Showing papers by "David Neely published in 2012"
TL;DR: The spectral features provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode and indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.
Abstract: The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 × 10(20) W cm(-2). The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5-10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.
270 citations
TL;DR: In this article, a new high-order harmonic generation mechanism reaching the "water window" spectral region was demonstrated with multiterawatt femtosecond lasers irradiating gas jets.
Abstract: We demonstrate a new high-order harmonic generation mechanism reaching the "water window" spectral region in experiments with multiterawatt femtosecond lasers irradiating gas jets. A few hundred harmonic orders are resolved, giving μJ/sr pulses. Harmonics are collectively emitted by an oscillating electron spike formed at the joint of the boundaries of a cavity and bow wave created by a relativistically self-focusing laser in underdense plasma. The spike sharpness and stability are explained by catastrophe theory. The mechanism is corroborated by particle-in-cell simulations.
72 citations
TL;DR: This Letter describes the first experimental demonstration of the guiding of a relativistic electron beam in a solid target using two colinear, relativistically intense, picosecond laser pulses, which could be of considerable benefit to fast-ignition inertial fusion.
Abstract: This Letter describes the first experimental demonstration of the guiding of a relativistic electron beam in a solid target using two colinear, relativistically intense, picosecond laser pulses. The first pulse creates a magnetic field that guides the higher-current, fast-electron beam generated by the second pulse. The effects of intensity ratio, delay, total energy, and intrinsic prepulse are examined. Thermal and K alpha imaging show reduced emission size, increased peak emission, and increased total emission at delays of 4-6 ps, an intensity ratio of 10:1 (second: first) and a total energy of 186 J. In comparison to a single, high-contrast shot, the inferred fast-electron divergence is reduced by 2.7 times, while the fast-electron current density is increased by a factor of 1.8. The enhancements are reproduced with modeling and are shown to be due to the self-generation of magnetic fields. Such a scheme could be of considerable benefit to fast-ignition inertial fusion.
48 citations
TL;DR: In this paper, a plasma half cavity target design for laser driven ion acceleration that enhances the laser to proton energy conversion efficiency and has been found to modify the low energy region of the proton spectrum is presented.
Abstract: We report on a plasma half cavity target design for laser driven ion acceleration that enhances the laser to proton energy conversion efficiency and has been found to modify the low energy region of the proton spectrum. The target design utilizes the high fraction of laser energy reflected from an ionized surface and refocuses it such that a double pulse interaction is attained. We report on numerical simulations and experimental results demonstrating that conversion efficiencies can be doubled, compared to planar foil interactions, when the secondary pulse is delivered within picoseconds of the primary pulse.
27 citations
TL;DR: In this paper, the influence of irradiated spot size on laser energy coupling to electrons, and subsequently to protons, in the interaction of intense laser pulses with foil targets is investigated experimentally.
Abstract: The influence of irradiated spot size on laser energy coupling to electrons, and subsequently to protons, in the interaction of intense laser pulses with foil targets is investigated experimentally. Proton acceleration is characterized for laser intensities ranging from 2 x 10(18) - 6 x 10(20) W/cm(2), by (1) variation of the laser energy for a fixed irradiated spot size, and (2) by variation of the spot size for a fixed energy. At a given laser pulse intensity, the maximum proton energy is higher under defocus illumination compared to tight focus and the results are explained in terms of geometrical changes to the hot electron injection. (C) 2012 American Institute of Physics. [doi:10.1063/1.3685615]
23 citations
TL;DR: In this article, a simple method to enhance ion generation with femtosecond ultraintense lasers is demonstrated experimentally by defocusing laser beams on target surface, and the authors find that the population of medium and low energy protons from ultra-thin foils is increased significantly while the proton cutoff energy is almost unchanged.
Abstract: A simple method to enhance ion generation with femtosecond ultraintense lasers is demonstrated experimentally by defocusing laser beams on target surface. When the laser is optimally defocused, we find that the population of medium and low energy protons from ultra-thin foils is increased significantly while the proton cutoff energy is almost unchanged. In this way, the total proton yield can be enhanced by more than 1 order, even though the peak laser intensity drops. The depression of the amplified spontaneous emission (ASE) effect and the population increase of moderate-energy electrons are believed to be the main reasons for the effective enhancement.
12 citations
TL;DR: In this article, a 7 MeV proton beam collimated to 16 mrad containing more than 106 particles is experimentally demonstrated by focusing a 2'J, 60 fs pulse of a Ti:sapphire laser onto targets of different materials and thicknesses placed in a millimeter scale conical holder.
Abstract: A 7 MeV proton beam collimated to 16 mrad containing more than 106 particles is experimentally demonstrated by focusing a 2 J, 60 fs pulse of a Ti:sapphire laser onto targets of different materials and thicknesses placed in a millimeter scale conical holder. The electric potential induced on the target holder by laser-driven electrons accelerates and dynamically controls a portion of a divergent quasi-thermal proton beam originated from the target, producing a quasi-monoenergetic “pencil” beam.
7 citations
Journal Article•
TL;DR: In this paper, the results of experiments conducted within the Work Package 10 (fusion experimental program) of the HiPER project were presented, where the physics relevant for advanced ignition schemes for inertial confinement fusion, i.e., fast ignition and the shock ignition, were studied.
Abstract: This paper presents the results of experiments conducted within the Work Package 10 (fusion experimental
programme) of the HiPER project. The aim of these experiments was to study the physics relevant for advanced ignition
schemes for inertial confinement fusion, i.e. the fast ignition and the shock ignition. Such schemes allow to achieve a higher
fusion gain compared to the indirect drive approach adopted in the National Ignition Facility in United States, which is
important for the future inertial fusion energy reactors and for realising the inertial fusion with smaller facilities.
5 citations
11 Jul 2012
TL;DR: Esirkepov et al. as mentioned in this paper demonstrate a new regime of high-order harmonics generation by relativistic-irradiance lasers in gas jet targets, where bright harmonics with both odd and even orders are emitted in the forward direction, while the base harmonic frequency is downshifted.
Abstract: We experimentally demonstrate a new regime of high-order harmonic generation by relativistic-irradiance lasers in gas jet targets. Bright harmonics with both odd and even orders, generated by linearly as well as circularly polarized pulses, are emitted in the forward direction, while the base harmonic frequency is downshifted. A 9 TW laser generates harmonics up to 360 eV, within the 'water window' spectral region. With a 120 TW laser producing 40 uJ/sr per harmonic at 120 eV, we demonstrate the photon number scalability. The observed harmonics cannot be explained by previously suggested scenarios. A novel high-order harmonics generation mechanism [T. Zh. Esirkepov et al., AIP Proceedings, this volume], which explains our experimental findings, is based on the phenomena inherent in the relativistic laser - underdense plasma interactions (self-focusing, cavity evacuation, and bow wave generation), mathematical catastrophe theory which explains formation of electron density singularities (cusps), and collective radiation due to nonlinear oscillations of a compact charge.
2 citations
11 Jul 2012
TL;DR: In this paper, a 7 MeV intense proton beam was controlled dynamically and energy selected by the self-induced quasi-static electric field on the target holder with the hole associated with the design of the well-known Wehnelt electrode of electron-gun.
Abstract: From the interaction between the high-contrast (∼more than 1010) 130 TW Ti:sapphire laser pulse and Stainless Steel-2.5 um-thick tape target, proton beam with energies up to 23 MeV with the conversion efficiency of ∼1% is obtained. After plasma mirror installation for contrast improvement, from the interaction between the 30 TW laser pulse and thin-foil target installed on the target holder with the hole whose shape is associated with the design of the well-known Wehnelt electrode of electron-gun, a 7 MeV intense proton beam is controlled dynamically and energy selected by the self-induced quasi-static electric field on the target holder. From the highly divergent beam having continuous spectrum, which are the typical features of the laser-driven proton beams from the interactions between the short-pulse laser and solid target, the spatial distribution of 7 MeV proton bunch is well manipulated to be focused to an small spots with an angular distribution of ∼10 mrad. The number of protons included in the bunch is >106.
1 citations
08 Jul 2012
TL;DR: In this paper, a novel spectrometer designed to enable simultaneous spectral and 1D imaging measurements of X-rays emitted in intense laser-plasma interactions is presented, which enables X-ray emission from a large region of the plasma to be characterised, facilitating for example temperature measurements over extended regions of the target.
Abstract: Summary form only given. A novel spectrometer designed to enable simultaneous spectral and 1-D imaging measurements of X-rays emitted in intense laser-plasma interactions is presented [1]. This new diagnostic enables X-ray emission from a large region of the plasma to be characterised, facilitating for example temperature measurements over extended regions of the target.
08 Jul 2012
TL;DR: In this paper, it was shown that ignition is possible with currently achievable proton temperatures, however conversion efficiencies of laser energy to protons must be increased beyond the few percent so far routinely achieved to upwards of ten percent for this to be a feasible concept.
Abstract: Summary form only given. Laser driven ion acceleration is particularly interesting due to its many potential applications, including (isochoric) heating of matter which has been proposed as an attractive method for heating nuclear fuel in fusion reactions. In theory ignition is predicted to be possible with currently achievable proton temperatures, however conversion efficiencies of laser energy to protons must be increased beyond the few percent so far routinely achieved to upwards of ten percent for this to be a feasible concept1.