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

Visual climate impact : Visual rhetoric in animated short film

Uppsatsens syfte är att upplysa kring hur filmer på YouTube fungerar retoriskt. Fokuset ligger ihur miljödiskursen representeras. Med hjälp av en semiologisk och retorisk analys undersöksfilmen The Turning Point (2020) av den brittiske animatören Steve Cutts. Detta sker med hjälpav semiologiska teorier samt sociopsykologiska teorier kopplade till visuell retorik. Resultatetav analysen blev att Cutts använder sig av kausala förhållanden mellan scener för att bygga ettargument. Filmen utnyttjar sedan tittarens tendens att identifiera sig med karaktärer för attövertyga. Resultatet blir relevant då det visar hur ett implicit, visuellt argument byggs upp i envideo på YouTube, en växande plattform för information och diskussion kring miljöfrågor. Minuppsats undersöker dock inte tittarens respons på materialet och ej heller YouTubeanvändaresinteraktion med övertalande visuella texter. Detta fordrar mer forskning påområdet där både responsstudier och diskursanalyser kan bli viktiga för att förstå hur åsikterformas på plattformen.The purpose of this essay is to inform about how videos on YouTube work rhetorically. The focus is on how the environmental discourse is represented. Using a semiological andrhetorical analysis, the film The Turning Point (2020) by the British animator Steve Cutts isexamined. This is done with semiological and socio-psychological theories linked to visualrhetoric. The results of the analysis showed that Cutts uses causal relationships betweenscenes to build an argument. The film then takes advantage of the viewer's tendency toidentify with characters to persuade. The result is relevant as it shows how an implicit, visualargument is built up in a video on YouTube, a growing platform for information anddiscussions on environmental issues. However, my essay does not examine the viewer'sresponse to the material, nor the interaction of YouTube users with persuasive visual texts.This requires more research in the area where both response studies and discourse analysis’scan be important for understanding how opinions are formed on the platform

Laser-driven particle acceleration - Experimental investigations

This thesis describes experimental studies of laser-driven particle acceleration. With the focused intensity of today’s high-power lasers exceeding 10^18 W/cm2,extremely high-gradient accelerators are possible. A number of experiments have been performed using two different laser systems, the Lund multi-TW laser and the Vulcan laser at CLF/RAL, UK, to study aspects ranging from laser system optimization and acceleration physics to detection and analysis methods. In one series of experiments, electrons were accelerated in underdense plasmas. As an intense laser pulse propagates in the plasma, a large-amplitude plasma wave is driven up in its wake. The longitudinal electric field in this wake can exceed 100GV/m. Relativistic electrons, injected from the background plasma, surf on the wave. By carefully controlling acceleration conditions, such as plasma density, laser pulse duration and focusing, short bursts of ~ 100MeV electrons with a small energy spread) were generated. Details of the acceleration mechanism were elucidated by studying the dependence of the electron beam on laser and plasma properties. It was found that for a range of plasma density–laser intensity combinations, quasi-mono-energetic beams are reliably produced. Using linearly polarized laser radiation, the spatial profile of the electrons acquires an elliptical shape that can be controlled by the direction of the laser polarization. This implies that there is a direct interaction between the laser pulse and the bunch of accelerated electrons, and that the electrons are originating from the small region occupied by the laser. It was also found that the contrast of the laser has a significant impact on the stability of the electron beam. In a second series of experiments, protons and other ions were accelerated from solid foil targets, a few μm thick, irradiated by a high-power laser. Since the protons are not relativistic, a stationary potential is needed to accelerate them. This was accomplished by allowing electrons, heated by the laser, to establish an electrostatic sheath on the back surface of the target foil. The strength of this field amounts to TV/m, accelerating protons to MeV energies over a very short distance. The laser contrast was again found to be an important parameter in the acceleration. The pedestal of amplified spontaneous emission (ASE) preceding the main pulse launches a shock wave into the target that could potentially destroy the back surface. However, by ontrolling the level and duration of the ASE, a regime in which the back surface is plastically deformed by the shock wave can be established, leading to an energy-dependent deflection of the proton beam. Using a plasma mirror to improve the contrast further, targets as thin as 20nm were used for proton acceleration. The conversion efficiency and maximum proton energy were found to increase significantly for these thin targets. In another experiment, the scaling of the maximum proton energy was studied at extreme laser intensities. The maximum energy was found to have a square root dependence on the laser intensity, but with a much lower energy than predicted by previous 1D plasma expansion models. Heating the target removes the protons and leads to efficient acceleration of heavier ions. Properties of these accelerated ions, such as charge state, energy, etc., were experimentally characterized. While not strictly a particle acceleration experiment, the final section of this thesis concerns a soft X-ray laser generated by transient excitation of ions in a laser-produced plasma. A technique called GRIP was employed to efficiently transfer energy to the desired ion charge state. The process involves setting up a smooth plasma gradient with one pulse, and pumping the plasma to population inversion with another pulse, impinging on the plasma at a grazing angle of incidence. The output energy of the soft X-ray laser was optimized with respect to a number of parameters. The details of high repetition rate operation of this laser were also studied, and the importance of the pointing stability of the driving lasers was established

Characterization of the arrival time jitter at the MAX-lab test-FEL using electro-optical spectral decoding

Electro-optical spectral decoding is used as an online diagnostic tool at the MAX-lab test-FEL to characterize the arrival time of electrons relative to the seed-laser pulse, measure the jitter between them and to measure the relative width of the electron bunch in order to optimize compression. Frequency characteristics of the jitter are presented. The measurements are used to get information on possible causes of the jitter and accompanying drifts. (C) 2010 Elsevier B.V. All rights reserved