Дисертації з теми "Intensité relativiste"

Cette thèse expérimentale s’est essentiellement déroulée au Laboratoire d’Optique Appliquée à Palaiseau (France), sur un système laser capable de générer des impulsions proches du cycle optique en durée avec des énergies de plusieurs mJ à une cadence de 1 kHz : la Salle Noire 2. Ce système laser Titane:Sapphire est double CPA avec un filtre non-linéaire entre les deux étages (basé sur la génération d’onde de polarisation croisée ou ‘XPW’) pour améliorer le contraste temporel, suivi d’un étage de post-compression dans une fibre flexible étirée à cœur creux. Grâce à ce système, nous étudions l’interaction laser-matière en régime relativiste à haute cadence. Nous parvenons, d’une part, dans des jets de gaz, à accélérer des électrons dans le sillage du laser jusqu’ à une énergie de quelques MeV; et d’autre part, par interaction avec des miroirs plasma, à générer des harmoniques d’ordres élevés qui sont associées dans le domaine temporel à des impulsions attosecondes. Malgré la prouesse technique de ces expériences, les propriétés des faisceaux XUV et d’électrons ainsi générés restent encore peu compatibles avec des applications phares en aval. À la suite de travaux précédents en Salle Noire 2, l’objectif de cette thèse était d’obtenir des faisceaux aux propriétés stables, ce qui a été accompli en rendant le système laser plus stable et fiable, ainsi qu’en implémentant une boucle de contrôle rapide de la phase enveloppe-porteuse des impulsions laser. En variant la phase enveloppe-porteuse, nous avons ainsi pu générer des impulsions attosecondes uniques en formant une porte temporelle d’intensité relativiste à la surface du miroir plasma, et aussi produire des faisceaux d’électrons stables en énergie et en direction, en contrôlant l’injection d’ électrons dans l’accélérateur laser-plasma. De plus, différents régime d’interaction avec les miroirs plasma ont été étudiés expérimentalement, tels que l’accélération d’électrons dans les longs gr adients de densité plasma, et l’accélération de protons en face avant de la cible (la face sur laquelle le laser est incident) le long de la direction normale à la cible, afin de mesurer de nouvelles observables (spectre d’énergie des électrons, divergence des faisceaux de protons) et ainsi mieux comprendre la dynamique d’interaction laser-plasma
This experimental thesis was essentially conducted at Laboratoire d’Optique Appliquée in Palaiseau (France), on a laser system capable of delivering near-single-cycle duration pulses containing a few mJ of energy at 1kHz repetition rate: the Salle Noire 2. This laser is a Titanium:Sapphire double CPA system with a nonlinear filter in between (based on the crossed polarized wave generation effect) for temporal contrast enhancement, followed by a stretched-flexible hollow-core-fiber based post-compression stage. Using this system, we study laser-matter interaction in the relativistic regime at high repetition rate. We can, on one hand, in gas jets, accelerate electrons in the wakefield of the laser up to several MeVs; and on the other hand, by interacting with plasma mirrors, generate high order harmonics which are associated to bright attosecond pulses in the time domain. Despite the technological prowess in these experiments, the properties of the XUV and electron beams thus generated remain scarcely compatible with the main applications downstream. Following up on previous works in Salle Noire 2, the objective of this thesis was to obtain beams with stable properties, which was achieved by making the laser system more stable and reliable, as well as implementing a fast carrier-envelope phase control loop. By varying the carrier-envelope phase of the laser pulses, we could generate XUV continua/isolated attosecond pulses by forming a relativistic-intensity temporal gate at the surface of the plasma mirror, and also produce electron beams exhibiting stable energy and angle of emission, by controlling the electron injection within the plasma accelerator. Additionally, different regimes of interaction with plasma mirrors were experimentally investigated, such as wakefield acceleration of electrons in long plasma density gradients, and the acceleration of protons on the target’s front side (onto which the laser impinges) along the target no rmal direction, in order to measure new observables (electron energy spectra, proton beam divergence) and thus gain deeper insights into the laser-plasma dynamics

Lors de la focalisation d’un laser femtoseconde ultra-intense [I>10¹⁶W/cm²] sur une cible solide, dès le début de l’impulsion le champ laser est suffisant pour totalement ioniser la surface de la cible. Le reste de l’impulsion est ensuite réfléchi dans la direction spéculaire par le plasma dense ainsi créé : c’est un miroir plasma. Le champ laser ultra-intense peut accélérer les électrons au sein du plasma à des vitesses relativistes. Certains sont éjectés vers le vide et ces miroirs plasmas sont ainsi des sources de faisceaux d’électrons énergétiques. De plus, ils rayonnent dans l’extrême ultra-violet (XUV) à chaque période laser, ce qui se traduit par de la génération d’harmoniques d’ordre élevé de la pulsation laser. L’objectif de cette thèse est de mieux comprendre l’interaction laser-plasma sur miroirs plasmas à l’aide de la caractérisation de ces deux observables physiques qui en sont issues : les faisceaux d’électrons relativistes et les faisceaux d’harmoniques d’ordre élevé. Une première partie traite de la mesure des faisceaux harmoniques. Du fait des conditions physiques extrêmes d’interaction, la détection ne peut se faire qu’à une distance macroscopique de la cible. Ainsi la caractérisation des propriétés angulaires de ces faisceaux (réalisée en fonction des conditions d’interaction au cours de travaux précédents) ne fournit que des informations partielles sur l’interaction en elle-même. La ptychographie, une technique de mesure par diffraction cohérente où une sonde est diffractée par un objet, est ici transposée à la génération d’harmoniques sur miroirs plasmas grâce à la micro-structuration optique du plasma à la surface de la cible. Les champs sources harmoniques sont ainsi reconstruits en amplitude et en phase spatiales directement dans le plan cible. Grâce à ces mesures dans différentes conditions d’interaction, des modèles théoriques analytiques d’interaction en régime non relativiste [I<10¹⁸W/cm²] et relativiste [I>10¹⁸W/cm²] développés précédemment sont validés expérimentalement. Une seconde partie de cette thèse est consacrée à l’étude expérimentale des propriétés angulaires et en énergie des faisceaux d’électrons relativistes issus des miroirs plasmas. Une étude théorique et numérique, permet de prouver que ces mesures sont la première observation claire de l’accélération d’électrons relativistes par laser dans le vide (VLA). Enfin, l’étude simultanée des efficacités de génération des faisceaux d’électrons et d’harmoniques montre une corrélation nette entre les deux processus en régime relativiste
When focusing an ultra-intense femtosecond laser pulse [I>10¹⁶W/cm²] onto a solid target, this target is ionized at the very beginning of the laser pulse. The resulting dense plasma then reflects the laser in the specular direction: it is a plasma mirror. The ultra-intense laser field can accelerate electrons within the plasma at relativistic speeds. Some are ejected towards the vacuum and these plasma mirrors are therefore sources of relativistic electron beams. Moreover, at each optical cycle they radiate in the form of extreme ultraviolet light, resulting in the generation of high-order harmonics of the laser frequency (HHG). The objective of this PhD is to understand laser-plasma interaction though the characterization of high-order harmonics and relativistic electron beams generated from plasma mirrors. The first part deals with harmonic beam measurement. Due to the extreme physical conditions during the interaction, detection can only be performed at macroscopic distance from target. Thus, the characterization of the harmonic beams’ angular properties (carried out as a function of interaction conditions in previous works) only provides partial information on the interaction itself. A technique of coherent diffraction imaging, named ptychography, which consists of diffracting a probe onto an object, is transposed to HHG on plasma mirrors by optically micro-structuring the plasma on a target surface. Harmonic fields are then reconstructed spatially in amplitude and phase directly in the target plane. Thanks to this measurement in different interaction conditions, previously developed theoretical analytical models in non-relativistic regime [I<10¹⁸W/cm²] and relativistic regime [I>10¹⁸W/cm²] are experimentally validated. The second part of the PhD is dedicated to the experimental characterization of angular and spectral properties of relativistic electron beams. A theoretical and numerical study shows that this constitutes the first clear observation of vacuum laser acceleration (VLA). Finally, a simultaneous study of harmonic and electron signals highlights a strong correlation between both processes in the relativistic regime

Lors de la réflexion d’un laser femtoseconde ultra-intense [Iʟ > 10¹⁶ W/cm²] sur une cible solide, celle-ci est ionisée dès les premiers cycles de l’impulsion. Un plasma se détend alors vers le vide avec un profil exponentiel de longueur caractéristique Lg. Pour de faibles longueurs de gradient Lg < λʟ, le gradient plasma est considéré comme raide, il réfléchit spéculairement l’impulsion incidente : c’est un miroir plasma. De tels plasmas, réfléchissant pour la lumière, sont aujourd’hui exploités dans différentes applications scientifiques, comme l’accélération de particules par laser ou encore la génération d’harmoniques d’ordre élevé, associées dans le domaine temporel à un train d’impulsions attosecondes. Néanmoins, pour favoriser ces émissions de lumière ou de particules, le transfert d’énergie entre l’impulsion laser incidente et le plasma est essentiel. L’objectif de cette thèse est de mieux comprendre ces interactions à l’aide de la caractérisation de ces deux observables physiques qui en sont issues : les émissions d’électrons relativistes et d’harmoniques d’ordre élevé. Tout d’abord, nous reportons dans ce manuscrit la première étude expérimentale et numérique détaillée des mécanismes de couplage laser-plasma dense impliqués en régime relativiste [Iʟ > 10¹⁸ W/cm²] en fonction notamment de la longueur caractéristique de gradient Lg. Cette étude a notamment permis d’identifier deux régimes distincts en fonction des conditions d’interaction, éclaircissant ainsi la physique régissant ces systèmes. Par ailleurs, au delà de cet aspect fondamental, le contrôle de ces sources est également essentiel pour de futures expériences. Pour cela, différentes approches permettant de mettre en forme spatialement et temporellement ces impulsions de lumière ultra-brèves ont été étudiées au cours de ce doctorat, ouvrant ainsi de nouvelles perspectives pour l’utilisation de ces sources. En particulier, nous démontrons qu’il est possible d’introduire un moment angulaire orbital aux impulsions XUV attosecondes via la mise en forme spatiale du faisceau IR femtoseconde incident ou bien de plasma dense créé à la surface de la cible mais également de contrôler la dynamique des électrons de surface du plasma à l’échelle attoseconde à l’aide d’un champ incident à deux couleurs. Finalement, une méthode novatrice basée sur des mesures de ptychographie dynamique a été développée afin de caractériser spatio-temporellement ces impulsions de lumière ultra-brèves, constituant un enjeu majeur pour la communauté
When an ultra-intense femtosecond laser beam [Iʟ > 10¹⁶ W/cm²] is focused on a solid target, the surface becomes completely ionized during the first optical cycles of the laser pulse. Due to their solid-like density and to their limited expansion into the vacuum such plasmas specularly reflect these pulses, just like ordinary mirrors do for low intensity. These plasmas are now used in many scientific applications like particle acceleration by laser light as well as high-order harmonic generation, associated to a train of attosecond pulses in the time domain. Nevertheless, to favor these emissions of light or particle, the energy transfert between the incident field and the dense plasma is crucial. The aim of this thesis is to better understand these interactions through the characterization of high-order harmonics and relativistic electron beams generated on plasma mirrors. We reported in this manuscript the first detailed experimental and numerical study of the coupling mechanisms involved between an ultra-intense laser light [Iʟ > 10¹⁸ W/cm²] and a dense plasma, and more specifically as a function of the gradient scale length Lg. These results enabled to identify two different regimes, clarifying some physical issues. Furthermore, beyond these fondamental aspects, the control of these sources is essential, particularly for futures pump-probe experiments or new spectroscopies. For that, several approaches have been studied to temporally and spatially shape these ultra-short light pulses, thus opening up new perspectives for these sources. We demonstrate in particular the generation of intense XUV vortex beam either by spatially shaping the incident IR field or the dense plasma created at the target surface as well as controlling the electron dynamics on the attosecond time scale with relativistic two-color waveforms. Finally, an innovative method based on in-situ ptychographic measurements has been developed to simultaneously characterize in time and space these ultrashort XUV light pulses, constituting one of the major challenges of the community

During the past decades, the development of laser technology has produced pulses with increasingly higher peak intensities. These can now be made such that their strength rivals, and even exceeds, the atomic potential at the typical distance of an electron from the nucleus. To understand the induced dynamics, one can not rely on perturbative methods and must instead try to get as close to the full machinery of quantum mechanics as practically possible. With increasing field strength, many exotic interactions such as magnetic, relativistic and higher order electric effects may start to play a significant role. To keep a problem tractable, only those effects that play a non-negligible role should be accounted for. In order to do this, a clear notion of their relative importance as a function of the pulse properties is needed.  In this thesis I study the interaction between atomic hydrogen and super-intense laser pulses, with the specific aim to contribute to the knowledge of the relative importance of different effects. I solve the time-dependent Schrödinger and Dirac equations, and compare the results to reveal relativistic effects. High order electromagnetic multipole effects are accounted for by including spatial variation in the laser pulse. The interaction is first described using minimal coupling. The spatial part of the pulse is accounted for by a series expansion of the vector potential and convergence with respect to the number of expansion terms is carefully checked. A significantly higher demand on the spatial description is found in the relativistic case, and its origin is explained. As a response to this demanding convergence behavior, an alternative interaction form for the relativistic case has been developed and presented. As a guide mark for relativistic effects, I use the classical concept of quiver velocity, vquiv, which is the peak velocity of a free electron in the polarization direction of a monochromatic electromagnetic plane wave that interacts with the electron. Relativistic effects are expected when vquiv reaches a substantial fraction of the speed of light c, and in this thesis I consider cases up to vquiv=0.19c. For the present cases, relativistic effects are found to emerge around vquiv=0.16c .

Nous étudions dans cette thèse théorique et numérique l'accélération d'électrons lors de l'interaction entre une impulsion laser d'intensité relativiste et un plasma surdense. Cette interaction est très sensible au profil de densité sur la face avant du plasma et deux régimes différents, correspondant à deux thématiques de recherche développées dans cette thèse, peuvent être considérés.Premièrement, pour des interfaces plasma-vide très abruptes, les mécanismes menant à l'émission d'électrons sont bien compris. Les électrons gagnent en particulier une grande quantité d'énergie lors de leur interaction dans le vide avec l'impulsion laser réfléchie. Nous proposons d'optimiser cette accélération en utilisant des faisceaux polarisés radialement, qui sont caractérisés par la présence d'un fort champ longitudinal, capable d'accélérer directement les électrons dans la direction de propagation du laser. Nous montrons que les plasmas surdenses conduisent à une accélération plus efficace que les autres méthodes existantes pour injecter des électrons dans une impulsion polarisée radialement. Ce résultat a été confirmé par des expériences effectuées récemment au CEA Saclay, au cours desquelles la possibilité d'accélérer des électrons dans la direction longitudinale, menant ce faisant à une diminution de la divergence angulaire du faisceau d'électrons, a été démontrée.Deuxièmement, pour des gradients de densité plasma plus grands, l'interaction n'est pas aussi bien comprise. Nous analysons des résultats expérimentaux obtenus récemment au LOA avec des impulsions de quelques cycles optiques et nous montrons que les électrons sont accélérés par une onde de sillage laser formée dans la partie quasi-critique du plasma. Ce processus ne se produit qu'avec des impulsions de quelques cycles optiques, en accord avec la condition de résonance, et se distingue par la rotation des ondes plasmas causée par le gradient de densité
This theoretical and numerical thesis is devoted to electron acceleration from the interaction between a relativistic intensity laser pulse and an overdense plasma. This interaction is very sensitive to the density profile at the plasma front surface and two different regimes, which correspond to two distinct lines of research investigated in this thesis, can be considered.First, for sharp plasma-vacuum interfaces, the mechanisms responsible for electron emission are well understood. The electrons receive in particular a large energy gain from their interaction in vacuum with the reflected laser. We propose to optimize the acceleration by using radially polarized beams, which exhibit a strong longitudinal electric field that can directly accelerate electrons in the laser propagation direction. We show that overdense plasmas lead to more efficient acceleration than other existing methods for injecting electrons into a radially polarized pulse. This result was confirmed by recent experiments performed at CEA Saclay, in which electron acceleration in the longitudinal direction, leading to a decrease in the electron beam angular spread, is demonstrated.Secondly, for larger plasma gradient scale lengths, the interaction is not as well understood. We analyze recent experiments performed in this regime at LOA with few-cycle pulses and find that electrons are accelerated by a laser wakefield formed in the near-critical part of the plasma. This process can only be driven by few-cycle pulses, by virtue of the resonant condition, and is characterized by the rotation of the plasma waves induced by the density gradient

Cette thèse porte sur l’étude théorique du transport d’un faisceau intense d’électrons relativistes dans une cible solide. Dans la première partie nous présentons les interprétations théoriques d’une partie des résultats d’une campagne d’expérience portant sur la production et le transport d’électrons relativistes dans une cible d’aluminium. Nous y démontrons la prédominance des e?ets collectifs sur les e?ets collisionels dans la première dizaine de microns de propagation grâce à des modèles de transports déjà existant au début de cette thèse. Ces modèles deviennent insu?sants dans le cas du transport de faisceau dans un isolant. Aussi, dans la deuxième partie, nous présentons un modèle de propagation du faisceau d’électrons relativistes dans un diélectrique incluant l’e?et de l’ionisation de la cible par le faisceau. Nous y quanti?ons les pertes d’énergies des électrons en fonction des paramètres du faisceau et du milieu environnant, et nous démontrons l’existence d’un régime de propagation pour lequel les électrons du plasma ne sont pas à l’équilibre thermodynamique local avec les ions. Ces résultats ont été comparés et con?rmés avec un code cinétique qui prend en compte l’ionisation par champ électrique et par collisions entre les électrons du plasma et les ions. Nous avons examiné la stabilité du faisceau et montré que ce dernier pouvait exciter deux types d’instabilités transverses sur des longueurs de propagation de l’ordre de 30 à 300 µm en fonction de la taille de la perturbation
This PhD thesis is a theoretical study of high-current relativistic electron beam transport through solid targets. In the ?rst part, we present an interpretation of a part of experimental results of laser– produced electron beam transport in aluminium foil targets. We have estimated the fast electron beam characteristics and we demonstrated that the collective e?ects dominate the transport in the ?rst tens of µm of propagation. These quantitative estimates were done with the transport models already existing at the beginning of this thesis. These models are no longer su?cient in the case a fast electron beam propagation in insulator targets. Thus, in the second part, we have developed a propagation model of the beam that includes the e?ects of electric ?eld ionization and the collisional ionization by the plasma electrons. We present estimates of the electron energy loss induced by the target ionization, and we discuss its dependence on the beam and target parameters. In the case of a relatively low fast electron density, we demonstrated that the beam creates a plasma where the electons are not in a local thermodynamic equilibrium with ions. We have examined the beam stability and we demonstrated that transverse instabilities can be excited by the relativistic electron beam over the propagation distances of 30 - 300 µm depending on the perturbation wavelength

In this thesis, the generation and transport of ultra-high intensity laser-driven relativistic electron beams in overdense plasma is investigated experimentally and numerically. The fast electron beam is experimentally diagnosed by means of a 2D Cu Ka imager and the TNSA-generated proton beam. Analytical models together with a 3D hybrid-PIC code are employed to simulate the beam properties in solids. The effects of the self-generated fields on the fast electron beam transport, the effect of the preplasma density scale length on the laser energy coupling to fast electrons and the influence of the laser spot size on the fast electron beam generation and transport, and on the subsequent proton beam, are reported. Fast electron injection and transport in metal foils irradiated at laser intensity up to 4 x 10²⁰ W/cm², is investigated . The beam transport is simulated over a wide range of beam source conditions and with or without inclusion of selfgenerated magnetic fields . The resulting hot electron beam properties are used in rear-surface plasma expansion calculations to compare with measurements of the beam of accelerated protons. An injection half-angle of ~ 50° - 70° is inferred, which is larger than that derived from previous experiments under similar conditions. The influence of laser spot size on laser energy coupling to electrons, and subsequently to the TNSA-generated protons, in foil targets is reported. Proton acceleration is characterized for laser intensities ranging from 2 x 10¹⁸ - 6 x 10²⁰ W/cm², by variation of the laser energy for a fixed spot size, and 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. The results are explained in terms of higher laser pulse energy and geometrical changes to the hot electron injection. The laser-to-electron energy conversion efficiency is investigated in metal foil s over a wide range of preplasma density scale lengths. A hybrid-PIC code is employed to model the fast electron beam transport in the solid, for a given hot electron source. The resulting fast electron density is used to infer the maximum proton energy for comparison with experimental results. It is shown, in agreement with previous published work, that some preplasma density scale length leads to an enhancement of the energy coupling efficiency of laser light to fast electrons.

The FIREBIRD mission (Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics) is a low earth orbit, space weather, CubeSat mission which is comprised of a two satellite constellation. This constellation is responsible for the measurement of relativistic electron microbursts with very fine spatial and temporal resolution. To achieve the spatial and temporal requirements of the mission, a global positioning system (GPS), for the purpose of navigation position and timing, is to be implemented on both satellites within the constellation. The integration and testing of this subsystem is integral to the mission's success. The GPS hardware must be capable of fulfilling the requirements of the mission in order for the science data to be interpreted reliably. This means that the GPS hardware must not only be accurate but precise as well. Also, a driver must be implemented in software in order for this data from the GPS hardware to be received, interpreted, and stored by the command and data handling subsystem.

There are at least two common models for calculating the photoemission of accelerated electrons. The 'extended-charge-distribution' method uses the quantum probability current (multiplied by the electron charge) as a source current for Maxwell's equations. The 'point-like-emitter' method treats the electron like a point particle instead of like a diffuse body of charge. Our goal is to differentiate between these two viewpoints empirically. To do this, we consider a large electron wave packet in a high-intensity laser field, in which case the two viewpoints predict measurable photoemission rates that differ by orders of magnitude. Under the treatment of the 'extended-charge-distribution' model, the strength of the radiated field is significantly limited by interferences between different portions of the oscillating charge density. Alternatively, no suppression of photoemission occurs under the 'point-like-emitter' model because the electron is depicted as having no spatial extent. We designed an experiment to characterize the photoemission rates of electrons accelerated in a relativistic laser focus. Free electron wave packets are produced through ionization by an intense laser pulse at the center of a large vacuum chamber. These quantum wave packets can become comparable in size to the laser wavelength through natural spreading and interactions with the sharp ponderomotive gradients of the laser focus. Electron radiation emitted out the side of the focus is collected by one-to-one imaging into a 105-micron gold-jacketed fiber, which carries the light to a single photon detector located outside the chamber. The electron radiation is red-shifted due to mild relativistic acceleration, and we use this signature to spectrally filter the outgoing light to discriminate against background. In addition, the temporal resolution of the electronics allows distinction between light that travels directly from the focus into the collection system and laser light that may scatter from the chamber wall.

An experiment currently underway at BYU is designed to test whether the size of a free electron wave packet affects the character of scattered radiation. Using a semi-classical argument wherein the wave packet is treated as a diffuse charge distribution, one would expect strong suppression of radiation in the direction perpendicular to the propagating field as the wave packet grows in size to be comparable to the wavelength of the driving field. If one disallows the interaction of the wave packet with itself, as is the case when calculating the rate of emission using QED, then regardless of size, the electron wave packet radiates with the strength of a point-like emitter. In support of this experiment, we explore a variety of physical parameters that impact the rate of scattered photons. We employ a classical model to characterize the exposure of electrons to high-intensity laser light in a situation where the electrons are driven by strong ponderomotive gradients. Free electrons are modeled as being donated by low-density helium, which undergoes strong-field ionization early on in the pulse or during a pre-pulse. When exposed to relativistic intensities (i.e. intensities sufficient to cause a Lorentz drift at a significant fraction of c), free electrons experience a Lorentz drift that causes redshifting of the scattered 800 nm laser light. This redshift can be used as a key signature to discern light scattered from the more intense regions of the focus. We characterize the focal volume of initial positions leading to significant redshifting, given a peak intensity of 2 x 10^18 W/cm 2 , which is sufficient to cause a redshift in scattered light of approximately 100 nm. Under this scenario, the beam waist needs to be larger than several wavelengths for a pulse duration of 35 fs in order to ensure free electrons remain in the focus sufficiently long to experience intensities near the peak pulse intensity despite strong ponderomotive gradients. We compute the rate of redshifted scattered photons from an ensemble of electrons distributed throughout the focus and relate the result to the scattered-photon rate of a single electron. We also estimate to what extent the ionization process may produce unwanted light in the redshifted spectral region that may confound the measurement of light scattered from electrons experiencing intensities greater than 1.5 x 10^18 W/cm^2.

碩士
國立高雄師範大學
資訊教育研究所
101
In today’s society, the rise of Information Techonology facilitate people to receive various news by a lot of pipelines,such as Social Network Sites.Everyone can express their own opinions in post-modern society.Therefore,the core of this study would realize the people who use the new techolonogy to watch news on Facebook,the types of personal moral judgement to influence on opinion sharing after news events have occured . This study created the online questionnaires survey, a total of 249 questionnaires returned, 232 questionnaires were available.The response rate was 93.6%.The instruments of this study include the Ethic Position Questionnaire(EPQ) developed by Forsyth, the Ethic decision-making scale developed by Rest,Moral intensity scale developed by Jones,Critical thinking intention Questionnaire developed by Yu-Chu Yeh,the knowledge-sharing scale developed by Abel Usoro et al. The result of this study are presented as follow: 1. Gender is significantly different to opinion sharing, male have a higher willingness to sharing his opinion on Facebook than females when the hot news events occurred. Also,gender is significantly different to moral judgement and intention, female have higher scores in moral judgement and intention than male.Gender is also significantly different to moral intensity,female have high sensitivity to feel something that caused someone to be hurt. 2. The various time of Facebook users whom watch news,the numbers of Facebook users whom join fan page,the frequency of Facebook users who response on Facebook fan page are significantly different to opinion sharing. 3. The moral philosophy is significantly correlated with critical thinking intention,ethic decision-making and moral intensity. 4. Critical thinking intention is positively significantly correlated with opinion sharing .Ethic decision-making and moral intensity are partially negtively significantly correlated with opinion sharing. Keyword: News Issue,Moral Philosophy, Ethic Decision-making, Moral Intensity, Critical Thinking Intention,Opinion Sharing, Facebook

High energy radiation is an important tool for many fields of research as it allows for the measurement of smaller structures and atomic interactions. The current best method of generating coherent and narrow bandwidth synchrotron radiation is with a free electron laser. It requires very high charge density, to start the amplification process and concurrently leads to its high level of coherency, and high energies (GeV to obtain keV photons). The stringent parameters on the electron bunch are met by linear accelerators. These are typically kilometre long straight structures that operate from tens to 100 Hz repetition rate. A novel design was proposed by the INFN Milan research group called MariX [1]. Here a LINAC is used in combination with a com- pression arc. This reduces the size of the facility, because the electron bunch can be accelerated twice by the same LINAC. As the electrons pass through dipoles in the compression arc the fields emanating from the particles in the bunch can cause deterioration to it. These fields, consisting out of the relativistic Coulomb-and radiation field, travel with the speeds of light, and thus originate from a point in the past. For this reason the behaviour of these retarded fields is investigated from first principles and developed into a 3D algorithm for calculating the forces within a bunch. An in depth overview is given on how the constituent fields behave over a large range of electron energies. Proportionality relations are given that determine which one is dominant. To reach unprecedented high energy photons is through the scattering of intense lasers with electron bunches; (inverse) Thomson or Compton scattering. Photon energies of keV can be reached with tens of MeV electrons, and MeV photons with GeV electrons. High repetition rate collisions are possible with cavity based laser systems. Currently the power in-side the cavity is several hundreds of kW with an intensity at the focus up to 1014−15 [W/cm2]. With these high powers the cavities can become degenerate, i.e. higher order transverse modes are excited, either by imperfections of the mirrors or deformations caused by heat dissipation. A short study provides insights to the observability of these modes in the Thomson spectrum. The general method for Thomson scattering is to have a (quasi) monochromatic laser pulse collide with an electron bunch with a very small energy spread. The latter usually leads to a reduction of the number of charges, and therefore the flux of scattered photons. The frequency of the scattered radiation is linearly dependent on that of the laser’s, and therefore the energy spread of the electrons could be compensated by including a frequency modulation. The highest intensity lasers obtained are by chirped pulse amplification and thus readily available. Two schemes have been investigated: longitudinal and transverse chirp. Both can reach the limit in bandwidth and number of photons scattered of the mono-energetic and mono-chromatic case. For ultra shorted pulses the carrier envelope phase becomes an important variable. Thomson scattering can be used to measure For intensities where non-linear effects dominate, because the scattered radiation contains the information of the laser pulse.. A model of its signature in the Thomson spectrum has been developed: it shifts the peaks of higher harmonics that overlap. This shift is also correlated to the emission direction of harmonics. A detailed analysis is given how to measure it experimentally.