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ABSTRACT: A novel active negative index metamaterial that derives its gain from an
electron beam is intro- duced. The metamaterial consists of a stack of
equidistant parallel metal plates perforated by a periodic array of holes
shaped as complementary split-ring resonators. It is shown that this structure
supports a negative-index transverse magnetic electromagnetic mode that can
resonantly interact with a relativistic electron beam. Such metamaterial can be
used as a coherent radiation source or a particle accelerator.
04/2012;
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ABSTRACT: Corrugated waveguide transmission lines are in use to transmit high power mm-wave radiation from gyrotrons to the plasma for
electron cyclotron plasma heating in tokamaks such as ITER. The coupling efficiency of the gyrotron output radiation formed
as a quasi-Gaussian beam to the waveguide mode is a critical issue. A hyperbolic corrugated horn serves as a converter of
the TEM00 Gaussian mode to the HE11 mode of a corrugated waveguide. We report the design of a hyperbolic horn for application in the ITER transmission line at
170GHz. The theoretical conversion efficiency of the horn is higher than 0.995.
KeywordsElectron cyclotron plasma heating–Gyrotron–Corrugated waveguide–Transmission line
Journal of infrared, millimeter and terahertz waves 04/2012; 32(3):283-294. · 0.74 Impact Factor
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ABSTRACT: In this communication, we report enhancements of nuclear spin polarization by dynamic nuclear polarization (DNP) in static and spinning solids at a magnetic field strength of 9T (250GHz for g=2 electrons, 380MHz for (1)H). In these experiments, (1)H enhancements of up to 170±50 have been observed in 1-(13)C-glycine dispersed in a 60:40 glycerol/water matrix at temperatures of 20K; in addition, we have observed significant enhancements in (15)N spectra of unoriented pf1-bacteriophage. Finally, enhancements of ∼17 have been obtained in two-dimensional (13)C-(13)C chemical shift correlation spectra of the amino acid U-(13)C, (15)N-proline during magic angle spinning (MAS), demonstrating the stability of the DNP experiment for sustained acquisition and for quantitative experiments incorporating dipolar recoupling. In all cases, we have exploited the thermal mixing DNP mechanism with the nitroxide radical 4-amino-TEMPO as the paramagnetic dopant. These are the highest frequency DNP experiments performed to date and indicate that significant signal enhancements can be realized using the thermal mixing mechanism even at elevated magnetic fields. In large measure, this is due to the high microwave power output of the 250GHz gyrotron oscillator used in these experiments.
Journal of Magnetic Resonance 12/2011; 213(2):404-9. · 2.14 Impact Factor
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ABSTRACT: 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.
Terahertz Science and Technology, IEEE Transactions on. 10/2011;
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ABSTRACT: Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from <; 0.1 to >; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.
Terahertz Science and Technology, IEEE Transactions on. 10/2011;
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ABSTRACT: The design and the operation of a frequency-tunable continuous-wave (CW) 330-GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency are reported. The gyrotron has generated 18 W of power from a 10.1-kV 190-mA electron beam working in a TE<sub>-4,3</sub> cylindrical mode, corresponding to an efficiency of 0.9%. The measured start oscillation current over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE<sub>-4,3,q</sub>, where q = 1-6. Moreover, the observed frequency range in the start current measurement is in reasonable agreement with the frequency range obtained from numerical simulations. The minimum start current was measured to be 33 mA. A continuous tuning range of 1.2 GHz was experimentally observed via a combination of magnetic, voltage, and thermal tuning. The gyrotron output power and frequency stabilities were assessed to be ±0.4% and ±3 ppm, respectively, during a 110-h uninterrupted CW run. Evaluation of the gyrotron output microwave beam pattern using a pyroelectric camera indicated a Gaussian-like mode content of 92% with an ellipticity of 28%. The gyrotron will be used for 500-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization.
IEEE Transactions on Electron Devices 09/2011; · 2.32 Impact Factor
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ABSTRACT: A set of constraints is derived for the operating characteristics of tokamak power reactors which are bulk-heated by electron cyclotron resonance heating (ECRH). Four heating modes are considered: ordinary wave heating at the electron cyclotron frequency, Ω, and at the second-harmonic frequency, 2Ω, and extraordinary-wave heating at Ω and at 2Ω. For ordinary-wave heating at Ω, which appears to be the most promising method, the wave frequency ω ≈ Ω must exceed the plasma frequency, ωp, for wave penetration into the plasma. The authors' main conclusion is that the need for high-density operation (n0 > 4 × 1020m−3) in moderate-size tokamak reactors, coupled with the wave accessibility condition Ω > ωp, leads to the requirement of frequencies in the 200-GHz range for ECRH of reactor plasmas. A further condition on the heating frequency may be derived by consideration of the ignition condition using empirical scaling laws for the energy confinement time. This latter condition does not increase the heating frequency requirement unless impurities are present or the energy confinement degrades with increasing temperature. It is also found that, for ordinary-wave heating at Ω, the average plasma β is limited to less than 0.039 for a central temperature below 15 keV, assuming parabolic density and temperature profiles. The use of extraordinary heating at Ω might lower the frequency requirement for ECRH of a reactor. However, it appears to be unattractive for reactor operation because, in order for the wave to penetrate to the centre of the plasma, the heating ports must be located on the inside of the torus. High-beta, lower field reactors can be heated from the outboard side of the torus with second-harmonic radiation. However, these devices will have to be heated at frequencies which are as high as or higher than those needed for devices which are heated with the ordinary wave at the fundamental frequency. Extrapolation of cyclotron resonance maser (gyrotron) technology to provide modular heating systems in the 200-GHz range is discussed.
Nuclear Fusion 01/2011; 19(3):389. · 4.09 Impact Factor
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ABSTRACT: A linearly polarized (LP<sub>mn</sub>) mode basis set for oversized, corrugated, metallic waveguides is derived for the special case of quarter-wavelength-depth circumferential corrugations. The relationship between the LP<sub>mn</sub> modes and the conventional modes (HE<sub>mn</sub>, EH<sub>mn</sub>, TE<sub>0n</sub>, TM<sub>0n</sub>) of the corrugated guide is shown. The loss in a gap or equivalent miter bend in the waveguide is calculated for single-mode and multimode propagation on the line. In the latter case, it is shown that modes of the same symmetry interfere with one another, causing enhanced or reduced loss, depending on the relative phase of the modes. If two modes with azimuthal (m) indexes that differ by one propagate in the waveguide, the resultant centroid and the tilt angle of radiation at the guide end are shown to be related through a constant of the motion. These results describe the propagation of high-power linearly polarized radiation in overmoded corrugated waveguides.
IEEE Transactions on Microwave Theory and Techniques 12/2010; · 1.85 Impact Factor
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ABSTRACT: We report amplification of pulses as short as 400 ps in a 1 kW, 140 GHz gyro-TWT amplifier. To our knowledge, this is the first observation of picosecond pulse amplification in a gyro-amplifier. The pulses show measurable broadening due to two distinct phenomena: group velocity dispersion and spectral narrowing due to the finite gain bandwidth. Theoretical analysis is in good agreement with the observed broadening of the pulses. Picosecond pulses are useful for applications in spectroscopy and advanced radar.
Infrared Millimeter and Terahertz Waves (IRMMW-THz), 2010 35th International Conference on; 10/2010
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ABSTRACT: An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro-TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz 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 show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.
Physical Review Letters 09/2010; 105(13):135101. · 7.37 Impact Factor
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ABSTRACT: The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE<sub>11, 2</sub> and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE<sub>11, 2,</sub> q . The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ± 6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%.
IEEE Transactions on Plasma Science 07/2010; · 1.17 Impact Factor
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ABSTRACT: We present experimental results on the operation of a 1.5 MW, 110 GHz, 3 μs pulsed gyrotron with an advanced internal mode converter (IMC). We have designed and installed an IMC that uses smooth curved mirrors rather than mirrors with phase correcting surfaces which are more sensitive to alignment. Hot and cold test experiments of the new IMC show over 95% Gaussian mode content in the output beam. We also explore the after cavity interaction (ACI) and study its effect on reducing the gyrotron's overall efficiency. Results show that the elimination of the ACI could yield as much as an 11% increase in efficiency.
Vacuum Electronics Conference (IVEC), 2010 IEEE International; 06/2010
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ABSTRACT: We report the operation of a frequency tunable continuous-wave (CW) 330 GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency. The gyrotron has generated 18 W of power from a 10.1-kV 190-mA electron beam working in a backward-wave TE<sub>-4,3</sub> cylindrical mode. The minimum start oscillation current of the device was measured to be 33 mA. A continuous tuning range of 1.2 GHz was achieved experimentally via a combination of magnetic, voltage, and thermal tuning. The gyrotron output power and frequency stabilities were assessed to be ±0.4% and ±003 ppm, respectively, during a 110-hour uninterrupted CW run. Evaluation of the gyrotron microwave output beam using a pyroelectric camera indicated a Gaussian-like mode content of 92% with an ellipticity of 28%. The gyrotron will be used as a source for 500 MHz nuclear magnetic resonance (NMR) experiments with sensitivity enhanced by dynamic nuclear polarization (DNP).
Vacuum Electronics Conference (IVEC), 2010 IEEE International; 06/2010
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ABSTRACT: We report the first experimental studies of picosecond pulse amplification and broadening in a gyrotron amplifier. These studies allow investigation of the amplification process in a new physical regime, where wave dispersion and finite gain bandwidth play a major role. The experiments were performed on a 140 GHz gyro-traveling wave tube (Gyro-TWT) operating in a higher order mode of a confocal waveguide. We observed significant pulse broadening for pulses shorter than 1 ns when the operating point of the amplifier was close to the waveguide cut-off. For example, a 580 ps pulse at 137.30 GHz, a frequency near cutoff, was measured to be broadened by 100 %, while pulses at higher frequency, further from cutoff, showed much smaller broadening. The results of these experimental studies show that for the picosecond pulse amplification necessary in applications such as Electron Paramagnetic Resonance (EPR), the operating point of the gyro-TWT should be chosen to be at least a few percent above the cut-off to avoid pulse distortion.
Vacuum Electronics Conference (IVEC), 2010 IEEE International; 06/2010
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ABSTRACT: The fields radiated from the waveguide were used to analyze propagating mode content in an oversized circular corrugated waveguide. The radiated fields were calculated by the Kirchhoff integral in the paraxial approximation. The irradiant waveguide modes of the radiated field are orthogonal within the paraxial approximation. The complex fractions in the irradiant mode expansion were deduced from an iteration process in which the reconstructed amplitude profile from the complex fractions coincided with the measured amplitude profile.
Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009
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ABSTRACT: The design and experimental study of a 140 GHz, 1 kW gyro-traveling wave tube (Gyro-TWT) operating in the HE<sub>06</sub> mode of a confocal waveguide is presented. A combination of quasioptical gain sections with dielectric loaded severs enables high gain operation with mode selectivity. A nanosecond-scale 120 mW pulse driver at 140 GHz was demonstrated. This input driver pulse will be amplified by the Gyro-TWT and the resulting output high-power short pulse will be transmitted to an EPR spectrometer probe. A quasioptical mode converter utilized to transform the higher order operating confocal mode into a Gaussian-like beam was designed and cold tested. The simulated and measured Gaussian beam patterns of the output mode converter are in good agreement.
Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009
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ABSTRACT: We report experimental testing of an active real-time imaging system useful for many practical applications, such as fast security check, food safety inspection, etc. The system consists of a 460-GHz gyrotron capable of producing 16 W in continuous wave operation and a pyroelectric array camera with 124-by-124 pixels. The detailed results obtained from the proof-of-concept experiment with the system will be presented.
Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009
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ABSTRACT: We report experimental results on a 1.5 MW, 110 GHz, 3 microsecond pulsed gyrotron with a single-stage depressed collector. A simplified mode converter with smooth mirror surfaces has been installed in the tube. The converter was designed with the code SURF3D. We present the hot and cold test results of the internal mode converter. The hot and cold test measurements show good agreement.
Infrared, Millimeter, and Terahertz Waves, 2009. IRMMW-THz 2009. 34th International Conference on; 10/2009
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ABSTRACT: The theory, design, and experimental results of a wideband 140-GHz 1-kW pulsed gyro-traveling-wave amplifier (gyro-TWA) are presented. The gyro-TWA operates in the HE <sub>06</sub> mode of an overmoded quasi-optical waveguide using a gyrating electron beam. The electromagnetic theory, interaction theory, design processes, and experimental procedures are described in detail. At 37.7 kV and a 2.7-A beam current, the experiment has produced over 820 W of peak power with a -3-dB bandwidth of 0.8 GHz and a linear gain of 34 dB at 34.7 kV. In addition, the amplifier produced a -3-dB bandwidth of over 1.5 GHz (1.1%) with a peak power of 570 W from a 38.5-kV 2.5-A electron beam. The electron beam is estimated to have a pitch factor of 0.55-0.6, a radius of 1.9 mm, and a calculated perpendicular momentum spread of approximately 9%. The gyro-amplifier was nominally operated at a pulselength of 2 mus but was tested to amplify pulses as short as 4 ns with no noticeable pulse broadening. Internal reflections in the amplifier were identified using these short pulses by time-domain reflectometry. The demonstrated performance of this amplifier shows that it can be applied to dynamic nuclear polarization and electron paramagnetic resonance spectroscopy.
IEEE Transactions on Electron Devices 06/2009; · 2.32 Impact Factor
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ABSTRACT: We report on the design and cold test validation of an overmoded TWT capable of producing power in excess of 100 Watts in the W-band and above. The TWT operates in the TM<sub>31</sub> mode of a rectangular cavity and has transverse dimensions three times larger than a conventional ladder TWT. Dielectric loading of a resonant cavity was utilized to suppress lower order modes and prevent parasitic oscillations. HFSS and MAGIC3D codes were used to predict performance. An X-Ku band scaled down version of the interaction structure was built and cold tests performed on it showed excellent agreement with HFSS simulations.
Vacuum Electronics Conference, 2009. IVEC '09. IEEE International; 05/2009
