Laser-driven beams of fast ions, relativistic electrons and coherent x-ray photons

This thesis presents experimental results on the development and optimization of novel and highly compact sources of beams of fast ions, relativistic electrons and coherent x-rays, driven by intense laser-plasma interactions. The rapid development of high-power, short-pulse laser systems have made available peak powers reaching the petawatt regime and focused intensities reaching 10^21 W/cm2. When interacting with matter, the extreme energy density (several GJ/cm3) associated with the focused laser pulses can create exceptionally high quasi-stationary electric fields, beyond several teravolt-per-meter (TV/m). By careful selection of the interaction conditions, electrons, protons or heavy ions can be accelerated to the multi-MeV kinetic energy level in distances ranging from a only a few micrometers up to several centimeters. The thesis addresses three important topics and summarizes results obtained using the multi-terawatt laser at the Lund Laser Centre in Sweden and the Vulcan Petawatt laser at the Rutherford-Appleton Laboratory in the United Kingdom. The thesis discusses laser-plasma acceleration of protons and heavy ions from thin foil metallic targets. The ion energy scalings with laser pulse and target parameters are investigated, and protons have been accelerated up to 55 MeV. Ultrathin targets, with thicknesses below 100 nm, and ultrahigh contrast laser pulses are shown to substantially enhance the proton maximum energy and laser-to-particle beam conversion efficiency. Shock waves, launched by the intrinsic laser prepulse, are shown to significantly influence the acceleration mechanisms. Novel schemes, involving multiple laser pulses, for active control of the spatial energy distribution of the accelerated ion beams are also presented. Results regarding the generation and optimization of quasi-monoenergetic electron beams are presented. Acceleration occurs in a plasma wave that is excited in the wake of an intense laser pulse in a tenuous plasma. Electrons are accelerated up to 200 MeV in less than 2 mm acceleration length. It is shown that, in the quasi-monoenergetic regime, electrons originate from the first plasma wave period. Current challenges such as electron beam stability are also specifically addressed. The thesis also reports the implementation of a laser in the soft x-ray regime. By using a grazing incidence pumping scheme, picosecond x-ray laser pulses with energies up to 3 uJ at a wavelength of 18.9 nm are produced at 10 Hz repetition rate, using Ni-like molybdenum ions as amplifying medium

Compact and high-quality gamma-ray source applied to 10 μm-range resolution radiography

International audienceGamma-ray beams with optimal and tuneable size, temperature, and dose are of great interest for a large variety of applications. These photons can be produced by the conversion of energetic electrons through the bremsstrahlung process in a dense material. This work presents the experimental demonstration of 30 μm resolution radiography of dense objects using an optimized gamma-ray source, produced with a high-quality electron beam delivered by a compact laser-plasma accelerator

Enhanced proton beams from ultrathin targets driven by high contrast laser pulses

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

Proton acceleration by a pair of successive ultraintense femtosecond laser pulses

We investigate the target normal sheath acceleration of protons in thin aluminum targets irradiated at relativistic intensity by two time-separated ultrashort (35 fs) laser pulses. For identical laser pulses and target thicknesses of 3 and 6 $\mu$m, we observe experimentally that the second pulse boosts the maximum energy and charge of the proton beam produced by the first pulse for time delays below $\sim0.6-1$ ps. By using two-dimensional particle-in-cell simulations we examine the variation of the proton energy spectra with respect to the time-delay between the two pulses. We demonstrate that the expansion of the target front surface caused by the first pulse significantly enhances the hot-electron generation by the second pulse arriving after a few hundreds of fs time delay. This enhancement, however, does not suffice to further accelerate the fastest protons driven by the first pulse once three-dimensional quenching effects have set in. This implies a limit to the maximum time delay that leads to proton energy enhancement, which we theoretically determine.Comment: 14 pages, 11 figure

Control of Laser Focusing using a Deformable Mirror and a Genetic Algorithm

In this thesis an adaptive optics system has been developed, implemented and evaluated at the Lund High Power Laser facility, Atomic Physics Division, Lund Institute of Technology. The laser system delivers ultrashort pulses with peak powers exceeding 40 TW at a repetition rate of 10 Hz. The pulses are focused to achieve extreme irradiance exceeding 1019 W/cm2 . In order to increase the peak intensity at the focal spot it is possible to either increase the pulse energy, to reduce the pulse length or to make the focal spot smaller. Cost and the laser bandwidth put limits to the two first alternatives. This thesis explores the third option. The aim for this project was to investigate the abilities of an adaptive optics system to precompensate for alignment or intrinsic optical errors that would degrade the focusing power of the laser system. A test system, using a smaller deformable mirror than required for the main laser system was developed and implemented by the author and comprise a deformable mirror, a detection system and an optimization algorithm. The deformable mirror was a Micromachined Membrane Deformable Mirror (MMDM), it had 37 electrostatic actuators and was coated with gold. The detection system measured the focal spot peak intensity and the mirror shape was optimized by a Genetic Algorithm (GA). Heavily astigmatic foci were routinely corrected and the MMDM was used to precompensate for the astigmatic errors introduced by off-axis focusing with a spherical mirror. At an off-axis angle of 17o , the focal spot size was improved from 6.5 to 1.3 times the diffraction limit. When the laser was focused with a well aligned parabolic mirror, the algorithm and the deformable mirror were still able to increase the focal spot peak intensity by 85%. The deformable mirror was also used to manipulate the focal spot intensity profile. Multiple focal spots of equal intensity were successfully generated from a single beam and a scheme that would allow tailored focal spots was tested

Third-order double-achromat bunch compressors for broadband beams

Many state-of-the-art applications for linear accelerators, such as free-electron lasers (FELs) and plasma-wakefield accelerators (PWFAs), require small normalized emittances, and PWFAs in particular are very sensitive to transverse slice offsets along the beam. Dispersive systems, such as bunch compressors, can cause different chromatic aberrations, one of which yields transverse slice offsets. In this paper, we show a design approach to double-achromat bunch compressors which greatly reduces different chromatic aberrations and mitigates coherent synchrotron radiation effects

Ultra-intense laser pulses in near-critical underdense plasmas - Radiation reaction and energy partitioning

Although, for current laser pulse energies, the weakly nonlinear regime of laser wakefield acceleration is known to be the optimal for reaching the highest possible electron energies, the capabilities of upcoming large laser systems will provide the possibility of running highly nonlinear regimes of laser pulse propagation in underdense or near-critical plasmas. Using an extended particle-in-cell (PIC) model that takes into account all the relevant physics, we show that such regimes can be implemented with external guiding for a relatively long distance of propagation and allow for the stable transformation of laser energy into other types of energy, including the kinetic energy of a large number of high energy electrons and their incoherent emission of photons. This is despite the fact that the high intensity of the laser pulse triggers a number of new mechanisms of energy depletion, which we investigate systematically

Experimental measurements of electron-bunch trains in a laser-plasma accelerator.

Spectral measurements of visible coherent transition radiation produced by a laser-plasma-accelerated electron beam are reported. The significant periodic modulations that are observed in the spectrum result from the interference of transition radiation produced by multiple bunches of electrons. A Fourier analysis of the spectral interference fringes reveals that electrons are injected and accelerated in multiple plasma wave periods, up to at least 10 periods behind the laser pulse. The bunch separation scales with the plasma wavelength when the plasma density is changed over a wide range. An analysis of the spectral fringe visibility indicates that the first bunch contains most of the charge

A tunable electron beam source using trapping of electrons in a density down-ramp in laser wakefield acceleration

One challenge in the development of laser wakefield accelerators is to demonstrate sufficient control and reproducibility of the parameters of the generated bunches of accelerated electrons. Here we report on a numerical study, where we demonstrate that trapping using density down-ramps allows for tuning of several electron bunch parameters by varying the properties of the density down-ramp. We show that the electron bunch length is determined by the difference in density before and after the ramp. Furthermore, the transverse emittance of the bunch is controlled by the steepness of the ramp. Finally, the amount of trapped charge depends both on the density difference and on the steepness of the ramp. We emphasize that both parameters of the density ramp are feasible to vary experimentally. We therefore conclude that this tunable electron accelerator makes it suitable for a wide range of applications, from those requiring short pulse length and low emittance, such as the free-electron lasers, to those requiring high-charge, large-emittance bunches to maximize betatron X-ray generation