A 250 GHz photonic band gap gyrotron amplifier

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 191-206).This thesis reports the theoretical and experimental investigation of a novel gyrotron traveling-wave-tube (TWT) amplifier at 250 GHz. The gyrotron amplifier designed and tested in this thesis has achieved a peak small signal gain of 38 dB at 247.7 GHz, with a 32 kV, 0.35 A electron beam and a 8.9 T magnetic field. The instantaneous -3 dB bandwidth of the amplifier at peak gain is 0.4 GHz. A peak output power of 45 W has been measured. The output power is not saturated but is limited by the 7.5 mW of available input power. The amplifier can be tuned for operation from 245- 256 GHz. With a gain of 24 dB and centered at 253.25 GHz the widest instantaneous -3 dB bandwidth of 4.5 GHz was observed for a 19 kV, 0.305 A electron beam. To achieve stable operation at these high frequencies, the amplifier uses a novel photonic band gap (PBG) interaction circuit. The PBG interaction circuit confines the TE₀₃-like mode which couples strongly to the electron beam. The PBG circuit provides stability from oscillations by supporting the propagation of TE modes in a narrow range of frequencies, allowing for the confinement of the operating TE₀₃-like mode while rejecting the excitation of oscillations at lower frequencies. Experimental results taken over a wide range of parameters, 15-30 kV and 0.25-0.5 A, show good agreement with a theoretical model. The theoretical model incorporates cold test measurements for the transmission line, input coupler, PBG waveguide and mode converter. This experiment achieved the highest frequency of operation (250 GHz) for a gyrotron amplifier. At present, there are no other amplifiers in this frequency range that are capable of producing either high gain or high-output power. With 38 dB of gain and 45 W this is also the highest gain observed above 94 GHz and the highest output power achieved above 140 GHz by any conventional-voltage vacuum electron device based amplifier. The output power, output beam pattern, instantaneous bandwidth, spectral purity and shot-to-shot stability of the amplified pulse meet the basic requirements for the implementation of this device on a pulsed dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectrometer.by Emilio A. Nanni.Ph.D

Application of Frustrated Total Internal Reflection of Millimeter Waves for Detection and Evaluation of Disbonds in Dielectric Joints

Millimeter waves penetrate inside of low loss dielectric materials and they are sensitive to the presence of internal interfaces and nonuniformities. This allows millimeter wave nondestructive inspection techniques to be utilized for inspecting dielectric composite structures. A disbond (a thin and extended airgap) in structures possessing adhesively bonded joints with complex geometries is commonly difficult to inspect. In this letter, we demonstrate the operational principle and the useful features of a millimeter wave technique, employing a frustrated total internal reflection of signals transmitted and received by dielectric waveguide probes for detecting and evaluating disbonds in such joints

A 250 GHz gyrotron with a 3 GHz tuning bandwidth for dynamic nuclear polarization

We describe the design and implementation of a novel tunable 250 GHz gyrotron oscillator with >10 W output power over most of a 3 GHz band and >35 W peak power. The tuning bandwidth and power are sufficient to generate a >1 MHz nutation frequency across the entire nitroxide EPR lineshape for cross effect DNP, as well as to excite solid effect transitions utilizing other radicals, without the need for sweeping the NMR magnetic field. Substantially improved tunability is achieved by implementing a long (23 mm) interaction cavity that can excite higher order axial modes by changing either the magnetic field of the gyrotron or the cathode potential. This interaction cavity excites the rotating TE[subscript 5,2,q] mode, and an internal mode converter outputs a high-quality microwave beam with >94% Gaussian content. The gyrotron was integrated into a DNP spectrometer, resulting in a measured DNP enhancement of 54 on the membrane protein bacteriorhodopsin.National Institutes of Health (U.S.) (Grant EB002804)National Institutes of Health (U.S.) (Grant EB003151)National Institutes of Health (U.S.) (Grant EB002026)National Institutes of Health (U.S.) (Grant EB001960)National Institutes of Health (U.S.) (Grant EB001035)National Institutes of Health (U.S.) (Grant EB001965)National Institutes of Health (U.S.) (Grant EB004866)National Science Foundation (U.S.). Graduate Research Fellowshi

Terahertz-driven linear electron acceleration

The cost, size and availability of electron accelerators is dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency (RF) accelerating structures operate with 30-50 MeV/m gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional RF structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators and suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here, we demonstrate the first linear acceleration of electrons with keV energy gain using optically-generated terahertz (THz) pulses. THz-driven accelerating structures enable high-gradient electron or proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. Increasing the operational frequency of accelerators into the THz band allows for greatly increased accelerating gradients due to reduced complications with respect to breakdown and pulsed heating. Electric fields in the GV/m range have been achieved in the THz frequency band using all optical methods. With recent advances in the generation of THz pulses via optical rectification of slightly sub-picosecond pulses, in particular improvements in conversion efficiency and multi-cycle pulses, increasing accelerating gradients by two orders of magnitude over conventional linear accelerators (LINACs) has become a possibility. These ultra-compact THz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, future linear particle colliders, ultra-fast electron diffraction, x-ray science, and medical therapy with x-rays and electron beams

AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven attosecond X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.United States. Air Force. Office of Scientific Research (Contract FA9550-12-1-0499)American Society for Engineering Education. National Defense Science and Engineering Graduate Fellowshi

Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe

We present a calculation of the microwave field distribution in a magic angle spinning (MAS) probe utilized in dynamic nuclear polarization (DNP) experiments. The microwave magnetic field (B[subscript 1S]) profile was obtained from simulations performed with the High Frequency Structure Simulator (HFSS) software suite, using a model that includes the launching antenna, the outer Kel-F stator housing coated with Ag, the RF coil, and the 4 mm diameter sapphire rotor containing the sample. The predicted average B[subscript 1S] field is 13 μT/W[superscript 1/2], where S denotes the electron spin. For a routinely achievable input power of 5 W the corresponding value is γ[subscript S]B[subscript 1S] = 0.84 MHz. The calculations provide insights into the coupling of the microwave power to the sample, including reflections from the RF coil and diffraction of the power transmitted through the coil. The variation of enhancement with rotor wall thickness was also successfully simulated. A second, simplified calculation was performed using a single pass model based on Gaussian beam propagation and Fresnel diffraction. This model provided additional physical insight and was in good agreement with the full HFSS simulation. These calculations indicate approaches to increasing the coupling of the microwave power to the sample, including the use of a converging lens and fine adjustment of the spacing of the windings of the RF coil. The present results should prove useful in optimizing the coupling of microwave power to the sample in future DNP experiments. Finally, the results of the simulation were used to predict the cross effect DNP enhancement (ϵ) vs. ω[subscript 1S]/(2π) for a sample of [superscript 13]C-urea dissolved in a 60:40 glycerol/water mixture containing the polarizing agent TOTAPOL; very good agreement was obtained between theory and experiment.National Institutes of Health (U.S.) (Grant EB002804)National Institutes of Health (U.S.) (Grant EB003151)National Institutes of Health (U.S.) (Grant EB002026)National Institutes of Health (U.S.) (Grant EB001960)National Institutes of Health (U.S.) (Grant EB001035)National Institutes of Health (U.S.) (Grant EB004866)National Science Foundation (U.S.). Graduate Research Fellowshi

Design of a two hundred fifty gigahertz gyrotron amplifier

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 105-109).A design is presented of a 250 GHz, 1 kW gyrotron traveling wave tube (gyro-TWT) amplifier with gain exceeding 50 dB. Calculations show that the amplifier will operate at 32 kV, 1 A with a saturated gain of 60 dB, an output power of 1 kW and a gain bandwidth of 3 GHz. The amplifier uses a novel photonic band gap (PBG) interaction circuit for stable single mode operation in an overmoded circuit. The design mode is a TE03-like mode confined by the PBG lattice with no nearby competing modes. The design of the input coupler, interaction circuit, output coupler and electron gun have also been completed. The amplifier design accounts for requirements imposed by the device's intended application of pulsed nuclear magnetic resonance spectroscopy with pulses as short as several hundred picoseconds. This design will be implemented at a later date for the construction of the highest frequency, kW power level amplifier based on vacuum electronics. To gain understanding of short pulse amplification in vacuum electron devices, an experimental study of sub-nanosecond pulse amplification in a gyro-TWT has been carried out on an existing experimental setup at 140 GHz. The gyro-TWT operates with 30 dB of small signal gain in the HE06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz. Experimental results taken over a wide range of parameters, with pulses as short as 400 ps, show good agreement with a theoretical model in the small signal gain regime.by Emilio A. Nanni.S.M

A Sustainability Roadmap for C3^3

The particle physics community has agreed that an electron-positron collider is the next step for continued progress in this field, giving a unique opportunity for a detailed study of the Higgs boson. Several proposals are current under evaluation of the international community. Any large particle accelerator will be an energy consumer and so, today, we must be concerned about its impact on the environment. This paper evaluates the carbon impact of the construction and operations of one of these Higgs factory proposals, the Cool Copper Collider (C$^{3}$). It introduces several strategies to lower the carbon impact of the accelerator. It proposes a metric to compare the carbon costs of Higgs factories, balancing physics reach, energy needs, and carbon footprint for both construction and operations, and compares the various Higgs factory proposals within this framework. For C$^{3}$, the compact 8 km footprint and the possibility for cut-and-cover construction greatly reduce the dominant contribution from embodied carbon.Comment: 20 Pages, 6 Figure

THz Dynamic Nuclear Polarization NMR

Dynamic nuclear polarization (DNP) increases the sensitivity of nuclear magnetic resonance (NMR) spectroscopy by using high frequency microwaves to transfer the polarization of the electrons to the nuclear spins. The enhancement in NMR sensitivity can amount to a factor of well above 100, enabling faster data acquisition and greatly improved NMR measurements. With the increasing magnetic fields (up to 23 T) used in NMR research, the required frequency for DNP falls into the THz band (140- 600 GHz ). Gyrotrons have been developed to meet the demanding specifications for DNP NMR, including power levels of tens of watts; frequency stability of a few megahertz; and power stability of 1% over runs that last for several days to weeks. Continuous gyrotron frequency tuning of over 1 GHz has also been demonstrated. The complete DNP NMR system must include a low loss transmission line; an optimized antenna; and a holder for efficient coupling of the THz radiation to the sample. This paper describes the DNP NMR process and illustrates the THz systems needed for this demanding spectroscopic application. THz DNP NMR is a rapidly developing, exciting area of THz science and technology.National Science Foundation (U.S.) (Graduate Research Fellowship)National Institutes of Health (U.S.) (grant EB001965)National Institutes of Health (U.S.) (grant EB004866)National Institutes of Health (U.S.) (grant EB001960)National Institutes of Health (U.S.) (grant EB002804)National Institutes of Health (U.S.) (grant EB003151)National Institutes of Health (U.S.) (grant EB002026

Near-Field Microwave Detection of Disbond in Carbon Fiber Reinforced Polymer Composites used for Strengthening Cement-Based Structures and Disbond Repair Verification

The use of carbon fiber reinforced polymer (CFRP) composites has been investigated in the past few years as a means to rehabilitate aging concrete structures. The detection of disbonds and delaminations between CFRP laminate and the concrete substrate is a critical issue in this area, as the presence of such defects adversely affects the effectiveness of the reinforcement. Thus, the development of a nondestructive testing (NDT) technique to detect these types of defects is of great interest in this field. Furthermore, any such method should also be capable of verifying the quality of the repair of such defects. Near-field microwave NDT techniques, utilizing open-ended rectangular waveguides, have shown great potential in the past for detecting delaminations and disbonds in complex composite structures. Microwave NDT techniques have the added advantages of providing real-time, noncontact, one-sided, rapid, and robust measurements. This paper presents the experimental results of detecting disbonds in a specially prepared CFRP reinforced mortar sample using near-field microwave NDT techniques, as well as the preliminary results showing the capability of this technique for verifying repair quality