| 1. |
- Alfvén, Hannes
(författare)
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Cosmology in the plasma universe
- 1988
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Ingår i: Laser and particle beams (Print). - : Cambridge University Press (CUP). - 0263-0346 .- 1469-803X. ; 6, s. 389-398
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Tidskriftsartikel (refereegranskat)abstract
- Space observations have opened the spectral regions of X-rays and γ-rays, which are produced by plasma processes. The Plasma Universe derived from observations in these regions is drastically different from the now generally accepted ‘Visual Light Universe’ based on visual light observations alone. Historically this transition can be compared only to the transition from the the geocentric to the heliocentric cosmology.The purpose of this paper is to discuss what criteria a cosmological theory must satisfy in order to be acceptable in the Plasma Universe.
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| 2. |
- Aurand, Bastian, et al.
(författare)
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A setup for studies of laser-driven proton acceleration at the Lund Laser Centre
- 2015
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 33:1, s. 59-64
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Tidskriftsartikel (refereegranskat)abstract
- We report on a setup for the investigation of proton acceleration in the regime of target normal sheath acceleration. The main interest here is to focus on stable laser beam parameters as well as a reliable target setup and diagnostics in order to do extensive and systematic studies on the acceleration mechanism. A motorized target alignment system in combination with large target mounts allows for up to 340 shots with high repetition rate without breaking the vacuum. This performance is used to conduct experiments with a split mirror setup exploring the effect of spatial and temporal separation between the pulses on the acceleration mechanism and on the resulting proton beam.
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| 3. |
- Badiei, Shahriar, 1969, et al.
(författare)
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Laser-driven nuclear fusion D+D in ultra-dense deuterium: MeV particles formed without ignition
- 2010
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 28:2, s. 313-317
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Tidskriftsartikel (refereegranskat)abstract
- The short D-D distance of 2.3 pm in the condensed material ultra-dense deuterium means that it is possible that only a small disturbance is required to give D+D fusion. This disturbance could be an intense laser pulse. The high excess kinetic energy of several hundred eV given to the deuterons by laser induced Coulomb explosions in the material increases the probability of spontaneous fusion without the need for a high plasma temperature. The temperature calculated from the normal kinetic energy of the deuterons of 630 eV from the Coulomb explosions is 7 MK, maybe a factor of 10 lower than required for ignition. We now report on experiments where several types of high-energy particles from laser impact on ultra-dense deuterium are detected by plastic scintillators. Fast particles with energy up to 2 MeV are detected at a time-of-flight as short as 60 ns, while neutrons are detected at 50 ns time-of-flight after passage through a steel plate. A strong signal peaking at 22.6 keV u-1 is interpreted as due to mainly T retarded by collisions with H atoms in the surrounding cloud of dense atomic hydrogen.
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| 4. |
- Borghesi, M., et al.
(författare)
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Progress in proton radiography for diagnosis of ICF-relevant plasmas
- 2010
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Ingår i: Laser and particle beams (Print). - 0263-0346 .- 1469-803X. ; 28:2, s. 277-284
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Tidskriftsartikel (refereegranskat)abstract
- Proton radiography using laser-driven sources has been developed as a diagnostic since the beginning of the decade, and applied successfully to a range of experimental situations. Multi-MeV protons driven from thin foils via the Target Normal Sheath Acceleration mechanism, offer, under optimal conditions, the possibility of probing laser-plasma interactions, and detecting electric and magnetic fields as well as plasma density gradients with similar to ps temporal resolution and similar to 5-10 mu m spatial resolution. In view of these advantages, the use of proton radiography as a diagnostic in experiments of relevance to Inertial Confinement Fusion is currently considered in the main fusion laboratories. This paper will discuss recent advances in the application of laser-driven radiography to experiments of relevance to Inertial Confinement Fusion. In particular we will discuss radiography of hohlraum and gasbag targets following the interaction of intense ns pulses. These experiments were carried out at the HELEN laser facility at AWE (UK), and proved the suitability of this diagnostic for studying, with unprecedented detail, laser-plasma interaction mechanisms of high relevance to Inertial Confinement Fusion. Non-linear solitary structures of relevance to space physics, namely phase space electron holes, have also been highlighted by the measurements. These measurements are discussed and compared to existing models.
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| 5. |
- Borgstrom, S, et al.
(författare)
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Time-resolved x-ray spectroscopy of optical-field-ionized plasmas
- 1995
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 13:4, s. 459-468
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Tidskriftsartikel (refereegranskat)abstract
- The time-dependent soft X-ray emission of helium and nitrogen plasmas generated by optical-field ionization is reported. The experiments were carried out by focusing pulses of the high-power Ti:sapphire laser of the Lund Institute of Technology (lambda = 796 nm, pulse duration 150 fs, pulse energy 150 mJ) to a 50-mu m diameter spot close to a nozzle, using He and N-2 as target gases. The emission on He+, N4+, and N3+ resonance lines was recorded by means of a flat-field grating spectrometer coupled to an X-ray streak camera. A pronounced difference in the temporal shape of the emission of the Lyman-alpha line of hydrogen-like helium and of the 2p-3d resonance lines of lithium-like and beryllium-like nitrogen was observed. The helium line exhibited an initial spike followed by a slow revival of the emission, whereas the nitrogen lines showed a slow decay after a fast initial rise. These observations are explained with the help of simulations.
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| 6. |
- Brenner, C. M., et al.
(författare)
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Dependence of laser accelerated protons on laser energy following the interaction of defocused, intense laser pulses with ultra-thin targets
- 2011
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 29:3, s. 345-351
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Tidskriftsartikel (refereegranskat)abstract
- The scaling of the flux and maximum energy of laser-driven sheath-accelerated protons has been investigated as a function of laser pulse energy in the range of 15-380 mJ at intensities of 10(16)-10(18) W/cm(2). The pulse duration and target thickness were fixed at 40 fs and 25 nm, respectively, while the laser focal spot size and drive energy were varied. Our results indicate that while the maximum proton energy is dependent on the laser energy and laser spot diameter, the proton flux is primarily related to the laser pulse energy under the conditions studied here. Our measurements show that increasing the laser energy by an order of magnitude results in a more than 500-fold increase in the observed proton flux. Whereas, an order of magnitude increase in the laser intensity generated by decreasing the laser focal spot size, at constant laser energy, gives rise to less than a tenfold increase in observed proton flux.
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| 7. |
- Bret, Antoine, et al.
(författare)
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Departure from MHD prescriptions in shock formation over a guiding magnetic field
- 2017
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Ingår i: Laser and particle beams (Print). - : Cambridge University Press. - 0263-0346 .- 1469-803X. ; 35, s. 513-519
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Tidskriftsartikel (refereegranskat)abstract
- In plasmas where the mean-free-path is much larger than the size of the system, shock waves can arise with a front much shorter than the mean-free path. These so-called "collisionless shocks" are mediated y collective plasma interactions. Studies conducted so far on these shocks found that although binary collisions are absent, the distribution functions are thermalized downstream by scattering on the fields, so that magnetohydrodynamic prescriptions may apply. Here we show a clear departure from this pattern in the case of Weibel shocks forming over a flow-aligned magnetic field. A micro-physical analysis of the particle motion in the Weibel filaments shows how they become unable to trap the flow in the presence of too strong a field, inhibiting the mechanism of shock formation. Particle-in-cell simulations confirm these results.
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| 8. |
- Bret, Antoine, et al.
(författare)
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Theory of the formation of a collisionless Weibel shock: pair vs. electron/proton plasmas
- 2016
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Ingår i: Laser and particle beams (Print). - 0263-0346 .- 1469-803X. ; 34:2, s. 362-367
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Tidskriftsartikel (refereegranskat)abstract
- Collisionless shocks are shocks in which the mean-free path is much larger than the shock front. They are ubiquitous in astrophysics and the object of much current attention as they are known to be excellent particle accelerators that could be the key to the cosmic rays enigma. While the scenario leading to the formation of a fluid shock is well known, less is known about the formation of a collisionless shock. We present theoretical and numerical results on the formation of such shocks when two relativistic and symmetric plasma shells (pair or electron/proton) collide. As the two shells start to interpenetrate, the overlapping region turns Weibel unstable. A key concept is the one of trapping time τp, which is the time when the turbulence in the central region has grown enough to trap the incoming flow. For the pair case, this time is simply the saturation time of the Weibel instability. For the electron/proton case, the filaments resulting from the growth of the electronic and protonic Weibel instabilities, need to grow further for the trapping time to be reached. In either case, the shock formation time is 2τp in two-dimensional (2D), and 3τp in 3D. Our results are successfully checked by particle-in-cell simulations and may help designing experiments aiming at producing such shocks in the laboratory.
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| 9. |
- Esmaeildoost, Niloofar, et al.
(författare)
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Effects of beam temperature and plasma frequency on the radiation growth rate of a FEL with a laser wiggler
- 2017
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Ingår i: Laser and particle beams (Print). - : Hindawi Limited. - 0263-0346 .- 1469-803X. ; 35:2, s. 241-251
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Tidskriftsartikel (refereegranskat)abstract
- A linearly polarized laser pulse has been employed as a wiggler in a free-electron laser (FEL) in the presence of a plasma background for generating short wavelength radiation down to the extreme ultraviolet ray and X-ray spectral regions. Introducing plasma background in the FEL interaction region would lessen the beam energy requirement and also enhance both the beam current and the electron-bunching process. This configuration affords the possibility of scaling the device to more compact FELs and would have a higher tunability by changing the plasma density and the temperature of the electron beam. Electron trajectories have been analyzed using single-particle dynamics. The effect of plasma density on electron orbits has been investigated. A polynomial dispersion relation considering longitudinal thermal motion has been derived, by employing perturbation analysis. Numerical studies indicate that by increasing plasma density, the growth rate for groups I and II decreases, while the growth rate for group III increases. In addition, the effect of beam temperature and cyclotron frequency on the growth rate has been discussed. It has been found that by increasing the thermal velocity of the electron beam, the growth rate for groups I and III trivially decreases, while it increases for group II orbits. Besides, an increase in cyclotron frequency cause growth enhancement for group I orbits, while it present a growth decrement for group II and III orbits.
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| 10. |
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| 11. |
- Haines, M. G., et al.
(författare)
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Fiber Z-pinch Experiments and Calculations in the Finite Larmor Radius Regime
- 1996
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Ingår i: Laser and particle beams (Print). - 0263-0346 .- 1469-803X. ; 14, s. 261-271
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Tidskriftsartikel (refereegranskat)abstract
- The dense Z-pinch project at Imperial College is aimed at achieving radiative collapse to high density in a hydrogen plasma, and also to study plasmas close to controlled fusion conditions. To this end, the MAGPIE generator (2.4 MV, 1.25, and 200 ns) has been built and tested, and is now giving preliminary experimental data at 60% of full voltage for carbon and CD2 fibers. These discharges are characterized by an initial radial expansion followed by the occurrence of m = 0 structures with transient X-ray emission from bright spots. Late in the discharge a disruption can occur, accompanied by hard X-ray emission from the anode due to an energetic electron beam and, in the case of CD2 fibers, a neutron burst. Concomitant theoretical studies have solved the linear stability problem for a Z-pinch with large ion Larmor radii, showing that a reduction in growth rate of m = 0 and m = 1 modes to about 20% of the magnetohydrodynamic (MHD) value can occur for a parabolic density profile when the Larmor radius is optimally 20% of the pinch radius. Two dimensional MHD simulations of Z-pinches in two extremes of focussed short-pulse laserplasma interactions and of galactic jets reveal a nonlinear stabilizing effect in the presence of sheared flow. One-dimensional simulations show that at low line density the lower hybrid drift instability can lead to coronal radial expansion of a Z-pinch plasma.
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| 12. |
- Hergott, JF, et al.
(författare)
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XUV interferometry using high-order harmonics: Application to plasma diagnostics
- 2001
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 19:1, s. 35-40
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e[minus sign/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations.
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| 13. |
- Holmlid, Leif, 1942
(författare)
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Direct observation of particles with energy >10 MeV/u from laser-induced processes with energy gain in ultra-dense deuterium
- 2013
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Ingår i: Laser and particle beams. - 0263-0346. ; 31:4, s. 715-722
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear fusion in ultra-dense deuterium D(-1) was reported previously to be induced by 0.2 J pulses with 5 ns pulse length, ejecting particles with energies in the MeV range. The ns-resolved signal from D(-1) to two in-line collectors at up to 1 m distance can be observed directly on an oscilloscope, showing particles with energies in the range 1–20 MeV u−1. They are probably mainly protons and deuterons in the form of neutral ultra-dense hydrogen H(-1) fragments. Electrons and photons give only small contributions to the fast signal. The observed signal at several mA peak current corresponds to 1 × 1013 particles released per laser shot and to an energy release >4 J assuming isotropic formation and average particle energy of 3 MeV. This corresponds to an energy gain of 30 in the process. A movable slit close to the laser target gives lateral resolution of the signal generation, showing almost only fast particles from the point of laser impact and penetrating photons from the plasma outside the laser impact point. The observation of multi-MeV particles indicates nuclear fusion, either as a source or as a result.
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| 14. |
- Kholodnaya, G., et al.
(författare)
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Study of the conditions for the effective initiation of plasma-chemical treatment of flue gas under the influence of a pulsed electron beam
- 2020
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Ingår i: Laser and particle beams (Print). - : CAMBRIDGE UNIV PRESS. - 0263-0346 .- 1469-803X. ; 38:3, s. 197-203
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the results of comprehensive studies of the efficiency of a pulsed electron beam transmission through a mixture of gases: nitrogen (83%), carbon dioxide (14%), and oxygen (2.6%) in the presence of ash and water vapor. The studied concentrations correspond to the concentrations of nitrogen, oxygen, and carbon dioxide in flue gas. The pressure and concentration of water vapor and ash in the drift chamber varied (375, 560, and 750 Torr; humidity 15 +/- 5% and 50 +/- 15%). The charge dissipation of a pulsed electron beam in the gas mixture in the presence of ash and water vapor was investigated, as well as the effect of the concentration of water vapor and ash on the geometric profile of the pulsed electron beam.
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| 15. |
- McKenna, P., et al.
(författare)
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Effects of front surface plasma expansion on proton acceleration in ultraintense laser irradiation of foil targets
- 2008
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 26:4, s. 591-596
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Tidskriftsartikel (refereegranskat)abstract
- The properties of beams of high energy protons accelerated during ultraintense, picosecond laser-irradiation of thin foil targets are investigated as a function of preplasma expansion at the target front surface. Significant enhancement in the maximum proton energy and laser-to-proton energy conversion efficiency is observed at optimum preplasma density gradients due, to self-focusing Of the incident laser pulse. For very long preplasma expansion, the propagating laser pulse is observed to filament, resulting in highly uniform proton beams, but with reduced flux and maximum energy.
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| 16. |
- Merdji, H., et al.
(författare)
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Coherence properties of high-order harmonics : Application to high-density laser-plasma diagnostic
- 2000
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 18:3, s. 495-502
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Tidskriftsartikel (refereegranskat)abstract
- We present two interferometry schemes in the extreme ultraviolet, based on either the wave-front division of a unique harmonic beam (1st scheme) or two spatially separated, phase-locked harmonic sources (2nd scheme). In the first scheme using a Fresnel bimirror interferometer, we measure the degree of spatial coherence of the 13 th harmonic generated in xenon, as a function of different parameters. A high degree of coherence, larger than 0.5, is found for the best conditions in almost the full section of the beam. Then, we demonstrate that the second scheme can be used for interferometry measurements with an ultrahigh time resolution. The 11th harmonic is used to study the spatial variation of the electron density of a laser-produced plasma. Electronic densities higher than 2.10 20 cm -3 are measured.
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| 17. |
- Mishra, Gaurav, et al.
(författare)
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Effect of laser pulse time profile on its absorption by argon clusters
- 2011
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Ingår i: Laser and particle beams (Print). - : Cambridge University Press. - 0263-0346 .- 1469-803X. ; 29:3, s. 305-313
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of medium sized Argon clusters (30 angstrom) with high-intensity femtosecond laser pulses (806 nm, 8 x 10(16) W/cm(2)) of durations ranging from 10 fs to 120 fs have been studied using a three-dimensional relativistic time dependent molecular dynamic approach. The dynamics of cluster expansion is explained in terms of temporal evolution of electron population in the cluster and snapshots of particle positions at various times. The effects of inter-cluster distance on ionization dynamics are presented. It is observed that the collisional ionization increases with decreasing inter-cluster distance. The effect of pulse duration on laser energy absorption is also studied. For a laser pulse of gaussian time profile, there exists an optimum pulse duration for maximum absorption. No such optimum exists for a nearly flat top (super-gaussian) laser pulse. Results indicate the existence of resonance absorption inside the cluster. It is also observed that the high energy component of ion emission from cluster is anisotropic, showing a preferential direction of emission along laser polarization while the low energy ions emerge almost isotropically.
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| 18. |
- Olofson, K. Frans G., 1976, et al.
(författare)
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Electron-positron pair production observed from laser-induced processes in ultra-dense deuterium D(-1)
- 2014
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Ingår i: Laser and particle beams. - 0263-0346. ; 32:4, s. 537-548
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Tidskriftsartikel (refereegranskat)abstract
- Laser-induced fusion in ultra-dense deuterium D(-1) is reported in several studies from our group, using ns- and ps-pulsed lasers. The ejection of ultra-dense hydrogen particles with thermal distributions and energy up to 20 MeV u−1 was studied previously by time-of-flight measurements. The investigations of the new processes continue now by studying the interaction of these particles with metal surfaces. In the present experiments, such particles penetrate in two steps through 1 mm of metal and reach three levels of collectors at distances up to 1 m. Only the fastest particles penetrate and move to the next level. The thermal time-of-flight distributions together with tests with strong magnetic fields exclude electrons as the particles observed. The sign of the signals to the metal collectors depends on the bias (negative bias gives positive signal and conversely) while the time variations of the signals for positive and negative bias are similar. The rapid variation of the signals indicates electrons and positrons ejected from the collectors, thus lepton-pair production. An increase in bias up to ± 400 V increases the peak signal up to 1 A with no observed limiting. A thick metal plate removes slow particles and most gamma photons. The number of lepton-pairs produced is > 4 × 1012 sr−1 in the forward direction per laser shot.
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| 19. |
- Osvay, Karoly, et al.
(författare)
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On the temporal contrast of high intensity femtosecond laser pulses
- 2005
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 23:3, s. 327-332
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Tidskriftsartikel (refereegranskat)abstract
- The temporal contrast is classified into two main regimes, the nanosecond-scale and the picosecond-scale contrast prior to the main pulse. The Lund terawatt laser system is shown to be improved on the nano- and picosecond-scale by a factor of 10 and 50, respectively, when it was optimized for contrast but not for energy. Calculations are also presented to emphasize the role of angular dispersion on the picosecond contrast. Finally we show a compromise between the duration and contrast of femtosecond laser pulses amplified in an optical parametric (chirped pulse) amplifier.
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| 20. |
- Svanberg, Sune, et al.
(författare)
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Generation, characterization, and medical utilization of laser-produced emission continua
- 2000
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 18:3, s. 563-570
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Tidskriftsartikel (refereegranskat)abstract
- Intense continua of electromagnetic radiation of very brief duration are formed in the interaction of focused ultra-short terawatt laser pulses with matter. Two different kinds of experiments, which have been performed utilizing the Lund 10 Hz titanium-doped sapphire terawatt laser system are being described, where visible radiation and X-rays, respectively, have been generated. Focusing into water leads to the generation of a light continuum through self-phase modulation. The propagation of the light through tissue was studied addressing questions related to optical mammography and specific chromophore absorption. When terawatt laser pulses are focused onto a solid target with high nuclear charge Z, intense X-ray radiation of few ps duration and with energies exceeding hundreds of keV is emitted. Biomedical applications of this radiation are described, including differential absorption and gated-viewing imaging.
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| 21. |
- Uhlig, Jens, et al.
(författare)
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Laser generated 300 keV electron beams from water
- 2011
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Ingår i: Laser and Particle Beams. - 0263-0346. ; 29:4, s. 415-424
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Tidskriftsartikel (refereegranskat)abstract
- Abstract in Undetermined300 keV electron beams with energy peaked in the range 280–390 keV were generated by focusing a high contrast ratio but temporally double pulsed 800 nm ultrafast laser onto a flowing water jet under both helium atmosphere at ambient pressure and water aspirator vacuum conditions, using laser intensities in the range 1015–1018 Wcm−2. Their characteristics have been investigated as functions of inter-pulse delay, incidence geometry and laser pulse chirp. Shot-to-shot variation of the beams' equatorial and azimuthal distributions was also recorded in real time. Measurements of the emitted charge and energy have been performed. Secondary X-ray emission arising from impingement of the electron beams on the target chamber walls and other parts of the apparatus have been identified. Preliminary results after transition to a high repetition rate laser system have shown similar behavior. Approaches for improvements and applications are suggested.
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