| 1. |
- Andreev, N. E., et al.
(författare)
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Analysis of laser wakefield dynamics in capillary tubes
- 2010
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- A general approach to the modifications of the spectrum of a laser pulse interacting with matter is elaborated and used for spectral diagnostics of laser wakefield generation in guiding structures. Analytical predictions of the laser frequency red shift due to the wakefield excited in a capillary waveguide are confirmed by self-consistent modeling results. The role of ionization blue shift, and nonlinear laser pulse and wakefield dynamics on the spectrum modification, is analyzed for recent experiments on plasma wave excitation by an intense laser pulse guided in hydrogen-filled glass capillary tubes up to 8 cm long. The dependence of the spectral frequency shift, measured as a function of filling pressure, capillary tube length and incident laser energy, is in excellent agreement with the simulation results, and the associated longitudinal accelerating field is in the range 1-10 GV m(-1).
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| 2. |
- Burza, Matthias, et al.
(författare)
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Dispersion and monochromatization of x-rays using a beryllium prism
- 2015
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Ingår i: Optics Express. - 1094-4087. ; 23:2, s. 620-627
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate experimentally and numerically that an x-ray prism made of beryllium can be used to disperse and monochromatize x-rays. A polished beryllium cuboid was employed as refractive and dispersive optics. The results of a proof-of-principle experiment and methods of performance optimization are presented. The spatial separation of undulator harmonics and their subsequent selection using a slit are described. A numerical study, assuming realistic beam and beamline parameters, suggests that undulator harmonics can be spatially separated in the range from 3 keV to beyond 20 keV, while maintaining throughput above 50%. Refractive optics is particularly suitable for low-repetition-rate sources such as free-electron lasers and other LINAC-based short-pulse sources. (C) 2015 Optical Society of America
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| 3. |
- Burza, Matthias, et al.
(författare)
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Hollow microspheres as targets for staged laser-driven proton acceleration
- 2011
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Ingår i: New Journal of Physics. - : Institute of Physics Publishing (IOPP). - 1367-2630. ; 13, s. 013030-
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Tidskriftsartikel (refereegranskat)abstract
- A coated hollow core microsphere is introduced as a novel targetin ultra-intense laser–matter interaction experiments. In particular, it facilitates staged laser-driven proton acceleration by combining conventional target normal sheath acceleration (TNSA), power recycling of hot laterally spreading electrons and staging in a very simple and cheap target geometry. During TNSA of protons from one area of the sphere surface, laterally spreading hot electrons form a charge wave. Due to the spherical geometry, this wave refocuses on the opposite side of the sphere, where an opening has been laser micromachined.This leads to a strong transient charge separation field being set up there, which can post-accelerate those TNSA protons passing through the hole at the right time. Experimentally, the feasibility of using such targets is demonstrated. A redistribution is encountered in the experimental proton energy spectra, as predicted by particle-in-cell simulations and attributed to transient fields set up by oscillating currents on the sphere surface.
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| 4. |
- Burza, Matthias
(författare)
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Laser-Driven Particle Acceleration - Improving Performance Through Smart Target Design
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Laser-driven particle acceleration makes use of sub-picosecond, pulsed, high-power laser systems, capable of producing intensities ~10^{19} W/cm^2 at the laser focus to form plasmas, and use ultra-relativistic and nonlinear dynamics to produce quasistatic acceleration fields. This allows electrons to be accelerated to ~100 MeV over sub-centimetre distances, while protons may be accelerated to the ~10 MeV regime. In addition, novel sources of x-ray radiation become available with these schemes. The topics covered in this thesis focus mainly on target normal sheath acceleration of protons in the overdense plasma regime and laser wakefield acceleration of electrons in the underdense regime. An experimental approach leads to novel acceleration concepts and investigations on properties of new target designs. In the overdense plasma regime, hollow microspheres were found to have the potential to enhance the conversion of laser energy into proton energy. The microscopic structure of the material used as target has impact on electron beam filamentation during electron transport through the target bulk. Long-range order was found to result in smoother beams of TNSA-produced protons as compared to amorphous structures. In addition it was demonstrated that short pulse (fs) laser-solid interactions produce magnetic fields, the strength of which can reach 10 kT, mimicking astrophysical conditions. In the underdense regime, it was found that when tailored appropriately, density ramps can provide means of dividing the laser wakefield acceleration process into four steps: nonlinear laser evolution, trapping, bunch transfer into the second bucket, and acceleration, resulting in beams with reduced relative energy spread and divergence compared to self-injection by a nonlinear plasma wave. It was further shown that capillaries can be used to improve efficiency by guiding and refocusing the laser light onto the central axis. Short bursts of soft x-rays were produced inside capillaries. Finally, the use of an asymmetric laser field at the focus facilitated off-axis electron injection into the accelerating phase of a plasma wake oscillation and enhanced x-ray emission.
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| 5. |
- Burza, Matthias, et al.
(författare)
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Laser wakefield acceleration using wire produced double density ramps
- 2013
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Ingår i: Physical Review Special Topics. Accelerators and Beams. - 1098-4402. ; 16:1
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Tidskriftsartikel (refereegranskat)abstract
- A novel approach to implement and control electron injection into the accelerating phase of a laser wakefield accelerator is presented. It utilizes a wire, which is introduced into the flow of a supersonic gas jet creating shock waves and three regions of differing plasma electron density. If tailored appropriately, the laser plasma interaction takes place in three stages: Laser self-compression, electron injection, and acceleration in the second plasma wave period. Compared to self-injection by wave breaking of a nonlinear plasma wave in a constant density plasma, this scheme increases beam charge by up to 1 order of magnitude in the quasimonoenergetic regime. Electron acceleration in the second plasma wave period reduces electron beam divergence by approximate to 25%, and the localized injection at the density downramps results in spectra with less than a few percent relative spread. DOI: 10.1103/PhysRevSTAB.16.011301
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| 6. |
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| 7. |
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| 8. |
- Coury, M., et al.
(författare)
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Influence of laser irradiated spot size on energetic electron injection and proton acceleration in foil targets
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:7
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Tidskriftsartikel (refereegranskat)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]
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| 9. |
- Coury, M., et al.
(författare)
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Injection and transport properties of fast electrons in ultraintense laser-solid interactions
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:4
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Tidskriftsartikel (refereegranskat)abstract
- Fast electron injection and transport in solid foils irradiated by sub-picosecond-duration laser pulses with peak intensity equal to 4 x 10(20)W/cm(2) is investigated experimentally and via 3D simulations. The simulations are performed using a hybrid-particle-in-cell (PIC) code for a range of fast electron beam injection conditions, with and without inclusion of self-generated resistive magnetic fields. The resulting fast electron beam transport properties are used in rear-surface plasma expansion calculations to compare with measurements of proton acceleration, as a function of target thickness. An injection half-angle of similar to 50 degrees - 70 degrees is inferred, which is significantly larger than that derived from previous experiments under similar conditions. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799726]
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| 10. |
- Enquist, Henrik, et al.
(författare)
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FemtoMAX - An X-ray beamline for structural dynamics at the short-pulse facility of MAX IV
- 2018
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Ingår i: Journal of Synchrotron Radiation. - 0909-0495. ; 25:2, s. 570-579
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Tidskriftsartikel (refereegranskat)abstract
- The FemtoMAX beamline facilitates studies of the structural dynamics of materials. Such studies are of fundamental importance for key scientific problems related to programming materials using light, enabling new storage media and new manufacturing techniques, obtaining sustainable energy by mimicking photosynthesis, and gleaning insights into chemical and biological functional dynamics. The FemtoMAX beamline utilizes the MAX IV linear accelerator as an electron source. The photon bursts have a pulse length of 100fs, which is on the timescale of molecular vibrations, and have wavelengths matching interatomic distances (Å). The uniqueness of the beamline has called for special beamline components. This paper presents the beamline design including ultrasensitive X-ray beam-position monitors based on thin Ce:YAG screens, efficient harmonic separators and novel timing tools.The FemtoMAX beamline facilitates studies of the structural dynamics of materials on the femtosecond timescale. The first commissioning results are presented.
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| 11. |
- Genoud, Guillaume, et al.
(författare)
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Active control of the pointing of a multi-terawatt laser.
- 2011
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 82:3
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Tidskriftsartikel (refereegranskat)abstract
- The beam pointing of a multi-terawatt laser wave laser is stabilized on a millisecond time scale using an active control system. Two piezo mirrors, two position sensing detectors, and a computer based optimization program ensure that both near- and far-field are stable, even during single shot operation. A standard deviation for the distribution of laser shots of 2.6 μ rad is achieved.
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| 12. |
- Genoud, Guillaume, et al.
(författare)
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Increasing energy coupling into plasma waves by tailoring the laser radial focal spot distribution in a laser wakefield accelerator
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:6
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Tidskriftsartikel (refereegranskat)abstract
- By controlling the focal spot quality with a deformable mirror, we are able to show that increasing the fraction of pulse energy contained within the central part of the focal spot, while keeping the total energy and central spot size constant, significantly increases the amount of energy transferred to the wakefield: Our measurements show that the laser loses significantly more laser energy and undergoes greater redshifting and that more charge is produced in the accelerated beam. Three dimensional particle in cell simulations performed with accurate representations of the measured focal spot intensity distribution confirm that energy in the wings of the focal spot is effectively wasted. Even though self-focusing occurs, energy in the wings of the focal spot distribution is not coupled into the wakefield, emphasising the vital importance of high quality focal spot profiles in experiments. (C) 2013 AIP Publishing LLC.
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| 13. |
- Genoud, Guillaume, et al.
(författare)
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Laser-plasma electron acceleration in dielectric capillary tubes
- 2011
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Ingår i: Applied Physics B. - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 105:2, s. 309-316
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Tidskriftsartikel (refereegranskat)abstract
- Electron beams and betatron X-ray radiation generated by laser wakefield acceleration in long plasma targets are studied. The targets consist of hydrogen filled dielectric capillary tubes of diameter 150 to 200 microns and length 6 to 20 mm. Electron beams are observed for peak laser intensities as low as 5x10(17) W/cm(2). It is found that the capillary collects energy outside the main peak of the focal spot and contributes to keep the beam self-focused over a distance longer than in a gas jet of similar density. This enables the pulse to evolve enough to reach the threshold for wavebreaking, and thus trap and accelerate electrons. No electrons were observed for capillaries of large diameter (250 mu m), confirming that the capillary influences the interaction and does not have the same behaviour as a gas cell. Finally, X-rays are used as a diagnostic of the interaction and, in particular, to estimate the position of the electrons trapping point inside the capillary.
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| 14. |
- Gopal, A., et al.
(författare)
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MegaGauss magnetic field generation by ultra-short pulses at relativistic intensities
- 2013
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 55:3
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Tidskriftsartikel (refereegranskat)abstract
- We report the experimental studies on megaGauss magnetic field generation using a 35 femtosecond laser at relativistic intensities. The polarization change of the self-generated harmonics was recorded to estimate the magnetic field. A parameter scan was performed by varying the input laser intensity as well as the contrast ratio. External optical probing diagnostics were performed using the second harmonic of the incident laser. Additionally, the optical transition radiation from the rear of the target was also recorded.
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| 15. |
- Gray, R. J., et al.
(författare)
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Laser pulse propagation and enhanced energy coupling to fast electrons in dense plasma gradients
- 2014
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 16
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Tidskriftsartikel (refereegranskat)abstract
- Laser energy absorption to fast electrons during the interaction of an ultra-intense (10(20) Wcm(-2)), picosecond laser pulse with a solid is investigated, experimentally and numerically, as a function of the plasma density scale length at the irradiated surface. It is shown that there is an optimum density gradient for efficient energy coupling to electrons and that this arises due to strong self-focusing and channeling driving energy absorption over an extended length in the preformed plasma. At longer density gradients the laser filaments, resulting in significantly lower overall energy coupling. As the scale length is further increased, a transition to a second laser energy absorption process is observed experimentally via multiple diagnostics. The results demonstrate that it is possible to significantly enhance laser energy absorption and coupling to fast electrons by dynamically controlling the plasma density gradient.
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| 16. |
- Ju, J., et al.
(författare)
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Analysis of x-ray emission and electron dynamics in a capillary-guided laser wakefield accelerator
- 2014
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Ingår i: Physical Review Special Topics. Accelerators and Beams. - 1098-4402. ; 17:5
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of electron acceleration driven by laser wakefield inside a 30.5 mm long dielectric capillary tube is analyzed using radiation emitted in the x-ray range. 3D particle-in-cell simulations, performed with parameters close to the experimental ones, show that in long plasmas, the accelerated electrons catch up and finally overrun the driving laser owing to a higher velocity of the electrons in the plasma. The electrons are then transversely scattered by the laser pulse, and penetrate the capillary wall where they generate bremsstrahlung radiation, modeled using geant4 simulations. The signature of bremsstrahlung radiation is detected using an x-ray camera, together with the betatron radiation emitted during electron acceleration in the plasma bubble. The reflection of betatron radiation from the inner capillary surface also accounts for a fraction of the observed signal on the x-ray camera. The simulation results are in agreement with the experimental ones and provide a detailed description of the electron and radiation properties, useful for the design of laser wakefield accelerators or radiation sources using long plasma media.
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| 17. |
- Ju, J., et al.
(författare)
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Enhancement of x-rays generated by a guided laser wakefield accelerator inside capillary tubes
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:19
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Tidskriftsartikel (refereegranskat)abstract
- Abstract in UndeterminedElectrons accelerated in the nonlinear regime in a laser wakefield accelerator experience transverse oscillations inside the plasma cavity, giving rise to ultra-short pulsed x-rays, also called the betatron radiation. We show that the fluence of x-ray can be enhanced by more than one order of magnitude when the laser is guided by a 10 mm long capillary tube instead of interacting with a 2 mm gas jet. X-rays with a synchrotron-like spectrum and associated critical energy ∼ 5 keV, with a peak brightness of ∼ 1×1021 ph/s/mm2/mrad2/0.1%BW, were achieved by employing 16 TW laser pulses.
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| 18. |
- Ju, J., et al.
(författare)
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Study of electron acceleration and x-ray radiation as a function of plasma density in capillary-guided laser wakefield accelerators
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1070-664X .- 1089-7674. ; 20:8
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Tidskriftsartikel (refereegranskat)abstract
- Laser wakefield electron acceleration in the blow-out regime and the associated betatron X-ray radiation were investigated experimentally as a function of the plasma density in a configuration where the laser is guided. Dielectric capillary tubes were employed to assist the laser keeping self-focused over a long distance by collecting the laser energy around its central focal spot. With a 40 fs, 16 TW pulsed laser, electron bunches with tens of pC charge were measured to be accelerated to an energy up to 300 MeV, accompanied by X-ray emission with a peak brightness of the order of 10 21 ph/s/mm(2)/mrad(2)/0.1%BW. Electron trapping and acceleration were studied using the emitted X-ray beam distribution to map the acceleration process; the number of betatron oscillations performed by the electrons was inferred from the correlation between measured X-ray fluence and beam charge. A study of the stability of electron and X-ray generation suggests that the fluctuation of X-ray emission can be reduced by stabilizing the beam charge. The experimental results are in good agreement with 3D particle-in-cell (PIC) simulation. (C) 2013 AIP Publishing LLC.
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| 19. |
- MacLellan, D. A., et al.
(författare)
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Annular Fast Electron Transport in Silicon Arising from Low-Temperature Resistivity
- 2013
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Ingår i: Physical Review Letters. - 1079-7114. ; 111:9
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Tidskriftsartikel (refereegranskat)abstract
- Fast electron transport in Si, driven by ultraintense laser pulses, is investigated experimentally and via 3D hybrid particle-in-cell simulations. A transition from a Gaussian-like to an annular fast electron beam profile is demonstrated and explained by resistively generated magnetic fields. The results highlight the potential to completely transform the beam transport pattern by tailoring the resistivity-temperature profile at temperatures as low as a few eV.
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| 20. |
- MacLellan, D A, et al.
(författare)
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Tunable mega-ampere electron current propagation in solids by dynamic control of lattice melt.
- 2014
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Ingår i: Physical Review Letters. - 1079-7114. ; 113:18
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Tidskriftsartikel (refereegranskat)abstract
- The influence of lattice-melt-induced resistivity gradients on the transport of mega-ampere currents of fast electrons in solids is investigated numerically and experimentally using laser-accelerated protons to induce isochoric heating. Tailoring the heating profile enables the resistive magnetic fields which strongly influence the current propagation to be manipulated. This tunable laser-driven process enables important fast electron beam properties, including the beam divergence, profile, and symmetry to be actively tailored, and without recourse to complex target manufacture.
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| 21. |
- Mangles, S. P. D., et al.
(författare)
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Controlling the spectrum of x-rays generated in a laser-plasma accelerator by tailoring the laser wavefront
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 95:18
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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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| 22. |
- Mangles, S. P. D., et al.
(författare)
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Self-injection threshold in self-guided laser wakefield accelerators
- 2012
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Ingår i: Physical Review Special Topics. Accelerators and Beams. - 1098-4402. ; 15:1
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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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| 23. |
- McKenna, P., et al.
(författare)
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Effect of Lattice Structure on Energetic Electron Transport in Solids Irradiated by Ultraintense Laser Pulses
- 2011
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Ingår i: Physical Review Letters. - 1079-7114. ; 106:18
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Tidskriftsartikel (refereegranskat)abstract
- The effect of lattice structure on the transport of energetic (MeV) electrons in solids irradiated by ultraintense laser pulses is investigated using various allotropes of carbon. We observe smooth electron transport in diamond, whereas beam filamentation is observed with less ordered forms of carbon. The highly ordered lattice structure of diamond is shown to result in a transient state of warm dense carbon with metalliclike conductivity, at temperatures of the order of 1-100 eV, leading to suppression of electron beam filamentation.
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| 24. |
- Wahlström, Claes-Göran, et al.
(författare)
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Supersonic jets of hydrogen and helium for laser wakefield acceleration
- 2016
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Ingår i: Physical Review Accelerators and Beams. - 2469-9888. ; 19
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Tidskriftsartikel (refereegranskat)abstract
- The properties of laser wakefield accelerated electrons in supersonic gas flows of hydrogen and helium are investigated. At identical backing pressure, we find that electron beams emerging from helium show large variations in their spectral and spatial distributions, whereas electron beams accelerated in hydrogen plasmas show a higher degree of reproducibility. In an experimental investigation of the relation between neutral gas density and backing pressure, it is found that the resulting number density for helium is ∼30% higher than for hydrogen at the same backing pressure. The observed differences in electron beam properties between the two gases can thus be explained by differences in plasma electron density. This interpretation is verified by repeating the laser wakefield acceleration experiment using similar plasma electron densities for the two gases, which then yielded electron beams with similar properties.
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| 25. |
- Welch, Ken, et al.
(författare)
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Environment-Induced Surface Dynamics of a Biomimetic Ionomer Studied Using in Situ Second Harmonic Generation
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:37, s. 11573-11579
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Tidskriftsartikel (refereegranskat)abstract
- The environmental-induced surface dynamics of the biomimetic phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) ionomer has been studied and compared to its unfunctionalized counterpart using in situ second harmonic generation measurements. Whereas the nonpolar liquid n-hexane did not induce any surface dynamic processes in the ionomer under study, the presence of water initiated a Debye-type dynamic reaction at the surface of the PC ionomer, which had no equivalent in the unfunctionalized material. This first-order reaction was attributed to a surface enrichment process of the functionalized ionomer in the hydrophilic environment involving movement of the PC endgroups from aggregates in the bulk to the surface. The time constant of the process was found to be about 6 min, and the corresponding activation energy was 0.4 eV. The dehydration process of the PC-functionalized ionomer in nitrogen gas atmosphere could be described by two time constants, one slightly below 1 min and the other one just above 13 min. The results presented in this work show that SHG measurements are well suited for the study of polymer surface restructuring dynamics in response to environmental changes. Such information is very important for the successful design and implementation of biomimetic polymers intended for biomedical applications.
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| 26. |
- Wojda, F., et al.
(författare)
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Laser-driven plasma waves in capillary tubes
- 2009
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Ingår i: Physical Review E (Statistical, Nonlinear, and Soft Matter Physics). - 1539-3755. ; 80:6
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Tidskriftsartikel (refereegranskat)abstract
- The excitation of plasma waves over a length of up to 8 cm is demonstrated using laser guiding of intense laser pulses through hydrogen-filled glass capillary tubes. The plasma waves are diagnosed by spectral analysis of the transmitted laser radiation. The dependence of the spectral redshift-measured as a function of filling pressure, capillary tube length, and incident laser energy-is in excellent agreement with simulation results. The longitudinal accelerating field inferred from the simulations is in the range of 1-10 GV/m.
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