Thesis (Ph. D.)--University of Washington, 1994. / Vita. Includes bibliographical references (leaves [141]-148).

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1991. / Vita. Abstract. Includes bibliographical references (leaves 186-203). Also available via the Internet

Thesis (Ph. D.)--University of Alberta, 2009. / Title from pdf file main screen (viewed on Jan. 5, 2010). "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics, Department of Physics, University of Alberta." Includes bibliographical references.

The acceleration of electrons in a laser produced plasma wave was studied experimentally. A plasma with a modulated density was produced by illuminating a grating with a ruby laser at an intensity of 10¹⁰ W/cm². The plasma expanding above the surface of the grating was diagnosed using interferometry, shadowgraphy and Raman-Nath scattering. The plasma density was found to be modulated with an amplitude of [formula omitted]/n=8% for grating spacings ranging from 6 to 35 µm. A CO₂ laser of intensity 7xlO¹¹ W/cm2 then irradiated this modulated plasma and generated plasma waves. The phase speeds of the plasma waves are v[formula omitted] = ±[formula omitted]k[formula omitted], where k[formula omitted] is the wavenumber of the grating and [formula omitted] is the frequency of the CO₂ laser. Electrons were injected at an energy of 25 keV in one of the plasma waves. In order for the phase speed of the wave to synchronize with the accelerating electrons, a grating with constantly increasing line spacing was used. No conclusive evidence of electron acceleration was obtained, even after the injection energy was increased to 92 keV. This lack of evidence was the result of a large electric field perpendicular to the surface of the grating, which deflected the electrons onto the grating. This detrimental electric field is produced when fast electrons are emitted by the plasma and leave it positively charged. At the low laser intensity used in this experiment, the origin of these electrons could not be identified. Some techniques to remedy this difficulty are proposed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis describes a detailed investigation into the process of laser-wakefield acceleration (LWFA) for the generation of high-energy electron beams using the hydrogen-filled capillary discharge waveguide. In only the second experiment to be performed using the newly commissioned Astra-Gemini laser at the Rutherford Appleton Laboratory, electron beams were accelerated to energies greater than 0.5 GeV by laser pulses of energy 2.5J and peak power of 30T\~T. The injec- tion and acceleration of electron beams was seen to depend on the state of the plasma channel for axial electron densities less than 2.5 x 1018 cm -3. With the aid of simulations performed using the code WAKE it was found that the plasma channel allows the laser pulse to maintain its self-focussed spot size along the length of the capillary even below the critical power for self-guiding. It was found that the threshold laser energy required for the production of elec- tron beams was reduced by the use of an aperture placed early in the laser system. This was attributed to the increased energy contained in the central part of the focal spot of the laser. A short paper on this work was published in Physical Review Special Topics - Accelerators and Beams and a longer paper was published in the New Journal of Physics. Transverse interferometry was used to measure the electron density of the plasma channel used in the Astra-Gemini experiments. An imaging system was devised which used cylindrical optics to increase the field of view of the capillary longitudinally, whilst maintaining the trans- verse resolution. The measured properties were consistent with previous measurements made by Gonsalves et al. [J]. The observed longitudinal variations in the plasma channel parameters were not found to be significant enough to affect the injection process.

Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 73-76).

Ph. D.

Coaxial plasma accelerators have been the subject of experimental and theoretical analysis since the 1950s. Theories have evolved that predict subsets of the measured data. This work separates coaxial plasma accelerator research into two broad categories classified by the ratio of accelerator discharge current to input gas flow rate. Devices that operate with this ratio above a particular threshold are called "starved" and the acceleration process is termed "“deflagration". Devices that operate below the threshold are called “over-fed" and the plasma undergoes a compressive energy conversion process termed "detonation". Over-fed (detonation) plasma accelerators add energy to the plasma through plasma heating and compression. The plasma exhaust velocity is limited to the magneto-sonic velocity which is nearly identical to the plasma Alfven velocity. Measured energy conversion efficiencies for detonation plasma accelerators have been typically less than 10%. Starved (deflagration) plasma accelerators add energy to the plasma by increasing the plasma kinetic energy. Thus, the plasma exhaust velocities measured in the deflagration accelerator exceed the plasma Alfven velocity by two orders of magnitude. Measured energy conversion efficiencies for the deflagration mode exceed 40%. Two additional sub-categories have been defined. The first is based on the number of acceleration stages. A single stage device processes neutral gas into the accelerated plasma. Multi-stage devices first ionize the neutral gas and then accelerate it to the final velocity. Finally, plasma accelerators with coaxial electrodes are classified by the interval in which the electrical energy is transformed into plasma energy. A new theory was developed to explain the deflagration plasma accelerator operation by examining the failures of previous magneto-hydro-dynamic based theories. The new theoretical treatment was used to develop a computer simulation of the deflagration plasma accelerator process. The theory and model were tested against experimental data for single and dual stage deflagration accelerator devices. With successful correlation achieved between the theory, computer model and experimental measurements, changes were made to the original accelerator, guided by modeling results. The new deflagration plasma accelerator was tested and the results closely matched the predictions for all key accelerator performance parameters. / Ph. D.

This thesis presents results from two strands of work towards realizing a free-electron laser (FEL) driven by electron bunches generated by a laser-wakefield accelerator (LWFA). The first strand focuses on selecting operating parameters for such a light source, on the basis of currently achievable bunch parameters as well as near-term projections. The viability of LWFA-driven incoherent undulator sources producing nanojoule-level pulses of femtosecond duration at wavelengths of 5 nm and 0.5 nm is demonstrated. A study on the prospective operation of an FEL at 32 nm is carried out, on the basis of scaling laws and full 3-D time-dependent simulations. A working point is selected, based on realistic bunch parameters. At that working point saturation is expected to occur within a length of 1.6 m with peak power at the 0.1 GW-level. This level, as well as the stability of the amplification process, can be improved significantly by seeding the FEL with an external radiation source. In the context of FEL seeding, we study the ability of conventional simulation codes to correctly handle seeds from high-harmonic generation (HHG) sources, which have a broad bandwidth and temporal structure on the attosecond scale. Namely, they violate the slowly-varying envelope approximation (SVEA) that underpins the governing equations in conventional codes. For this purpose we develop a 1-D simulation code that works outside the SVEA. We carry out a set of benchmarks that lead us to conclude that conventional codes are adequately capable of simulating seeding with broadband radiation, which is in line with an analytical treatment of the interaction. The second strand of work is experimental, and focuses on on the use of coherent transition radiation (CTR) as an electron bunch diagnostic. The thesis presents results from two experimental campaigns at the MPI für Quantenoptik in Garching, Germany. We present the first set of single-shot measurements of CTR over a continuous wavelength range from 420 nm to 7 μm. Data over such a broad spectral range allows for the first reconstruction of the longitudinal profiles of electron bunches from a laser-wakefield accelerator, indicating full-width at half-maximum bunch lengths around 1.4 μm (4.7 fs), corresponding to peak currents of several kiloampères. The bunch profiles are reconstructed through the application of phase reconstruction algorithms that were initially developed for studying x-ray diffraction data, and are adapted here for the first time to the analysis of CTR data. The measurements allow for an analysis of acceleration dynamics, and suggest that upon depletion of the driving laser the accelerated bunch can itself drive a wake in which electrons are injected. High levels of coherence at optical wavelengths indicate the presence of an interaction between the bunch and the driving laser pulse.

Electromagnetic acceleration has the potential for various applications stemming from space electric propulsion systems to future air breathing hypersonic augmentation.<p>Electromagnetic acceleration uses electromagnetic body force produced by the interactions of currents carried in plasma which is either externally applied or self-induced magnetic fields to accelerate the whole body of gas. Historically, these plasmas sources have been arc jets, shock tube and microwaves. Never has an electromagnetic accelerator been powered by an inductively coupled plasma (ICP) source.<p>The von Karman Institute has experimentally investigated the acceleration of an electrically conductive fluid produce by a subsonic ICP source. This ICP source was powered with a 15 kW and 27.1 MHz radio frequency facility called the Minitorch. The electromagnetic acceleration was accomplished with the design, fabrication and testing of a linear Hall current magnetofluiddynamic accelerator (MFDA) channel. The channel was geometrically orientated into the Hall configuration to accounts for the large Hall Effect. This channel used a single pair of copper annulus electrodes powered by a 10 kW direct current power supply. The channel was water cooled and contained various diagnostics to provide greater insight to the electromagnetic acceleration process. This was the first successful magnetofluiddynamic acceleration of an ICP source and validates the proof of concept.<p>One-dimensional MFD modeling was formulated and used to determine the necessary performance requirements of the MFDA channel E and B field subsystems. An interaction parameter of approximately 2.25 was required for the doubling of an inlet velocity of 300 m/sec. The required subsystem need to provide a current density was 6 Amps/cm2 with a magnetic field strength of 0.50 Tesla over an acceleration length of 0.1 meters. Additional the most critical constraint was the thermal management subsystem which was designed to overcome large heat transfer fluxes to achieve a steady state condition over a test run of 10 minutes.<p>The dynamic pressure measured increase the inlet velocity 101% for an argon plasma flowing at 1.01 g/s at a magnetic field strength of 0.49 Tesla. his strong acceleration of the plasma was most notable near the region of the electrodes at the exit of the 0.1 m long channel. The central region of the plasma has less dynamic pressure increase corresponding to only a maximum of 15% increase in velocity at a magnetic strength of 0.49 Tesla. Experimental results showed that axial discharge voltages increased with increased magnetic fields, indicating a strong Hall Effect in the accelerator as expected.<p>Theoretical analysis was accomplished using the one-dimensional equation of motion and was compared to utilizing only the momentum equation. Experimental force fluxes were compared to the calculated values of the one-dimensional equation of motion and momentum equation. The reference area for the current density was selected from intensity measurement using a high speed camera with the MFDA channel on. There was significant error in the analysis concerning using the momentum Lorentz force only versus the one-dimensional equations of motion; which included joule heating. This analysis summarized the necessity to include joule heating in the formulation of the problem. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished