We propose to use photonic band gap (PBG) structures for constructing traveling wave tubes (TWTs) at 100 GHz, a completely novel approach. Using a PBG fiber allows us to create an all-dielectric slow-wave structure with very large band width and low losses in the mm-wave regime, compared to TWTs made out of metals. Additional capabilities such as mode selectivity are also achievable. We designed two 100 GHz pencil beam PBG TWTs using Ansoft's HFSS, 3D electromagnetic simulation software for high frequency applications. The first design is a periodic array of vacuum rods in a dielectric matrix, with a smaller vacuum rod forming the line defect. A fiber drawing procedure is being utilized to construct this design out of fused silica. The second structure is a periodic array of dielectric rods in a vacuum matrix, surrounding a thick hollow dielectric tube that accommodates the electron beam. This model is being fabricated out of silicon by means of high-pressure laser chemical vapor deposition (HP-LCVD), a versatile approach to synthesize fibers from the vapor phase. Additionally, a scaled 10 GHz cold test made from alumina rods is being produced for design confirmation purposes, and a 100 GHz sheet beam PBG TWT is being investigated for even greater power generation.
Development work is underway on a high current, mildly relativistic, L-band klystron source. The intense beam used in this tube presents unique design complications for the output circuit. Various considerations are discussed such as the trade-offs between beam potential and kinetic energies, optimum shunt impedances, and advantages of discrete versus distributed circuits. Three output circuit designs are presented. A description of the experimental hardware is given along with power extraction results from two output circuits.
KEYWORDS: Modulation, Microwave radiation, Medium wave, L band, 3D modeling, Nose, Photonic integrated circuits, Electron beams, Signal detection, Amplifiers
Work is continuing on a high-current relativistic klystron amplifier (RKA) with the goal of producing 1 kJ per pulse with a 1 microsecond(s) pulsewidth and a peak power of 1 GW. The three cavity tube has already produced pulses with more than 150 J and over 450 MW peak power. The original output cavity was thought to be limiting the performance, and a new cavity has been designed, built, and is now on-line and being conditioned up towards high power. Current experimental results are presented.
Recent experimental results, supporting simulations, and design modeling are presented from a developmental effort to produce a long pulse (approximately 1 microsecond(s) ) J-band (5.85 - 8.2 GHz) relativistic klystron amplifier (RKA) of the high current NRL genealogy. This RKA is designed to operate at approximately 6.6 GHz, with a desired rf output approximately 700 MW. Conversion of electron beam energy to microwave energy is obtained by a mock magnetically insulated coaxial converter which, in various incarnations, can be made to be either a cavity gap extractor or an inverse cathode.
KEYWORDS: Signal detection, Modulation, Medium wave, Microwave radiation, L band, Photonic integrated circuits, Iris, 3D modeling, Picosecond phenomena, Amplifiers
A high-current relativistic klystron amplifier (RKA) is being developed with the goal of producing 1 kJ per pulse with a 1 microsecond(s) pulsewidth and a peak power of 1 GW. The three cavity tube is fully assembled and is undergoing high power testing. Peak power levels as high as 400 MW have been produced so far. Current experimental results are presented.
Experimental results to-date will be presented from a developmental effort to a produce a J- band (5.85 - 8.2 GHz) relativistic klystron amplifier (RKA) of the high current Naval Research Laboratory (NRL) genealogy. The nominal experimental parameters of this RKA are: V0 approximately equals 600 kV; I0 approximately equals 2 - 4 kA; Bz approximately equals 1.5 T; (tau) beam approximately equals 300 ns; vin approximately equals 6.6 GHz; Pin <EQ 500 kW. Because of the smaller component sizes which accompany this frequency ((lambda) approximately equals 4.5 cm as compared with (lambda) approximately equals 30 cm for the bulk of other RKA research efforts), much of the discussion will concentrate on the physical principles, fabrication issues, and experimental pitfalls associated with scaling the RKA design.
We discuss basic Relativistic Klystron Amplifier physics. We show that in the intense space- charge regime the maximum power extraction does not coincide with the maximum harmonic bunching. In addition, we show that as the beam is bunched, the additional power stored in the Coulomb fields does not add significantly to the overall power extraction. Because of these effects, the power extraction at 1.3 GHz for a 500 kV, 5 kA beam with reasonable beam-to- wall spacing is limited to around 35%.
The goal of this research effort is to develop a long-pulse relativistic klystron amplifier (RKA) by extending the pulse length of this gigawatt-class device by an order of magnitude beyond the current state-of-the-art (100 ns) to one microsecond. A research approach is described for obtaining kilojoule microwave pulses at 1.3 GHz. Achieving kilojoule microwave pulses requires extending the electron beam pulse duration beyond one microsecond without diode closure, and maximizing the microwave extraction efficiency at the fundamental frequency. Our earliest experiments have produced a modulated electron beam for one microsecond with a peak rf current of 0.9 kA and a voltage of 350 - 400 kV. In some cases we have observed beam modulation in excess of 2 microseconds. The component of beam power at the microwave drive frequency (1.3 GHz) was approximately 350 MW. Although only a small effort has been put forth to address the output coupling issues, approximately 50 - 70 MW was coupled into dominant mode rectangular waveguide. Recently, an electron beam diode has been tested that delivers peak currents in excess of 5 kA for a monotonically increasing current pulse exceeding durations of 1 microsecond(s) at beam kinetic energies above 500 keV. to achieve this result close attention was given to minimizing the current losses from the diode and maximizing the beam current transmission through the RKA.
Brian Newnam, Roger Warren, Steven Conradson, John Goldstein, Brian McVey, Mark Schmitt, C. James Elliott, Michael Burns, Bruce Carlsten, Kwok-Chi Chan, W. Johnson, Tai-San Wang, Richard Sheffield, Karl Meier, R. Olsher, Marion Scott, J. Griggs
Los Alamos has designed and proposes to establish an XUV-IR free-electron laser (FEL) user facility for scientific research and industrial applications based on coherent radiation ranging from soft x rays as short as 1 nm to far-infrared wavelengths as long as 100 micrometers . As the next-generation light source beyond low-emittance storage rings with undulator insertion devices, this proposed national FEL user facility should make available to researchers broadly tunable, picosecond-pulse, coherent radiation with 104 to 107 greater spectral flux and brightness. The facility design is based on two series of FEL oscillators including one regenerative amplifier. The primary series of seven FEL oscillators, driven by a single, 1-GeV rf linac, spans the short-wavelength range from 1 to 600 nm. A second 60-MeV rf linac, synchronized with the first, drives a series of three Vis/IR FEL oscillators to cover the 0.5 to 100-micrometers range. This paper presents the motivation for such a facility arising from its inherently high power per unit bandwidth and its potential use for an array of scientific and industrial applications, describes the facility design, output parameters, and user laboratories, makes comparisons with synchrotron radiation sources, and summarizes recent technical progress that supports the technical feasibility.
Lester Thode, Bruce Carlsten, Kwok-Chi Chan, Richard Cooper, C. James Elliott, Steven Gitomer, John Goldstein, Michael Jones, Brian McVey, Mark Schmitt, H. Takeda, Robert Tokar, Tai-San Wang, Lloyd Young
KEYWORDS: Free electron lasers, Electron beams, Magnetism, Oscillators, Diagnostics, Physics, Resonators, 3D modeling, Particles, Algorithm development
Even at the conceptual level, the strong coupling between the laser subsystem elements, such as the accelerator, wiggler, optics, and control, greatly complicates the understanding and design of a free-electron laser (FEL). Given the requirements for a high-performance FEL, the coupling between the laser subsystems must be included in the design approach. To address the subsystem coupling, we implemented the concept of an integrated numerical experiment (INEX). Unique features of the INEX approach are consistency and numerical equivalence of experimental diagnostics. The equivalent numerical diagnostics mitigate the major problem of misinterpretation that often occurs when theoretical and experimental data are compared. A complete INEX model has been applied to the 10-micrometers high-extraction-efficiency experiment at Los Alamos and the 0.6-micrometers Burst Mode experiment at Boeing Aerospace. In addition, the agreement between INEX and the experiments is very good. With the INEX approach, it now appears possible to design high-performance FELs for numerous applications. The first full-scale test of the INEX approach is the Los Alamos High-Brightness Accelerator FEL (HIBAF) experiment. Implementation and experimental validation of the INEX concept are discussed.
The 10-pm Los Alamos free-electron laser (FEL) facility is being upgraded. The conventional electron gun and bunchers have been replaced with a much more compact 6-MeV photoinjector accelerator. By adding existing parts from previous experiments, the primary beam energy will be doubled to 40 MeV. With the existing 1-mS wiggler (Xv, 2.7 cm) and resonator, the facility can produce photons with wavelengths from 3 to 10 im when lasing on the fundamental mode and produce photons in the visible spectrum with short period wigglers or harmonic operation. After installation of a 150° bend, a second wiggler will be added as an amplifier. The installation of laser transport tubes between the accelerator vault and an upstairs laboratory will provide experimenters with a radiation free environment for experiments. At the time of writing (Jan. 1990), the injector plus one additional tank has been installed and tested with beam to an energy of 17 MeV.
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