Relativistic electron-cyclotron resonances in laser Wakefield acceleration

In this thesis, the magnetized, relativistic plasma that overlaps the pump laser in Laser Wakefield Acceleration (LWFA) was investigated. The Jeti 40 laser was used to drive the plasma wave and a transverse, few-cycle probe pulse in the visible to near-infrared spectrum was implemented to image the laser-plasma interaction. The recorded shadowgrams were sorted depending on the properties of the accelerated electron bunches, and subsequently stitched together based on the timing delay between the pump and probe beams. The resulting data showed two signatures unique to the relativistic, magnetized plasma near the pump pulse. Firstly, a significant change in the brightness modulation of the shadowgrams, coinciding with the location of the pump pulse, shows a strong dependence on the pump’s propagation length and the probe’s spectrum and polarization. Secondly, after ~1.5 mm of propagation in the plasma, polarization-dependent diffraction rings appear in front of the plasma wave. A mathematical model using relativistic corrections to the Appleton-Hartree equation was developed to explain these signals. By combining the model with data from 2D Particle-in-Cell (PIC) simulations using the VSim code, the plasma’s birefringent refractive index distribution was investigated. Simulated shadowgrams of a 3D PIC simulation using the EPOCH code were also analyzed with respect to the aforementioned signals. The results of the study present a compelling description of the pump-plasma interaction. The previously unknown signals arise from relativistic, electron-cyclotron motion originating in the 10s of kilotesla strong magnetic fields of the pump pulse. Advantageously, a VIS-NIR probe is resonant with the cyclotron frequencies at the peak of the pump. With further refinement, the measurement of this phenomenon could allow for the non-invasive experimental visualization of the pump laser’s spatio-temporal energy distribution and evolution during propagation through the plasma

Plasma Radiation Model of Fast Radio Bursts from Magnetars

We propose a novel idea for the coherent intense millisecond radio emission of cosmic fast radio bursts (FRBs), which have recently been identified with flares from a magnetar. Motivated by the conventional paradigm of Type III solar radio bursts, we will explore the emission of coherent plasma line radiation at the electron plasma frequency and its harmonic as potential candidates of the coherent FRB emissions associated with magnetar flares. We discuss the emissions region parameters in relativistic strongly magnetized plasmas consisting of electrons, positrons and protons. The goal is to make observable predictions of this model to confront the multi-wavelength observations of FRBs from magnetars. These results will impact both observational radio astronomy and space-based astrophysicsComment: 13 pages 7 figure

Plasma source characterization for plasma-based acceleration experiments

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

LWFA diagnostics development for ATLAS-300 and nonlinear plasma wavelength scalings

LWFA is the abbreviation of Laser WakeField Acceleration (or Accelerator), which is an emerging technology promising a drastic reduction of particle accelerators' size and cost. LWFA relies on the large (~GV/cm) electric field in the plasma wave excited by laser pulses with a relativistic peak intensity in excess of 10^18 W/cm^2. During the course of this thesis work, diagnostic tools, for electrons and for the plasma medium, specially adapted to the conditions of LWFA research were developed for the ATLAS-300 Ti:sapphire laser at the Laboratory for Extreme (LEX) Photonics at Ludwig-Maximilians-Universität München (LMU). For electron diagnostics, the implementation and characterization of scintillating screens and permanent dipole magnets were carried out. On the plasma diagnostics side, a few-cycle probe beam for producing fs-snapshots of the plasma wave was developed. Specifically, the light-emitting efficiency of nine commonly used scintillating screen types were calibrated with electron beams from the ELBE Linac at Helmholtz Zentrum Dresden Rossendorf. Existing methods for transferring the absolute calibration results into cross-calibration with a constant light source were improved upon. Regarding the dipole magnet, the B-field distribution near the gap central plane was measured with a Hall sensor and electron tracking was performed in General Particle Tracer using the measured field map. Scintillating screens and the dipole magnet comprised the electron spectrometer, allowing for energy-resolved detection of electron beams generated in the LWFA. Self-injected electron beams with several hundreds pC of charge and peak energy above 1 GeV were accelerated in a hydrogen-filled varying-length gas cell. Injecting with shock fronts in supersonic gas jets, stable electron beams with spectral density beyond 10 pC/MeV was achieved thanks to a percent-level energy spread. The highlight of this work is the construction and application of a hollow-core fiber based pulse compression setup, serving as the probe beam during the experiments. This setup delivered sub-10 fs probe pulses, with which, shadowgraphic snapshots of the laser plasma interaction were recorded. In particular, laser-driven plasma waves were resolved owing to the ultrashort probe pulse duration. A portion of the probe beam was split out to illuminate a Nomarski interferometer, enabling independent measurement of the average plasma density with Abel inversion technique. A systematic measurement of plasma wavelength at varying densities or laser intensities revealed insufficiency of the current understanding of nonlinear plasma wavelength scaling. An empirical scaling was proposed based on a set particle-in-cell simulations, which relates the nonlinear plasma wavelength not only to the drive laser's peak strength, but also to its spot-size-to-pulse-length aspect ratio. Excellent agreement was found between the measurement and the new scaling law.LWFA steht für Laser WakeField Acceleration (oder Accelerator), einer neuen Technologie, die eine drastische Reduzierung der Größe und Kosten von Teilchenbeschleunigern verspricht. LWFA beruht auf dem großen (~ GV/cm) elektrischen Feld in der Plasmawelle, die durch Laserpulse mit relativistischer Spitzenintensität (10^18 W/cm^2) angeregt wird. Im Rahmen dieser Dissertation wurden die für die LWFA-Forschung am Ti:Saphir Laser ATLAS-300 am Laboratory for Extreme Photonics der Ludwig-Maximilians-Universität München geeignete Diagnostiken entwickelt, sowohl für Charakterisierung der Elektronen als auch zur Untersuchung des Plasmamediums. Zur Elektronendiagnostik wurden Implementierung und Charakterisierung von Szintillationsschirmen und Permanentdipolmagneten durchgeführt. Zur Plasmadiagnose wurde ein Probestrahl mit wenigen Zyklen entwickelt, mit dessen Hilfe fs-Schnappschüsse der Plasmawelle ermöglicht wurden. Konkret wurde die Lichtemissionseffizienz von neun gängigen Szintillationsschirmtypen mit Elektronenstrahlen des ELBE Linac am Helmholtz Zentrum Dresden-Rossendorf kalibriert. Bestehende Verfahren zur Übertragung der absoluten Kalibrierergebnisse in eine Kreuzkalibrierung mit einer Konstantlichtquelle wurden entscheidend verbessert. Bezüglich des Dipolmagneten wurde die Verteilung des B-Feldes in der Nähe der Mittelebene des Spalts mit einem Hall-Sensor gemessen und die Elektronen wurde in General Particle Tracer mit der gemessenen Feldverteilung getrackt. Szintillationsschirme und der Dipolmagnet bildeten das Elektronenspektrometer, das eine energieaufgelöste Nachweis von im LWFA erzeugten Elektronenstrahlen ermöglichte. Selbstinjizierte Elektronenstrahlen mit mehreren hundert pC Ladung und Spitzenenergien über 1 GeV wurden in einer mit Wasserstoff gefüllten Gaszelle unterschiedlicher Länge beschleunigt. Durch die Injektion an Stoßfronten in überschallen Gasstrahlen wurden stabile Elektronenstrahlen mit einer spektralen Dichte über 10 pC/MeV dank einer prozentualen Energieverteilung erreicht. Das Highlight dieser Arbeit ist der Aufbau und die Anwendung eines auf Hohlfasern basierenden Pulskompressionsaufbaus, der während der Experimente als Probestrahl dient. Dieser Aufbau lieferte sub-10 fs Probepulse, mit denen Phasenkontrast-Schnappschüsse der Laser-Plasma-Wechselwirkung aufgezeichnet wurden. Insbesondere wurden lasergetriebene Plasmawellen aufgrund der ultrakurzen Probepulsdauer aufgelöst. Ein Teil des Probestrahls wurde aufgespalten, um ein Nomarski-Interferometer zu beleuchten, was eine unabhängige Messung der durchschnittlichen Plasmadichte mit der Abel-Inversion ermöglicht. Eine systematische Messung der Plasmawellenlänge bei unterschiedlichen Dichten oder Laserintensitäten zeigte, dass das derzeitige Verständnis der Skalierung von nichtlinearen Plasmawellen-länge unzureichend ist. Basierend auf einer Reihe von Particle-in-Cell Simulationen wurde eine empirische Skalierung vorgeschlagen, die die nichtlineare Plasmawellenlänge nicht nur mit der Spitzenstärke des Treiblasers, sondern auch mit seinem Aspektverhältnis von Punktgröße zu Pulslänge in Beziehung setzt. Es wurde eine hervorragende Übereinstimmung zwischen der Messung und dem neuen Skalierungsgesetz festgestellt

COMPACT LASER DRIVEN ELECTRON AND PROTON ACCELERATION WITH LOW ENERGY LASERS

Laser-driven particle accelerators offer many advantages over conventional particle accelerators. The most significant of these is the magnitude of the accelerating gradient and, consequently, the compactness of the accelerating structure. In this dissertation, experimental and computational advances in laser-based particle acceleration in three intensity regimes are presented. All mechanisms investigated herein are accessible by “tabletop” ultrashort terawatt-class laser systems found in many university labs, with the intention of making them available to more compact and high repetition rate laser systems. The first mechanism considered is the acceleration of electrons in a preformed plasma “slow-wave” guiding structure. Experimental advances in the generation of these plasma guiding structures are presented. The second mechanism is the laser-wakefield acceleration of electrons in the self-modulated regime. A high-density gas target is implemented experimentally leading to electron acceleration at low laser pulse energy. Consequences of operating in this regime are investigated numerically. The third mechanism is the acceleration of protons by a laser-generated magnetic structure. A numerical investigation is performed identifying operating regimes for experimental realizations of this mechanism. The key advances presented in this dissertation are: The development and demonstration of modulated plasma waveguide generation using both mechanical obstruction and an interferometric laser patterning method The acceleration of electrons to MeV energy scales in a high-density hydrogen target with sub-terawatt laser pulses and the generation of bright, ultra-broadband optical pules from the interaction region 3D particle-in-cell (PIC) simulations of self-modulated laser wakefield acceleration in a plasma, showing the generation of broadband radiation, and the role of “direct laser acceleration” in this regime 3D PIC simulations of laser wakefield acceleration in the resonant regime, identifying spatio-temporal optical vortices in a laser-plasma system 3D PIC simulations of proton acceleration by magnetic vortex acceleration using TW-class laser pulse

Physics of Laser-driven plasma-based acceleration

The physics of plasma-based accelerators driven by short-pulse lasers is reviewed. This includes the laser wake-field accelerator, the plasma beat wave accelerator, the self-modulated laser wake-field accelerator, and plasma waves driven by multiple laser pulses. The properties of linear and nonlinear plasma waves are discussed, as well as electron acceleration in plasma waves. Methods for injecting and trapping plasma electrons in plasma waves are also discussed. Limits to the electron energy gain are summarized, including laser pulse direction, electron dephasing, laser pulse energy depletion, as well as beam loading limitations. The basic physics of laser pulse evolution in underdense plasmas is also reviewed. This includes the propagation, self-focusing, and guiding of laser pulses in uniform plasmas and plasmas with preformed density channels. Instabilities relevant to intense short-pulse laser-plasma interactions, such as Raman, self-modulation, and hose instabilities, are discussed. Recent experimental results are summarized

HIGH REPETITION RATE LASER-DRIVEN ELECTRON ACCELERATION TO MEGA-ELECTRON-VOLT ENERGIES

Laser-driven particle accelerators have the potential to be a compact and cost-effective replacement for conventional accelerators. Despite the significant achievements of laser wakefield acceleration in the last two decades, more work is required to improve the beam parameters such as the energy spread, emittance, repetition rate, and maximum achievable energy delivered by these advanced accelerators to values comparable to what conventional accelerators offer for various applications. The goal of this dissertation is to lower the threshold of the laser pulse energy required for driving a wakefield and in turn enable higher repetition rate particle acceleration with current laser technology. In the first set of electron acceleration experiments presented in this dissertation, we show that the use of a thin gas jet target with near critical plasma density lowers the critical power for relativistic self-focusing and leads to electron acceleration to the ~MeV range with ~1pC charge per shot, using only ~1mJ energy drive laser pulses delivered by a 1kHz repetition rate laser system. These electron beams accelerated in the self-modulated laser wakefield acceleration (SM-LWFA) regime have a thermal energy distribution and a rather large divergence angle (>150mrad). In order to improve the energy spread and the divergence, in the second set of the experiments, we employ a few-cycle laser pulse with a ~7fs duration and ~2.5mJ energy as the driver, to perform wakefield acceleration in the bubble regime using near critical plasma density targets. The results show a significant improvement in the energy spread and divergence of the beam. The electron bunches from this experiment have a quasi-monoenergetic peak at ~5MeV with an energy spread of ΔE/E≃0.4 and divergence angle of ~15mrad. These results bring us closer to the use of tabletop advanced accelerators for various scientific, medical, and industrial applications