New developments in energy transfer and transport studies in relativistic laser-plasma interactions (original) (raw)
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Recent fast electron energy transport experiments relevant to fast ignition inertial fusion
A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5–5 ps and a focused intensity of up to 1021Wcm−2. Adependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particlein- cell simulations and has been found to be an intrinsic property of the laser–plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15–30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines’ relativistic absorption model.
A number of experiments have been undertaken at the Rutherford Appleton Laboratory that were designed to investigate the physics of fast electron transport relevant to fast ignition inertial fusion. The laser, operating at a wavelength of 1054 nm, provided pulses of up to 350 J of energy on target in a duration that varied in the range 0.5–5 ps and a focused intensity of up to 10 21 W cm −2. A dependence of the divergence of the fast electron beam with intensity on target has been identified for the first time. This dependence is reproduced in two-dimensional particle-in-cell simulations and has been found to be an intrinsic property of the laser–plasma interaction. A number of ideas to control the divergence of the fast electron beam are described. The fractional energy transfer to the fast electron beam has been obtained from calibrated, time-resolved, target rear-surface radiation temperature measurements. It is in the range 15–30%, increasing with incident laser energy on target. The fast electron temperature has been measured to be lower than the ponderomotive potential energy and is well described by Haines' relativistic absorption model.
2006
This thesis is related to inertial fusion research, and particularly concerns the approach to fast ignition, which is based on the use of ultra-intense laser pulses to ignite the thermonuclear fuel. Until now, the feasibility of this scheme has not been proven and depends on many fundamental aspects of the underlying physics, which are not yet fully understood and which are also very far from controls. The main purpose of this thesis is the experimental study of transport processes in the material over-dense (solid) and under-dense (gas jet) of a beam of fast electrons produced by pulse laser at a intensity of some 1019 Wcm-2.
Review of physics and applications of relativistic plasmas driven by ultra-intense lasers
Physics of Plasmas, 2001
As tabletop lasers continue to reach record levels of peak power, the interaction of light with matter has crossed a new threshold, in which plasma electrons at the laser focus oscillate at relativistic velocities. The highest forces ever exerted by light have been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. Not only is this acceleration gradient up to a thousand times greater than in radio-frequency-based sources, but the transverse emittance of the particle beams is comparable or lower. Additionally, laser-based accelerators have been demonstrated to work at a repetition rate of 10 Hz, an improvement of a factor of 1000 over their best performance of just a couple of years ago. Anticipated improvements in energy spread may allow these novel compact laser-based radiation sources to be useful someday for cancer radiotherapy and as injectors into conventional accelerators, which are critical tools for x-ray and nuclear physics research. They might also be used as a spark to ignite controlled thermonuclear fusion. The ultrashort pulse duration of these particle bursts and the x rays they can produce, hold great promise as well to resolve chemical, biological or physical reactions on ultrafast ͑femtosecond͒ time scales and on the spatial scale of atoms. Even laser-accelerated protons are soon expected to become relativistic. The dense electron-positron plasmas and vast array of nuclear reactions predicted to occur in this case might even help bring astrophysical phenomena down to Earth, into university laboratories. This paper reviews the many recent advances in this emerging discipline, called high-field science. new field of physics, known as high-field science. It is not intended to be comprehensive, but rather to be restricted to a discussion of some of the highlights, mainly over the last 5 years, in the relativistic regime of laser-plasma interactions. Several reviews have already been published on highintensity laser development and applications, 2-4 relativistic nonlinear optics, 4,5 laser accelerators, 6 and intense laserplasma interactions. The paper is organized as follows. A brief basic theoretical overview of relativistic laser-plasmas interactions, with references only to early work, is presented in Sec. II. Recent results and references to more recent theoretical and numerical work are discussed in Sec. III A; experimental results are presented in Sec. III B. Prospects and applications are reviewed in Sec. IV.
Fast electron generation and transport in laser-irradiated targets at relativistic intensities
The transport of relativistic electrons in solid targets irradiated by a short laser pulse at relativistic intensities has been studied both experimentally and numerically. A Monte-Carlo collision code takes into account individual collisions with the ions and electrons in the target. A 3D-hybrid code takes into account these collisions as well as the generation of electric and magnetic fields and the self-consistent motion of the electrons in these fields. It predicts a magnetic guiding of a fraction of the fast electron current over long distances and a localized heating of the material along the propagation axis. In experiments performed at LULI on the 100 TW laser facility, several diagnostics have been implemented to diagnose the geometry of the fast electron transport and the target heating. The typical conditions were: E1 less-than-or-equal 20 J, lambda = 1 mum, tau approximately 300 fs, I approximately 1018-5.1019W/cm2. The results indicate a modest heating of the target (ty...
Relativistic laser channeling in plasmas for fast ignition
Physical Review E, 2007
We report an experimental observation suggesting plasma channel formation by focusing a relativistic laser pulse into a long-scale-length preformed plasma. The channel direction coincides with the laser axis. Laser light transmittance measurement indicates laser channeling into the high-density plasma with relativistic selffocusing. A three-dimensional particle-in-cell simulation reproduces the plasma channel and reveals that the collimated hot-electron beam is generated along the laser axis in the laser channeling. These findings hold the promising possibility of fast heating a dense fuel plasma with a relativistic laser pulse.
It is demonstated experimentally that the presence of a long-pulse laser created backplasma on the target backside can focus the relativistic electrons produced by short-pulse laser interaction with the front of a solid target. Comparison to that without the backplasma, the number density of the fast electrons is increased by one order of magnitude and their divergence angle is reduced five fold. The effect can be attributed to the absence of the backside sheath electric field and the collimation effect of the megagauss baroclinic magnetic field there.
Generation and Transport of Fast Electrons in Laser Irradiated Targets at Relativistic Intensities
AIP Conference Proceedings, 2002
The transport of relativistic electrons in solid targets irradiated by a short laser pulse at relativistic intensities has been studied both experimentally and numerically. A Monte-Carlo collision code takes into account individual collisions with the ions and electrons in the target. A 3D-hybrid code takes into account these collisions as well as the generation of electric and magnetic fields and the self-consistent motion of the electrons in these fields. It predicts a magnetic guiding of a fraction of the fast electron current over long distances and a localized heating of the material along the propagation axis. In experiments performed at LULI on the 100 TW laser facility, several diagnostics have been implemented to diagnose the geometry of the fast electron transport and the target heating. The typical conditions were: £/<20 J, A= 1 um, r~300fs, 7~ 10 18-5.10 19 W/cm 2. The results indicate a modest heating of the target (typically 20-40 eV over 20 |um to 50 urn), consistent with an acceleration of the electrons inside a wide aperture cone along the laser axis.
Electron and photon production from relativistic laser–plasma interactions
Nuclear Fusion, 2003
The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser-plasma interaction and electron transport numerical simulations.