Letters. - 1079-7114. ; 96:21
  • Tidskriftsartikel (refereegranskat)abstract
    • Beam profile measurements of laser-wakefield accelerated electron bunches reveal that in the monoenergetic regime the electrons are injected and accelerated at the back of the first period of the plasma wave. With pulse durations c tau >=lambda(p), we observe an elliptical beam profile with the axis of the ellipse parallel to the axis of the laser polarization. This increase in divergence in the laser polarization direction indicates that the electrons are accelerated within the laser pulse. Reducing the plasma density (decreasing c tau/lambda(p)) leads to a beam profile with less ellipticity, implying that the self-injection occurs at the rear of the first period of the plasma wave. This also demonstrates that the electron bunches are less than a plasma wavelength long, i.e., have a duration < 25 fs. This interpretation is supported by 3D particle-in-cell simulations.
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46.
  • Mangles, S. P. D., et al. (författare)
  • Controlling the spectrum of x-rays generated in a laser-plasma accelerator by tailoring the laser wavefront
  • 2009
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 95:18
  • Tidskriftsartikel (refereegranskat)abstract
    • By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam's betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotronlike x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield.
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47.
  • Mangles, S. P. D., et al. (författare)
  • On the stability of laser wakefield electron accelerators in the monoenergetic regime
  • 2007
  • Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 14:5
  • Tidskriftsartikel (refereegranskat)abstract
    • The effects of plasma density and laser energy on the stability of laser produced monoenergetic electron beams are investigated. Fluctuations in the principal beam parameters, namely, electron energy, energy-spread, charge, and pointing, are demonstrated to be minimized at low densities. This improvement in stability is attributed to the reduced time for pulse evolution required before self-injection occurs; i.e., that the pulse is closest to the matched conditions for these densities. It is also observed that electrons are only consistently produced above a density-dependent energy threshold. These observations are consistent with there being a threshold intensity (a(0)greater than or similar to 3) required for the occurrence of self-injection after accounting for pulse compression. (C) 2007 American Institute of Physics.
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48.
  • Mangles, S. P. D., et al. (författare)
  • Self-injection threshold in self-guided laser wakefield accelerators
  • 2012
  • Ingår i: Physical Review Special Topics. Accelerators and Beams. - 1098-4402. ; 15:1
  • Tidskriftsartikel (refereegranskat)abstract
    • A laser pulse traveling through a plasma can excite large amplitude plasma waves that can be used to accelerate relativistic electron beams in a very short distance-a technique called laser wakefield acceleration. Many wakefield acceleration experiments rely on the process of wave breaking, or self-injection, to inject electrons into the wave, while other injection techniques rely on operation without self-injection. We present an experimental study into the parameters, including the pulse energy, focal spot quality, and pulse power, that determine whether or not a wakefield accelerator will self-inject. By taking into account the processes of self-focusing and pulse compression we are able to extend a previously described theoretical model, where the minimum bubble size k(p)r(b) required for trapping is not constant but varies slowly with density and find excellent agreement with this model.
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49.
  • McKenna, P, et al. (författare)
  • High-intensity laser-driven proton acceleration: influence of pulse contrast
  • 2006
  • Ingår i: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Science. - : The Royal Society. - 1364-503X .- 1471-2962. ; 364:1840, s. 711-723
  • Tidskriftsartikel (refereegranskat)abstract
    • Proton acceleration from the interaction of ultra-short laser pulses with thin foil targets at intensities greater than 10(18) W cm(-2) is discussed. An overview of the physical processes giving rise to the generation of protons with multi-MeV energies, in well defined beams with excellent spatial quality, is presented. Specifically, the discussion centres on the influence of laser pulse contrast on the spatial and energy distributions of accelerated proton beams. Results from an ongoing experimental investigation of proton acceleration using the 10 Hz multi-terawatt Ti : sapphire laser (35 fs, 35 TW) at the Lund Laser Centre are discussed. It is demonstrated that a window of amplified spontaneous emission (ASE) conditions exist, for which the direction of proton emission is sensitive to the ASE-pedestal preceding the peak of the laser pulse, and that by significantly improving the temporal contrast, using plasma mirrors, efficient proton acceleration is observed from target foils with thickness less than 50 nm.
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50.
  • Neely, D., et al. (författare)
  • Enhanced proton beams from ultrathin targets driven by high contrast laser pulses
  • 2006
  • Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 89:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The generation of proton beams from ultrathin targets, down to 20 nm in thickness, driven with ultrahigh contrast laser pulses is explored. the conversion efficiency from laser energy into protons increases as the foil thickness is decreased, with good beam quality and high efficiencies of 1% being achieved, for protons with kinetic energy exceeding 0.9 MeV, for 100 nm thick aluminum foils at intensities of 10(19) W/cm(2) with 33 fs, 0.3 J pulses. To minimize amplified spontaneous emission (ASE) induced effects disrupting the acceleration mechanism, exceptional laser to ASE intensity contrasts of up to 1010 are achieved by introducing a plasma mirror to the high contrast 10 Hz multiterawatt laser at the Lund Laser Centre. It is shown that for a given laser energy on target, regimes of higher laser-to-proton energy conversion efficiency. can be accessed with increasing contrast. The increasing efficiency as the target thickness decreases is closely correlated to an increasing proton temperature. (c) 2006 American Institute of Physics.
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