Dissertations / Theses on the topic 'Plasma-Based Accelerator'

Laser-plasma wakefield accelerator (LWFA) is a promising novel technology that is introducing miniaturization to the accelerator world: the unprecedented gradient of acceleration shrinks the accelerator down to table-top size. Moreover, the LWFA comes with an embedded light source: electrons, while accelerating, undergo betatron oscillatory motion that results in synchrotron radiation emitted in a narrow cone along the direction of propagation. In this thesis we study theoretically and we prove experimentally a new regime of betatron oscillation that occurs when electrons experience the electromagnetic field of the laser during acceleration and oscillate resonantly at the laser frequency or its sub-harmonics. The signature of the harmonically resonant betatron (HRB) regime is a large oscillation amplitude and consequently prolific emission of high energy photons up to the MeV range. The HRB source has unique properties: very short pulse length (~10 fs), small source size (few microns), high peak brightness of the order of 1023 photons/s mm2 mrad2 0.1% B.W., which is comparable with a third generation light source. These properties make the source particularly appealing for the life sciences and medical and security applications. As a part of a future applications project, we give the scaling of the photon energy as a function of laser intensity and plasma density, which could extend toward tens of MeV. The thesis also investigates another gamma-ray source that utilises beams from the LWFA: bremsstrahlung radiation from high energy electrons interacting with metal targets. We study the electron beam and target parameters to optimize the emission process and the gamma-ray beam properties to match potential application requirements, such as radioisotope generation via photonuclear process. The results of a proof of concept experiment are presented and compared with simulations. Finally, we investigate numerically the possibility of generating a converging gamma ray beam based on the bremsstrahlung process. The results are encouraging, and the potential impact of a compact converging gamma-ray beam source is discussed with particular attention to medical applications in cancer treatment.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 149-162).
Plasma-material interactions (PMI) in magnetic fusion devices such as fuel retention, material erosion and redeposition, and material mixing present significant scientific and engineering challenges, particularly for the next generation of devices that will move towards reactor-relevant conditions. Achieving an integrated understanding of PMI, however, is severely hindered by a dearth of in-situ diagnosis of the plasma-facing component (PFC) surfaces. To address this critical need, this thesis presents an accelerator-based diagnostic that nondestructively measures the evolution of PFC surfaces in-situ. The diagnostic aims to remotely generate isotopic concentration maps that cover a large fraction of the PFC surfaces on a plasma shot-to-shot timescale. The diagnostic uses a compact, high-current radio-frequency quadrupole accelerator to inject 0.9 MeV deuterons into the Alcator C-Mod tokamak. The tokamak magnetic fields in between plasma shots are used to steer the deuterons to PFCs where the deuterons cause high-Q nuclear reactions with low-Z isotopes ~5 [mu]m into the material. Scintillation detectors measure the induced neutrons and gammas; energy spectra analysis provides quantitative reconstruction of surface concentrations. An overview of the diagnostic technique, known as accelerator-based in-situ materials surveillance (AIMS), and the first AIMS diagnostic on the Alcator C-Mod is given; a description of the complementary simulation tools is also provided. Experimental validation is shown to demonstrate the optimized beam injection into the tokamak, the quantification of PFC surfaces isotopes, and the measurement localization provided by magnetic beam steering. Finally, the first AIMS measurements of fusion fuel retention are presented, demonstrating the local erosion and codeposition of deuterium-saturated boron surface films. The finding confirms that deuterium codeposition with boron is insufficient to account for the net fuel retention in Alcator C-Mod.
by Zachary Seth Hartwig.
Ph. D.

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 214-218).
Plasma-material interactions (PMI) in magnetic fusion devices pose significant scientific and engineering challenges for the development of steady-state fusion power reactors. Understanding PMI is crucial for the develpment of magnetic fusion devices because fusion plasmas can significantly modify plasma facing components (PFC) which can be severely detrimental to material longevity and plasma impurity control. In addition, the retention of tritium (T) fuel in PFCs or plasma co-deposited material can disrupt the fuel cycle of the reactor while contributing to radiological and regulatory issues. The current state of the art for PMI research involves using accelerator based ion beam analysis (IBA) techniques in order to provide quantitative measurement of the modification to plasma-facing surfaces. Accelerated ~MeV ion beams are used to induce nuclear reactions or scattering, and by spectroscopic analysis of the resulting high energy particles (s', p, n, a, etc.), the material composition can be determined. PFCs can be analyzed to observe erosion and deposition patterns along their surfaces which can be measured with spatial resolution down to the -1 mm scale on depth scales of 10 - 100 pim. These techniques however are inherently ex-situ and can only be performed on PFCs that have been removed from tokamaks, thus limiting analysis to the cumulative PMI effects of months or years of plasma experiments. While ex-situ analysis is a powerful tool for studying the net effects of PMI, ex-situ analysis cannot address the fundamental challenge of correlating the plasma conditions of each experiment to the material surface evolution. This therefore motivates the development of the in-situ diagnostics to study surfaces with comparable diagnostic quality to IBA in order resolve the time evolution of these surface conditions. To address this fundamental diagnostic need, the Accelerator-Based In-Situ Materials Surveillance (AIMS) diagnostic [22] was developed to, for the first time, provide in-situ, spatially resolved IBA measurements inside of the Alcator C-Mod tokamak. The work presented in this thesis provided major technical and scientific contributions to the development and first demonstration AIMS. This included accelerator development, advanced simulation methods, and in-situ measurement of PFC surface properties and their evolution. The AIMS diagnostic was successfully implemented on Alcator C-Mod yielding the first spatially resolved and quantitative in-situ measurements of surface properties in a tokamak, with thin boron films on molybdenum PFCs being the analyzed surface in C-Mod. By combining AIMS neutron and gamma measurements, time resolved and spatially resolved measurements of boron were made, spanning the entire AIMS run campaign which included lower single null plasma discharges, inboard limited plasma discharges, a disruption, and C-Mod wall conditioning procedures. These measurements demonstrated the capability to perform inter shot measurements at a single location, and spatially resolved measurements over longer timescales. This demonstration showed the first in-situ measurements of surfaces in a magnetic fusion device with spatial and temporal resolution which constitutes a major step forward in fusion PMI science. In addition, an external ion beam system was implemented to perform ex-situ ion beam analysis (IBA) for components from Alcator C-Mod Tokamak. This project involved the refurbishment of a 1.7 MV tandem linear accelerator and the creation of a linear accelerator facility to provide IBA capabilities for MIT Plasma Science and Fusion Center. The external beam system was used to perform particle induced gamma emission (PIGE) analysis on tile modules removed after the AIMS measurement campaign in order to validate the AIMS using the well established PIGE technique. From these external PIGE measurements, a spatially resolved map of boron areal density was constructed for a section of C-Mod inner wall tiles that overlapped with the AIMS measurement locations. These measurements showed the complexity of the poloidal and toroidal variation of boron areal density between PFC tiles on the inner wall ranging from 0 to 3pm of boron. Using these well characterized ex-situ measurements to corroborate the in-situ measurements, AIMS showed reasonable agreement with PIGE, thus validating the quantitative surface analysis capability of the AIMS technique.
by Harold Salvadore Barnard.
Sc. D.

Le plus fréquemment, l’interaction laser-matière est étudiée avec des lasers ayant des longueurs d’onde dans l’infrarouge proche (PIR), car ce sont les lasers qui peuvent générer les impulsions les plus intenses. Pour ces lasers, des cibles de densité allant de 0,05 à 2,5 fois la densité critique sont difficiles à créer mais elles offrent des perspectives intéressantes. Dans cette thèse, des jets d’hydrogène ayant de densité dans ce domaine sont utilisées dans le contexte de deux applications :Premièrement, des ions sont accélérées par choc non-collisionnel (collisionless shock acceleration, CSA). Lors de l’interaction d’une impulsion laser PIR avec une cible légè- rement sur-critique, un faisceau de protons est généré. Il est collimé, dirigé vers l’avant et quasiment monoénergetique. Des simulations indiquent que cela est lié à la formation d’un choc non-collisionnel et à l’accélération des protons par ce choc, en sus de leur accélération par le processus standard dit ”target normal sheath acceleration (TNSA)” qui est effectif en face arrière de la cible. Pour beaucoup d’applications, ces faisceaux de particules quasi-monoénergetiques sont plus appropriés que ceux à spectre large qui sont générés de façon routinière par TNSA.Deuxièmement, de l’énergie est transférée d’une impulsion laser (pump) vers une autre en contrepropagation (seed), par rétrodiffusion Brillouin stimulée, dans le régime de couplage fort (strong coupling-SBS), à des densités entre 0,05 et 0,2 fois la densité critique. Pour des impulsions à large bande (60 nanomètres), le rôle de la pré-ionisation sur la propagation et la rétrodiffusion Brillouin spontanée et stimulée est étudié, en incluant l’influence du chirp. Pour des lasers à bande plus étroite, il est démontré que l’impulsion seed peut être amplifiée par des dizaines de milliJoules, et des signatures d’amplification efficace et d’affaiblissement de l’impulsion laser pompe sont trouvées. Ce concept vise à l’amplification des impulsions laser à des puissances au-delà du seuil de dommage des amplificateurs laser basés sur des matériaux solides
Laser-matter interaction is studied mostly with near-infrared (NIR) lasers as they can generate the most intense pulses. For these lasers, targets between 0.05 to 2.5 times the critical density are challenging to create but offer interesting prospects. In this thesis, novel high-density Hydrogen gas jet targets with densities in this range are used in view of two applications:First, ions are accelerated by collisionless shock acceleration (CSA). Upon interaction of a NIR laser with a slightly overcritical gas jet target, a collimated, quasi-monoenergetic proton beam is generated in forward direction. Simulations indicate the formation of a collisionless shock and acceleration of protons both by the shock and target normal sheath acceleration (TNSA) on the target rear surface under these conditions. These directed, monoenergetic particle bunches are more suitable for many applications than the broadband particle beams already generated routinely.Second, at densities between 0.05 and 0.2 times the critical density, energy is transferred from one laser pulse (pump) to a counterpropagating pulse (seed), via Stimulated Brillouin Backscattering in the strongly-coupled regime (sc-SBS). For the case of broad- band (60 nanometers) pulses, the role of the preionization for pulse propagation and both spontaneous and stimulated Brillouin backscattering are studied, including the influence of the chirp. It is shown that for narrower bandwidths, the seed pulse is ampli- fied by tens of millijoules, and signatures of efficient amplification and pump depletion are found. This concept aims at amplifying laser pulses to powers above the damage thresholds of solid state amplifiers

Recently, ultra-intense laser-driven ion acceleration has turned out to be an extremely interesting phenomenon, capable to produce ion beams which could potentially be suitable for applications as hadron therapy or dense matter diagnostics. The present PhD thesis is addressed to the study of Target Normal Sheath Acceleration (TNSA), namely the laser-based ion acceleration mechanism which dominates the presently accessible experimental conditions. The work is focused in particular on the theoretical modeling of TNSA, motivated by the need for an effective description which, by adopting proper approximations that can limit the required computational efforts, is capable to provide reliable predictions on the resulting ion beam features, given an initial laser-target configuration. Indeed, the development of a robust TNSA theoretical model would mean a deeper comprehension of the key physical factors governing the process, allowing at the same time to draw guidelines for potential experiments in the next future. In this dissertation, in order to achieve a significant advancement in the TNSA modeling field, the results of two original works are reported, the first is focused on a critical, quantitative analysis of existing descriptions, and the second, starting from the conclusions of such an analysis, is dedicated to the extension of a specific model, aiming at the inclusion of further, crucial, TNSA aspects. The quantitative analysis consists in the comparison of six well-known published descriptions, relying on their capability in estimating the maximum ion energy, which is tested over an extensive database of published TNSA experimental results, covering a wide range of laser-target conditions. Such a comparative study, despite the technical issues to be faced in order to reduce the arbitrariness of the results, allows to draw some interesting conclusions about the effectiveness of the six models considered and about TNSA effective modeling in general. According to the results, the quasi-static model proposed by M. Passoni and M. Lontano turns out to be the most reliable in predicting the ion cut-off energy, at the same time achieving such estimates through a self-consistent treatment of the accelerating potential. This work highlights also the limits of such a TNSA model, and of the main approximations usually adopted to obtain the different maximum ion energy estimates. Thus, starting from such considerations, an extension of this Passoni-Lontano model is proposed, including new crucial elements of TNSA physics within the description. In particular, further insights of the hot electron population dynamics are implemented, leading to a refined maximum energy prediction, which exhibits more solid theoretical bases, and which broadens the predicting capability of the original model to a larger range of system parameters. The resulting estimates are validated by means of literature experimental data and numerical simulations, demonstrating a remarkable agreement in most of the cases. The achieved model turns out to be particularly suitable in reproducing the maximum ion energy dependence on the target thickness, while some promising insights are obtained in the Mass Limited Targets (MLT) case. Nonetheless, further theoretical work is still required to attain a quantitative agreement with recently published experimental results on MLTs.

La microscopie électronique et la diffraction d’électrons ont permis de comprendre l’organisation des atomes au sein de la matière. En utilisant une source courte temporellement, il devient possible de mesurer les déplacements atomiques ou les modifications de la distribution électronique dans des matériaux. A ce jour, les sources ultra-brèves pour les expériences de diffraction d’électrons ne permettent pas d’atteindre une résolution temporelle inférieure à la centaine de femtosecondes (fs). Les accélérateurs laser-plasma sont de bons candidats pour atteindre une résolution temporelle de l’ordre de la femtoseconde. De plus, ces accélérateurs peuvent fonctionner à haut taux de répétition, permettant d’accumuler un grand nombre de données.Dans cette thèse, un accélérateur laser-plasma fonctionnant au kHz a été développé et construit. Cette source accélère des électrons à une énergie de 100 keV environ à partir d’impulsions laser d’énergie 3 mJ et de durée 25 fs. La physique de l’accélération a été étudiée, démontrant entre autres l’effet du front d’onde laser sur la distribution transverse des électrons.Les premières expériences de diffraction avec ce type de sources ont été réalisées. Une expérience de preuve de principe a montré que la qualité de la source est suffisante pour obtenir de belles images de diffraction sur des feuilles d’or et de silicium. Dans un second temps, la dynamique structurelle d’un échantillon de Silicium a été étudiée avec une résolution temporelle de quelques picosecondes, démontrant le potentiel de ce type de sources.Pour augmenter la résolution temporelle à sub-10 fs, il est nécessaire d’accélérer les électrons à des énergies relativistes de quelques MeV. Une étude numérique a montré que l’on peut accélérer des paquets d’électrons ultra-courts grâce à des impulsions laser de 5 mJ et 5 fs. Il serait alors possible d’atteindre une résolution temporelle de l’ordre de la femtoseconde. Finalement, une expérience de post-compression des impulsions laser due à l’ionisation d’un gaz a été réalisée. La durée du laser a pu être réduite d’un facteur deux, et l’homogénéité de ce processus a été étudiée expérimentalement et numériquement
Electronic microscopy and electron diffraction allowed the understanding of the organization of atoms in matter. Using a temporally short source, one can measure atomic displacements or modifications of the electronic distribution in matter. To date, the best temporal resolution for time resolved diffraction experiments is of the order of a hundred femtoseconds (fs). Laser-plasma accelerators are good candidates to reach the femtosecond temporal resolution in electron diffraction experiments. Moreover, these accelerators can operate at a high repetition rate, allowing the accumulation of a large amount of data.In this thesis, a laser-plasma accelerator operating at the kHz repetition rate was developed and built. This source generate electron bunches at 100 keV from 3 mJ and 25 fs laser pulses. The physics of the acceleration has been studied, and the effect of the laser wavefront on the electron transverse distribution has been demonstrated.The first electron diffraction experiments with such a source have been realized. An experiment, which was a proof of concept, showed that the quality of the source permits to record nice diffraction patterns on gold and silicium foils. In a second experiment, the structural dynamics of a silicium sample has been studied with a temporal resolution of the order of a few picoseconds.The electron bunches must be accelerated to relativistic energies, at a few MeV, to reach a sub-10 fs temporal resolution. A numerical study showed that ultra-short electron bunches can be accelerated using 5 fs and 5 mJ laser pulses. A temporal resolution of the order of the femtosecond could be reached using such bunches for electron diffraction experiments. Finally, an experiment of the ionization-induced compression of the laser pulses has been realized. The pulse duration was shorten by a factor of 2, and the homogeneity of the process has been studied experimentally and numerically

Plasma-based accelerators aim to efficiently generate relativistic electrons by exciting plasma waves using a laser or particle beam driver, and "surfing" electrons on the resulting wakefields. In the blowout regime of such wakefield acceleration techniques, the intense laser radiation pressure or beam fields expel all of the plasma electrons transversely, forming a region completely devoid of electrons ("bubble") that co-propagates behind the driver. Injection, where initially quiescent background plasma electrons become trapped inside of the plasma bubble, can be caused by a variety of mechanisms such as bubble expansion, field ionization or collision between pump and injector pulses. This work will present a study of the injection phenomenon through analytic modeling and particle-in-cell (PIC) simulations. First, an idealized model of a slowly expanding spherical bubble propagating at relativistic speeds is used to demonstrate the importance of the bubble's structural dynamics in self-injection. This physical picture of injection is verified though a reduced PIC approach which makes possible the modeling of problem sizes intractable to first-principles codes. A more realistic analytic model which takes into account the effects of the detailed structure of the fields surrounding the bubble in the injection process is also derived. Bubble expansion rates sufficient to cause injection are characterized. A new mechanism for generation of quasi-monoenergetic electron beams through field ionization induced injection is presented, and simulation results are compared to recent experimental results. Finally, a technique for frequency-domain holographic imaging of the evolving bubble is analyzed using PIC as well as a novel simulation method for laser probe beam propagation.
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This thesis shows the characterization of the plasma sources needed for the plasma-based experiments of SPARC_LAB. During this thesis work, I have studied and implemented the tools needed to measure the plasma density into both gas-filled and laser trigger ablative capillaries. The diagnostic system, based on the analysis of the Stark broadening of the emitted spectral lines, allowed to measure in a single shot the evolution of the plasma density variation along the entire capillary length in steps of 100 ns. As far as we know, this is the first single-shot, longitudinally-resolved measurement based on the Stark broadening analysis to measure low density plasma evolution (10^17 cm^-3) in a capillary discharge. By knowing the temporal evolution of the plasma density, it is possible to chose the correct working point for the accelerator and to check its stability and reliability. Moreover, the versatility of the system allows to verify online the proper functioning of the acceleration process, monitoring the variation of plasma density distribution along the acceleration path. This system has been implemented in the SPARC bunker and it has been used to characterize hydrogen filled capillary discharge. To complete the characterization of these capillaries, the discharge current profile has been characterized. The same diagnostic tool has been used to study how to proper engineering of the longitudinal plasma density can be performed with 3D printed laser trigger ablative capillaries whose prototyping cost is negligible, thanks to relatively fast manufacturing processes and their cheap materials. This investigation leads to measure the effect of the tapering of the capillary on the plasma density distribution along the whole capillary length. Tailoring the density from the beginning to the end of the interaction let to preserve the beam quality after the acceleration, but also it ensures the matching between the beams and the plasma. Finally, I implemented a Mach-Zehnder interferometer to detect the plasma density along the propagation length of a laser pulse in a gas-jet for self injection LWFA experiments performed at SPARC_LAB.

博士
國立中央大學
物理學系
101
Laser driven wakefield is capable of sustaining field in excess of 1 GV/cm, making it a promising candidate for the next-generation electron accelerator. The development of chirp pulse amplification boosts the power of laser pulse to terawatt (TW) or even petawatt (PW) level, making it possible to drive a large-amplitude plasma wave to accelerate electrons by the poderomotive force of the intense focused laser pulse. This technique has achieved GeV electron energy in centimeter scale and is being investigated as a driver for generating bright photons with spectrum ranging from THz to X-rays. The most important issues in this field are injection and acceleration process in the driven plasma wave, which determines the quality and shot-to-shot stability of the electron beam and the highest energy gain one can obtain from the accelerator. This thesis reports the efforts and accomplishments on the development of a plasma-waveguide-based Laser wakefield accelerator with two kinds of new electron injection scheme and channel betatron radiated X-ray source. In the first part of my work, a systematic experimental study on injection of electrons in a gas-jet-based laser wakefield accelerator via ionization of dopant was conducted. The pump-pulse threshold energy for producing a quasimonoenergetic electron beam was significantly reduced by doping the hydrogen gas jet with argon atoms, resulting in a much better spatial contrast of the electron beam. Furthermore, laser wakefield electron acceleration in an optically preformed plasma waveguide based on the axicon-ignitor-heater scheme was achieved. It was found that doping with argon atoms can also lower the pump-pulse threshold energy in the case with a plasma waveguide. In the second part of my work, a variable three-dimensionally structured plasma waveguide was fabricated by adding a transverse heater pulse into the axicon-ignitor-heater-scheme. With this technique, induction of electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a monoenergetic electron beam. The injection is correlated with a section of expanding cross-section in the plasma waveguide. Moreover, the intensity of the X-ray beam produced by the electron bunch in betatron oscillation was greatly enhanced with a transversely shifted section in the plasma waveguide. The technique opens the route to a compact hard x-ray pulse source.

碩士
國立中正大學
物理學系暨研究所
100
Laser wakefield electron accelerator has been demonstrated to have the capability of generating GeV-energy, low-emittance, high-stability electron pulses in centimeter scale length. It has shown great potential to become the key technology for next-generation TeV collider and various table-top ultrashort-pulse photon sources of wavelength ranging from THz to gamma-ray. These applications require an electron beam with high quality and stability, both of which are determined by finely controlled injection and acceleration processes in a laser-driven plasma wave. Here we report demonstration of production of a low-energy-spread electron beam via injection by longitudinal wave-breaking induced in a density down ramp in a plasma waveguide. In this scheme, the plasma waveguide is generated by using the axicon-ignitor-heater scheme, and an additional transverse heater pulse passing through a knife edge is used to produce longitudinal density variation in the plasma waveguide. This technique allows us to freely control the position and slope of the density ramp and to observe them with probing interferometry. It was observed that generation of a high-energy quasi-monoenergetic electron beam occurs only when the transverse heater pulse produces a density down ramp, and the probability of production varies with the position of density ramp. Good guiding of the pump laser pump was still maintained under the condition of presence of density ramps and high pump-pulse energy. The energy spread of the produced electron beam can be as low as 1%. The tunability of the density ramp allows us to clarify the ramp injection process and to optimize the quality of the electron beam. With this technique of fabrication of three-dimensional plasma density structure, integration of electron injector, accelerator, and x-ray free-electron laser in a single plasma waveguide may be achieved.

碩士
國立臺灣大學
物理研究所
101
Here we demonstrate the use of laser-driven plasma accelerators, which accelerate high-charge electron beams to high energy in short distances. The particles being accelerated in the plasma accelerator also undergo transverse (betatron) oscillations to intrinsically ultrafast beams of hard X-rays. The experiment was performed at National Central University, Taiwan, using a 100-TW-class Ti:sapphire laser system with 10-Hz pulse repetition rate. Five laser beams from this system were used for the experiment. The axicon ignitor and heater pulses were used to fabricate a ∼ 1-cm length in full width at half maximum (FWHM) uniform plasma waveguide in a gas jet. The 1.1-J, 40-fs pump pulse was coupled into the plasma waveguide at a delay of 1.9 ns with respect to the axicon heater pulse to excite a plasma wave (plasma bubble) which can accelerate electrons. The focal spot size was 10 μm in FWHM. By adding a transverse heater 1 pulse into the axicon ignitor-heater scheme for producing a plasma waveguide, a variable three-dimensionally structured plasma waveguide can be fabricated. With this technique, electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of electron beam energy large than 200 MeV. Then a 116-mJ, 210-ps transverse heater 2 pulse was used to introduce lower density spatial gaps between uniform plasma density sections that behind the fabricated plasma waveguide. When accelerated electrons that enter the depression at the proper phase in their betatron oscillation will increase their transverse displacement, thus exiting the depression with larger betatron amplitude. This technique opens a route to a compact hard-X-ray pulse source. We could produce x-ray with photon energies in the range of 1–10 keV by using lower density spatial gaps between uniform plasma density sections. If we compare laser- driven betatron-radiation x-ray source with conventional particle accelerator facilities, we find the size is significantly smaller and the cost is much cheaper. This reduces the size of the synchrotron source from the tens of meters to the centimeter scale, simultaneously accelerating and wiggling the electron beam. Then the betatron radiation has intrinsically striking features for ultra-fast imaging. Therefore laser- driven betatron-radiation x-ray source has the potential to be a table top hard-X-ray pulse source.

Accurate single-shot visualization of laser wakefield structures can improve our fundamental understanding of plasma-based accelerators. Previously, frequency domain holography (FDH) was used to visualize weakly nonlinear sinusoidal wakes in plasmas of density n[subscript e] < 0.6 × 10¹⁹/cm³ that produced few or no relativistic electrons. Here, I address the more challenging task of visualizing highly nonlinear wakes in plasmas of density n[subscript e] ~ 1 to 3× 10¹⁹/cm³ that can produce high-quality relativistic electron beams. Nonlinear wakes were driven by 30 TW, 30 fs, 800 nm pump pulses. When bubbles formed, part of a 400 nm, co-propagating, overlapping probe pulse became trapped inside them, creating a light packet of plasma wavelength dimensions--that is, an optical "bullet"--that I reconstruct by FDH methods. As ne increased, the bullets first appeared at 0.8 × 10¹⁹/cm³, the first observation of bubble formation below the electron capture threshold. WAKE simulations confirmed bubble formation without electron capture and the trapping of optical bullets at this density. At n[subscript] >1× 10¹⁹/cm³, bullets appeared with high shot-to-shot stability together with quasi-monoenergetic relativistic electrons. I also directly observed the temporal walk-off of the optical bullet from the beam-loaded plasma bubble revealed by FDH phase shift data, providing unprecedented visualization of the electron injection and beam loading processes. There are five chapters in this thesis. Chapter 1 introduces general laser plasma- based accelerators (LPA). Chapter 2 discusses the FDH imaging technique, including the setup and reconstruction process. In 2006, Dr. N. H. Matlis used FDH to image a linear plasma wakefield. His work is also presented in Chapter 2 but with new analyses. Chapter 3, the main part of the thesis, discusses the visualization of LPAs in the bubble regime. Chapter 4 presents the concept of frequency domain tomography. Chapter 5 suggests future directions for research in FDH.
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Plasma-based particle acceleration holds the promise to make the applications that revolve around accelerators more affordable. The central unifying theme of this dissertation is the modeling of certain non-linear and relativistic phenomena in plasma dynamics to devise mechanisms that benefit plasma-accelerators. Plasma acceleration presented here has two distinct flavors depending upon the accelerated particle mass which dictates the acceleration structure velocity and potential. The first deals with ion acceleration, where acceleration structure velocities are a significant fraction of the speed of light, with major applications in medicine. The second focusses on the acceleration of electrons and positrons for light-sources and colliders where the acceleration structures are wakefields with phase-velocities near the speed of light.

The increasing Lorentz factor of the laser-driven electron quiver momentum forms the basis of Relativistically Induced Transparency Acceleration (RITA) scheme of ion acceleration. Lighter ions are accelerated by reflecting off a propagating acceleration structure, referred to as a snowplow, formed by the compression of ponderomotively driven critical layer electrons excited in front of a high intensity laser pulse in a fixed-ion plasma. Its velocity is controlled by tailoring the laser pulse rise-time and rising density gradient scale-length. We analytically model its induced transparency driven propagation with a 1-D model based on the linearized dispersion relation. The model is shown to be in good agreement with the weakly non-linear simulations. As the density compression rises into the strongly non-linear regime, the scaling law predictions remain accurate but the model does not exactly predict the RITA velocity or the accelerated ion-energy. Multi-dimensional plasma effects modify the laser radial envelope by self-focussing in the rising density gradient which can be integrated into our model and filamentation which is mitigated by a matched laser focal spot-size. We show that the critical layer motion in RITA compares favorably to the bulk-plasma motion driven by radiation pressure or collision-less shocks.

Non-linear mixing of the laser, incident on and reflected off the propagating critical layer modulates its envelope affecting the acceleration structure velocity and potential, in the process setting up a feedback loop. For long pulses the envelope distortion grows with time, disrupting the accelerated ion-beam spectral shape. We model the Chirp Induced Transparency Acceleration (ChITA) mechanism that over- comes this effect by introducing decoherence through a frequency chirp in the laser.

In a rising density gradient, the non-linearity of electron trajectories leads to the phase-mixing self-injection of electrons into high phase-velocity plasma wakefields. The onset of trapping depends upon the wake amplitude and the density gradient scale-length. This self-injection mechanism is also applicable to controlling the spuriously accelerated electrons that affect the beam-quality.

Non-linear ion dynamics behind a train of asymmetric electron-wake excites a cylindrical ion-soliton similar to the solution of the cylindrical Korteweg-de Vries (cKdV) equation. This non-linear ion-wake establishes an upper limit on the repetition rate of the future plasma colliders. The soliton is excited at the non-linear electron wake radius due to the time-asymmetry of its radial fields. In a non-equilibrium wake heated plasma the radial electron temperature gradient drives the soliton. Its radially outwards propagation leaves behind a partially-filled ion-wake channel.

We show positron-beam driven wakefield acceleration in the ion-wake channel. Optimal positron-wakefield acceleration with linear focussing fields is shown to require a matched hollow-plasma channel of a radius that depends upon the beam properties.

Dissertation