Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / Includes bibliographical references (p. [155]-159). / The advantages of elliptic (or sheet) beams have been known for many years, but their inherent three-dimensional nature presents significant theoretical, design, and experimental challenges in the development of elliptic beam systems. The present work provides a framework for the design of elliptic cross-section charged-particle beam formation and transport systems. An effective mathematical formalism for describing accelerating elliptic cross-section beams is developed in which the particle distribution function for an elliptic beam is associated with a hyperellipsoid in phase space, and the evolution equations for the particle distribution hyperellipsoid are obtained. A novel methodology is presented for the design of elliptic beam-forming diodes utilizing an analytic prescription for the surfaces of three-dimensional electrodes which generate, accelerate, and confine a highly laminar elliptic beam. Three-dimensional simulations and tolerance studies are performed, confirming the theoretical predictions that a near-ideal beam can be produced. Focusing systems are described for elliptic beams in coasting, accelerating, and compressing regions with analytic prescriptions for the applied electric and magnetic fields required to maintain a laminar flow profile for particles within the beam. Numerical phase-space evolution and 3D simulations confirm that self-consistent laminar flow profiles are maintained by the theoretically-designed applied fields. The traditional approach to charged-particle dynamics problems involves extensive numerical optimization over the space of initial and boundary conditions in order to obtain desired charged-particle trajectories. The approach taken in the present work is to obtain analytic inverses wherever possible in order to minimize any necessary numerical optimization. Desired trajectories are assumed, and the applied fields and electrode geometries are then determined in a manner consistent with the assumed trajectories. / by Ronak Jayant Bhatt. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references (pages 463-490). / 21 cm cosmology, the statistical observation of the high redshift universe using the hyperfine transition of neutral hydrogen, has the potential to revolutionize our understanding of cosmology and the astrophysical processes that underlie the formation of the first stars, galaxies, and black holes during the "Cosmic Dawn." By making tomographic maps with low frequency radio interferometers, we can study the evolution of the 21 cm signal with time and spatial scale and use it to understand the density, temperature, and ionization evolution of the intergalactic medium over this dramatic period in the history of the universe. For my Ph.D. thesis, I explore a number of advancements toward detecting and characterizing the 21 cm signal from the Cosmic Dawn, especially during its final stage, the epoch of reionization. In seven different previously published or currently submitted papers, I explore new techniques for the statistical analysis of interferometric measurements, apply them to data from current generation telescopes like the Murchison Widefield Array, and look forward to what we might measure with the next generation of 21 cm observatories. I focus in particular on estimating the power spectrum of 21 cm brightness temperature fluctuations in the presence enormous astrophysical foregrounds and how those measurements may constrain the physics of the Cosmic Dawn. / by Joshua Shane Dillon. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2006. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / In title on t.p., "[gamma]" appears as the lower-case Greek letter; "[right arrow]" appears as the symbol; and, "[Delta]" appears as the upper-case Greek letter. Vita. / Includes bibliographical references (p. 213-220). / The first excited state of the proton, the Delta, can be reached through a magnetic dipole spin flip of one of the quarks (M1) or through electric and Coulomb quadrupole terms (E2 and C2) which indicate a deviation from spherical symmetry. The quark models using the color hyperfine interaction underestimate the size of the quadrupole terms by more than an order of magnitude. Models using the pion cloud do a much better job of describing the data. This is expected due to the spontaneous breaking of chiral symmetry which leads to a cloud of virtual p wave pions which introduce the non-spherical amplitudes. The data presented in this work fill gaps in the low Q2, long distance region where the pion cloud is expected to dominate and to produce significant Q2 variation. The p(e, e'p)7r0 reaction was measured in the A region at Q2 = 0.060 (GeV/c)2, the lowest Q2 to date for pion electroproduction, utilizing out-of-plane magnetic spectrometers at the Mainz Microtron in Germany. / (cont.) This work reports results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios obtained from fitting the new data with models using a three parameter, resonant multipole fit: M3/2 = (40.33 i 0.63stat+syst ± 0.61model) (10-3/m,r+), E2/M1 = Re(E3/2/M3/2) = (-2.28 i 0.29stat+syst ± 0.20modeI)%, and C2/M1 = Re(S3/M+3/2) = (-4.81 ± 0.27stat+syst i 0.26model)%. These new results for the transition multi-poles disagree with predictions of the quark models but are in reasonable agreement with a chiral extrapolation of lattice QCD, chiral effective field theory and dynamical model results confirming the dominance and general Q2 variation of the long range pionic contribution. While there is qualitative agreement with the models, there is no quantitative agreement thus indicating the need for further improvement of the models. / by Sean C. Stave. / Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (page 43). / Hot Jupiters raise strong tides in their host stars due to their large masses (around a Jupiter mass) and tight orbits (orbital periods P </~ 3 days). These tides remove orbital energy and angular momentum, causing the planet's orbit to decay. The rate of decay depends on the detailed structure of the host star through the coefficients of nonlinear tidal coupling. Here we calculate these coefficients over a range of stellar mass and age on the main sequence (0.8 </= M/M < 1.2). These coefficients provide crucial input for future studies of nonlinear tidal dissipation; our analysis enables these studies to be extended to systems with non-solar-type hosts. / by Brent Belland. / S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (leaf 79). / In the summer of 2004, the BEC 2 lab of Wolfgang Ketterle's group at MIT started a new research direction of studying degenerate fermionic Lithium atoms in optical lattices. The major contributions to the new experimental setup are the Lithium laser system, a new imaging technique, and an advanced experiment control system. First, a tapered amplifier laser diode system is discussed. The laser is locked using an error signal generated using frequency modulation of saturated absorption spectroscopy of Lithium Vapor. The laser is then locked using a PID controller. Second, a new imaging system is developed. The imaging system is designed to eliminate noise generated by the mechanical vibrations of the machine. The system relies on taking multiple images with time scale shorter than typical vibration periods. Third, a new experiment control system is developed. The new control system replaced the outdated 8 years old control system by providing us with more channels and speed, combined with an automation feature. The new system is designed to accommodate more complex experiments in the future. / by Widagdo Setiawan. / S.B.

Massachusetts Institute of Technology. Dept. of Physics. Thesis. 1965. Ph.D. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 1997. / Includes bibliographical references (p. 125-129). / by Takuji Kimura. / Ph.D.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2005. / Includes bibliographical references (p. 85-87). / The gas electron multiplier (GEM) is a novel charge amplification element for use in gaseous particle detectors. Because of their high gain ([approx.] 10⁵ when cascaded) and ability to withstand harsh radiation conditions, GEMs have been selected for use in the time projection chambers of the latest high-energy experiments, including COMPASS, STAR, and the planned ILC. Foil quality has been found to be critical for optimal GEM performance. If an irregularity in just one of the roughly 600, 000 amplification holes of a 10 x 10 cm GEM induces a spark discharge, the entire GEM is useless. Computer simulations have also shown that amplification behavior is strongly dependent on hole diameter and shape. To improve foil quality control, an automated scanner has been constructed to optically examine every hole in a GEM. The machine can measure each hole's outer copper diameter and inner Kapton diameter and locate and geometrically classify any foil imperfections. The scanner has been used to characterize GEMs recently manufactured by Tech-Etch. These scans indicate a general absence of etching defects, but measured diameter inhomogeneity may result in gain variations as large as i20% over the active foil area. / by Brian Paul Tamm. / S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007. / Includes bibliographical references (p. 185-193). / High power, high frequency gyrotrons used in plasma heating must achieve the highest possible efficiency in order to reduce system size and cost and to minimize thermal and mechanical problems. This thesis presents an experimental study of efficiency enhancement in a 1.5 MW, 110 GHz gyrotron oscillator, which operated at 50 % efficiency with a single-stage depressed collector. We present the design and detailed experimental results of a new low ohmic loss cavity, a four-mirror internal mode converter and a single-stage depressed collector. The low ohmic loss TE22,6 cavity, designated "V-2005", was designed using the code MAGY to have a Q factor of 830, which would be suitable for CW operation in an industrial gyrotron. The cavity was first tested in the axial configuration, in which the output waveguide also serves as the electron beam collector. In 3 microsecond pulsed operation at 97 kV and 40 A, an output power of 1.67 MW at an efficiency of 42 % was obtained without a depressed collector. The V-2005 cavity efficiency exceeds that of the older "V-2003" cavity by 5 percentage points. The enhanced efficiency of the V-2005 cavity may be understood by analyzing the start-up scenario of the cavities. During start-up, the V-2003 cavity suffers from strong mode competition with the TE19,7 mode, resulting in a relatively low efficiency, while the V-2005 cavity has an absence of such mode competition. The experimental mode maps (regions of oscillation vs. magnetic field) obtained for the two cavities are in excellent agreement with the start-up simulations. Following the axial configuration experiments, the experiment was rebuilt with an internal mode converter consisting of a launcher and 4 mirrors, and with a single-stage depressed collector. An output power of 1.5 MW was measured. The internal mode converter operated at 90 % efficiency. When the depressed collector was run at 25 kV, an overall efficiency of 50 % was achieved. An aftercavity interaction (ACI) was investigated as a possible cause of efficiency reduction in the gyrotron. / (cont.) The ACI occurs when the spent electron beam interacts at cyclotron resonance with the traveling output microwave beam in a region of lower magnetic field just after the cavity. The presence of the ACI was identified from the comparison between simulation results and the measurement of the depression voltage as a function of beam current. Future research should consider ways of eliminating the ACI, ways of improving the internal mode converter, and the use of a two-stage depressed collector. / by Eunmi Choi. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2014. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / In this thesis I study the generation of density perturbations in two classes of inflationary models: hybrid inflation and multifield inflation with non-minimal coupling to gravity. In the case of hybrid inflation, we developed a new method of treating these perturbations that does not rely on a classical trajectory for the fields. A characteristic of the spectrum is the appearance of a spike at small length scales, which could conceivably seed the formation of black holes that can evolve to become the supermassive black holes found at the centers of galaxies. Apart from numerically calculating the resulting spectrum, we derived an expansion in the number of waterfall fields, which makes the calculation easier and more intuitive. In the case of multifield inflation, we studied models where the scalar fields are coupled non-minimally to gravity. We developed a covariant formalism and examined the prediction for non-Gaussianities in these models, arguing that they are absent except in the case of fine-tuned initial conditions. We have also applied our formalism to Higgs inflation and found that multifield effects are too small to be observable. We compared these models to the early data of the Planck satellite mission, finding excellent agreement for the spectral index and tensor to scalar ratio and promising agreement for the existence of iso-curvature modes. / by Evangelos I. Sfakianakis. / Ph. D.